ALBERT

Description: Two mafic eruptive products from Vesuvius, a tephrite and a trachybasalt, have been crystallized in the laboratory to constrain the nature of primitive Vesuvius magmas and their crustal evolution. Experiments were performed at high temperatures (from 1000 to ≥1200°C) and both at 0·1 MPa and at high pressures (from 50 to 200 MPa) under H 2 O-bearing fluid-absent and H 2 O- and CO 2 -bearing fluid-present conditions. Experiments started from glass except for a few that started from glass plus San Carlos olivine crystals to force olivine saturation. Melt H 2 O concentrations reached a maximum of 6·0 wt % and experimental f O 2 ranged from NNO – 0·1 to NNO + 3·4 (where NNO is nickel–nickel oxide buffer). Clinopyroxene (Mg# up to 93) is the liquidus phase for the two investigated samples; it is followed by leucite for H 2 O in melt 〈3 wt %, and by phlogopite (Mg# up to 81) for H 2 O in melt 〉3 wt %. Olivine (Fo 85 ) crystallized spontaneously in only one experimental charge. Plagioclase was not found. Upon progressive crystallization of clinopyroxene, glass K 2 O and Al 2 O 3 contents strongly increase whereas MgO, CaO and CaO/Al 2 O 3 decrease; the residual melts follow the evolution of Vesuvius whole-rocks from trachybasalt to tephrite, phonotephrite and to tephriphonolite. Concentrations of H 2 O and CO 2 in near-liquidus 200 MPa glasses and primitive melt inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was reconstructed by the olivine-added experiments. They show that the primitive Vesuvius melts are trachybasalts (K 2 O ~ 4·5–5·5 wt %, MgO = 8–9 wt %, Mg# = 75–80, CaO/Al 2 O 3 = 0·9–0·95) that crystallize Fo-rich olivine (90–91) as the liquidus phase between 1150 and 1200°C and from 300 to 〈200 MPa. Primitive Vesuvius melts are volatile-rich (1·5–4·5 wt % H 2 O and 600–4500 ppm CO 2 in primitive melt inclusions) and oxidized (from NNO + 0·4 to NNO + 1·2). Assimilation of carbonate wall-rocks by ascending primitive magmas can account for the disappearance of olivine from crystallization sequences and explains the lack of rocks representative of olivine-crystallizing magmas. A correlation between carbonate assimilation and the type of feeding system is proposed: carbonate assimilation is promoted for primitive magma batches of small volumes. In contrast, for longer-lived, large-volume, less frequently recharged, hence more evolved, cooler reservoirs, magma–carbonate interaction is limited. Primitive magmas from Vesuvius and other Campanian volcanoes have similar redox states. However, the Cr# of Vesuvius spinels is distinctive and therefore the peridotitic component in the mantle source of Vesuvius differs from that of the other Campanian magmas.

Description: Peridotite xenoliths exhumed by Quaternary alkaline magmatism in the Tahalgha district, southern Hoggar, represent fragments of the subcontinental lithospheric mantle beneath the boundary between the two major structural domains of the Tuareg Shield: the ‘Polycyclic Central Hoggar’ to the east and the ‘Western Hoggar’, or ‘Pharusian Belt’, to the west. Samples were collected from volcanic centres located on both sides of a major lithospheric shear zone at 4°35' separating these two domains. Although showing substantial variations in their deformation microstructures, equilibrium temperatures and modal and chemical compositions, the studied samples do not display any systematic changes of these features across the 4°35' fault. The observed variations rather record small-scale heterogeneities distributed throughout the study area and reflecting the widespread occurrence of vein conduits and metasomatized wall-rocks related to trans-lithospheric melt circulation during the Cenozoic. These features include partial annealing of pre-existing deformation microstructures, post-deformation metasomatic reactions, and trace-element enrichment, coupled with heating from 750–900°C (low-temperature lherzolites) to 900–1150°C (intermediate- T lherzolites and high- T harzburgites and wehrlites). Trace-element modelling confirms that the range of rare earth element (REE) variations observed in the Tahalgha clinopyroxenes may be accounted for by reactive porous flow involving a single stage of basaltic melt infiltration into a light REE (LREE)-depleted protolith. Whole-rock compositions record the final entrapment of disequilibrium metasomatic melts upon thermal relaxation of the veins–wall-rock system. The striking correlations between equilibrium temperatures and trace-element enrichment favor a scenario in which the high-temperature peridotites record advective heat transport along melt conduits, whereas the intermediate- and low-temperature lherzolites reflect conductive heating of the host Mechanical Boundary Layer. This indicates that the lithosphere did not reach thermal equilibrium, suggesting that the inferred heating event was transient and was rapidly erased by thermal relaxation down to the relatively low-temperature present-day geotherm. The low- T (〈900°C) deformed lherzolites (porphyroclastic to equigranular) are characterized by only incipient annealing and LREE-depleted clinopyroxene compositions. They were only weakly affected by the Cenozoic events and could represent relatively well-preserved samples from rejuvenated Pan-African lithosphere. Extensive lithospheric rejuvenation occurred either regionally during the Pan-African orogeny, as a result of lithospheric delamination or thermomechanical erosion after thickening, or more locally along the meridional shear zones. The low- T Tahalgha lherzolites are comparable with lherzolites from Etang de Lherz, southern France, interpreted as lithospheric mantle rejuvenated by melt-induced refertilization during a late stage of the Variscan orogeny.

Description: Hydrothermal experiments were conducted at 200 MPa and 900–1018°C to determine the solubilities, fluid(s)–melt partitioning, and mixing properties of H 2 O, CO 2 , S, Cl, and F in phonolitic–trachytic melts saturated in vapor, vapor plus saline liquid, or saline liquid. The bulk compositions and S, Cl, and F concentrations of the run-product glasses were determined by electron microprobe and the H 2 O and CO 2 contents by Fourier-transform infrared spectroscopy (FTIR). A new parameterization was developed to calculate molar absorption coefficients for FTIR analysis of carbonate in glasses and applied to the run-product glasses. The concentrations of volatiles in the fluid(s) were determined by mass-balance calculations and checked with chloridometer analysis and gravimetry. The range in oxygen fugacity of these experiments is NNO to NNO + 2 (where NNO is nickel–nickel oxide buffer). The phonolitic–trachytic melts dissolved up to 7·5 wt % H 2 O, 0·94 wt % Cl, 0·73 wt % CO 2 , 0·75 wt % F, and 0·16 wt % S, and the integrated bulk fluid(s) contained up to 99 mol % H 2 O, 34 mol % Cl, 82 mol % CO 2 , 1·7 mol % F, and 3·7 mol % S. The mixing relationships of H 2 O, CO 2 , and Cl in melt versus fluid(s) are complex and strongly non-ideal at these pressure–temperature conditions, particularly with two fluid phases stable. The concentrations of H 2 O and CO 2 in melt change with the addition of Cl ± S to the system, and the solubility of Cl in melt varies with S. The reductions in H 2 O and CO 2 solubility in melt exceed those resulting from simple dilution of the coexisting fluid(s) owing to addition of other volatiles. The partitioning of H 2 O and CO 2 between fluid(s) and melt varies as a function of fluid(s) and melt composition. The experimental data are applied to phonolitic and related magmas of Mt. Somma–Vesuvius, Italy, Mt. Erebus, Antarctica, and Cripple Creek, USA, to better interpret processes of fluid(s) exsolution in eruptive and mineralizing systems. Application of the experimental results also provides constraints on eruptive and mineralizing fluid(s) compositions.

Description: The early stages of magmatic processes operating at mantle depths beneath continental arcs are poorly known. The chemical compositions of minerals and rocks, mineral Sr–Nd–Hf–O isotopes and zircon U–Pb ages of garnet clinopyroxenite dykes from the Shenglikou peridotite massif (North Qaidam Orogen, NE Tibet, China) were studied to constrain their sources and genesis, and the dynamic processes that controlled pyroxenite formation beneath an early Paleozoic active continental margin. Major-element compositions of bulkrocks suggest that the pyroxenitic protoliths were cumulates segregated from a melt, which was extracted from a peridotite-dominated mantle source. Bulk-rock and mineral trace-element patterns show strong enrichment in fluid-mobile elements (e.g. Cs, Rb, Ba, Th, U, K, Pb and Li) and marked negative anomalies in the high field strength elements relative to rare earth elements, similar to the characteristics of melts derived from a volatile-rich sub-arc mantle. Enriched Sr and Nd initial isotopic compositions at 500 Ma ( 87 Sr/ 86 Sr of 0·70919–0·71774 and Nd of –16·3 to –3·4) are in contrast to the highly radiogenic Hf isotope compositions (similar to those of the depleted-mantle reservoir) and to the uncontaminated upper-mantle 18 O V-SMOW (garnet: 5·6 ± 0·3, 2SD, n = 61; zircon: 5·9 ± 0·3, 2SD, n = 28). These decoupled isotopic signatures suggest that the melt source was located in a convective mantle wedge (controlling the Hf and O isotopes) that had been pervasively metasomatized by fluids from a subducted Proto-Tethys oceanic slab (controlling the Sr–Nd isotopes and highly incompatible elements). Zircons with two groups of U–Pb ages (430 ± 5 Ma and 401 ± 7 Ma) were generated by recrystallization events, corresponding to UHP metamorphism and a major uplift stage during the North Qaidam orogeny, respectively. The combined evidence reveals a picture of continental arc magmatism at mantle depths and subsequent continental collision. The subduction of the Proto-Tethys oceanic slab beneath the southern Qilian margin triggered flux melting of the metasomatized convective mantle wedge and generated hydrous arc magmas. These primitive magmas intruded into the overlying lithospheric mantle and segregated the cumulates parental to the Shenglikou pyroxenites. Subsequent continental subduction incorporated fragments of the mantle-wedge peridotite (containing pyroxenite dykes) at ~430 Ma and carried them to shallow depths during exhumation at ~400 Ma.

Description: Using a new high-resolution dataset, this study presents evidence for short length scale 18 O/ 16 O heterogeneity in the mantle source region of young (age 12 ka bp ) Icelandic basalts. The dataset comprises secondary ion mass spectrometry determinations of 18 O/ 16 O in single compositional zones of plagioclase crystals from the primitive Borgarhraun flow in northern Iceland, along with trace and major element data from the same zones. The presence of mantle under Iceland with 18 O below typical mid-ocean ridge basalt (MORB) values of ~5·5 ± 0·3 (VSMOW) has previously been disputed, because variability in 18 O in many Icelandic basalts is also known to be caused by the interaction of basaltic melts with crustal lithologies that have been altered by low- 18 O meteoric water. Primitive basalt flows, such as Borgarhraun, and their macrocrysts are the most likely candidates to retain a mantle 18 O signature. However, the role of crustal processes in generating the low 18 O in olivine crystals from these flows has not unequivocally been ruled out. By making intra-crystal analyses in Borgarhraun plagioclase it has been possible in this study to obtain a detailed record of the chemical and isotopic compositions of the melts that crystallized the plagioclase zones. The variability observed in trace element compositions of the early crystallized anorthitic plagioclase zones (80·9–89·4 mol % anorthite) is firstly shown to arise from melt compositional variability, and equilibrium melt concentrations of Sr, La and Y are then calculated from the crystal concentrations of these elements using carefully selected partition coefficients. The ranges of incompatible trace element ratios (La/Y, Sr/Y) in these equilibrium melts reflect a range of compositions of fractional mantle melts, a result that is in agreement with previous proposals for the cause of variability in trace element indices of Borgarhraun olivine-hosted melt inclusions and clinopyroxene compositional zones. Correlations observed between La/Y and Sr/Y in the melts in equilibrium with the Borgarhraun plagioclase zones and the 18 O of these zones therefore support the hypothesis that the mantle under Iceland is heterogeneous in 18 O/ 16 O. Such correlations have not previously been observed in intra-crystal data from Iceland, and provide strong evidence that mantle material with abnormally low 18 O may exist in the form of readily fusible heterogeneities alongside ambient mantle with MORB-like 18 O (+5·5) on a length scale of 〈100 km. The lowest 18 O of plagioclase that is attributed to a mantle origin in this study is 4·5 ± 0·4, equating to a melt equivalent value of 4·3 ± 0·5 or an olivine equivalent value of 3·8 ± 0·5.

Description: Mafic to ultramafic intrusions of the Qullinaaraaluk suite (Q-suite) were emplaced into the Ungava craton of the Northeastern Superior Province during an episode of intense igneous activity and crustal reworking from c. 2·74 to 2·70 Ga. Orthopyroxene-rich Q-suite intrusions from the Hudson Bay Terrane and southwestern Rivière Arnaud Terrane, and orthopyroxene-poor Q-suite intrusions from the north–central Rivière Arnaud Terrane indicate the existence of at least two Q-suite magma types: a subalkaline magma parental to the orthopyroxene-rich intrusions and a transitional magma parental to the orthopyroxene-poor intrusions. Both types of intrusions are characterized by light rare earth element (LREE)-enriched, high field strength element (HFSE)-depleted trace element profiles that reflect, in large part, contamination by the tonalite–trondhjemite–granodiorite-dominated crust. Near-chondritic to strongly sub-chondritic initial Nd (2·72 Ga) values (+2 to –10) of the Q-suite intrusions reflect the combined effects of both the amount of crustal contamination and the age-dependent isotopic composition of the contaminant. The inferred trace element profiles of the uncontaminated Q-suite magmas were probably flat to LREE-depleted. The transitional magmas that produced the least evolved dunitic cumulates of the Q-suite were ferropicrites (MgO ~14 wt %, FeO TOT ~17 wt %). In contrast, the magmas parental to the primitive Q-suite harzburgites were Fe-rich, high-Mg basalts (MgO ~11 wt %; FeO ~14 wt %). The high Fe contents of the Q-suite magmas are incompatible with derivation from a pyrolitic mantle [Mg-number ~0·90, Mg/(Mg + Fe TOT )] and require sources significantly enriched in iron (Mg-number ≤0·79). Both magma types are also characterized by relatively low Ni contents suggesting derivation from source regions depleted in Ni relative to pyrolitic mantle peridotite. Differences in the major element compositions of the subalkaline and transitional parental magmas may reflect compositional diversity among the Fe-rich mantle sources. Comparisons with melting experiments on compositions analogous to the Martian mantle suggest that the Q-suite magmas may rather be generated by different degrees of melting of a common source with an Fe content slightly lower than that of the Homestead L5 ordinary chondrite (Mg-number = 0·77). The Fe-rich picritic to high-Mg basaltic magmas last equilibrated with garnet-free harzburgitic to lherzolitic residues at upper mantle pressures (≤5 GPa). The craton-wide occurrence of c. 2·72–2·70 Ga Q-suite mafic to ultramafic plutons suggests that underplating by Fe-rich mantle melts may have had a key role in the c. 2·74–2·70 Ga cratonization of the Northeastern Superior Province.

Description: Arc basalts are more oxidized than mid-ocean ridge basalts, but it is unclear whether this difference is due to differentiation processes in the Earth’s crust or to a fundamental difference in the oxygen fugacity of their mantle sources. Distinguishing between these two hypotheses is important for understanding redox-sensitive processes related to arc magmatism, and thus more broadly how Earth materials cycle globally. We present major, volatile, and trace element concentrations in combination with Fe 3+ /Fe ratios determined in olivine-hosted glass inclusions and submarine glasses from five Mariana arc volcanoes and two regions of the Mariana Trough. For single eruptions, Fe 3+ /Fe ratios vary along liquid lines of descent that are either slightly oxidizing (olivine + clinopyroxene + plagioclase fractionation, CO 2 ± H 2 O degassing) or reducing (olivine + clinopyroxene + plagioclase ± magnetite fractionation, CO 2 + H 2 O + S degassing). Mariana samples are consistent with a global relationship between calc-alkaline affinity and both magmatic H 2 O and magmatic oxygen fugacity, where wetter, higher oxygen fugacity magmas display greater affinity for calc-alkaline differentiation. We find, however, that low-pressure differentiation cannot explain the majority of variations observed in Fe 3+ /Fe ratios for Mariana arc basalts, requiring primary differences in magmatic oxygen fugacity. Calculated oxygen fugacities of primary mantle melts at the pressures and temperatures of melt segregation are significantly oxidized relative to mid-ocean ridge basalts (~QFM, where QFM is quartz–fayalite–magnetite buffer), ranging from QFM + 1·0 to QFM + 1·6 for Mariana arc basalts, whereas back-arc related samples record primary oxygen fugacities that range from QFM + 0·1 to QFM + 0·5. This Mariana arc sample suite includes a diversity of subduction influences, from lesser influence of a homogeneous H 2 O-rich component in the back-arc, to sediment melt- and fluid-dominated influences along the arc. Primary melt oxygen fugacity does not correlate significantly with sediment melt contributions (e.g. Th/La), nor can it be attributed to previous melt extraction in the back-arc. Primary melt oxygen fugacity correlates strongly with indices of slab fluids (e.g. Ba/La) from the Mariana Trough through the Mariana arc, increasing by 1·5 orders of magnitude as Ba/La increases by a factor of 10 relative to mid-ocean ridge basalts. These results suggest that contributions from the slab to the mantle wedge may be responsible for the elevated oxygen fugacity recorded by Mariana arc basalts and that slab fluids are potentially very oxidized.

Description: Magma mixing and crystal mush disaggregation are important processes in basaltic magma reservoirs. We carried out a detailed petrological and geochemical study on a highly plagioclase-phyric eruption within the Eastern Volcanic Zone of Iceland—the Skuggafjöll eruption—to investigate crystal storage and transport processes within a single magmatic system. Crystal content and phase proportions vary between samples: the least phyric samples have phase proportions similar to the low-pressure, three-phase gabbro eutectic (plg:cpx:ol ~ 11:6:3), whereas highly phyric samples are strongly enriched in plagioclase (plg:cpx:ol ~ 8:1:1). Statistically significant geochemical variability in 28 whole-rock samples collected across the eruption can be accounted for by variable accumulation of a troctolitic assemblage containing plagioclase and olivine in an approximately 9:1 ratio. Two macrocryst assemblages are defined using compositional and textural information recorded in QEMSCAN® images: a primitive assemblage of high-anorthite plagioclase (An 〉83 ) and high-forsterite olivine (Fo 〉84 ), and an evolved assemblage of low-anorthite plagioclase (An 〈79 ), low-forsterite olivine (Fo 〈82 ) and clinopyroxene (Mg# ~ 82). Plagioclase and olivine have strongly bimodal composition distributions whereas the composition distribution of clinopyroxene is unimodal. The mean trace element composition of melt inclusions hosted within high-forsterite olivine and high-anorthite plagioclase macrocrysts is the same (mean Ce/Y ~ 0·47–0·48), confirming that both primitive macrocryst phases crystallized from the same distribution of melts. Clinopyroxene macrocrysts and matrix glasses are in Ce/Yb equilibrium with each other, indicating that the evolved assemblage crystallized from melts with a more incompatible trace element-enriched composition (mean Ce/Y ~ 0·65–71) than the primitive assemblage. Variability in whole-rock, macrocryst and melt inclusion compositions suggests that the Skuggafjöll magma experienced two stages of crystallization. Primitive macrocrysts crystallized first from incompatible trace element-depleted melts within a shallow crustal magma reservoir. These primitive macrocrysts were subsequently stored in crystal mushes that ultimately disaggregated into an evolved and incompatible trace element-enriched magma from which the evolved assemblage crystallized. On average, ~17% of the erupted magma at Skuggafjöll is composed of accumulated macrocrysts entrained from crystal mushes. The timescale between mush disaggregation and eruption, during which crystal accumulation occurred, was short—of the order of years—according to simple diffusion calculations. Striking petrological similarities between Skuggafjöll and other highly phyric eruptions both in Iceland and along mid-ocean ridges indicate that crystal accumulation by mush disaggregation is likely to be an important mechanism for generating highly phyric magmas in basaltic plumbing systems.

