ALBERT

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: 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: We report the results of experiments on two natural marine sediments with different carbonate contents (calcareous clay: CO 2 = 6·1 wt %; marl: CO 2 = 16·2 wt %) at subduction-zone conditions (3 GPa, 750–1200°C). Water (7–15 wt %) was added to the starting materials to simulate the effects of external water addition from within the subducting slab. The onset of melting is at 760°C in water-rich experiments; melt becomes abundant by 800°C. In contrast, the onset of melting in published, water-poor experiments occurs at variable temperatures with the production of significant melt fractions being restricted to more than 900°C (phengite-out). The different solidus temperatures ( T solidus ) can be ascribed to variable fluid X H2O [H 2 O/(CO 2 + H 2 O)], which, in turn, depends on bulk K 2 O, H 2 O and CO 2 . Partial melts in equilibrium with residual garnet, carbonate, quartz/coesite, epidote, rutile, kyanite, phengite, and clinopyroxene are granitic in composition, with substantial dissolved volatiles. Supersolidus runs always contain both silicate melt and solute-rich fluid, indicating that experimental conditions lie below the second critical endpoint in the granite–H 2 O–CO 2 system. Carbonatite melt coexists with silicate melt and solute-rich fluid above 1100°C in the marl. The persistence of carbonate to high temperature, in equilibrium with CO 2 -rich hydrous melts, provides a mechanism to both supply CO 2 to arc magmas and recycle carbon into the deep Earth. The trace element compositions of the experimental glasses constrain the potential contribution of calcareous sediment to arc magmas. The presence of residual epidote and carbonate confers different trace element characteristics when compared with the trace element signal of Ca-poor marine sediments (e.g. pelagic clays). Notably, epidote retains Th and light rare earth elements, such that some melts derived from calcareous sediments have elevated Ba/Th and U/Th, and low La/Sm PUM , thereby resembling fluids conventionally ascribed to altered oceanic crust. Our results emphasize the importance of residual mineralogy, rather than source lithology, in controlling the trace element characteristics of slab-derived fluids.

Description: Major ± trace element and Sr–Nd–Pb–Hf–He isotope data are presented for more than 300 geochemically diverse basalt samples collected by submersible from the Inflated Central Endeavour Segment of the Juan de Fuca Ridge. Seven chemically distinct basalt types are present, from depleted (D-) to enriched mid-ocean ridge basalt (E-MORB). By combining the geochemical data with high-resolution bathymetry and age determinations, the detailed spatial and temporal scale of on-axis mantle-derived basalt heterogeneity is determined. The basalts define binary mixing arrays in all isotope plots that are usual in their correlations, but unusual in the limited range of Sr–Nd–Hf isotope compositions for D- to E-MORB, and greater range in Pb isotopes. The basalts also define two different styles of enrichment of moderately incompatible elements. Geochemical enrichment began when the currently inflated axial ridge formed 〈10 5 years ago. One enrichment style (the Inflated Ridge Trend) characterizes basalts erupted across the ~5 km wide ridge from 〉10^000 to ~4000 years ago, whereas the other enrichment style (the Graben Trend) characterizes most basalt types erupted within the axial graben after it formed ~2300 years ago. We attribute the Inflated Ridge Trend to a relatively high proportion of pyroxenite (or melt derived therefrom) to enriched peridotite in the mantle during a phase of ridge inflation that lasted at least 6000 years. The Graben Trend reflects the reduced effect of pyroxenite after the axial graben formed. Because at least 14 different samplings of mantle components occurred within 〈1 km of ridge length and width during a time when 〈1 km of upwelling occurred, we infer that the scale of mantle heterogeneity far from a plume is 〈 1 km. The enriched mantle component at Endeavour is young with 206 Pb/ 204 Pb ~19·0; Hf and He isotope ratios trend toward HIMU characteristics. These traits are regionally widespread and are shared with the next two ridge segments to the north (West Valley and Explorer).

Description: The Talbot Sub-basin is one of several bimodal volcanic depositional centres of the Mesoproterozoic Bentley Basin in central Australia. It is dominated by rocks of rhyolitic composition and includes ignimbrites, some forming large to super-eruption size deposits. Ferroan, incompatible trace element enriched, A-type compositions, anhydrous mineralogy and clear evidence for local rheomorphism indicate high eruption temperatures, with apparent zircon-saturation temperatures suggesting crystallization at 〉900°C. Comagmatic basalt is of mantle origin with minor Proterozoic basement contamination. The rhyolites cover the same range of Nd isotope compositions ( Nd(1070) +1·24 to –0·96) and La/Nb ratios (1·2–2·1) as the basalts ( Nd(1070) +2·1 to –1·1: La/Nb 1·2–2·3) and are compositionally far removed from all older basement and country-rock components (average Nd(1070) = –4, La/Nb = 10). The rhyolites and basalts are cogenetic through a process probably involving both fractional crystallization of mafic magmas and partial melting of recently crystallized mafic rock in a lower crustal intraplate, extraction of dacitic magmas to a voluminous upper crustal chamber system, and separation of rhyolite by processes involving rejuvenation and cannibalization of earlier chamber material. More than 230 000 km 3 of parental basalt is required to form the 〉22 000 km 3 of preserved juvenile rhyolite in the Talbot Sub-basin alone, which represents one of the most voluminous known felsic juvenile additions to intracontinental crust. Zircon U–Pb age components are complex and distinct from those of basement and country rock and contain antecrystic components reflecting dissolution–regrowth processes during periodic rejuvenation of earlier-emplaced chamber material without any significant interaction with country rock. The overall duration of magmatism was 〉30 Myr but can be divided into between two and four separate intervals, each probably of a few hundred thousand years’ duration and each probably reflecting one of the distinct lithostratigraphic groups defined in the sub-basin. Neither the composition nor style of felsic and mafic volcanism changes in any significant way from one volcanic event to the next and the range of zircon U–Pb ages indicates that each period utilized and cannibalized the same magma chamber. This volcanism forms a component of the 1090–1040 Ma Giles Event in central Australia, associated with magma-dominated extension at the nexus of the cratonic elements of Proterozoic Australia. This event cannot be reasonably reconciled with any putative plume activity but rather reflects the 〉200 Myr legacy of enhanced crustal geotherms that followed the final cratonic amalgamation of central Australia.