Description: Studies of both naturally quenched and experimentally reheated melt inclusions have established that they can lose or gain H 2 O after entrapment in their host mineral, before or during eruption. Here we report nanoSIMS analyses of H 2 O, Cl and F in olivine around melt inclusions from two natural basaltic samples: one from the Sommata cinder cone on Vulcano Island in the Aeolian arc and the other from the Jorullo cinder cone in the Trans-Mexican Volcanic Belt. Our results constrain olivine/basaltic melt partition coefficients and allow assessment of mechanisms of volatile loss from melt inclusions in natural samples. Cl contents in olivine from both samples are mostly below detection limits (≤0·03 ± 0·01 ppm), with no detectable variation close to the melt inclusions. Assuming a maximum Cl content of 0·03 ppm for all olivines, maximum estimates for Cl partition coefficients between olivine and glass are 0·00002 ± 0·00002. Olivines from the two localities display contrasting H 2 O and F compositions: Sommata olivines contain 27 ± 11 ppm H 2 O and 0·28 ± 0·07 ppm F, whereas Jorullo olivines have lower and proportionately more variable H 2 O and F (11 ± 12 ppm and 0·12 ± 0·09 ppm, respectively; uncertainties are two standard deviations for the entire population). The variations of H 2 O and F contents in the olivines exhibit clear zonation patterns, increasing with proximity to melt inclusions. This pattern was most probably generated during transfer of volatiles out of the inclusions through the host olivine. H 2 O concentration gradients surrounding melt inclusions are roughly concentric, but significantly elongated parallel to the crystallographic a-a xis of olivine. Because of this preferential crystallographic orientation, this pattern is consistent with H 2 O loss that is rate-limited by the ‘proton–polaron’ mechanism of H diffusion in olivine. Partition coefficients based on olivine compositions immediately adjacent to melt inclusions are 0·0007 ± 0·0003 for H 2 O and 0·0005 ± 0·0003 for F. The H 2 O and F diffusion profiles most probably formed in response to a decrease in the respective fugacities in the external melt, owing to either degassing or mixing with volatile-poor melt. Volatile transport out of inclusions might also have been driven in part by increases in the fugacity within the inclusion owing to post-entrapment crystallization. In the case of F, because of the lack of data on F diffusion in olivine, any interpretation of the measured F gradients is speculative. In the case of H 2 O, we model the concentration gradients using a numerical model of three-dimensional anisotropic diffusion of H, where initial conditions include both H 2 O decrease in the external melt and post-entrapment enrichment of H 2 O in the inclusions. The model confirms that external degassing is the dominant driving force, showing that the orientation of the anisotropy in H diffusion is consistent with the proton–polaron diffusion mechanism in olivine. The model also yields an estimate of the initial H 2 O content of the Sommata melt inclusions before diffusive loss of 6 wt % H 2 O. The findings provide new insights on rapid H 2 O loss during magma ascent and improve our ability to assess the fidelity of the H 2 O record from melt inclusions.

Description: Ulleung Island is the top of a 3000 m (from sea floor) intraplate alkalic volcanic edifice in the East Sea/Sea of Japan. The emergent 950 m consist of a basaltic lava and agglomerate succession (Stage 1, 1·37–0·97 Ma), intruded and overlain by a sequence of trachytic lavas and domes, which erupted in two episodes (Stage 2, 0·83–0·77 Ma; Stage 3, 0·73–0·24 Ma). The youngest eruptions, post 20 ka bp, were explosive, generating thick tephra sequences of phonolitic composition (Stage 4), which also entrained phaneritic, porphyritic and cumulate accidental lithics. Major element chemistry of the evolved products shows a continuous spectrum of trachyte to phonolite compositions, but these have discordant trace element trends and distinct isotopic characteristics, excluding a direct genetic relationship between the two end-members. Despite this, the Stage 3 trachytes and some porphyritic accidental lithics have chemical characteristics transitional between Stage 2 trachytes and Stage 4 phonolites. Within the phonolitic Stage 4 tephras three subgroups can be distinguished. The oldest, Tephra 5, is considerably enriched in incompatible elements and chondrite-normalized rare earth element (REE) patterns display negative Eu anomalies. The later tephras, Tephras 4–2, have compositions intermediate between the early units and the trachyte samples, and their REE patterns do not have significant Eu anomalies. The last erupted, Tephra 1, from a small intra-caldera structure, has a distinct tephriphonolite composition. Trace element and isotopic chemistry as well as textural characteristics suggest a genetic relationship between the phaneritic lithics and their host phonolitic pumices. The Stage 4 tephras are not related to earlier phases of basaltic to trachytic magmatism (Stages 1–3). They have distinct isotopic compositions and cannot be reliably modelled by fractional crystallization processes. The differences between the explosive phonolitic (Stage 4) and effusive trachytic (Stage 2–3) eruptions are mainly due to different pre-eruptive pressures and temperatures, causing closed- versus open-system degassing. Based on thermodynamic and thermobarometric modelling, the phonolites were derived from deeper (subcrustal) magma storage and rose quickly, with volatiles trapped until eruption. By contrast, the trachytes were stored at shallower crustal levels for longer periods, allowing open-system volatile exsolution and degassing before eruption.

Description: Mantle polymict breccias sampled by kimberlite magmas are complex mixtures of mantle minerals and rock clasts, cemented together by olivine, phlogopite, orthopyroxene, ilmenite, rutile and sulphides. Because of the kimberlite-like texture (i.e. mineral clasts of diverse origin and composition set in a magmatic matrix) and the large geochemical heterogeneity preserved in polymict breccias, these rocks are believed to derive from primitive or precursor kimberlite magmas. Therefore, the study of such xenoliths can provide constraints on the processes occurring in the mantle during the early stages of kimberlite ascent, and possibly on the composition of kimberlite melts. To constrain the petrogenesis of these unusual rocks, we have studied two samples of polymict breccia from the Bultfontein kimberlite (Kimberley, South Africa) and compared our results with published data for other polymict breccias. The most abundant phase in the matrix of the studied samples is olivine with a narrow range in Mg# (~88–89), but variable Ni–Mn–Ca contents. Similar compositions are characteristic of magmatic olivine in the Bultfontein and nearby De Beers kimberlites. Orthopyroxene is the dominant phase in the matrix of polymict breccias surrounding clinopyroxene clasts, which, like the other silicate mineral clasts, are highly resorbed. The matrix orthopyroxene exhibits variable compositions, with significant enrichment in Ca, Na, Cr, Sr, Ba and light rare earth elements in the grains adjacent to clinopyroxene. The other main matrix phases (phlogopite, ilmenite and rutile) also display variable compositions. Matrix olivine hosts primary carbonate-rich inclusions similar to those observed in polymict breccia ilmenite. These inclusions were previously interpreted as an alkali-carbonate melt trapped during ilmenite growth. This alkali-carbonate melt may represent the parental melt to the matrix minerals of the polymict breccias. The variable composition of the matrix minerals is attributed to rapid, small-scale (centimetre to millimetre) variations in the melt composition owing to clast dissolution, possibly coupled with wall-rock assimilation, closely followed by fast cooling. Partial digestion of silicate porphyroclasts increased the Si content of the matrix melt, thus allowing crystallization of orthopyroxene. Further arguments in favour of a genetic relationship between polymict breccias and kimberlite magmas are provided by (1) similar Hf isotope compositions of polymict breccia ilmenite and South African kimberlites, (2) overlapping olivine compositions in polymict breccias and the host Bultfontein kimberlite, and (3) the occurrence of alkali-carbonate inclusions in polymict breccia and kimberlite minerals. Polymict breccias are interpreted as failed kimberlite intrusions, which metasomatized the magmatic conduit through which subsequent pulses of kimberlite magmas ascended. These wall-rock interactions would limit reactions between later pulses of kimberlite melt and mantle wall-rocks, thus enhancing the ability of kimberlite magmas to reach the surface.

Description: High-Mg diorites enriched in incompatible elements and their extrusive equivalents are rare subduction-related rock types that have been found in modern arc settings and in Late Archean sequences, where they are associated with trondhjemite–tonalite–granodiorite (TTG) suites. Archean rocks with these geochemical features are known as sanukitoids and, despite their limited abundance, are considered to be the indicators of the onset of modern plate tectonics because of their similarities to modern subduction-related high-Mg andesites and diorites. Understanding the genesis of sanukitoid rocks is thus an essential step towards understanding crustal growth processes. The accepted petrogenetic models for modern, enriched, high-Mg andesites and their Archean equivalents, the sanukitoids, consist of metasomatic enrichment of the mantle wedge by slab components and its subsequent partial melting, or the modification of siliceous slab components through continuous reaction with mantle peridotite during their ascent through the mantle wedge. We present new data on the petrography, mineral chemistry and whole-rock geochemistry (major and trace elements and Sr–Nd–Pb isotopes) of andesitic rocks from an ~30 ka lava flow (Yuyos flow) from the Chacana Caldera Complex, Eastern Cordillera of Ecuador. These rocks show a remarkable geochemical affinity with Archean sanukitoids, including high magnesium numbers (0·58–0·63) accompanied by high contents of incompatible elements (e.g. Th 17–23 ppm, U 6–7·5 ppm, Ba 1600–1800 ppm, Sr 1430–1565 ppm, La 74–94 ppm). Additionally, the sanukitoid-like andesites of Yuyos are associated with predominant silica-rich (adakite-like) andesites, which are widespread throughout the Quaternary arc of Ecuador. This makes the Quaternary Ecuadorian magmatic province a close equivalent of the Archean TTG–sanukitoid association. The bulk-rock geochemistry, petrography and mineral chemistry data indicate that the sanukitoid-like features of the andesites of the Yuyos flow derive from intracrustal recycling of the felsic–intermediate to mafic–ultramafic roots of the Quaternary volcanic arc of Ecuador by ‘normal’ mantle-derived basaltic magmas with the geochemical characteristics of continental arc basalts or high-alumina basalts. In view of the similarities between the Yuyos andesites and Archean sanukitoids in terms of geochemistry and lithological association, we suggest that genetic models should consider the possibility of intracrustal recycling as a process responsible for the peculiar signatures of both Archean sanukitoids and modern enriched high-Mg andesites.

Description: Multichronometric analyses were performed on samples from a transect in the French–Italian Western Alps crossing nappes derived from the Briançonnais terrane and the Piemonte–Liguria Ocean, in an endeavour to date both high-pressure (HP) metamorphism and retrogression history. Twelve samples of white mica were analysed by 39 Ar– 40 Ar stepwise heating, complemented by two samples from the Monte Rosa nappe 100 km to the NE and also attributed to the Briançonnais terrane. One Sm–Nd and three Lu–Hf garnet ages from eclogites were also obtained. White mica ages decrease from c. 300 Ma in the westernmost samples (Zone Houillère), reaching c. 300°C during Alpine metamorphism, to 〈48 Ma in the internal units to the east, which reached c. 500°C during the Alpine orogeny. The spatial pattern of Eocene K–Ar ages demonstrates that Si-rich HP white mica records the age of crystallization at 47–48 Ma and retains Ar at temperatures of around 500°C. Paleocene–early Eocene Lu–Hf and Sm–Nd ages, recording prograde garnet growth before the HP peak, confirm eclogitization in Eocene times. Petrological and microstructural features reveal important mineralogical differences along the transect. All samples contain mixtures of detrital, syn-D 1 and syn-D 2 mica, and retrogression phases (D 3 ) in greatly varying proportions according to local variations in the evolution of pressure–temperature–fluid activity–deformation ( P–T–a–D ) conditions. Samples from the Zone Houillère mostly contain detrital mica. The abundance of white mica with Si 〉 6·45 atoms per formula unit increases eastward. Across the whole traverse, phengitic mica grown during HP metamorphism defines the D 1 foliation. Syn-D 2 mica is more Si-poor and associated with nappe stacking, exhumation, and hydrous retrogression under greenschist-facies conditions. Syn-D 1 phengite is very often corroded, overgrown by, or intergrown with, syn-D 2 muscovite. Most importantly, syn-D 2 recrystallization is not limited to S 2 schistosity domains; micrometre-scale chemical fingerprinting reveals muscovite pseudomorphs after phengite crystals, which could be mistaken for syn-D 1 mica based on microstructural arguments alone. The Cl/K ratio in white mica is a useful discriminator, as D 2 retrogression was associated with a less saline fluid than eclogitization. As petrology exerts the main control on the isotope record, constraining the petrological and microstructural framework is necessary to correctly interpret the geochronological data, described in both the present study and the literature. Our approach, which ties geochronology to detailed geochemical, petrological and microstructural investigations, identifies 47–48 Ma as the age of HP formation of syn-D 1 mica along the studied transect and in the Monte Rosa area. Cretaceous apparent mica ages, which were proposed to date eclogitization by earlier studies based on conventional ‘thermochronology’, are due to Ar inheritance in incompletely recrystallized detrital mica grains. The inferred age of the probably locally diachronous, greenschist-facies, low-Si, syn-D 2 mica ranges from 39 to 43 Ma. Coexistence of D 1 and D 2 ages, and the constancy of non-reset D 1 ages along the entire transect, provides strong evidence that the D 1 white mica ages closely approximate formation ages. Volume diffusion of Ar in white mica (activation energy E = 250 kJ mol –1 ; pressure-adjusted diffusion coefficient D' 0 〈 0·03 cm 2 s –1 ) has a subordinate effect on mineral ages compared with both prograde and retrograde recrystallization in most samples.

Description: Cerro Uturuncu is a long-dormant, compositionally monotonous, effusive dacitic volcano in the Altiplano–Puna Volcanic Complex (APVC) of SW Bolivia. The volcano recently gained attention following the discovery of an ~70 km diameter Interferometric Synthetic Aperture Radar (InSAR) anomaly roughly centred on its edifice. Uturuncu dacites, erupted over the past ~1 Myr, invariably have a phase assemblage of plagioclase–orthopyroxene–biotite–ilmenite–magnetite–apatite–zircon and rhyolite glass. To better constrain storage conditions of the dacite magmas and to help understand their relationship with the observed deformation, petrological experiments were performed in cold-seal hydrothermal vessels. Volatile-saturated ( P H2O = P TOTAL and P H2O + P CO2 = P TOTAL ) phase equilibria experiments were run between 50 and 250 MPa and 760 and 900°C at f O 2 ~ Ni–NiO. Two synthetic starting compositions were investigated based on a typical Uturuncu dacite whole-rock and its rhyolitic groundmass glass. Pre-eruptive magma storage conditions have been estimated by comparing results from the experiments with natural phase assemblages, modes, and mineral and glass compositions. H 2 O-saturated experiments constrain storage pressures to 100 ± 50 MPa, equivalent to 1·9–5·7 km below surface. In the dacite, natural phase assemblages are reproduced at 870°C, 100 MPa with both orthopyroxene and biotite stabilized concurrently. Natural glass chemistries are most closely replicated at 50 MPa at 870°C, reflecting the role of decompression crystallization prior to eruption. In H 2 O-saturated rhyolite experiments the natural phase assemblage is most closely replicated at 870°C, 50 MPa. Isothermal, mixed volatile dacite experiments at 870°C further constrain storage pressures to 110 ± 10 MPa. Assuming that there has been no dramatic change in the eruptive behaviour of Uturuncu in the last 270 kyr, pre-eruptive storage of dacite magmas at ~100 MPa precludes their role in producing the large diameter deformation anomaly. If deformation is a result of magmatic intrusion, then intrusion of less evolved magmas into deeper, mid-crustal storage regions is a more probable explanation. Intrusion within the Altiplano–Puna Magma Body (APMB), the extent of which is roughly coincident with the APVC, is most likely. It is proposed that dacite magmas form from andesitic parents, via fractionation and/or assimilation, within the APMB. Dacites then rise buoyantly to shallow storage levels where they stall and crystallize prior to eruption. Microlites form during subsequent ascent from the storage region to the surface.

Description: Major and trace element and Sr–Nd–Pb isotope data for whole-rocks and major element data for minerals within basalt samples from the Chugaryeong volcano, an intra-plate back-arc volcanic centre in the central part of the Korean Peninsula, are used to address the process of magma genesis in the deep back-arc region of eastern Asia. There are two lava flow units at Chugaryeong volcano: the Chongok (0·50 Ma) and the Chatan (0·15 Ma) basalts. These basalts have similar MgO (9·1–10·4 wt %) but exhibit differences in their major and trace element and isotope compositions. The Chongok basalt has higher TiO 2 , Al 2 O 3 , Na 2 O, K 2 O, P 2 O 5 , Cr 2 O 3 , large ion lithophile elements (LILE), high field strength elements (HFSE), and rare earth elements (REE), and lower FeO*, SiO 2 , and CaO than the Chatan basalt. In addition, the Chongok basalt has more radiogenic 143 Nd/ 144 Nd and 206 Pb/ 204 Pb, and less radiogenic 87 Sr/ 86 Sr and 208 Pb/ 204 Pb than the Chatan basalt. Chi-square tests for the major elements indicate that crystal fractionation can explain the chemical variations within each basalt suite; intra-crustal processes, including crystal fractionation and assimilation of continental crust, cannot result in the formation of one basalt suite from the other. The Sr–Nd–Pb isotopic compositions of the Chongok and Chatan basalts plot on mixing hyperbolae between peridotite mantle xenoliths from the area and a fluid flux derived from a mixture of ancient and recent sediments. The trace element compositions of the estimated primary melts for the two basalt suites suggest different degrees of partial melting of a common enriched mantle source that was metasomatized by a Ba-, K-, Pb-, and Sr-rich fluid. The estimated degree of melting increased with time from ~7·5% for the Chongok basalt to ~10% for the Chatan basalt. The source mantle for the Chatan basalt is more enriched in Ba and Pb, indicating a greater fluid flux than for the Chongok basalt. This suggests that melting of the source mantle increased with time, sustained by an increased sediment-derived fluid flux from the deeper upper mantle.

Description: At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750–850°C and pressures 〈3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ~800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth’s mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting.

Description: A novel application of X-ray micro-computed tomography is described, which can be used to rapidly characterize chemical populations of natural olivine crystals in erupted basalts. This technique can be deployed during volcanic crises to directly track changes in magma components of an erupting system in near-real-time. Such changes are fundamental in controlling eruption style, duration and intensity. We demonstrate a method that can generate data from hundreds of crystals within hours, which allows time-series petrological data to be recorded and interpreted alongside various complementary monitoring techniques (e.g. seismicity, ground deformation). Our direct-detection will allow greater understanding of the dynamics of sub-volcanic magma plumbing systems, and can provide important insights into how an eruption may evolve. The same technique can also be used to generate rich baseline datasets from eruption sequences in the geological record in a more efficient manner than conventional methods allow.

Description: Field evidence from the Baladiyah complex in the northern part of the Arabian–Nubian Shield of Saudi Arabia indicates several erosional unconformities separating different high- and medium-grade metasedimentary sequences. This suggests that the collision between East and West Gondwana involved several cycles of exhumation and burial, providing a unique opportunity to study the multiple stages of this orogeny. A mineral equilibria approach and thermodynamic modeling are used to place constraints on the formation conditions of each of these cycles. It is shown that the complex is characterized by three regional metamorphic events followed by a fourth metamorphic event related to shear heating owing to the thrusting of post-tectonic granites. During the first metamorphic event peak metamorphism was at around 705–715°C and 5·2–5·6 kbar followed by subsequent decompression to the Earth’s surface. Subsequently deposited sediments attained 635–670°C and 4·2–5 kbar during a second metamorphic event, followed by exhumation, erosion and deposition of molasse sediments. The rocks were then buried for a third time and metamorphosed to greenschist-facies metamorphic conditions (330 ± 30°C and 3·6–4·6 kbar) under the load of the molasse sediments. Finally, post-tectonic granites were intruded and thrust during the final Pan-African exhumation, causing a fourth metamorphic event (700 ± 25°C). Correlation of this metamorphic evolution with the deformation history shows that the first and the second metamorphic events occurred in a compressional regime (D 1 and D 2 ), interpreted to be related to the first (750 Ma) and the second (676 Ma) collision stages between East and West Gondwana, respectively. The third deformation phase began with a compressional regime causing the third metamorphic event, and then turned into an oblique transpressive regime, which led to escape tectonics and the development of the large-scale Najd strike-slip shear zone system.

Description: Two types of blue halo (types I and II) composed of blueschist-facies minerals are centered around a brittle, normal shear zone in greenschist-facies rocks on the island of Syros, Aegean Sea, Greece. The shear zone is steeply dipping and cuts a near-horizontal layer of greenschist-facies rocks (albite + epidote + actinolite + chlorite + quartz). Type I and II blue haloes are 0·3 m and c . 1 m wide respectively, and are seen on both sides of the shear zone. The inner type I haloes are composed of nearly pure glaucophane schist and were formed by metasomatic addition of Na 2 O and SiO 2 , and to a lesser extent of K 2 O and large ion lithophile elements (LILE), coupled with loss of CaO, Al 2 O 3 and MnO. The outer type II haloes consist of a carbonated blueschist-facies assemblage (glaucophane + calcite + phengite + epidote + garnet + quartz). These experienced only slight metasomatic changes (i.e. addition of K 2 O and LILE), which cannot alone explain halo formation. We present petrological, geochemical and thermodynamic evidence that this assemblage was preserved at greenschist-facies conditions because X CO2 was elevated by flow of a CO 2 -bearing fluid along the shear zone, which was approximately contemporaneous with greenschist-facies hydration in the surrounding rocks. We further note that the flux of CO 2 -bearing fluid along the shear zone was rapid with respect to the fluid flux in the surrounding rocks. Mass-balance calculations reveal that the fluid flux within the shear zone was at least 100–2000 times greater than the fluid flux within the surrounding rocks. Mineral textures show greenschist-facies minerals replacing blueschist minerals in the type II haloes, supporting our interpretation that the blueschist-facies minerals were preserved during greenschist-facies retrogression. A simplified P–T vs X CO2 pseudosection confirms that preservation of carbonated blueschist can occur at greenschist-facies conditions in the presence of a CO 2 -bearing fluid.

Description: The c. 673 Ma (U–Pb rutile) Amon kimberlites located in northern Baffin Island intruded Late Archean basement rocks of the Rae craton as a subhorizontal sill complex. The Amon sills are part of widespread low-volume, volatile-rich ultramafic magmatic province that occurred along the northern and eastern margins of Laurentia, demarcating the temporal and spatial breakout from the Rodinia supercontinent during the Late Neoproterozoic. Numerous other known kimberlite forward that are related to these rifting events between c. 680 and 540 Ma are located in mainland Nunavut, Ontario, Quebec, Labrador, and West Greenland. The magmas that fed the Amon sills are archetypal Group-I kimberlites, based on groundmass mineralogy (e.g. phlogopite, spinel, ilmenite) and bulk-rock compositions, including moderately depleted Sr–Nd–Hf isotope ratios. However, a wide compositional range, together with observed flowage textures, indicates that some magma differentiation occurred during sill emplacement. The Amon samples that are interpreted as parental kimberlite magma compositions overlap published compositions of experimentally derived, near-solidus partial melts of carbonated peridotite between 5 and 10 GPa; that is, equivalent to an origin from 150 to 300 km depth. Furthermore, the Amon kimberlites are characterized by moderately depleted Nd ( Nd(i) = +1·5 to +3·5) and Hf ( Hf(i) = +1·1 to +8·7) isotope compositions, without pronounced isotope decoupling as known from other kimberlite occurrences worldwide. Among the studied Late Neoproterozoic volatile-rich ultramafic magmatic rocks in Laurentia, the Amon kimberlites have Nd–Hf isotope systematics that are similar to those of a previously identified, carbonate-rich, depleted end-member component. This common component is suggested to represent a widespread near-solidus partial melt of volatile-fluxed fertile peridotite within the uppermost convecting mantle beneath the rifting supercraton. Our preferred model for Late Neoproterozoic kimberlite and related magmatism along the rifted margins of Laurentia invokes a combination of redox- and decompression-related low-degree partial melting of convecting upper mantle material that flows beneath rugged topography at the base of thick continental lithosphere. Provided that carbonate metasomatism of lower cratonic mantle is ubiquitous on a global scale, we argue that proto-kimberlitic melt is likely to be constantly present beneath the cratonic roots of supercontinents, and that it is most efficiently extracted during fast and changing plate motions, such as during the assembly and break-up of supercontinents. This idea is supported by the known kimberlite emplacement patterns of the Gondwana–Pangea (510 Ma–Recent) and Rodinia (1300–550 Ma) supercontinent cycles, but it remains difficult to test for older kimberlites and related rocks dating back to c. 3 Ga.

Description: Zircon, monazite, garnet and feldspar trace element microanalysis, zircon and monazite U–Pb isotopic ages and monazite Th–U–Pb chemical dating, determined in situ from a leucogranitic vein in migmatitic gneiss at Kulappara in the Kerala Khondalite Belt (Trivandrum Block), India, demonstrate accessory mineral crystallization from an evolving melt-bearing system from c. 545 to 535 Ma at temperatures of greater than 780°C. Marked changes in zircon chemistry to lower Th/U and Yb n /Gd n occurred during its growth in the evolving melt, correlated with a dramatic change in microtexture from initial ‘hopper’-like feathered-core and outer planar sector domains to darker planar zones and elliptical to lobate infilling and replacement zircon. Initially high Th/U, high heavy rare earth element (HREE) zircon crystallized rapidly from melt from 544 ± 9 Ma under open-system conditions in which the host-rock mineralogy had no chemical impact. This zircon precipitated prior to crystallization of significant garnet, trapping melt as inclusions that later crystallized to cryptogranite within zircon cores. Further zircon crystallization ensued under localized closed-system conditions leading to the establishment of zircon–garnet REE equilibrium at least on local (i.e. millimetre to centimetre) scales, consistent with melt entrapment, at 542 ± 6 Ma. Monazite crystallized in this fractionated melt by 535 ± 6 Ma. These results demonstrate that zircon can be a sensitive indicator of changing conditions and scales of melt transfer and interaction in high-temperature migmatites, recording in this instance a transition from melt-dominated open-system behaviour to closed-system crystallization and mineral–melt interaction at T 〉 780°C in the deep crust of a hot orogenic belt.

Description: Felsic metagranitoids form a major part of the crust, but the metamorphic story they record is difficult to decipher because of a lack of index minerals. The microstructures and metamorphic assemblages of felsic gneisses and metadolerite dykes from the Lewisian Gneiss complex, NW Scotland, have been examined to estimate the pressure–temperature–time (P–T–t) history of the region. Characteristic geometries and compositions of zoned epidote and plagioclase from the gneisses and amphibole from the dykes provide key information. Bulk-rock compositions are modelled to constrain the likely metamorphic conditions experienced by the rocks. P–T–t paths are refined using a novel model for fractionation during grain-recycling of the plagioclase. In the gneisses, plagioclase grains have relatively albitic cores (An 10–12 ) grading to more anorthitic rims (An 20–30 ). The equant grain shapes of the plagioclase and asymmetry of the zoning across grain boundaries are consistent with the zoning having formed during coarsening, or grain-recycling, following deformation. The increase in anorthite content is due to the breakdown of epidote to release Ca and Al. Sharp boundaries between Fe-poor cores and Fe-rich rims in epidote result from the resorption of epidote whilst the plagioclase is growing followed by later regrowth. The possible P–T conditions for the end of deformation and start of grain-recycling are restricted to those that occur along the plagioclase isopleth with the same value as the core compositions. These starting conditions are explored along with P–T–t path orientations over a range of values. The results are compared with the observed compositions and grain sizes to determine the best-fit P–T–t path. Most of the best-fit paths are dominated by decompression rather than heating (both of which result in epidote breakdown). Starting conditions are probably between 9 and 10·2 kbar at around 580°C along a quasi-linear path ending at around 7·5–8 kbar at 600–620°C. The timescale of decompression is poorly constrained owing to the uncertainty in the grain-recycling parameters. Rates of exhumation are between 0·14 and 2 mm a –1 , which are reasonable within the range of present-day processes. Scourie dyke assemblages and mineral zoning broadly corroborate this P–T–t path. The path is similar to those recorded in Phanerozoic orogenic cycles but the significance of this work lies in our new methods for elucidating the metamorphic histories of metagranitoids.

Description: The role of water in the uppermost mantle has been explored to 6 GPa (~200 km) by a novel experimental approach in which the silicate melting solidus, the stability of hydrous phases and the H 2 O contents in nominally anhydrous minerals (NAMs) were determined. The composition studied is a fertile lherzolite modelled as a source for mid-ocean ridge basalts (MORB). The use of crushed olivine as traps for melt or fluid inclusions allows a distinction to be made between quenched hydrous silicate melt and quench material from water-rich vapour phase. The vapor-saturated solidus (water-rich vapor) of fertile lherzolite increases in temperature ( T ) from a minimum of 970°C at 1·5 GPa (~50 km) to 1375°C at 6 GPa. The Ca-rich amphibole pargasite is stable to the vapour-saturated solidus to 3 GPa (~100 km). Based on normative components, at 2·5 GPa the near-solidus melt (1–2%) in mantle with very low H 2 O content is transitional between sodic–dolomitic carbonatite and olivine melilitite. With higher melt fraction (~5%) at higher T or higher H 2 O content it is olivine-rich basanite. Both immediately below and above the solidus, the H 2 O content in residual lherzolite is ~200 ppm retained in NAMs at 2·5 and 4 GPa. The experimentally determined vapour-saturated solidus corrects recent numerical models of melting of lherzolite + H 2 O based on inferred high solubilities of H 2 O in NAMs and accounts for a discrepant experimental determination of the vapour-saturated solidus in which very high water/rock ratios were used. At 2·5 ± 0·1 GPa, the water content of experimental charges was varied from 0·05 to 14·5 wt %. Below the solidus and with increasing water content from 0·05 to 2·9 wt %, pargasite decreases in K 2 O and Na 2 O content and is absent in experiments with 7·25 and 14·5 wt % H 2 O. Also with increasing water content from 0·05 to 14·5 wt % H 2 O, the Na 2 O content of clinopyroxene decreases from 1·6 wt % to below the limit of detection (0·2 wt %). The destabilization of pargasite and change of clinopyroxene composition at 2·5 GPa and 1000°C are attributed to the leaching role (Na 2 O and K 2 O particularly) of the water-rich vapour at high water/rock ratios. The hydrous mineral pargasite is the major site of H 2 O storage in fertile uppermost mantle lherzolite but pargasite is unstable at pressures ( P ) 〉3 GPa (~100 km depth), causing a sharp drop in the water storage capacity of the upper mantle from 〉2000 to ~200 ppm. For small H 2 O contents (〈2000 ppm approximately), the temperature of the vapour-undersaturated solidus of fertile upper mantle lherzolite decreases sharply with increasing P at ~90 km depth. The negative d T /d P for the vapour-undersaturated solidus has important rheological and geodynamic consequences. In oceanic intraplate settings, the geotherm passes from subsolidus pargasite-bearing lherzolite to garnet lherzolite with incipient melting, creating the rheological boundary at ~90 km depth, between lithosphere and asthenosphere. The asthenosphere becomes geochemically zoned with the ‘enriched’ intraplate basalt source (〉500 ppm H 2 O) overlying the ‘depleted’ MORB source (~200 ppm H 2 O) in the deeper asthenosphere. Water also plays a significant role at convergent margins, where hydrous silicate melting in the mantle wedge is initiated at the vapour-saturated solidus. Melting of lherzolite at or near the vapour-saturated solidus does not fully dehydrate residual lherzolite or harzburgite. Residual lithosphere returned to the upper mantle may carry ~100–200 ppm H 2 O. At 6 GPa the low K/Na model mantle composition (MORB-source mantle) with 〉200 ppm H 2 O has normal rather than supercritical melting behaviour with the solidus at 1375°C, which is ~350°C below the C + H-free solidus.

Description: Graciosa is the westernmost island of the Terceira Rift, located to the east of the Mid-Atlantic Ridge, in the Azores archipelago. The three volcanic complexes recognized in the island have been characterized, including the study of lava flows, gabbroic xenoliths (alkaline and subalkaline), syenites and dunites. A detailed study of mineral compositions and whole-rock major and trace element concentrations has allowed the characterization of volcanic units and the investigation of the magmatic and temporal evolution of the island. Lava flows from the three volcanic complexes, alkaline gabbros (cumulate origin) and syenites (bulk trachyte liquid origin) can be related by polybaric fractional crystallization starting at ~15 km depth; to explain the alternation of mafic and evolved products with time, magma chamber replenishment by primitive melts is needed. In contrast, the origin of the subalkaline gabbros, found for the first time in the Azores, appears to be related to fractionation of highly refractory melts at shallower levels (~3 km). The lack of significant crustal assimilation in most Graciosa samples suggests that the range in Sr–Nd–Pb isotope compositions probably reflects those of a predominantly FOZO-like mantle source. Enriched (EMI- and EMII-type) mantle source components, similar to those of São Miguel and Faial, have also been recognized for the first time in Graciosa.

Description: Chemical compositions of hydrous melts, compatible with those that would form by incipient melting of upper mantle peridotite at 180 km depth, have been determined using a series of iterative crystallization experiments. Experiments were performed in a multianvil apparatus at 6 GPa and 1400°C and a melt was ultimately produced that was saturated in a residual peridotite assemblage (olivine + clinopyroxene + garnet ± orthopyroxene). The multiply saturated hydrous melts have higher (Mg + Fe)/Si and Al/Ca compared with hydrous melts produced at lower pressures. The melt compositions are similar to those determined near the dry peridotite solidus at ~1700°C, when compared on an H 2 O-free basis. Melt H 2 O contents were determined to be ~11 wt % using mass balance, and these estimates were made more accurate by maintaining a large proportion of melt (〉70 wt %) in each experiment. If the geophysically inferred seismic low-velocity zone is caused by the presence of H 2 O-rich melt then at the base of this zone, at ~220 km, these results imply that the melt must contain 15–16 wt % H 2 O. The hydrous melt compositions, when compared on a volatile-free basis, are found to be similar to those of group II kimberlites (orangeites). The low FeO and Na 2 O but enriched K 2 O concentrations in group II magmas imply their derivation from melt-depleted cratonic lithosphere enriched by the metasomatic addition principally of K 2 O and H 2 O. A simple model is proposed in which this enrichment occurs by the addition of phlogopite to the source peridotite. Using determined K 2 O and H 2 O partition coefficients and assuming that the ratio of both components in the source is controlled by their ratio in phlogopite, group II kimberlite magmas can be constrained as being the product of ~0·2 wt % melting of a garnet peridotite source rock enriched with 1·7 wt % phlogopite, undergoing melting at near-adiabatic temperatures.

Description: The last Plinian-type eruption of Volcán de Colima, Mexico, occurred in 1913; this resulted in the removal of the top 100 m of the edifice and the deposition of a tephra layer that blanketed the slopes of the Colima Volcanic Complex (CVC). Road-cuts on the flanks of the nearby Nevado de Colima edifice expose pre-1913 air-fall tephra, pyroclastic flow and ash-rich surge deposits resulting from numerous highly explosive events throughout the Holocene. The majority of the pumice and scoria fallout deposits are medium-K subalkaline basaltic andesite and andesite in composition, defining a clear major element differentiation trend. In contrast, three newly discovered scoria fallout deposits are high-K subalkaline, transitional to alkaline, basaltic andesite in composition and are characterized by the presence of phlogopite; these deposits have high MgO (up to 7·9 wt %), K 2 O (up to 2·6 wt %) and P 2 O 5 (up to 0·67 wt %) contents. They are also strongly enriched in fluid- and melt-mobile large ion lithophile elements (LILE; Rb, Ba, K, Sr and Th) and light REE (LREE; La, Ce, Pr and Nd) relative to the majority of the Colima tephra fallout deposits. Strontium and Nd isotope systematics reveal that the high-K mafic scoria have more radiogenic Sr ( 87 Sr/ 86 Sr = 0·70365–0·70408) and less radiogenic Nd ( 143 Nd/ 144 Nd = 0·51279–0·51294) compared with the majority of the subalkaline tephras ( 87 Sr/ 86 Sr = 0·70338–0·70371 and 143 Nd/ 144 Nd = 0·51290–0·51295). Two-component mixing models, using whole-rock geochemical data, indicate the importance of magma mixing in the petrogenesis of the Colima magmas, with addition of up to 50% by volume of an alkaline mafic magma component in the most potassic magmas. This is supported by mineral chemistry and textural data, which reveal multiple episodes of decompression and magma mingling within a shallow crustal magma storage region. The presence of these potassic tephra fall deposits among the otherwise prevailing medium-K subalkaline stratigraphy indicates that pulses of K-rich alkaline mafic magmas periodically enter the CVC plumbing system on timescales of a few thousand years and may trigger Plinian explosive eruptions.

Description: Olivine-hosted melt inclusions from both Fernandina and Santiago islands in the Galapagos Archipelago have compositions indicating that plagioclase played an important role in the magmatic evolution of these volcanic islands. The major and trace element chemistry of the Santiago melt inclusions indicates simple plagioclase assimilation. In contrast, Fernandina inclusions have compositions for which the plagioclase appears to be present only as a ‘ghost’ trace element signature (i.e. ‘ghost plagioclase’ signature). Two competing hypotheses have been proposed to explain this unique signature: (1) incorporation of an ancient recycled plagioclase-rich cumulate into the mantle; (2) shallow-level interaction between melts and plagioclase-rich cumulates in the present-day lower oceanic crust. Here we present new Pb isotope measurements for olivine-hosted melt inclusions from Fernandina and Santiago islands to distinguish between the two models. The new Pb isotope data are within the range previously reported for whole-rock basalts from those islands. Melting and mixing models involving ancient (~0·5–1 Ga) recycled plagioclase-rich cumulates cannot reproduce the observed trace element and Pb isotopic characteristics of the Fernandina melt inclusions with a ghost plagioclase signature. Shallow-level diffusive interactions between basalt and present-day plagioclase-rich cumulates provide the simplest explanation for the observed trace element compositions and Pb isotope ratios of melt inclusions from Fernandina and Santiago islands.

Description: To constrain effects of chloride-bearing H 2 O–CO 2 fluids on complex natural assemblages during high-grade metamorphism and anatexis, we report the results of experiments on the interaction of biotite–hornblende tonalitic gneiss from the Sand River Formation (Limpopo Complex, South Africa) with H 2 O–CO 2 , H 2 O–CO 2 –KCl, H 2 O–CO 2 –NaCl, and H 2 O–CO 2 –(K, Na)Cl fluids at 550 MPa, 750 and 800°C, and varying chloride/(H 2 O + CO 2 ) ratios with molar CO 2 /(CO 2 + H 2 O) = 0·5. Heating of solid cylinders of gneiss at both temperatures in the absence of a free fluid phase produced no changes in the gneiss phase assemblage. The equimolar H 2 O–CO 2 fluid at 750°C also did not significantly influence the phase assemblage. Addition of KCl to the fluid at 750°C resulted in formation of the clinopyroxene + K-feldspar (+ ilmenite/titanite) assemblage after biotite, hornblende and plagioclase. Orthopyroxene accompanied by amphibole appeared only at 800°C as a result of biotite breakdown in the presence of H 2 O–CO 2 and low-salinity H 2 O–CO 2 –KCl fluids. Increase in the KCl content in the fluid at 800°C resulted in the production of a clinopyroxene-bearing assemblage. Increase of the NaCl content stabilized amphibole in an assemblage with either orthopyroxene (at low NaCl concentrations) or clinopyroxene. Nevertheless, clinopyroxene (+ albite) is stable only at high salt concentrations. Comparison of the experimental results with the results of thermodynamic modeling using the Gibbs free energy minimization method (PERPLE_X software) showed that mineral reactions and assemblages in the run products were governed by the activities of alkali components imposed by KCl and NaCl in the H 2 O–CO 2 fluids, and decrease of the water activity served as an additional factor stabilizing anhydrous assemblages. No melts formed at 750°C in the presence of the H 2 O–CO 2 –KCl fluids. These fluids provoked melting only at 800°C with formation of rhyolitic melts. With increasing KCl content of the fluid, the melt composition changed to potassic rhyolitic with Al 2 O 3 〈 13·5 wt %, CaO 〈 2 wt %, K 2 O + Na 2 O 〉 7 wt %, FeO/(FeO + MgO) 〉 0·8, K 2 O/Na 2 O 〉 1, and moderate enrichment in Cl (0·2–0·6 wt %). Increasing NaCl content caused melting at 750°C and shifted the melt composition towards trachytic and trachyandesitic compositions at both 750 and 800°C. The experiments support a model for the formation of ferroan A-type granite–syenite complexes via crustal melting in the presence of H 2 O–CO 2 –salt fluids in extensional tectonic settings. They demonstrate a possible link between A-type granitoids and mid-crustal dehydration zones in amphibolite- to granulite-facies terrains and allow a new interpretation of mineral assemblages within these zones in terms of variations in fluid salinity.