Description: Ophiolites allow spatial and temporal assessment of the causes and length-scales of upper mantle compositional heterogeneity because they permit field-based observations to be coupled with geochemical investigations of upper mantle lithologies. The ~497 Ma Leka Ophiolite Complex (Norway) comprises a section of early Palaeozoic (Iapetus) oceanic lithosphere with well-exposed mantle and lower crustal sections and generally low degrees of serpentinization. The Leka upper mantle section is heterogeneous at the centimetre to metre scale, manifested by abundant dunite lenses and sheets in harzburgitic host-rock, especially within ~500 m of the palaeo Moho. Abundant chromitite (≥60 vol. % Cr-spinel) and pyroxenite lenses and layers also occur in the uppermost 200–300 m of the mantle section. These diverse mantle lithologies probably developed in a suprasubduction-zone (SSZ) setting, as a result of fluid-assisted melt extraction, offering an opportunity to interrogate the nature of chemical heterogeneities developed in such rocks. At ~497 Ma, the Os isotopic compositions of Leka harzburgites averaged ~2% more radiogenic than the projected average for abyssal peridotites at that time, yet they exhibit nearly chondritic relative abundances of the highly siderophile elements (HSE). Several of the harzburgites are characterized by low initial 187 Os/ 188 Os (〈0·121), reflecting Proterozoic melt depletion. Preservation of Os isotopic compositions consistent with ancient (〈0·5 to 2 Ga) melt depletion episodes is a common characteristic of melt-depleted oceanic peridotites. There is no clear evidence that SSZ melt extraction had a discernible impact on the bulk Os isotopic composition of the Iapetus oceanic mantle, as represented by the Leka harzburgites. By contrast, non-harzburgitic lithologies are generally characterized by more radiogenic initial 187 Os/ 188 Os and more variable HSE abundances. The dunites, chromitites and pyroxenites of the LOC can be separated into two groups on the basis of their trace element geochemistry and the Re-Os isotope errorchrons that they define, yielding ages of 485 ± 32 Ma and 589 ± 15 Ma, respectively. The former age corresponds, within error, to the accepted age of the ophiolite (497 ± 2 Ma). The meaning of the latter age is uncertain, but possibly corresponds to the early stages of Iapetus opening. The Leka ophiolite reveals the importance of oceanic lithosphere formation processes for mantle heterogeneity at metre to kilometre scales, but also emphasizes the robustness of Os isotopes in recording older melt-depletion events.

Description: Regionally metamorphosed pelitic rocks at Campolungo, Central Alps, contain biotite, muscovite, garnet, staurolite, kyanite, and quartz, and the minor minerals tourmaline, plagioclase, chlorite, rutile, and ilmenite. Accessory allanite, apatite, monazite, potassium feldspar, xenotime, and zircon have also been identified. The bulk-rock chemical composition is similar to that of shales, and indicates that the protolith was deposited in an active continental margin setting. Element distribution maps, electron microprobe analyses and in situ UV–laser ablation inductively coupled plasma mass spectrometry data document a pronounced zoning in garnet and tourmaline porphyroblasts. Garnet displays a typical bell-shaped MnO zoning profile, with a maximum (~3 wt %) in the euhedral core. Cores are also rich in Y and heavy rare earth elements (HREE; e.g. 2150 ppm Y). In their broad rim, all garnet crystals display a subhedral annulus (10–15 µm wide), which is distinctly enriched in Ca, Sr, Y, and HREE, and which probably resulted from the breakdown of allanite (at ~550°C, ~6·4 kbar). Another characteristic feature of garnet rims is their sinusoidal chondrite-normalized REE pattern, which may represent partial equilibration with a light REE-enriched medium, probably generated through the breakdown of metamorphic allanite. Similar REE patterns are exhibited by a Ca-poor internal zone (inside the annulus), which may represent an earlier partial equilibration following the breakdown of detrital monazite. The large tourmaline crystals exhibit an optically visible three-stage zoning, which comprises: a euhedral core; a continuously zoned inner rim with a prominent euhedral Ca-rich annulus; and an outer rim, which also displays a distinct Ca-rich annulus and is separated from the inner rim by a sutured boundary. This boundary represents a marked chemical discontinuity, characterized for example by a decrease in the Zn concentration from 250 ppm (inner rim) to 20 ppm (outer rim). This change in Zn content reflects staurolite growth, which started after resorption of the inner rim of tourmaline and after a major deformation event. This chemical and textural discontinuity coincides with a marked shift in 18 O, which increases by ~0·8 across the inner rim–outer rim boundary. Our thermodynamic models suggest that resorption of the inner rim of tourmaline may be associated with small amounts (5–7 vol. %) of melt formed at ~650°C and 8·5 kbar. By using detailed textural observations, major and trace element zoning patterns and thermodynamic data, it was possible to model the metamorphic evolution of these rocks in considerable detail and, specifically, to correlate the growth and breakdown of major and accessory minerals.