Description: Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi wehrlite intrusion is composed mainly of olivine (Fo69–75), clinopyroxene (Mg# = 75–84), intercumulus plagioclase (An53–86) and Fe–Ti oxides. Both petrography and mineral compositions suggest that olivine and clinopyroxene crystallized earlier than plagioclase and Fe–Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35–39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive Nd i values (4·3–4·8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi wehrlite intrusion and dykes. The Xiaohaizi wehrlite intrusion is characterized by Sr–Nd isotopic disequilibrium between clinopyroxene and plagioclase separates: 87 Sr/ 86 Sr i (0·7038–0·7041) and Nd i (1·0–1·9) of clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase ( 87 Sr/ 86 Sr i = 0·7042–0·7043, Nd i = 0·4–1·0). The 87 Sr/ 86 Sr i and Nd i of clinopyroxene separates correlate positively and negatively with Zr/Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi wehrlite intrusion. 87 Sr/ 86 Sr i increases and Nd i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and Nd i of clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean–Neoproterozoic basement and carbonates as contaminants.

Description: Olivine textures are potentially important recorders of olivine origin and crystallization conditions. Primary magmatic and xenocrystic origins are commonly considered for olivine from ultramafic to intermediate magmas, whereas secondary olivine origins (i.e. crystals formed by recrystallization or peritectic reaction) are rarely considered in the interpretation of magmatic phenocrysts. The main aim of our study was to determine textures that are characteristic for secondary magmatic olivine and non-characteristic, or at least rare, for primary magmatic olivine. To characterize the textures of the different olivine types, we review previous experimental work and present new textural data for olivine from four melanorite melting and eight basalt crystallization experiments. We qualitatively characterize olivine textures using transmitted light and back-scattered electron microscopy, and semi-quantitatively characterize the 2D surface area of olivine branches and single crystals, their 2D and calculated 3D shapes, and 2D grain boundary segment lengths. Olivine recrystallization yields crystals with dendritic branches, whereas peritectic reaction produces olivine clusters with randomly oriented crystals. In agreement with previous studies, we find that olivine crystal and branch size, grain boundary segmentation, and inclusion relations cannot unequivocally distinguish between olivine of secondary and primary origins. However, recrystallized olivine typically has short prismatic branches, whereas primary magmatic olivine dendrites commonly have elongated branches. The peritectic crystals are closely comparable with glomerocrysts and single polyhedral primary magmatic crystals, but they commonly form small groups of touching crystals and clusters with crystal-poor cores. Spatial association and a comparison with xenocryst textures may further guide the interpretation of their origin, but detailed analyses of core compositions and zoning patterns appear necessary to firmly distinguish peritectic from primary magmatic crystals. Our comparison of experimental and natural samples suggests that dendritic olivine crystals and clusters of polyhedral olivine that commonly occur in mafic and ultramafic igneous rocks should be evaluated for possible primary as well as secondary magmatic origins.

Description: Troctolitic intrusive rocks poor in augite are common in certain Proterozoic anorthosite complexes and related rocks. The Lower Zone of the Kiglapait intrusion, Labrador, consists of ~1570 km 3 troctolite today and possibly ~2900 km 3 before erosion. In this augite-free Lower Zone, plagioclase fractionation is as low as 1·6% An km –1 of cumulate thickness and averages 3% An km –1 . When augite crystallizes after 84% of the magma has crystallized, the fractionation becomes as much as 17% An km –1 of cumulate. Why such a difference? It is clear from first principles of phase equilibria that fractionation accelerates with saturation in augite, but not so clear that the difference should be so great. The answer is to be found in the silica-poor nature of troctolitic magma that is critically undersaturated in silica. This low-silica effect reduces the activity of the NaSi albite component relative to the CaAl anorthite component in the plagioclase, thereby favoring the An component of the liquid and crystals and weakening the fractionation process. As the normative augite component in the magma rises from the base of the Lower Zone to the base of the Upper Zone, the activity of silica also rises slightly and its consequent effect on plagioclase composition tends to diminish. Liquid fractionation paths derived from observed crystal paths, when plotted in the system Diopside–Anorthite–Albite, rise across the liquidus fractionation lines toward diopside and reach augite saturation near the 1 atm cotectic. They produce plagioclase compositions 10 mol % higher in An than pure liquidus fractionation lines predict. The key criterion for the troctolitic fractionation of plagioclase composition is the absence of Ca-poor pyroxene from the rocks. Noritic magmas, by contrast, have higher activities of silica and more effective fractionation of plagioclase. A parallel fractionation of olivine is also retarded in the Lower Zone by the accumulation of ferric iron in the liquid until augite and titanomagnetite crystallize in the Upper Zone.

Description: Oxidation state is a sensitive indicator of geochemical processes within the upper mantle. Here we report results of a regional study of the oxidation state of spinel peridotite xenoliths from 45 volcanic centers distributed over ~20 000 km 2 in the Massif Central, France. The log f O 2 values relative to the fayalite–magnetite–quartz oxygen buffer (FMQ) were determined from the equilibrium between the Fe-bearing components in olivine, orthopyroxene and spinel, with the Fe 3+ content of spinel measured either by Mössbauer spectroscopy or by electron microprobe using secondary spinel standards. For the entire suite of samples, log f O 2 values range between FMQ – 0·47 and FMQ + 1·66. Our data confirm the presence of two distinct lithospheric mantle domains, previously reported in the literature, lying north and south of 45°30'N, respectively. The northern domain, with its refractory bulk composition, tends to record more oxidized conditions, having log f O 2 values mostly at or above FMQ + 1. The log f O 2 in the southern domain is more variable, including values below FMQ. Assuming that increasing equilibration temperatures among xenoliths reflect increasing depths of origin, samples from the northern domain suggest that the shallower part of the subcontinental lithospheric mantle (SCLM) is somewhat more oxidized than at deeper levels. On the other hand, such a general observation cannot be made for the southern domain. The high log f O 2 values of harzburgites suggest that they are more sensitive to resetting of their oxidation state by metasomatism than lherzolites. In terms of modally metasomatized xenoliths, the ‘melt’ leading to the addition of clinopyroxene apparently had a higher oxidation state (log f O 2 〉 FMQ + 1) than the agent responsible for crystallization of amphibole (log f O 2 ~ FMQ + 0·6). Furthermore, amphibole-bearing and amphibole-free peridotites exhibit the same range in f O 2 . Cryptic metasomatism can also reset oxidation state, sometimes very effectively. Metasomatic processes are probably the reason why the xenolith suite from the Massif Central records relatively high log f O 2 values compared with ‘normal’ non-cratonic SCLM. This study demonstrates the utility of using oxidation state to help characterize and delineate domains in the lithospheric mantle.

Description: To constrain the effect of redox state on sulfur transport from subducting crust to mantle wedge during fluid-present melting and the stability of sulfur-bearing phases in the downgoing ocean crust, here we report high-pressure phase equilibria experiments on a H 2 O-saturated mid-ocean ridge basalt with 1 wt % S at variable oxygen fugacity ( ). Double-capsule experiments were conducted at 2·0 and 3·0 GPa and 950–1050°C, using Co–CoO, Ni–NiO, Ni x Pd 1– x –NiO, and Fe 2 O 3 –Fe 3 O 4 external buffers. Sulfur content at sulfide saturation (SCSS) or sulfur content at sulfate saturation (SCAS) of experimental hydrous partial melts was measured by electron microprobe. All experiments were fluid-saturated and produced either pyrrhotite- or anhydrite-saturated assemblages of silicate glass, clinopyroxene, garnet, and rutile or titanomagnetite, ± amphibole ± quartz ± orthopyroxene. The silicate partial melt composition evolves from rhyolitic at 950°C to trachydacitic and trachyandesitic at 1050°C with increasing . At pyrrhotite saturation, melt S contents range from ~30 ppm S at 〈 FMQ – 1 to ~500 ppm S at FMQ 〈 ≤ FMQ + 1·1, whereas at anhydrite saturation ( ≥ FMQ + 2·5) melt S concentrations range from ~700 ppm S to 0·3 wt % S. Mass-balance calculations suggest that the aqueous fluid phase at equilibrium may contain as much as ~15 wt % S at 1050°C at pyrrhotite saturation ( ≤ FMQ + 1·1), in agreement with previous estimates, and up to 8 wt % S at anhydrite saturation. Our data also show that decreases markedly with increasing at pyrrhotite saturation, from several thousand at 〈 FMQ – 1 to ~ 200–400 at FMQ 〈 ≤ FMQ + 1·1, owing to the increase of melt S content. At anhydrite saturation, is very low (〈100) but increases with decreasing temperature, in an opposite way to previous observations at pyrrhotite saturation. As a consequence, at T ≤ 900°C, might be in the range 200 ± 100, irrespective of . The present study confirms that slab partial melts saturated with pyrrhotite are unable to efficiently transport S from slab to mantle wedge, and suggests that slab partial melts in equilibrium with anhydrite also have very limited power to enrich the mantle wedge in S. Importantly, slab-derived aqueous fluids appear to be efficient vectors for the transport of sulfur from slab to mantle wedge at all . Therefore, S transfer from ocean crust to wedge mantle is not dependent and could take place over a range of conditions, and oxidized slab conditions are not necessarily required to enrich the mantle wedge in S. Finally, depending on the initial amount of sulfur in the slab, the proportion of residual anhydrite and pyrrhotite in the dehydrated slab below the region of formation of arc magmas is likely to be significant and may efficiently be recycled into the deep mantle.

Description: Intra-plate Cenozoic volcanism in Kita-Matsuura, northwestern Kyushu, Japan, shows systematic spatio-temporal changes in geochemistry that can be explained by partial melting followed by melt segregation in a region of upwelling mantle. We have examined the thermal and melting history of the upwelling mantle by quantitatively estimating melt water contents and melting conditions. The water content of a spectrum of primary melts is estimated to range from 0·5 to 1·5 wt % based on a combination of a plagioclase-liquid and olivine-saturated liquid geohygrometers and MELTS calculations. The estimated melt segregation temperature ranges from 1330 to 1500°C, at pressures from 1·7 to 2·8 GPa under hydrous conditions. Melting temperature and pressure decreased with time, whereas the water content of the primary melts increased. Corresponding temporal decreases in high field strength element (HFSE) abundances and HFSE/large ion lithophile element (LILE) ratios require progressive melt extraction and aggregation from a melting mantle with a continuous and gradually increasing input of H 2 O-rich fluid or melt into the melting system. The estimated isotope composition of influxed fluid lies on a mixing line between the sediment and altered oceanic crust of the Philippine Sea Plate, with strong affinity to the sediment composition. Based on the temporal variation of the magmas and the melting model, we propose small-scale upwelling ( c. 70 km in diameter) of a dry mantle peridotite that interacts progressively with the overlying wet mantle wedge. The wet mantle wedge was previously hydrated by fluids from sediments from the subducted Philippine Sea Plate, whereas the deep and dry mantle could have been derived from the mantle beneath the subducted Pacific Plate through a slab window.

Description: Cosigüina volcano, in northwestern Nicaragua, erupted violently on 20–24 January 1835, producing pumice, scoria, ash fall deposits, and pyroclastic flows with a bulk tephra volume of ~6 km 3 . New geochemical data are presented for bulk-rocks, matrix glasses, melt inclusions and minerals from the 1835 deposits and a pre-1835 basaltic andesite tephra, with the aim of shedding light on the magmatic processes and associated timescales that led to the eruption. Our results reveal that the 1835 eruption was fed by a compositionally and thermally zoned magma reservoir situated ~4 km ( P H2O ~100 MPa) beneath the volcano. Small volumes of crystal-poor dacite (〈10 wt % phenocrysts, 63·8–64·8 wt % SiO 2 , ~950°C) and silicic andesite (〈10 wt % phenocrysts, 62·2 wt % SiO 2 , 960–1010°C) were erupted first, followed by relatively crystal-rich andesite (15–30 wt % phenocrysts, 57·4–58·8 wt % SiO 2 , 960–1010°C), which accounts for ~90% of the erupted magma. The pre-1835 basaltic andesite (~20 wt % phenocrysts, 52·4 wt % SiO 2 , 1110–1170°C) represents a mafic end-member for Cosigüina. The major and trace element compositions of the bulk-rocks, melt inclusions and matrix glasses suggest that (1) the pre-1835 basaltic andesite is a plausible parent for the 1835 magmas, (2) the 1835 andesite bulk-rocks do not represent true melts, but instead mixtures of silicic andesite liquid and a component of accumulated crystals dominated by plagioclase, and (3) the silicic andesite and dacite formed from the andesite magma through liquid extraction followed by fractional crystallization. Observed bimodal to trimodal crystal populations are consistent with a multi-stage, polybaric differentiation process, with calcic plagioclase (An 75–90 , An 90–95 ) and magnesian clinopyroxene (Mg# = 67–75), plus olivine and magnetite, forming from mafic andesite, basaltic andesite and basalt in the lower crust. The calcic plagioclase exhibits sieve textures, which may be the result of H 2 O-undersaturated decompression during magma ascent to the upper crust; An 50–65 plagioclase lacking a sieve texture, orthopyroxene (Mg# = 61 and 63–72), clinopyroxene (Mg# = 67), magnetite and apatite crystallized from andesite to dacite liquids in the shallow magma reservoir. An 75–90 plagioclase comprising entire phenocrysts or cores with An 50–65 rims in the 1835 magmas is cognate from earlier stages of differentiation and shows evidence of extensive diffusion of Mg when compared with similar An 75–95 crystals hosted in the pre-1835 basaltic andesite. Using plagioclase–melt Mg partitioning and modelling of the Mg diffusion process, we constrain the residence time of these crystals in the silicic liquids to more than 100 years and less than 2000 years, with detailed analysis of three crystals yielding ~400 years. We propose that magma reservoir zonation occurred on timescales of 10 2 –10 3 years at Cosigüina. The occurrence of H 2 O-rich fluid inclusions in all 1835 samples and volatile element systematics in melt inclusions imply that the magmas were saturated with a vapour phase (H 2 O, S, ± CO 2 ) during much of their evolution in the upper crust. Accumulation of free gas at the top of the magma reservoir may have led to overpressurization of the system, triggering the eruption. Catastrophic release of this exsolved vapour and syn-eruptive devolatilization of the melt injected several teragrams of S into the atmosphere. Our data, coupled with independent evidence from ice cores and tree rings, indicate that the Cosigüina eruption had a sizeable atmospheric impact comparable with or larger than that of the 1991 Pinatubo eruption. Stratigraphic evidence shows that Cosigüina has produced 〉15 compositionally zoned explosive eruptions in the past, suggesting that similar future eruptions are likely. The products of the 1835 eruption of Cosigüina share many features with compositionally zoned eruptive sequences elsewhere, such as the climactic eruption of Mount Mazama, the ad 79 ‘Pompei’ eruption of Vesuvius and the 1912 eruption of Novarupta–Katmai.

Description: Tests of models of melt generation and mantle source variations beneath mid-ocean ridges require a definitive set of mid-ocean ridge basalt (MORB) compositions corrected for shallow-level processes. Here we provide such a dataset, with both single sample and segment means for 241 segments from every ocean basin, which span the entire range of spreading rate, axial depth, and MORB chemical composition. Particular attention is paid to methods of fractionation correction. Values corrected to 8 wt % MgO are robust as they are within the range of the data. Extrapolation to equilibrium with mantle olivine is a non-unique procedure that is critically dependent on the MgO content where plagioclase first appears. MORB data, trace element ratios and calculated liquid lines of descent provide consistent evidence that plagioclase fractionation primarily occurs between 8 and 9 wt % MgO, with the exception of hydrous magmas mainly from back-arc segments. Varying the MgO content of plagioclase appearance over large ranges does not produce the observed systematics at 8 wt % MgO, but may contribute to the spread of the data. Data were evaluated individually for each segment to ensure reliable fractionation correction, and segment means are reported normalized both to MgO of 8 wt % and also to a constant Mg/(Mg + Fe) in equilibrium with Fo 90 olivine. Both sets of corrected compositions show large variations in Na 2 O and FeO, good correlations with segment depth, and systematic relationships among the major elements. A particularly good correlation exists between Al 90 and Fe 90 . These new data are not in agreement with the presentation of Niu & O’Hara (Journal of Petrology 49, 633–664, 2008), whose results relied on an inaccurate fractionation correction procedure, which led them to large errors for high- and low-FeO magmas. The entire dataset is provided in both raw and normalized form so as to have a uniform basis for future evaluations. The new data compilation permits tests of competing models for the primary causes of variations in MORB parental magmas: variations in mantle composition, mantle temperature, reactive crystallization or lithospheric thickness. The principal component of chemical variation among segment mean compositions is remarkably consistent with variations in mantle temperature of some 200°C beneath global ocean ridges. Comparisons with experimental data, pMELTS and other calculations show that variations in mantle fertility at constant mantle potential temperature produce trends that are largely orthogonal to the observations. At the same time, there is clear evidence for mantle major element heterogeneity beneath and around some hotspots and beneath back-arc basins. Super slow-spreading ridges display a characteristic chemical signature of elevated Na 90 and Al 90 and lowered Si 90 relative to faster-spreading ridges. If this signature were produced by reactive crystallization, Si 90 should be higher rather than lower in these environments owing to the thicker lithosphere and lower temperatures of mantle–melt reaction. Instead, the data are consistent with lower extents of mantle melting beneath a thicker lithosphere. Hence, variations in extent of melting appear to be the dominant control on the major element compositions of MORB parental magmas. Trace elements, in contrast, require a large component of mantle heterogeneity, apparent in the factor of 50 variation in K 90 . Such variations do not correlate with the other major elements, showing that major element and trace element (and isotope) heterogeneity reflect different processes. This supports the model of movement of low-degree melts for the creation of trace element and isotope mantle heterogeneity, and is inconsistent with large variations in the amount of recycled crust in most ocean ridge mantle sources.

Description: The Outer Bytownite Gabbros of the Cuillin Centre on the Isle of Skye include a 175 m rhythmically layered sequence consisting of alternating layers of coarse-grained massive gabbro and finer-grained laminated gabbro. We infer that the massive layers formed from repeated influxes of plagioclase-phyric magmas, which flowed into an existing magma chamber and deposited their entrained phenocrysts on the chamber floor. The phenocrysts accumulated rapidly, trapping large amounts of pore liquid in the cumulus pile. During the intervals between influxes, laminated gabbro layers formed by crystallization of the resident magma; these layers accumulated more slowly, by crystal settling or in situ growth, allowing sufficient time for their pore liquids to be expelled by compaction or adcumulus crystallization. The resulting fractions of trapped pore liquid were 〈10% in laminated gabbros and 20–40% in massive gabbros. High porosity in the massive layers is attributable to rapid accumulation of a low-density cumulus assemblage, with trapping of pore liquids facilitated by the confining effects of impermeable laminated cumulates above and below. Among layers, whole-rock Sr and Nd isotope ratios reveal variable degrees of contamination by the Archean crust beneath Skye. In the lower part of the section, influxes of plagioclase-phyric magma are associated with a shift to more primitive mineral compositions, lower 87 Sr/ 86 Sr, and higher 143 Nd/ 144 Nd, indicating that the replenishing magmas were initially more primitive and less contaminated than those already residing in the chamber. In accordance with this interpretation, massive layers have lower 87 Sr/ 86 Sr than laminated layers, consistent with derivation of the plagioclase phenocrysts from a less contaminated source. In the upper part of the section, the shift toward more primitive and less contaminated compositions is reversed, suggesting that over time the replenishing magmas underwent increasing degrees of fractionation and contamination within the magmatic plumbing system that fed the intrusion. These observations and inferences, coupled with published evidence for other isotopic reversals, support a model for growth of the Cuillin Centre by frequent replenishment of a small-volume magma chamber with pulses of magma that underwent complex histories of fractionation and contamination during their transit through the Archean crust; periods dominated by assimilation and fractional crystallization within the feeder system alternated with periods of vigorous recharge by relatively primitive and less contaminated magmas. These cycles were superimposed on an overall trend toward less contamination through time, consistent with increasing dominance of the system by uncontaminated tholeiitic magmas as the Hebridean magmatic province matured.

Description: The Rogart and Strontian high Ba–Sr plutons (Northern Highlands, Scotland) comprise a range of lithologies from felsic to ultramafic rocks. The latter are mantle-derived and their differentiation to produce the felsic components of the plutons is the result of fractional crystallization and variable assimilation of the surrounding Moine metasediments. New results presented here demonstrate that accessory mineral chemistry can provide further insight into their petrogenesis and highlight the petrological potential of apatite and titanite. The main accessory minerals titanite, apatite and zircon contain most of the rare earth elements (REE) in the high Ba–Sr plutons. Results for apatite and titanite show that careful imaging and in situ trace element analysis provide constraints on the petrogenetic history of the host-rock. In both plutons, apatite and titanite record in situ crystallization and fractionation. In Strontian, both apatite and titanite from the granitoids record a mixing event with mafic magma in their rim compositions. Apatite and titanite chemistries are sensitive to the nature of their host-rocks (felsic versus ultramafic) and some elements (e.g. Sr, V) closely reflect whole-rock chemistry and the degree of fractionation. In some cases, whole-rock trace element concentrations can be calculated based on accessory mineral chemistry. Thus, trace elements in accessory minerals can give direct access to the nature and crystallization history of plutonic rocks. This petrological tool may be helpful in provenance studies using accessory minerals, and because high Ba–Sr plutons have recently been equated with Archaean sanukitoids, this might also be important in constraining the temporal distribution of this important magma type.

Description: The Tertiary Skaergaard intrusion, East Greenland, intruded at the shallow crustal unconformity between Precambrian amphibolite-facies gneisses and overlying Tertiary Plateau Basalts. Maximum contact metamorphic temperatures in quartzo-feldspathic gneisses were determined in two sample traverses across the aureole on the western contact of the intrusion using a combination of microstructural observations (both optical and cathodoluminescence) and the titanium-in-quartz (TitaniQ) thermometer. The onset of recrystallization of the quartz in the gneisses occurred between 390 and 340 m from the contact whereas H 2 O-fluxed melting occurred in gneisses closer than 130 m from the contact (where T 〉 ~ 675°C). The maximum temperature recorded by quartz at the contact is ~865 ± 70°C. Melt fractions reach 50–60 vol. % in some samples although the melt is heterogeneously distributed on all scales. Minor bands of amphibolite-facies mafic gneiss are extensively reacted to an anhydrous pyroxene-bearing hornfels close to the contact, whereas those further than ~130 m are overprinted by a greenschist-facies assemblage. Discrepancies between the expected temperature for the amphibolite- to greenschist-facies reaction and temperatures obtained from adjacent quartzo-feldspathic gneisses are consistent with the formation of the anhydrous pyroxene hornfels directly from the mafic gneiss, with the lower-grade greenschist-facies assemblage forming on the retrograde path after the establishment of limited hydrothermal activity. It is unlikely that devolatilization reactions in the gneiss produced sufficient H 2 O to account for the pegmatitic features formed in the Marginal Border Series in the intrusion. A simple one-dimensional thermal model, neglecting any advection of heat by hydrothermal circulation, was fitted to the profile of maximum temperature through the aureole. The generally lower temperatures seen in the gneiss compared with those previously reported for the contact metamorphosed basalts higher up the walls of the intrusion are consistent with a heterogeneous release of latent heat of crystallization.

Description: Two composite dykes containing abundant mafic enclaves within a felsic host crop out in the Maladeta Plutonic Complex, Pyrenees, Spain. Field, petrographic and geochemical criteria reveal mixing between gabbroic and aplitic magmas, giving rise to a variety of hybrid compositions. The rocks contain spongy plagioclase, quartz ocelli and amphibole–biotite clots that are interpreted as early crystals destabilized by reaction with the hybrid melts. Spongy plagioclase and quartz ocelli were mechanically transferred from the felsic to the mafic magma, whereas amphibole–biotite clots are former pyroxene crystals from the mafic magma. Multivariate statistics (principal component analysis) have been used to examine variations in the mineral trace element compositions, which are best explained by the crystal transfer process. This study shows that crystal transfer represents a mixing mechanism that can overcome some of the physical limitations of interaction between rheologically contrasting magmas and explain deviations of the hybrid compositions from the theoretical mixed chemical composition. In particular, hybrid whole-rock compositions show non-linear correlations in inter-element variation diagrams for elements that are enriched or depleted in preferentially transferred crystals. This effect has been quantified by extending magma mixing modelling to include crystal transfer. The composite dykes studied could be regarded as scale models of the behaviour of larger-scale magmatic systems, so this investigation has important implications for interpreting the petrogenesis of igneous suites by magma mixing.

Description: The Taupo Volcanic Zone (TVZ) is well known for its extraordinary rate of rhyolitic magma generation and caldera-forming eruptions. Less is known about how large volumes of rhyolitic magma are extracted and stored prior to eruption, and the role tectonics might play in the process of melt extraction and control of caldera eruption(s). Here we present a new model for the extraction, storage and simultaneous eruption of the 〉245 km 3 paired Mamaku and Ohakuri magmas sourced from calderas centred ~30 km apart (the Rotorua and Ohakuri calderas, respectively) in the central TVZ. The Mamaku and Ohakuri ignimbrites share a similar bulk pumice composition and the same phenocryst assemblage; however, bulk-rock compositions suggest several poorly mixed magma types in each erupted volume, which are randomly distributed throughout the eruptive deposits. To refine models of the pre-eruptive geometry of the magmatic system and discuss a possible origin for triggering of each eruption, we present an expanded database of matrix glass and quartz-hosted melt inclusion compositions along with the existing bulk-rock and mineral compositions. Major and trace element compositions show that the region produced five different magma batches, extracted from the same source region, and a continuous intermediate mush zone beneath the Mamaku–Ohakuri region is suggested here. These magma batches were most probably juxtaposed but isolated from each other in the upper crust, and evolved separately until eruption. The observed geochemical differences between the batches are likely to be generated by different extraction conditions of the rhyolitic melt from a slightly heterogeneous mush. The lack of evidence for more mafic recharge prior to eruption (for example, there are no bright cathodoluminescence rims on quartz crystals) suggests that a magmatic input is unlikely to be an eruption trigger. However, tectonic activity could be an efficient way to trigger the eruption of isolated magma batches, with the evacuation of one magma batch causing a disturbance to the local stress field and activating regionally linked faults, which then lead to the eruption of additional magma batches and associated caldera subsidence. In addition, the extensional tectonic regime coupled with a high heat flux could be the controlling factor in the emplacement of some of the shallowest and most SiO 2 -rich magmas on Earth.

Description: Cumulate and crystal mush disruption and reactivation are difficult to recognize in coarse-grained, shallow plutonic rocks. Mafic minerals included in hornblende and zoned plagioclase provide snapshots of early crystallization and cumulate formation, but are difficult to interpret in terms of the dynamics of magma ascent and possible links between silicic and mafic rock emplacement. This study presents the field relations, the microtextures and the mineral chemistry of the Miocene mafic sill complex of the Torres del Paine intrusive complex (Patagonia, Chile) and its subvertical feeder zone. We summarize a number of observations that occur in structurally different, shallow, plutonic rocks, as follows. (1) The mafic sill complex was built up by a succession of braided sills of shoshonitic and high-K calc-alkaline porphyritic hornblende-gabbro and fine-grained monzodiorite sills. Local diapiric structures and felsic magma accumulation between sills indicate limited separation of intercumulus liquid from the mafic sills. Anhedral hornblende cores, with olivine + clinopyroxene ± plagioclase ± apatite inclusions, crystallized at temperatures 〉900°C and pressures of ~300 to ~400 MPa. The corresponding rims and monzodiorite matrix crystallized at 〈830°C, ~70 MPa. This abrupt compositional variation suggests stability and instability of hornblende during recycling of the mafic roots of the complex and subsequent decompression. (2) The near lack of intercumulus crystals in the subvertical feeder zone layered gabbronorite and pyroxene–hornblende gabbronorite stocks testifies that melt is more efficiently extracted than in sills, resulting in a cumulate signature in the feeding system. Granitic liquids were extracted at a higher temperature ( T 〉950°C) than estimated from the composition of the granite minimum. We show that hornblende–plagioclase thermobarometry is a useful monitor for the determination of the segregation conditions of granitic magmas from gabbroic crystal mushes, and for monitoring the evolution of shallow crustal magmatic crystallization, decompression and cooling .

Description: Multiphase solid inclusions within cumulus silicates, particularly olivine, in Fe–Ti oxide ores from the Lower Zone of the Baima intrusion, Emeishan large igneous province, SW China, have been identified for the first time using 2-D scanning electron microscope and 3-D high-resolution X-ray computed tomography. These inclusions are spherical to subspherical and range from 100 to 300 µm in diameter. They are composed dominantly of titanomagnetite and ilmenite with minor apatite, hornblende, phlogopite and pyrrhotite. The titanomagnetite in the inclusions has a low Cr content (〈700 ppm) similar to the interstitial titanomagnetite, suggesting that these inclusions cannot be early crystallized mineral aggregates. In contrast, the spherical shape of these inclusions provides evidence of early trapped liquids from which these minerals crystallized. Based on the composition and modal proportions of the daughter mineral phases within the inclusions, the trapped liquids are estimated to have 82·1–59·6 wt % FeO T , 11·4–18·5 wt % TiO 2 , 2·69–6·12 wt % Al 2 O 3 , 1·40–4·47 wt % MgO, 0·87–4·93 wt % SiO 2 and ~1 wt % volatiles including F, S, Cl, P and H 2 O. Such a liquid composition deviates substantially from that of the slightly evolved ferrobasaltic magmas inferred to be parental to the Fe–Ti–(V) oxide-bearing mafic–ultramafic intrusions of the Emeishan large igneous province. It is thus speculated that these trapped liquids are immiscible Fe–Ti-rich melts formed upon cooling of ferrobasaltic magma. The net-textured and disseminated oxide ores have titanomagnetite compositions similar to those in the inclusions, suggesting that the oxide ores of the Baima intrusion also formed from Fe–Ti-rich melts immiscibly separated from ferrobasaltic magmas. We propose that immiscible Fe–Ti-rich liquids with high density percolated down through crystal-bearing silicate magma and crystallized an interconnected Fe–Ti oxide network interstitial to olivine, plagioclase and clinopyroxene. This study highlights that immiscible separation of Fe–Ti-rich liquids from ferrobasaltic magmas is an important mechanism in the formation of magmatic Fe–Ti–(V) oxide deposits hosted in mafic–ultramafic layered intrusions.

Description: Mafic to intermediate volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100 000 km 2 Hamersley Basin on the southern margin of the Archaean Pilbara Craton, Western Australia. These represent one of the oldest (2·8–2·7 Ga) known examples of a continental flood basalt sequence. A regional burial metamorphic gradient extends across the basin from prehnite–pumpellyite facies in the north to epidote–actinolite greenschist facies in the south and west. Superimposed on this metamorphic gradient, regional-scale metasomatism has affected extensive areas of the Fortescue Group. Metasomatized mafic lavas are characterized by well-developed assemblages dominated by pumpellyite–quartz or epidote–quartz associations. The mineral associations of metasomatic domain types broadly match the distribution of metamorphic isograd indicator minerals with a southward and westward increase in the proportion of epidote. A continuum exists between least altered rocks that preserve the regional metamorphic signature and the most intensely altered metasomatized rocks. Metasomatism is essentially continuous over a stratigraphic strike length of 100 km and across a strike width of 20 km. Regionally, metasomatically altered volcanic rocks occur widely across the Hamersley Basin and its outliers, over an area of some 450 km by 200 km. Metasomatic alteration is most conspicuous in the lower-grade metamorphic zones because pumpellyite- and epidote-rich rocks are green to yellow–green in outcrop. Whole-rock geochemical data indicate that metasomatism is associated with strong depletions in alkalis (Na, K, Li, Rb), alkali earths (Mg, Sr, Be, Ba) and heavy first transition series metals (Mn, Fe, Co, Ni, Cu, Zn), with a significant enrichment in Si. Calcium shows more variable mobility. Such geochemical trends, particularly depletions in Fe, Mn and base metals (Co, Ni, Cu, Zn), together with the metasomatic mineral assemblages, are comparable with those associated with the sub-sea-floor circulation of seawater, particularly in relation to the metal-depleted root zones of base metal deposits. Petrographic features indicate that the development of metasomatic mineral associations post-dates the formation of regional metamorphic assemblages. Consequently, it is interpreted that the hydrothermal fluid flowed through the buried pile after sufficient time to allow for the metamorphic mineral assemblages to approach equilibrium with ambient P–T conditions. The hydrothermal depletion in Fe observed across the Fortescue Group mafic lavas is intriguing given the abundance of iron ore in the overlying Hamersley Group, and a possible connection cannot be ignored. Furthermore, the scale of fluid flow observed in the Fortescue Group, which occurred through zones of inherent permeability, such as vesicular and brecciated lava flow tops, without the aid of major cross-cutting structures, has significant implications for the size of metasomatic systems in other mafic volcanic terranes, with potential consequences for the exploration for hydrothermal mineral deposits.

Description: The 3 km long Bellevue Core was recovered from the Upper Zone (UZ) and Main Zone (MZ) of the Northern Limb of the Bushveld Complex—the largest known layered mafic intrusion. We present abundance data for 57 trace elements hosted in plagioclase, clinopyroxene, low-Ca pyroxene and olivine, obtained by laser ablation inductively coupled plasma mass spectrometry. These data permit the first comprehensive investigation of trace element distribution in the Bushveld Complex on a mineral-by-mineral basis, and show that the Cr content of pyroxene is the only proxy that records three cryptic magma injections, coincident with two pyroxenite horizons and the UZ–MZ boundary. On the basis of Cr abundance, we infer that an ‘anomalous’ troctolitic horizon at the base of the Bellevue Core was related to injection of relatively primitive magma, analogous to the Lower or Critical Zone magmas in the Eastern and Western Limbs of the Bushveld Complex; this horizon defines the greatest inconsistency of magmatic stratigraphy between the Northern Limb and the Eastern and Western Limbs of the complex, and is prospective for platinum-group element exploration. However, the most significant finding of this study is that plagioclase, pyroxene and olivine have undergone extensive diffusive modification. The major and trace element geochemistry of these minerals records sub-solidus equilibration during the prolonged cooling of the Bushveld Complex, controlled by diffusion rates and local partitioning between adjacent minerals. By analogy, texturally mature cumulates in any layered intrusion are also likely to be chemically re-equilibrated.

Description: The Monviso ophiolite Lago Superiore Unit (LSU; Western Alps) constitutes a well-preserved, largely coherent fragment of eclogitic upper oceanic lithosphere subducted to c. 80 km depth (between 50 and 40 Ma) and exhumed along the subduction interface. Within-slab, 10–100 m thick, eclogite-facies shear zones cut this section; the Intermediate Shear Zone (ISZ) follows the boundary between gabbroic and basaltic eclogites (associated with minor calcschist lenses), and the Lower Shear Zone (LSZ) marks the contact between gabbroic eclogites and the antigorite serpentinite sole. Up to 10 m fragments of mylonitic gabbroic eclogites were transported within serpentinite schists from the LSZ during eclogite-facies deformation. Metasomatic rinds, formed on these fragments during peak to early retrograde lawsonite-eclogite-facies metamorphism (c. 550°C and 2·6 GPa), document episodic, prominent rock–fluid interaction along intra-slab, channelized fluid migration pathways associated with deformation. We present new petrological and geochemical data on hydrous eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and serpentinite-derived ultrabasic schists from block rinds. Bulk-rock compositions, laser ablation inductively coupled plasma mass spectrometry mineral analyses and X-ray Cr and Mg maps of garnet and clinopyroxene demonstrate that these samples underwent significant enrichments in Mg, Cr, Ni, ± large ion lithophile elements and prominent depletions in Fe and V during eclogite-facies metasomatism. Boron isotope data for phengite ( 11 B = 0 to + 7; 80 〈 B 〈 130 µg g –1 ), clinopyroxene and chlorite ( 11 B = –7 to + 4; B 〈 3 µg g –1 ), and antigorite ( 11 B = –4 to 0; 20 〈 B 〈 30 µg g –1 ) suggest that the metasomatic block rinds formed during interaction with serpentinite-derived fluids. These compositional patterns point to focused, fluid-mediated element transfer through the subducted slab. Serpentinite-derived fluids, via antigorite breakdown some 15–20 km deeper than the maximum depth reached by these eclogites, thereby equilibrate with fluids derived from oceanic crust and/or sedimentary material. Although slab components of diverse flavour have long been recognized to be central in triggering island arc magmatism, this example is among the first to document synkinematic, long-range, large-scale, channelized dehydration fluid flow within subducted oceanic slabs.

Description: Miocene basanite from Krzeniów (SW Poland, eastern part of Cenozoic Central European Volcanic Province) contains scarce, small (usually 〈4 cm in diameter) spinel harzburgite and dunite mantle xenoliths. Two groups are defined based on the forsterite (Fo) content in olivine: group A (Fo 90·4–91·7 ) and group B (Fo 88·0–89·8 ). Both group A and B rocks are either clinopyroxene-free or clinopyroxene-poor. The group A orthopyroxene (mg# 0·91–0·92) is Al-poor and strongly light rare earth element (LREE) depleted [(La/Lu) N 〈 0·42]. The mg# of group A clinopyroxene varies from 0·92 to 0·94 and is negatively correlated with its Al content. The trace element compositions of clinopyroxene define two patterns: A1, U-shaped [(La/Lu) N = 20·23–64·73]; A2, almost linear LREE enriched [(La/Lu) N = 11·91–14·00]. The group B orthopyroxene is also Al poor and LREE depleted [(La/Lu) N = 0·03–0·04], but its mg# is lower than that in group A orthopyroxene (0·90–0·92). The mg# of group B clinopyroxene (0·90–0·92) is lower than that in group A, whereas the Al content is similar. The REE patterns of group B clinopyroxene mimic those of subgroup A2 [(La/Lu) N = 5·64–11·30]. The group A harzburgites and dunites represent spinel-facies lithospheric mantle that underwent metasomatism by CO 2 -bearing silicate melts subjected to chromatographic fractionation. Locally carbonatite–silicate immiscible melts were generated. The peridotites in the distal parts of the chromatographic system were little affected by metasomatism and preserve much of their pre-metasomatic, depleted characteristics. The group B rocks are similar to those of group A except for the lower mg# of orthopyroxene and olivine. They were affected by percolating alkaline silicate melt undersaturated in clinopyroxene, which lowered the mg# of the olivine and orthopyroxene (‘Fe metasomatism’). The A and B peridotites are representative of the Miocene lithospheric mantle close to the margins of the neighbouring Eger Rift.