Description: The Taupo Volcanic Zone (TVZ), located in the North Island of New Zealand, represents part of a magmatic arc that is at present undergoing active extension. Around 0·9 Myr ago, an acceleration in rifting was followed by a progressive transition in the composition of volcanic products (until ~0·7 Ma) from typical arc-type andesite into overwhelmingly large, caldera-forming rhyolitic eruptions with subordinate basalt and dacite in the Central TVZ. Despite an obvious compositional gap in the erupted products in the Central TVZ within the last 0·7 Myr (little to no erupted products with SiO 2 contents between 55 and 65 wt %), phenocryst minerals (plagioclase, amphibole, pyroxene) show an uninterrupted compositional record that suggests crystallization from a continuum of melt compositions. Coupled with radiogenic isotope evidence, the whole-rock and mineral chemistry data are consistent with magmatic differentiation controlled by crystal fractionation of primary mantle-derived magmas accompanied by some assimilation of local wall-rocks. In the Southern TVZ and in the early part of the Central TVZ, magmatic differentiation was dominated by the lower crustal evolution of relatively dry (~1 wt % H 2 O) arc basalts, crystallizing a pyroxene–plagioclase-dominated assemblage. However, the conditions of crystallization in the lower crust appear to have changed within the last million years in the Central TVZ, with amphibole and oxides appearing earlier in the crystallization sequence. In this framework and using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs, we show that melts extracted from crystal mushes within an optimal ‘extraction window’ (~50 and 80 vol. % crystals) match those erupted at the surface. Lower crustal mushes fed by basalt with 1 wt % H 2 O (dominated by a pyroxene–plagioclase assemblage) release andesitic melts at the extraction window. These melts then erupt at the surface to form the observed andesitic part of the arc. With a slightly higher water content (~2 wt %) in the basalt, the melt composition at the extraction window from lower crustal mushes is dacitic rather than andesitic. Although some dacitic melts will reach the surface, most will be trapped in the upper crust and crystallize to form a silicic mush. Extraction of the interstitial liquid after 〉50% crystallization from this upper crustal reservoir produces the large volumes of rhyolitic magma erupted over the past 0·7 Myr (〉4000 km 3 from ignimbrite-forming eruptions).

Description: The Musgrave Province lies at the convergence of major structural trends formed during the Proterozoic amalgamation of the North, West and South Australian Cratons prior to c. 1290 Ma. The Musgrave Orogeny, one of three Mesoproterozoic orogenies to affect the province, produced the granites of the Pitjantjatjara Supersuite, which dominate the outcrop. This orogeny was an intracontinental and dominantly extensional event in which ultrahigh-temperature (UHT) conditions persisted from c. 1220 to c. 1120 Ma. The onset of UHT conditions is heralded by a change from low-Yb granites to voluminous Yb-enriched granites, reflecting a rapid decrease in crustal thickness. The Pitjantjatjara granites are ferroan, calc-alkalic to alkali-calcic rocks and include charnockites with an orthopyroxene-bearing primary mineralogy. They were emplaced at temperatures ≥1000°C from c. 1220 to c. 1150 Ma. Their geochemical and Nd and Hf isotopic homogeneity over a scale of 〉15 000 km 2 reflects a similarly homogeneous source. This source included an old enriched felsic crustal component. However, the bulk source was mafic to intermediate in composition. The long-lived UHT regime, and thermal limits on the amount of crust sustainable below the level of intrusion, indicates a significant (〉50%) mantle-derived source component. However, a positive correlation between Mg-number and F suggests that many Pitjantjatjara granites formed through the breakdown of F-rich biotite in a crustal granulite. We suggest that under- and intraplated mafic magmas assimilated the limited available felsic crust into lower crustal MASH (melting, assimilation, storage, homogenization) domains. These partially cooled but were remobilized during subsequent under- and intra-plating events to produce the Pitjantjatjara granites. The duration of UHT conditions is inconsistent with a mantle plume. It reflects an intracontinental lithospheric architecture where the Musgrave Province was rigidly fixed at the nexus of three thick cratonic masses. This ensured that any asthenospheric upwelling was focused beneath the province, providing a constant supply of both heat and mantle-derived magma.