Description: The Tertiary Austurhorn intrusive complex in SE Iceland represents an exhumed magma chamber that has recorded an extensive history of magma mixing and mingling. The basal part of the intrusion consists predominantly of granophyres that have been intensively and repeatedly intruded by more mafic magma. This association of granophyres, basic and hybrid rocks at Austurhorn is referred to in the literature as a ‘net-veined’ complex, but field relations suggest a much more complex emplacement history. Here we present petrological and physical constraints on the various processes that resulted in magma mixing and mingling and the formation of different generations of hybrid rocks at Austurhorn. The complexity of the mixing and mingling processes increases towards the inferred centre of the intrusion, where chaotic hybrid rocks dominate the exposed lithology. Complex cross-cutting relations between different hybrid generations strongly suggest multiple magma injection and reheating events in the basal part of the shallow magma chamber. Model calculations employing distribution coefficients based on rare earth element concentrations reveal that early stage hybrid magma generations formed by pure endmember mixing between felsic and mafic magma with about 10% mafic fraction in the hybrids. With repeated injections of mafic magma into the base of the magma chamber, the intruding magma interacted to a greater extent with pre-existing hybrids. This led to the formation of hybrid magma compositions that are shifted towards the mafic endmember over time, with up to 30% of the mafic fraction in the hybrids. These mixing processes are recorded in the zonation patterns of clinopyroxene and plagioclase phenocrysts; the latter have been divided into four main groups by cross-correlation analysis. Melt viscosity calculations were performed to constrain the possible conditions of magma mixing and the results indicate that the interaction of the contrasting magmas most probably occurred at temperatures of approximately 1000°C up to 1120°C. This suggests that the initiation of effective magma mixing requires local superheating of the felsic magmas, thereby confining the process to areas of localized, substantial mafic magma injection.

Description: Pliocene to Quaternary magmatism in the Karacadağ Volcanic Complex in SE Turkey occurred in the foreland region of the Arabia–Eurasia collision and can be divided into two phases. The earlier Karacadağ phase formed a north–south-trending volcanic ridge that erupted three groups of lavas. The same range of mantle sources contributed to the younger Ovabağ phase lavas, which were erupted from monogenetic cones to the east of the Karacadağ fissure. As at several other intraplate localities across the northern Arabian Plate this magmatism represents mixtures of melt from shallow, isotopically enriched mantle and from deeper, more depleted mantle. The deep source is similar to the depleted mantle invoked for other northern Arabian intraplate volcanic fields but at Karacadağ this source contained phlogopite. This source could be located in the shallow convecting mantle or may represent a metasomatic layer in the base of the lithosphere. There is no evidence for a contribution from the Afar mantle plume, as has been proposed elsewhere in northern Arabia. Melting during the Karacadağ and Ovabağ phases could have resulted from a combination of upwelling beneath weak or thinned lithosphere and restricted local extension of that weakened lithosphere as it collided with Eurasia. Tension associated with the collision focused magma of the Karacadağ phase into the elongate shield volcano of Mt. Karacadağ. The northern end of the fissure accommodated more extensive differentiation of magma, with isolated cases of crustal contamination, consistent with greater stress in the lithosphere closest to the collision. Most magma batches of the Karacadağ and Ovabağ phases differentiated by fractional crystallization at ~5 MPa, near the boundary between the upper and lower crust. Magma batches dominated by melt from garnet lherzolite show evidence for restricted amounts of differentiation at ~22·5 MPa, which is close to the base of the lithospheric mantle.

Description: To better understand the significance of the chemical compositions of forsterite-hosted melt inclusions, dynamic crystallization experiments were performed at atmospheric pressure and a low cooling rate (2°C h –1 ) on a starting glass material with a composition in the CMAS system (12·21 wt % CaO, 28·48 wt % MgO, 11·96 wt % Al 2 O 3 , 47·32 wt % SiO 2 ). Experiments were quenched at various temperatures, both above and below the theoretical solidus. Olivine is the liquidus phase and the mesostasis may exhibit a second crystalline phase corresponding to a metastable Al-rich pyroxene. Olivine crystal morphologies change from polyhedral to skeletal, depending on the quenching temperature. Regardless of the quenching temperature, the chemical compositions of liquids trapped in melt (now glass) inclusions in the olivines do not contain the boundary layer that should result from rapid growth of these crystals. The liquid compositions also show that down to temperatures far below the theoretical solidus, olivine is the only phase that crystallizes on the walls of these inclusions. Glass inclusion analyses can therefore be used to determine the chemical evolution of a liquid in metastable equilibrium with the host olivine, irrespective of the quenching temperature. Melt inclusions therefore follow their own liquid line of descent distinct from that of the whole charge. This experimental study suggests that it should be possible to (1) draw the metastable extensions of the liquidus from natural glass inclusions and (2) determine the real supersaturation degrees of the residual magmas.

Description: Major-element, lithophile trace element, and Sm–Nd and U–Pb zircon isotopic data are presented for Palaeoproterozoic mid-ocean ridge basalt (MORB)-type tholeiitic dikes ranging in age from 2140 ± 3 to 2126 ± 5 Ma studied at six localities within three terranes in the Karelian Craton, eastern Fennoscandian Shield. All the studied dikes have remarkably uniform geochemical and isotope characteristics. They are tholeiitic basalts with low contents of large ion lithophile elements, high field strength elements, and rare earth elements (REE), nearly flat chondrite-normalized REE patterns [(La/Sm) n = 0·9–1·2, (Gd/Yb) n = 1·0–1·2], and positive Ti, Nb, and Zr anomalies in the primitive mantle-normalized diagrams. The dikes also show relatively uniform initial Nd isotope compositions, with Nd values ranging from +1·4 to +3·0, despite the occurrence of these dikes within Archaean terranes with different crustal history. According to the results of U–Pb (zircon) and Sm–Nd internal isochron dating the crystallization age of the dikes is constrained to be c . 2·14 Ga. The studied MORB-type tholeiitic dikes are probably comagmatic with Palaeoproterozoic MORB-type basalts that have previously been recognized in the Karelian Craton, and might represent relicts of their magma feeder system. The uniformity of ages and geochemical and isotope characteristics of the MORB-type dikes and volcanic rocks suggest that they are probably related to a common magmatic event. This event was near-contemporaneous with the eruption of high-Ti plume-related basalts and intrusion of dikes in the c . 2·1 Ga Jatulian continental flood basalt province. Geochemical modelling indicates that the chemical and isotopic compositions of the dikes are best explained by derivation of their parental magmas by partial melting of a uniformly depleted mantle source in the spinel peridotite stability field, followed by fractional crystallization and minor (〈6%) assimilation of continental crustal material. This suggests that magma-storage processes in upper crustal chambers were very short-lived; this could be the result of rapid extension and fast transport of melts through the relatively thin lithosphere of the Karelian Craton. Indirect evidence for the formation of the studied dikes in an extensional tectonic setting is provided by the established presence of extensional tectonics in the eastern part of the Fennoscandian Shield at c . 2·1 Ga associated with the opening of the Lapland–Kola and Svecofennian oceans. The studied continental MORB-type tholeiites, therefore, may play an important role as indicators of the timing of continental breakup. Palaeoproterozoic MORB-type tholeiitic dikes and basalts show significant geochemical similarities to Phanerozoic syn-breakup continental flood basalts of the North Atlantic and Afar provinces; this adds further support to the indicative role of continental MORB-type tholeiites in the reconstruction of continental breakup processes in the Precambrian.

Description: The Magma Chamber Simulator quantifies the impact of simultaneous recharge, assimilation and crystallization through mass and enthalpy balance in a multicomponent–multiphase (melt + solids ± fluid) composite system. As a rigorous thermodynamic model, the Magma Chamber Simulator computes phase equilibria and geochemical evolution self-consistently in resident magma, recharge magma and wallrock, all of which are connected by specified thermodynamic boundaries, to model an evolving open-system magma body. In a simulation, magma cools from its liquidus temperature, and crystals ± fluid are incrementally fractionated to a separate cumulate reservoir. Enthalpy from cooling, crystallization, and possible magma recharge heats wallrock from its initial subsolidus temperature. Assimilation begins when a critical wallrock melt volume fraction (0·04–0·12) in a range consistent with the rheology of partially molten rock systems is achieved. The mass of melt above this limit is removed from the wallrock and homogenized with the magma body melt. New equilibrium states for magma and wallrock are calculated that reflect conservation of total mass, mass of each element and enthalpy. Magma cooling and crystallization, addition of recharge magma and anatectic melt to the magma body (where appropriate), and heating and partial melting of wallrock continue until magma and wallrock reach thermal equilibrium. For each simulation step, mass and energy balance and thermodynamic assessment of phase relations provide major and trace element concentrations, isotopic characteristics, masses, and thermal constraints for all phases (melt + solids ± fluid) in the composite system. Model input includes initial compositional, thermal and mass information relevant to each subsystem, as well as solid–melt and solid–fluid partition coefficients for all phases. Magma Chamber Simulator results of an assimilation–fractional crystallization (AFC) scenario in which dioritic wallrock at 0·1 GPa contaminates high-alumina basalt are compared with results in which no assimilation occurs [fractional crystallization only (FC-only)]. Key comparisons underscore the need for multicomponent–multiphase energy-constrained thermodynamic modeling of open systems, as follows. (1) Partial melting of dioritic wallrock yields cooler silicic melt that contaminates hotter magma. Magma responds by cooling, but a pulse of crystallization, possibly expected based on thermal arguments, does not occur because assimilation suppresses crystallization by modifying the topology of multicomponent phase saturation surfaces. As a consequence, contaminated magma composition and crystallizing solids are distinct compared with the FC-only case. (2) At similar stages of evolution, contaminated melt is more voluminous (~3·5 x ) than melt formed by FC-only. (3) In AFC, some trace element concentrations are lower than their FC-only counterparts at the same stage of evolution. Elements that typically behave incompatibly in mafic and intermediate magmas (e.g. La, Nd, Ba) may not be ‘enriched’ by crustal contamination, and the most ‘crustal’ isotope signatures may not correlate with the highest concentrations of such elements. (4) The proportion of an element contributed by anatectic melt to resident magma is typically different for each element, and thus the extent of mass exchange between crust and magma should be quantified using total mass rather than the mass of a single element. Based on these sometimes unexpected results, it can be argued that progress in quantifying the origin and evolution of open magmatic systems and documenting how mantle-derived magmas and the crust interact rely not only on improvements in instrumentation and generation of larger datasets, but also on continued development of computational tools that couple thermodynamic assessment of phase equilibria in multicomponent systems with energy and mass conservation.

Description: The Jurassic Attawapiskat kimberlites allow the study of the association of a world-class primary diamond deposit (Victor Mine) with a post-Archaean rift system, the Midcontinent Rift, which affected the southern Superior Craton at ~1·1 Ga. Peridotite xenoliths and xenocrysts from the Attawapiskat kimberlites have been analysed to understand the processes of craton formation and modification in the Superior lithospheric mantle. Chondrite-normalized platinum group element (PGE N ) signatures in olivine are complex and highly variable, but correlate with Os isotopic compositions. The existence of a depleted mantle reservoir beneath the Attawapiskat area since the Palaeoarchaean is indicated by ~3·6 Ga T RD ages preserved in peridotitic olivine. An Mg# up to 93·6 in these olivines requires that protolith formation involved high degrees of partial melting, leading to harzburgitic and dunitic residues. Cr-rich garnets with positive slopes in depleted chondrite-normalized heavy rare earth elements (HREE N ) are consistent with fractional polybaric melt extraction continuing from the garnet into the spinel stability field. T RD ages of ~2·7 Ga in olivines with residual PGE N patterns probably reflect residual PGE alloy or refractory PGE sulphide inclusions and indicate that additional melting occurred in the mantle at the time of subduction–accretion, with hydrous melts infiltrating the overlying mantle wedge leading to iridium-group PGE (I-PGE) alloy formation. Metasomatic melts related to the Midcontinent Rift (1·1 Ga) interacted with variably depleted peridotite, leading to platinum-group PGE (P-PGE) enrichment and Mesoproterozoic T RD ages. Older depleted domains are, however, preserved (e.g. sinusoidal REE N patterns in lherzolitic garnet). After the thermal impact of the rift subsided, diamond-stable conditions were extended to shallower depths in the lithosphere via cooling, and diamonds sampled by post-rift kimberlites, such as Victor (~180 Ma), must have formed after the Midcontinent Rift. These diamonds are likely to be of mixed parageneses: high-pressure compositions indicative of diamond stability are observed in both lherzolitic and high-Mg eclogitic to pyroxenitic garnets at Victor.

Description: New zircon U–Th model-age and trace element datasets are presented from Taupo volcano (New Zealand), which are used to investigate the timescales and broad-scale magmatic processes involving zircon crystallization after the caldera-forming 25·4 ka Oruanui supereruption. Detailed 14 C-based chronologies and controls on vent locations allow the timing and location of post-caldera eruptions to be spatially and temporally constrained to an extent not possible for any other supervolcano. After ~5 kyr of post-Oruanui quiescence, Taupo erupted three dacitic units, followed by another ~5 kyr break, and then a sequence of rhyolitic units in three subgroups (SG1–SG3) from 12 ka onwards. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U–Th zircon model ages in Taupo SG1 rhyolites (erupted from 12 to 10 ka) indicate only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains in some units. U–Pb dating of these older grains shows that both 300–450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units that were vented within the outline of the Whakamaru caldera. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolite (Units B and C) have notably high concentrations of U, Th, P, Y + (REE) 3+ and Nb but with only minor variations in Hf and Ti. SG2 zircons typically have higher Sc contents, reflecting large-scale changes in melt chemistry and crystallizing mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U–Th ages from single post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallization, acting within a crustal protolith chemically independent of that which was dominant in the Oruanui system. These periods of cooling and crystallization alternate with times of rejuvenation and eruption, sometimes demonstrably accompanying syn-eruptive regional rifting and mafic magma injection. Not only were the processes that developed the supersized Oruanui magma body rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale.

Description: Caledonian amphibolite-facies shear zones developed in granulite-facies anorthosites and anorthositic gabbros of the Bergen Arcs, western Norway, allow a detailed study of the relationships between fluid-infiltration, mineral reactions, the evolution of microstructure and deformation mechanisms. A sequence of rocks from the relatively pristine granulites into a shear zone has been studied by optical microscopy, electron microprobe microanalysis (EMPA), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), focusing on the progressive development of microstructure in the plagioclase feldspars, leading up to their deformation in the shear zone. At the outcrop scale, fluid infiltration into the granulites is marked by a distinct colour change in the plagioclase from lilac–brown to white. This is associated with the breakdown of the intermediate composition plagioclase (~An 50 ) in the granulite to a complex intergrowth of Na-rich and Ca-rich domains. EBSD analysis shows that this intergrowth retains the crystallographic orientation of the parent feldspar, but that the Ca-rich domains contain low-angle grain boundaries. Within the shear zone, this complex intergrowth coarsens by grain boundary migration, annihilating these grain boundaries but retaining the Na-rich and Ca-rich zoning pattern. Analysis of nearest-neighbour misorientations of feldspar grains in the shear zone demonstrates that local crystallographic preferred orientation (CPO) is inherited from the parent granulite grain orientations. Random pair misorientation angle distributions show that there is no CPO in the shear zone as a whole, nor is there significant shape preferred orientation (SPO) in single grains. These observations are interpreted in terms of fluid-induced weakening and deformation by dissolution–precipitation (pressure solution) creep.

Description: The Kamchatka Peninsula is home to some of the most frequent and prolific subduction-related volcanic activity in the world, with the largest number of caldera-forming eruptions per length of the volcanic arc. Among those, Gorely volcano has a topographically prominent Late Pleistocene caldera (13 km x 12 km, estimated to have produced 〉100 km 3 of magma), which is now almost completely filled by a central cone. We report new 40 Ar/ 39 Ar ages and geochemical and isotopic data for newly recognized Mid-Pleistocene ignimbrite units of large but unknown volume sourced from the Gorely eruptive center, most of which were deposited in marginal glacial conditions. These ignimbrites have crystallinities of 9–24% and most are quartz-, amphibole-, and zircon-undersaturated. Additionally, we studied 32 eruptive units, including stratigraphically constrained Holocene tephra, and pre- and post-caldera lava sequences, to understand the petrogenetic and temporal evolution of this long-lived, multi-cyclic, arc volcano. Material erupted prior to the formation of the modern Gorely edifice, including the voluminous ignimbrites and eruptions of the ‘pra-Gorely’ stage, consist primarily of dacite and andesite, whereas sequences of the modern Gorely edifice are represented by basalt to basaltic andesite. MELTS and EC-AFC modeling shows that it is possible to obtain silicic compositions near those of the evolved ignimbrite compositions through 60–75% fractional crystallization at 1 kbar and nickel–nickel oxide (NNO) oxygen fugacity. However, our newly compiled major and trace element data for Gorely yield two separate bimodal peaks in a SiO 2 –frequency diagram, showing a prominent Daly Gap, with a deficiency in andesite. Trace element concentrations define two separate trends, one for more silicic and another for more mafic sequences. Additionally, 18 O melt values reconstructed from coexisting plagioclase and clinopyroxene phenocrysts range from a low value of 4·85 to a normal value of 6·22. The low 18 O values range throughout the known lifespan of Gorely, with the lowest value being from the first known ignimbrite to erupt, indicating episodic but temporally decreasing crustal assimilation of previously hydrothermally altered material. 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios show wide ranges from 0·70328 to 0·70351 and from 0·51303 to 0·51309 respectively, also suggesting incorporation of surrounding crust, although there are less clear trends throughout the lifespan of Gorely. The combination of light and diverse 18 O values with elevated 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd ratios suggests contamination by older and isotopically diverse, low- 18 O country-rocks, such as the neighboring 11 Ma Akhomten granitic massif, which shows ranges in 18 O, 87 Sr/ 86 Sr, and 144 Nd/ 143 Nd values overlapping with the Gorely magmas. In addition, the presence of glomerocrysts and mafic enclaves in the majority of Gorely dacites indicates a period of crystal settling and subsequent intrusion of hot, primitive basalt that probably triggered eruption. Finally, elevated Nb concentrations relative to other Kamchatkan volcanoes suggest that Gorely magmas may involve an enriched component, probably caused by delamination of a lower crustal root. Our results argue for an incremental view of silicic magma generation at so-called ‘long-term eruptive centers’, in Kamchatka and worldwide, consisting of alternating episodes of magmatic and hydrothermal activity, and glacial advances and retreats. We demonstrate that large-volume, isotopically distinct, silicic magma can be generated rapidly between cone-building phases of volcanic activity through a combination of fractional crystallization, assimilation of older country rocks, and shallow assimilation of hydrothermally altered but otherwise petrochemically similar older intracaldera tuffs and intrusions. These transient shallow silicic magma chambers empty nearly completely in ignimbrite-forming eruptions after 10 3 –10 5 years of assembly, partially triggered by glacial surface dynamics.

Description: Monogenetic volcanism can produce eruptive suites showing considerable complexity in compositional features and pre-eruptive magma evolution. The ~5 ka Mount Gambier Volcanic Complex (MGVC), a monogenetic volcanic centre in SE Australia’s Newer Volcanics Province (NVP), is a good example. It displays a complex stratigraphy of interbedded deposits related to different eruption styles from a multi-vent system. Formation of the MGVC proceeded through simultaneous eruption of two alkali basaltic magma batches: a more alkaline and light rare earth element enriched basanite batch (Mg# 58–62) in the west and a trachybasalt batch (Mg# 58–64) enriched in SiO 2 and CaO in the east. Trace element modelling suggests an origin of both magma batches from a single parental melt formed by 4–5% partial melting of a metasomatized lherzolite source in the asthenospheric mantle (2·2 GPa; ~80 km). At the base of the lithosphere, part of this parental melt interacted with a deep-seated pyroxenite contaminant to form the trachybasaltic suite. Further modification of either magma batch at crustal levels appears to have been negligible. Isotope and trace element signatures are consistent with the inferred asthenospheric magma source; Pb isotopes in particular suggest a source with mixed Indian mid-ocean ridge basalt–Enriched Mantle 2 affinities, the latter perhaps related to metasomatic overprinting. It is argued that Cenozoic NVP volcanism in SE Australia is not necessarily related to a mantle plume but can be explained by other models involving asthenospheric upwelling. Fast magma ascent rates in the lithosphere evidenced by the presence of mantle xenoliths may reflect reactivation of lithospheric structures that provide magma pathways to the surface.

Description: Granulite xenoliths entrained within Quaternary rift basalts in northern Tanzania document the composition, equilibration conditions, age and petrogenesis of the present-day lower crust beneath the eastern margin of the Tanzanian Craton (Labait) and the adjacent Mozambique Belt (Lashaine and Naibor Soito). Mafic to intermediate Archean lithologies dominate throughout the suites (~2·66 Ga based on U–Pb zircon ages), demonstrating that deep-seated Archean lithosphere extends far to the east of the margin of the Tanzanian Craton within the Mozambique Belt. There is no evidence for significant additions to the crust via magmatic underplating since that time. Many of the lower crustal xenoliths share compositional similarities with lavas from the greenstone belts of the Tanzanian Craton, suggesting that they crystallized from similar magmas. Extreme depletions of highly incompatible elements (e.g. Cs, Rb, Th and U) in the granulites, relative to the lavas, coupled with unradiogenic 143 Nd/ 144 Nd (0·5114–0·5122) and 87 Sr/ 86 Sr (0·7040–0·7051), suggest that these depletions occurred coincident with or shortly after the rocks crystallized, possibly through partial melting associated with metamorphism. These samples may thus represent high-grade lower crustal complements to the greenstone belt lavas. Compared with the craton-margin samples, granulite xenoliths from within the Mozambique Belt record very high peak equilibration pressures at moderate temperatures (〉1·2 to 〉1·7 GPa, 750–960°C, based on pseudosections), documenting their equilibration deep within thickened continental crust during the East African Orogeny, c . 560 Myr ago. These samples therefore offer an unusual window into the deepest reaches of the crust in a continental orogen. Despite the fact that the Mozambique Belt experienced significant crustal thickening followed by post-orogenic collapse, there is no evidence for loss of the deep lithosphere associated with these processes.

Description: The 2007 caldera-forming eruption of Piton de la Fournaise (PdF) erupted the largest volume of magma (210 Mm 3 ) recorded at this volcano in at least three centuries. Major and trace element and Sr–Nd isotope data for bulk-rocks, groundmasses and olivine phenocrysts have been combined with melt inclusion data (major, trace and volatile elements) to track magma evolution over the whole eruptive sequence. We show that each eruptive phase had a distinctive geochemical and petrological signature and that caldera collapse on 5 April was preceded by a marked shift in bulk magma composition and crystal content and size. Aphyric basalt erupted at the beginning of the sequence (February 2007) had relatively high Sr isotope ratio ( 87 Sr/ 86 Sr = 0·70420–0·704180) and low Nd isotopic ratio ( 143 Nd/ 144 Nd = 0·51285–0·51286). Olivine-basalts extruded on 2–5 April just before caldera collapse are less enriched in radiogenic Sr ( 87 Sr/ 86 Sr = 0·70412–0·70416), but characterized by the same Nd isotopic composition. This magma is interpreted as a new deep input, which pressurized the shallow PdF plumbing system and triggered the 2007 activity. Post-collapse oceanite lavas represent the main volume of magma extruded in 2007. Their bulk-rocks and groundmasses have 87 Sr/ 86 Sr (~0·70418) intermediate between those of February and 5 April, and similar to those of the March 2007 and 2001–2006 lavas. We show that the Steady State Basalts (SSB) commonly erupted at PdF are hybrid melts, which result from multistep mixing between ‘alkaline’ and ‘transitional’ end-members. Our results lead us to propose a new model of the PdF plumbing system to reconcile the petrological, geochemical and geophysical observations: (1) the shallow portion (above sea level) of the PdF plumbing system hosts several small sills, in which magma experiences variable degrees of degassing, cooling and crystallization; (2) oceanite lavas result from the withdrawal of shallow harrisitic mushes stored at low pressures (〈48 MPa; 〈1800–2400 m depth) below both the volcano summit and its eastern flank; (3) water degassing plays a major role in fast magma crystallization at shallow depths. Multistep ascent and periodic extrusion of the shallow magmas is promoted by injections of deeper and hotter basaltic magma, containing up to 1·3 wt % H 2 O and 1630 ppm S. In 2007, the new deep input was the ultimate source of the large excess in sulfur degassing detected by satellites. Lateral draining and intrusion of magma below the eastern flank of the volcano are the cause of major volcano deformation, flank sliding and summit caldera collapse.

Description: New compositional profiles across plagioclase grains from the Layered Series (LS), Marginal Border Series (MBS) and Upper Border Series (UBS) of the Skaergaard intrusion are used to understand the mechanisms of cumulate rock solidification and the fate of the interstitial liquid. The data show that plagioclase crystals display three types of compositional profile over the whole intrusion, as follows. (1) Grains with normal zoning, which dominate the MBS and UBS. These are interpreted as having formed at the top of a crystal mush and then buried in the cumulate pile. Crystallization of the interstitial melt resulted in liquid differentiation and produced normally zoned rims on plagioclase cores. (2) Unzoned crystals, which dominate the upper part of the LS, also crystallized at the top of the mush and were then buried in mush with a low interstitial liquid fraction or one experiencing convective movements that kept the liquid to a constant composition. (3) Crystals with a mantle of decreasing An content followed by a rim of constant composition. Grains showing this complex zoning mostly occur in the lower parts of the LS. Depending on the stratigraphic position within the intrusion, the composition of the rim can be An 56 , An 51 or An 40 . In the main magma body, these compositions (An contents) correspond to those of plagioclase primocrysts (e.g. cores) at the appearance of cumulus clinopyroxene (An 56 ), Fe–Ti oxides (An 51 ) and apatite (An 40 ). Compositional buffering of plagioclase rims is interpreted as being a consequence of enhanced release of latent heat of crystallization at the appearance of new interstitial phases in the crystal mush. When a new phase saturates, the latent heat contribution to the global enthalpy budget of the system becomes sufficiently high to keep the interstitial melt at its liquidus temperature for a period of time that could exceed thousands of years. Under these conditions, equilibrium, adcumulus growth together with diffusion and possibly advection of chemical components result in the formation of plagioclase rims of constant composition (An content). Efficient thermal buffering of the mush liquid depends on the porosity (i.e. fraction of liquid within the mush) and the degree of compositional homogeneity of the mush. In a heterogeneous and highly porous mush, saturation of the new phase occurs in the coldest part of the mush and the enhanced latent heat release at the saturation of this phase is quickly dissipated to the whole volume of liquid, including the warmest part that is not yet saturated in a new phase. As a consequence, no thermal buffering occurs and interstitial crystallization produces grains with normal zoning. The distribution of the various types of plagioclase grains throughout the Skaergaard intrusion can therefore be used to infer the spatial variability in the physical properties of the crystal mush, such as the residual porosity, both at an intrusion-wide scale and at a millimetre- to centimetre-scale.

Description: Grenada is the southernmost island in the Lesser Antilles arc, a chain of subduction-related volcanoes distinguished by its diversity of magma composition and unusually abundant plutonic xenoliths, many with cumulate textures. We have determined the mineral compositions of a newly collected, extensive suite of plutonic xenoliths from Grenada and examined their relationship with the lavas in an attempt to explore the role of intra-crustal processes on magmatic evolution. The plutonic assemblages are dominated by mafic phases with abundant hornblende and clinopyroxene, and include the only known plagioclase-free examples in the Lesser Antilles. Bulk compositions are unlike those of natural silicate melts and are consistent with the majority of the xenoliths having a cumulate origin. Experimental and thermobarometric evidence shows that the entire cumulate suite can be generated in a narrow pressure range (0·2–0·5 GPa) with different assemblages resulting from small variations in melt chemistry and temperature. Temperature estimates are consistent with the observed crystallization sequence of olivine -〉 clinopyroxene -〉 hornblende -〉 plagioclase. A spinel phase is present throughout ranging from Cr- to Fe 3+ -rich. The crystallization sequence requires elevated magmatic H 2 O contents (~7 wt % H 2 O) sufficient both to suppress plagioclase crystallization and to render this phase extremely rich in anorthite upon appearance; this is a characteristic of many island arc settings. Studied lavas from the M- and C-series span picrites and ankaramites to hornblende- and orthopyroxene-bearing andesites. MELTS modelling confirms experimental hypotheses that the two lava series can be derived from a common picritic magma, with M-series differentiation occurring in the uppermost mantle (~1·4–1·8 GPa) and C-series in the shallow crust (~0·2 GPa). Plutonic xenoliths from Grenada are notably different from those of the neighbouring island of St Vincent, the respective assemblages and mineral chemistry demonstrating the effect of small-scale changes in melt composition and magma storage conditions between these two islands. We suggest that the unusual petrological and geochemical characteristics of Grenada magmas are a result of proximity to the South American continent and associated localized thickening of the oceanic lithosphere. This increases the depth of magma generation and is reflected in the elevated LREE/HREE of the Grenada lavas, indicating that last equilibration with a garnet lherzolite source occurred at a depth of ≥60 km.

Description: Convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel. Studies of high-pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to mid-crustal levels, but the details of such dynamics are controversial. To understand the dynamics of a subduction channel we report the results of a petrochronological study from the central Sesia Zone, a key element of the internal Western Alps. This comprises two polymetamorphic basement complexes (Eclogitic Micaschist Complex and Gneiss Minuti Complex) and a thin, dismembered cover sequence (Scalaro Unit) associated with pre-Alpine metagabbros and metasediments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro–Bonze Units) yield unequivocal petrological and geochronological evidence of two distinct high-pressure stages. Ages (U–Th–Pb) of growth zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples. Two independent tectono-metamorphic ‘slices’ showing a coherent metamorphic evolution during a given time interval have been recognized: the Fondo slice (which includes Scalaro and Bonze rocks) and the Druer slice (belonging to the Eclogitic Micaschist Complex). The new data indicate separate stages of deformation at eclogite-facies conditions for each recognized independent kilometer-sized tectono-metamorphic slice, between ~85 and 60 Ma, with evidence of intermittent decompression ( P ~ 0·5 GPa) within only the Fondo slice. The evolution path of the Druer slice indicates a different P–T– time evolution with prolonged eclogite-facies metamorphism between ~85 and 75 Ma. Our approach, combining structural, petrological and geochronological techniques, yields field-based constraints on the duration and rates of dynamics within a subduction channel.

Description: The intensive variables of dacitic–rhyodacitic magmas prior to four large Plinian eruptions of Santorini Volcano over the last 200 kyr (Minoan, Cape Riva, Lower Pumice 2 and Lower Pumice 1) were determined by combining crystallization experiments with study of the natural products, including the volatile contents of melt inclusions trapped in phenocrysts. Phase equilibria of the silicic magmas were determined at pressures of 1, 2 and 4 kbar, temperatures of 850–900°C, fluid (H 2 O + CO 2 )-saturation, X H 2 O [= molar H 2 O/(H 2 O + CO 2 )] between 0·6 and 1 (melt H 2 O contents of 2–10 wt %), and redox conditions of FMQ (fayalite–magnetite–quartz buffer) or NNO + 1 (where NNO is Ni–NiO buffer). Experiments were generally successful in reproducing the phenocryst assemblage of the natural products. The phase relationships vary significantly among the investigated compositions, revealing a sensitivity to small variations in whole-rock compositions. Our results show that the pre-eruptive storage conditions of the four silicic magmas were all very similar. The magmas were stored at T = 850–900°C and P ≥ 2 kbar, under moderately reduced conditions (NNO = –0·9 to –0·1), and were poor in fluorine (~500–800 ppm) and sulphur (≤100 ppm), but rich in water and chlorine (5–6 wt % and 2500–3500 ppm, respectively). In all cases, the melts were slightly undersaturated with respect to H 2 O, but most probably saturated with respect to H 2 O + Cl ± CO 2 and a brine. The Santorini magma plumbing system appears to be dominated by a large, long-lived (≥200 kyr) predominantly silicic magma storage region situated at ≥8 km depth, from which crystal-poor melt batches were extracted during the largest caldera-forming eruptions of the volcanic system.

Description: We present analyses of Sr isotope zoning by microdrilling and thermal ionization mass spectrometry in plagioclase crystals from Parinacota volcano (Central Volcanic Zone, northern Chile), which were analysed for major and minor element zoning in a previous study. Although the isotopic range of the bulk-rock samples is small at this volcano (0·7067–0·7070, except for one flow of mafic andesite at 0·7061), significant variations are seen (0·70649–0·70700) within and between plagioclase crystals. A general negative correlation is observed between Sr isotope composition and Sr concentration in the liquid in equilibrium with each plagioclase zone, as calculated from chemical zoning data and partition coefficients. Additional scatter is superimposed on this general trend, indicating a decoupling between isotopic and chemical variations for Sr. In one dacite sample a detailed isotopic profile shows increasing contamination during crystal growth, except for an abrupt decrease correlated with a dissolution surface and interpreted as a recharge event. We apply energy-constrained recharge, assimilation and fractional crystallization modelling to the melt evolution recorded in the chemical and isotopic zoning in this crystal. Results suggest 20% assimilation of the local wall-rock gneiss, at high initial temperatures. The isotopic data confirm the involvement of two contrasting mafic magmas, which are sampled at flank cinder cone vents. One (Lower Ajata) has a low Sr content with high 87 Sr/ 86 Sr, the other (Upper Ajata) has a high Sr content with lower 87 Sr/ 86 Sr. In some samples from Parinacota, the isotopic composition of plagioclase crystal rims or groundmass crystals is significantly higher than that of the high 87 Sr/ 86 Sr mafic magma. In others, where chemical zoning profiles suggest that recharge was from the low 87 Sr/ 86 Sr magma, the 87 Sr/ 86 Sr of the groundmass and crystal rims is higher than expected. This indicates either additional parent magmas to the two previously identified, or further crustal assimilation, either at lower crustal depths, before crystallization of plagioclase, or just after the last recharge. Our results illustrate the complexity of magma–crust interaction beneath Parinacota, which is likely to be representative of many other Central Andean volcanoes formed on thick crust. Such complex interactions can be revealed by combined study of chemical and isotopic zoning in plagioclase (in a textural petrographic context), despite a small whole-rock isotopic range. The distinct contamination patterns of various samples suggest an important role for the geometry, location and evolution of the magma plumbing system and, in general, variations of the thermal and compositional structure of the crust underneath the volcano.

Description: The northern Cascade arc is an end-member ‘hot’ subduction zone where slab dehydration may be essentially complete prior to reaching sub-arc depths, presenting a potential problem for a flux melting origin for arc basalts. Nevertheless, mafic lavas from the Mt. Baker volcanic field, the most magmatically productive volcano in the northern Cascade arc, record subduction input and compositions typical of calc-alkaline magmas. Relative to normal mid-ocean ridge basalt, the most primitive lavas at Mt. Baker show elevated abundances of large ion lithophile elements (LILE), Pb and light rare earth elements (LREE). High field strength elements (HFSE) and heavy REE (HREE) are depleted relative to LILE. Reconstructed primary magmas define three groups: Group I calc-alkaline basalts (Sulphur Creek, Lake Shannon, Hogback) have lower SiO 2 , (La/Yb) N and LILE/HFSE, and are olivine- and plagioclase-phyric; Group II high-Mg basaltic andesites (Tarn Plateau, Cathedral Crag) also contain clinopyroxene phenocrysts and have higher H 2 O, SiO 2 , (La/Yb) N and LILE/HFSE; Group III low-K olivine tholeiite (Park Butte) has the lowest (La/Yb) N but highest LILE/HFSE. The redox states of all groups lie between QFM (quartz–fayalite–magnetite) and QFM +1·4. Pb, Sr and Nd isotope compositions are similar among all the analysed samples and are consistent with a depleted mantle source modified by input from the subducting slab. Trace element–isotope modeling indicates a subduction component composed predominantly of metabasalt-derived fluid with lesser amounts of sediment melt and metabasalt melt. Group I basalts record the smallest melt fractions (~5–7%), lowest water contents (1·5–2·1 wt %), and highest temperatures and pressures of mantle segregation (up to 1354°C, 1·5 GPa) from lherzolitic (± spinel) residues. Group II basaltic andesites show the greatest extents of mantle metasomatism, the highest water contents (2·7–3·7 wt %) and partial melt fractions (10–12%), and segregated from harzburgite at ~1270°C, 1 GPa, consistent with pooling of melts at the Moho. Group III records P–T conditions similar to Group I (~1·4 GPa, 1326°C) but melt fractions (12%) and mantle residues (harzburgite) are more similar to Group II, and H 2 O contents (~2·1 wt %) are intermediate. Melting beneath Mt. Baker was initiated by dehydration melting of amphibole peridotite at ~95 km and 1020°C, within the stability field of garnet lherzolite. Initial melt fractions were small (~1–2%) and near water-saturation. Phase equilibria and trace element modeling show no evidence for garnet-bearing mantle residues, indicating that progressive melting during ascent of diapirs through the hot core of the convecting mantle wedge reduced H 2 O contents and erased any residual garnet signature. Because fluid release from the slab is restricted to the forearc, mantle hydrated at shallow depths in the serpentine and/or chlorite stability fields must be down-dragged to the region of amphibole stability to initiate dehydration melting.

Description: Detailed structural and petrological mapping in the Beni Bousera peridotite (Rif Belt, northern Morocco) shows that this orogenic peridotite massif is composed of four tectono-metamorphic domains with consistent kinematics, marked by a pervasive, shallowly dipping foliation with a NW–SE stretching lineation that progressively rotates towards a NNE–SSW orientation in the lowermost part of the massif. From top to bottom, these domains are garnet–spinel mylonites, Ariègite subfacies fine-grained porphyroclastic spinel peridotites, Ariègite–Seiland subfacies porphyroclastic, and Seiland subfacies coarse-porphyroclastic to coarse-granular spinel peridotites. Microstructures and crystal preferred orientations point to deformation dominantly by dislocation creep in all domains, but the continuous increase in average olivine grain size indicates decreasing plastic work rates from top to bottom. This evolution in deformation conditions is consistent with the change in synkinematic pressure and temperature conditions, from 900°C at 2·0 GPa in garnet–spinel mylonites to 1150°C at 1·8 GPa in the Seiland domain. A pervasive diffuse dunitic–websteritic layering subparallel to the foliation suggests deformation in the presence of melt in the Seiland domain. Gravitational instabilities owing to local melt accumulation may account for 〈200 m wide areas exhibiting a vertical lineation in this domain. To account for the consistent kinematics and the tectono-metamorphic evolution, which implies a temperature gradient of c. 125°C km –1 preserved across the Beni Bousera massif, we propose that the entire massif records the functioning of a low-angle shear zone, a few kilometres wide, which accommodated exhumation of the base of the lithosphere from ~90 to ~60 km depth. Partial melting in the Seiland domain may be explained by fast decompression of the footwall, without the need for exotic heat sources. Moreover, if the present-day orientation of the shear zone is similar to that when it was active in the mantle, the stretching lineations at high angle to the metamorphic gradient imply that shearing parallel to the trend of the belt accompanied thinning; that is, a transtensional deformation of the margin.

Description: Textural investigations of the ad 79 Vesuvius pumice emphasize the complexity of magma degassing and crystallization during the eruption, which emitted two types of pumice (white and gray) associated with different magma bodies of phonolitic and tephriphonolitic compositions respectively. These studies proposed that velocity gradients caused spatial variations in degassing within the ascending magma column at both the conduit and the localized scale. To validate this hypothesis, analyses of volatiles (Cl, H 2 O) and major elements in pumice glasses and melt inclusions were performed using high spatial resolution tools (microRaman spectrometry and electron microprobe) and combined with major element and volatile concentration profiles and maps. The results indicate that the melt phase differentiated through degassing-induced crystallization of leucite, and that the gray pumice magma was efficiently homogenized prior to degassing. Because Cl diffuses more slowly than H 2 O during fast ascent, it behaves as an incompatible element and can be used as a tracer of crystallization and H 2 O degassing. We emphasize the importance of strain localization in generating zones of preferential exsolution and permeable pathways for gases, and establish degassing scenarios that incorporate the effects of shear-zones.

Description: The 1·60 Ma Otowi Member of the Bandelier Tuff is a chemically and isotopically zoned high-silica rhyolite ignimbrite with subchondritic concentrations of Sr. Sanidine phenocrysts and glasses from early erupted to late-erupted tuff exhibit systematic large variations in 87 Sr/ 86 Sr (0·70519–0·70947) and small ranges in Pb isotope ratios ( 206 Pb/ 204 Pb = 17·795–17·835). In all but the earliest-erupted tuff, sanidine phenocrysts are zoned with relatively Sr- and Ba-rich overgrowths on Sr- and Ba-poor cores. We estimate sodic sanidine/melt partition coefficients for Rb, Cs, Sr, Ba and Pb to aid in understanding the origins of the elemental and isotopic variations. Isotopic heterogeneity is observed within single pieces of pumice, single quartz–sanidine glomerocrysts and single crystals. At the observed high values of 87 Rb/ 86 Sr (≤1000) caused by the extreme Sr depletion, radiogenic ingrowth or contamination of the magma by small amounts of country-rock could both be responsible for elevated 87 Sr/ 86 Sr. The Pb isotope variations cannot be due to ingrowth and unequivocally indicate open-system behavior. Among the sanidine phenocrysts, positive correlations of 87 Sr/ 86 Sr vs 87 Rb/ 86 Sr superficially resemble isochrons but are better modelled as mixing arrays; at least three such arrays, corresponding to three distinct magmatic recharge events, can be identified. The latest event may have played a role in destabilizing the system and triggering eruption. In all cases the recharging magma was another high-silica rhyolite only slightly less depleted in Sr. These relationships are explained by a model that starts with a crystal-poor high-silica rhyolite melt lens overlying a sanidine + quartz crystal pile. Successive melting events within the crystal pile and mixing of new and old melts (recharge) causes isotopic and chemical zoning in the main melt lens and short length scale disequilibrium in the crystal pile. A late episode of quartz crystallization may have captured inclusions of melt that had been additionally contaminated by Precambrian granitoid country-rock as the magma began the upward journey that culminated in eruption.

Description: The concept of an ‘Azores mantle plume’ has been widely debated, and the existence of an Azores hotspot questioned. In an effort to shed new light on this controversy, we present He isotope and major, trace and volatile element compositions for basaltic scoriae from five monogenetic cones emplaced along the fissure zone of Pico Island, the youngest island of the Azores archipelago. The bulk scoriae and lavas are moderately alkaline basalts, and their He isotope ratios, determined on olivine crystals, vary between 10·2 and 11·1 ± 0·1 R a . In contrast, melt inclusions hosted in olivine (Fo 76–83·5 ) span a large range of compositions (K 2 O = 0·7–1·7 wt %; Ce = 32–65 ppm; Nb = 21–94 ppm), which extends the compositional field of lavas erupted along the Pico fissure zone. This chemical evolution is predominantly controlled by polybaric fractional crystallization. Most melt inclusions share similar enrichments in large ion lithophile and light rare earth elements, and trace element ratios (La/Sm, La/Yb, Sr/Nd, Ta/Th, Zr/Y) with their bulk-rocks. Only a few of them differ in their lower contents of incompatible elements and La/Sm, Li/Ta and Na/K ratios, a feature that is ascribed to distinct conditions of melting. As a whole, the melt inclusions preserve high and variable volatile contents, and contain up to 1·8–2·0 wt % of H 2 O and 0·4 wt % of CO 2 . The total fluid pressures, retrieved from the dissolved CO 2 and H 2 O concentrations, and the P CO2 from fluid inclusions, indicate magma ponding and crystallization at the crust–mantle boundary (ca. 18 km deep). The H 2 O/Cl and H 2 O/Ce ratios in the inferred parental undegassed basalts of the Pico fissure zone average 0·036 ± 0·006 and 259 ± 21, respectively. The latter value is significantly higher than that reported for typical mid-ocean ridge basalts from the southern Mid-Atlantic Ridge, but is similar to published ratios for submarine undegassed basalts from the Azores platform. Combining the calculated compositions of Pico primary magmas formed by low degrees of melting with recent geophysical data for the Azores, we propose a model for Azores magma generation involving the decompression melting of a water-enriched mantle domain (H 2 O = 680–570 ppm) with an estimated temperature excess of ≤120°C with respect to the Mid-Atlantic Ridge.

Description: Pyroxenites are an essential component in petrological and geochemical models for melt formation at mid-ocean ridges and ocean islands. Despite their rarity, their origin has been widely discussed and various processes have been invoked for their formation. Here, we present a detailed study of the microtextures and major, minor and trace element compositions of relatively fresh pyroxenites and associated harzburgites from the ultraslow-spreading Lena Trough, Arctic Ocean. Microtextural and geochemical characteristics suggest an origin by magmatic assimilation–fractional crystallization with a high ratio of mass crystallized to mass assimilated. The major element compositions of pyroxenes suggest that this process occurred at high pressures (〉0·7 GPa), although interstitial plagioclase in two of the pyroxenites indicates that melt–rock reaction continued at lower pressures. The parental melt to the pyroxenites was most probably depleted mid-ocean ridge basalt similar to basalts from the North Lena Trough and westernmost Gakkel Ridge; basalts from the Central Lena Trough cannot have functioned as parental melts. The melt was generated close to the garnet–spinel facies transition by variable degrees of partial melting and reacted with the local refractory harzburgite. Pyroxenites from this study provide further evidence, together with plagioclase-bearing and vein-bearing peridotites, for significant melt stagnation below the Lena Trough that occurred over a range of depths, either continuously or stepwise. Comparison with abyssal pyroxenites reveals common characteristics, suggesting that, consistent with results of high-pressure crystallization experiments, they mark the onset of (reactive) crystallization of melts passing through the deeper parts of the mid-ocean ridge plumbing system.

Description: We present zircon textural, trace element and U–Pb age data obtained by secondary ion mass spectrometry (SIMS) (SHRIMP-RG: sensitive high-resolution ion microprobe, reverse geometry) from 15 stratigraphically controlled Bishop Tuff samples and two Glass Mountain (GM) lava samples (domes OD and YA). Bishop zircon textures divide into four suites: (a) dominant sector-zoned grains, with (b) subordinate grains showing bright rims [lower U, Th, rare earth elements (REE)] in CL imaging, (c) sparse GM-type grains (texturally similar to zircons from GM dome YA) and (d) sparse Mesozoic xenocrysts from Sierran granitoid country rocks. All Bishop zircons from suites (a)–(c) combined have a weighted mean age of 777·9 ± 2·2 ka (95% confidence) and a tail back to ~845 ka. Our eruption age estimate using the weighted mean of 166 rim ages of 766·6 ± 3·1 ka (95% confidence) is identical within uncertainty to published estimates from isotope-dilution thermal ionization mass spectrometry (ID-TIMS) (767·1 ± 0·9 ka, 2) and 40 Ar/ 39 Ar (767·4 ± 2·2 ka, 2) techniques, the latter using the 28·172 Ma age for the Fish Canyon sanidine standard. We estimate also an eruption age for GM dome YA of 862 ± 23 ka (95% confidence), significantly older than the currently accepted 790 ± 20 ka K–Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean age of 838·5 ± 8·8 ka (95% confidence), implying that the Bishop Tuff system was active for only ~80 kyr, and had effectively no temporal overlap with the GM system. Trace element variations in Bishop zircons are influenced strongly for many elements by sector zoning, producing up to 3 x concentration differences between sides and tips within the same growth zone. Contrasting trends in molar (Sc + Y + REE 3+ )/P ratios between sides and tips indicate contrasting mechanisms of substitution in different sectors of the same crystal. Concentrations of Ti in tips are double those in the sides of crystals, hindering applicability of the Ti-in-zircon thermometer, in addition to variations inherent to the 0·15–0·67 range in values proposed for aTiO 2 . The bright-rim portions of grains are inferred to have crystallized from the same magma as that which generated the bright rims seen under cathodoluminescence or back-scattered electron imaging on quartz and feldspar, respectively. This less evolved, slightly hotter magma invaded the deeper parts of the chamber represented in the late-erupted northern units possibly up to ~10 kyr prior to eruption, but invaded shallower levels only very shortly before eruption, as shown by our textural information and previously proposed from the sharp delineation of quartz bright rims. By obtaining a large number of analyses from zircon separates that systematically cover the entire Bishop Tuff eruption sequence we can produce an eruption age estimate using SIMS to the same precision and accuracy as ID-TIMS and 40 Ar/ 39 Ar techniques.

Description: Paleoproterozoic eclogite xenoliths from the Jericho and Muskox kimberlites located in the northern Slave craton, Canada, display geochemical and isotopic evidence of a recycled oceanic lithosphere origin. The eclogites comprise a diverse suite of kyanite- and diamond-bearing and bimineralic xenoliths that have a wide range of geochemical compositions. Calculated whole-rock eclogite compositions are broadly basaltic to picritic, ranging from 5 to 13 wt % MgO. Calculated whole-rock trace-element compositions for the eclogites show three overall REE patterns: the first pattern resembles that of oceanic gabbros with prominent positive Eu anomalies and flat heavy REE; the second pattern is similar to the first but displays marked light REE depletion that is interpreted to record partial melt loss; the third pattern lacks Eu anomalies and instead displays both gradually increasing REE contents from La to Lu similar to boninitic melts and ‘humped’ middle REE patterns similar to those observed in eclogites formed by reaction with subduction fluids. Several eclogites have garnet and/or clinopyroxene 18 O values 〉6, which coupled with the trace-element systematics of all the eclogites is interpreted to reflect derivation from recycled oceanic lithosphere protoliths. Lead isotope data for eclogite clinopyroxenes form a broad array that extends from highly radiogenic isotope compositions similar to the host Jericho kimberlite to an intersection with the Stacey–Kramers terrestrial two-stage Pb evolution curve at c . 2·2 Ga. The slope of this data array defines a secondary Pb isochron with an associated age of 1·7 ± 0·3 Ga. Although it is possible that the Pb isotope array is actually a mixing line between the eclogite protolith and the host Jericho kimberlite, both the secondary isochron age and the c . 2·2 Ga Stacey–Kramers model age are consistent with Paleoproterozoic protolith oceanic crust formation. Eclogite xenoliths and eclogitic diamond inclusions from the Slave craton almost exclusively yield c . 2 Ga ages, which are broadly compatible with formation during subduction associated with the c . 1·9–1·8 Ga Wopmay orogeny at the western margin of the Slave craton. Trends in the Sr–Pb isotopic compositions of the eclogite clinopyroxene show that seawater alteration of the oceanic crust protoliths was an important process during protolith formation, and cryptic metasomatism further increased the Pb and Sr isotope values. This metasomatism is probably associated with the Jurassic kimberlite magmatism; however, contributions from the Mesoproterozoic Mackenzie event cannot be completely excluded.

Description: We have carried out experimental studies of the rate at which water is consumed by hydration reactions under mid-crustal conditions. Both pelitic and mafic assemblages are susceptible to extensive hydration in the laboratory on a time scale of weeks to months. Quantitative hydration rate determinations were made using enstatite–oligoclase ± diopside powder mixtures and a natural hypersthene hornfels. Under all conditions, the main hydration product was saponite clay with variable amounts of talc according to the initial proportion of enstatite to plagioclase. The experiments yield consistent rates for water consumption of around 10 –8 g H 2 O per m 2 of mineral surface per second at 400°C and 300 MPa (3 kbar). Additional experiments were run at 300°C and 500°C and at lower pressures (40 MPa), as well as with NaCl; rates appear to be faster at higher temperatures and in the presence of salt, but slower at low pressure. Comparison of powder and core experiments on the natural hornfels indicates that it is primarily the outer surface of the rock core that is available for hydration, with only minor infiltration along grain boundaries. The hydration rates reported here appear to be typical for the types of lithology that demonstrate moderate to high degrees of retrogression along joints and deformation zones in crystalline rocks of the upper crust. Assuming that the surface roughness and damage effects in a natural fault zone are comparable with those of the materials used here in the experiments, the measured hydration rates imply that a natural fracture in crystalline rocks of the middle crust that becomes filled with a water film 0·2 mm in thickness will dry out through incorporation of the water into hydrous phases on a time scale of the order of 10–100 years. This clearly implies that free water has only a short residence time in crystalline rocks of the middle crust or deeper, provided they have cooled below their original temperature of formation and therefore have the potential to undergo retrograde hydration. We infer that the strength of retrograde shear zones in the middle to lower crust will fluctuate through time, with episodes of water infiltration resulting in short periods of water weakening before the water is fully consumed and the rocks become stronger once more.

Description: The growth and dissolution behaviour of detrital, metamorphic and magmatic monazite and zircon during granulite-facies anatexis in pelitic and psammo-pelitic granulites and in garnetiferous granite from the southern margin of the Central Indian Tectonic Zone (CITZ) have been investigated using reconstructed metamorphic reaction history, monazite electron microprobe dating and sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon geochronology. Whereas the pelitic granulites record medium-pressure granulite-facies metamorphism (BM 1 stage), the psammo-pelitic granulite reached ultrahigh temperatures ( T Max 〉 880°C at 8·7 kbar). The meta-psammite additionally records two stages of granulite-facies recrystallization (BM 2 and BM 3 ). Irrespective of variations in the bulk-rock compositions and peak metamorphic conditions, monazite is highly reactive during the BM 1 event, producing complex, chemically zoned crystals. Textural, compositional and chemical ages of these grains indicate the stability of six compositional domains (CD1 to CD6 in the paragenetic sequence), of which CD1 represents pre-metamorphic detrital cores of Paleoproterozoic age. CD2 and CD3 (combined mean age of 1612 ± 14 Ma) mark two stages of recrystallization of detrital monazite cores during prograde events. Rims of CD4 monazite (ages between 1615 ± 14 and 1586 ± 14 Ma) on partially to completely equilibrated cores indicate melt crystallization at, or immediately following, peak BM 1P metamorphism. CD5 monazite (age of 1574 ± 7 Ma) is restricted to the psammo-pelitic granulites, and marks final melt crystallization at the solidus during post-peak cooling (BM 1R stage, where R represents retrograde metamorphism). The metamorphic rim of CD6 monazite (age of 1539 ± 24 Ma) around partially resorbed CD5 domains is linked to the decomposition of BM 1 garnet during the terminal hydration event as part of a granulite-facies recrystallization event. Compositionally homogeneous monazite and rims of chemically zoned monazite grains in granite together record a magmatic crystallization age of 1604 ± 9 Ma. SHRIMP U–Pb zircon dating of the psammo-pelitic granulite and garnetiferous granite indicates detrital or inherited cores of Paleo- to Neoarchean age (3584 ± 3 to 2530 ± 3 Ma), which have been variously recrystallized and overgrown by new zircon: (1) at 1658 ± 12 Ma; (2) between 1595 ± 5 and 1590 ± 6 Ma; (3) at 1574 ± 9 Ma. These zircon dates are correlated with the timing of the following: (1) the protoliths of precursor sediments of the metasedimentary granulites, deposited between 2530 and 1658 Ma; (2) a short-lived high-grade event ~65–70 Myr before the culmination of the BM 1 granulite-facies event; (3) a high- T anatectic event, corresponding to the peak BM 1P metamorphism at T Max 〉 900°C; (4) final crystallization of anatectic melt at the solidus (cf. BM 1R metamorphic stage). These chronological constraints from monazite and zircon, when integrated with the metamorphic reaction history and published geochronological data, allow recognition of three episodes of granulite-facies metamorphism in the CITZ at 1658 Ma (pre-BM 1 event), between 1612 and 1574 Ma (BM 1 event), and between 1572 and 1539 Ma (combined BM 2 and BM 3 events), as part of a latest Paleoproterozoic to Early Mesoproterozoic orogenic event. This orogeny is linked to the growth of the Proto-Greater Indian Landmass.

Description: Magma fluxes in the crust control the thermal viability and mechanical stability of magma chambers. We estimated the magma fluxes required to generate the negative seismic velocity anomaly observed below Soufrière Hills volcano, Montserrat. Growth of a magma body by accretion of andesitic sills was simulated numerically and the resulting temperatures and melt fractions were used to calculate a synthetic anomaly of seismic wave velocity, which was filtered to be comparable with the velocity anomaly obtained from a tomographic experiment. Petrology indicates that before it was reheated, remobilized and erupted, the temperature of the magma residing in the chamber was about 850°C. We ran simulations where convection is assumed to be low and heat transfer is mostly by conduction and simulations where convection is assumed to be vigorous enough to rapidly cool the magma chamber to 850°C. In both cases, magma chamber growth over the last 350 years results in tomography anomalies that are too strong, unless the magma was emplaced at an unlikely low melt fraction (〈0·5). Good fits between the modelled and the observed velocity anomaly were obtained with sills 2–5 km in radius emplaced over 6000–150 000 years, depending on the temperature and melt fraction of the emplaced magma. Because of a trade-off between intrusion dimensions and emplacement durations, the volumetric magma fluxes are restricted to 7 x 10 –4 and 5 x 10 –3 km 3 a –1 . The velocity anomaly can be reproduced with a chamber containing high melt-fraction magma or with a mush of crystals and melt. The range of magma ages in the modelled magma chamber is much wider than the crystal residence time of the erupted andesite. This suggests that the eruption taps small pockets of recently assembled magma and that the velocity anomaly is mostly due to a non-eruptible mush.

Description: In New Caledonia, blueschist and eclogite preserve, as inclusions in porphyroblastic minerals, a record of sulfide present during prograde subduction processes. Sulfide inclusions in prograde garnet and lawsonite became chemically isolated from the matrix whereas sulfide minerals in the matrix continued to equilibrate with matrix fluids, or grew later, during retrogression. Cu–Fe sulfide mineral inclusions have been found across metamorphic grade within silicate-defined metamorphic mineral zones spanning a crustal profile of ~30 km. Bulk area scans of sulfide inclusions provide compositions that represent mixtures of the solid sulfide that were included as the host silicate minerals grew. In general, single sulfide inclusion compositions and aggregate sulfide assemblages are distinct from those of matrix phases. High Cu contents in sulfide inclusions are interpreted to be a consequence of Fe lost from sulfide to growing garnet, rather than the result of intrinsically high Cu in the bulk-rock. The distribution of sulfide inclusion compositions across metamorphic grade, considered together with the available thermodynamic data, suggests that covellite/nukundamite-bearing inclusions in lawsonite, high in both Cu and S, disappear at higher grades as these sulfide minerals are no longer stable. Similarly, clustering near the ratio Fe:Cu = 1:1 may cease with increasing grade owing to the replacement of chalcopyrite/intermediate solid solution (iss) by the denser assemblage pyrite + pyrrhotite + bornite/digenite.