ALBERT

Description: At fast-spreading mid-ocean ridges the existence of a near steady-state axial magma lens indicates that melt differentiation is an open-system process. Field relations in ophiolites and tectonic windows at fast-spreading ridges, together with some chemical characteristics of mid-ocean ridge basalts (MORB) and oceanic plutonic rocks, indicate that assimilation is a common process in and around the axial magma lens. Magma and mush zone mixing and mingling is indicated by the petrology of MORB and oceanic plutonic rocks; mush disaggregation provides an efficient mechanism for return of interstitial melt to an eruptible reservoir—a form of in situ crystallization. Despite such copious evidence to the contrary, MORB differentiation is generally modeled assuming perfect fractional crystallization (Rayleigh distillation). Here we present a simple open-system model for MORB differentiation that includes assimilation and in situ crystallization that can be used to generate synthetic basalt datasets to compare with natural sample suites. Inversion of the model allows the parental melt compositions to be estimated quantitatively. We use a numerical Bayesian inversion scheme to determine the parental melt compositions for three large (〉150 samples in each) normal-MORB suites from the East Pacific Rise. The parental melt compositions determined this way differ significantly from those that would be calculated assuming closed-system fractional crystallization. Parental MORB are more depleted than commonly assumed, suggesting that the upper mantle is more depleted than generally believed and/or that the extent of melting is larger (for example, with melts poorly focused to the ridge axis). The more depleted character of parental than erupted melts has important implications for using basalt trace element systematics in chemical geodynamic models. For example, the Sm/Nd of parental MORB are significantly lower than those of erupted MORB and this needs considering in models of the Nd-isotope evolution of the mantle.

Description: Evidence for chemical and lithological heterogeneity in the Earth’s convecting mantle is widely acknowledged, yet the major element signature imparted on mantle melts by this heterogeneity is still poorly resolved. In this study, a recent thermodynamic melting model is tested on a range of compositions that correspond to potential mantle lithologies (harzburgitic to pyroxenitic), to demonstrate its applicability over this compositional range, in particular for pyroxenite melting. Our results show that, despite the model’s calibration in peridotitic systems, it effectively reproduces experimental partial melt compositions for both Si-deficient and Si-excess pyroxenites. Importantly, the model accurately predicts the presence of a free silica phase at high pressures in Si-excess pyroxenites, indicating the activation of the pyroxene–garnet thermal divide. This thermal divide has a dominant control on solidus temperature, melt productivity and partial melt composition. The model is used to make new inferences on the link between mantle composition and melting behaviour. In silica-deficient and low-pressure (olivine-bearing) lithologies, melt composition is not very sensitive to source composition. Linearly varying the source composition between peridotite and basaltic pyroxenite, we find that the concentration of oxides in the melt tends to be buffered by the increased stability of more fusible phases, causing partial melts of even highly fertile lithologies to be similar to those of peridotite. An exception to this behaviour is FeO, which is elevated in partial melts of silica-deficient pyroxenite even if the bulk composition does not have a high FeO content relative to peridotite. Melt Al2O3 and MgO vary predominantly as a function of melting depth rather than bulk composition. We have applied the thermodynamic model to test the hypothesis that Fe-rich mantle melts such as ferropicrites are derived by partial melting of Si-deficient pyroxenite at elevated mantle potential temperatures. We show that the conspicuously high FeO in ferropicrites at a given MgO content does not require a high-Fe mantle source and is indeed best matched by model results involving around 0–20% melting of silica-deficient pyroxenite. A pyroxenite source lithology also accounts for the low CaO content of ferropicrites, whereas their characteristic low Al2O3 is a function of their high pressure of formation. Phanerozoic ferropicrites are exclusively located in continental flood basalt (CFB) provinces and this model of formation confirms that lithological heterogeneity (perhaps recycled oceanic crust) is present in CFB mantle sources.

Description: As the major component, Archean granitoids provide us with an insight into the formation of the early continental crust. We report the study of a series of Neoarchean granitoids, including tonalite–trondhjemite–granodiorite (TTG) and potassic granitoids, in the Xingcheng region of the eastern North China Craton. Zircon U–Pb dating shows that the TTG granitoids were emplaced in the Neoarchean within a 75 Myr period (2595–2520 Ma), with coeval mafic magmatic enclaves, followed by intrusion of potassic granitoids. The geochemistry of the TTG granitoids is consistent with partial melting of Mesoarchean enriched mafic crustal sources at different depths (up to 10–12 kbar equivalent pressure) during a continental collision event. The potassic granitoids are derived from either low-degree melting of Mesoarchean enriched mafic crustal sources or remelting of Mesoarchean TTGs in response to post-collisional extension, and were hybridized with Neoarchean mantle-derived mafic melts to various degrees. The TTG and potassic granitoids in the Xingcheng region record the evolution from collision of micro-continental blocks to post-collisional extension, consistent with other studies, suggesting that the amalgamation of micro-continental blocks is what gave rise to the cratonization of the North China Craton at the end of the Archean. The rock assemblage of these granitoids resembles those of syn- and post-collisional magmatism in Phanerozoic orogenic belts, and the estimated average composition is similar to that of the present-day upper continental crust, suggesting that a prototype upper continental crust might have been developed at the end of the Archean from a mixture of TTG and potassic granitoids. Together with concurrent high-grade metamorphism in the North China Craton, we conclude that collisional orogenesis is responsible for continental cratonization at the end of the Archean in the North China Craton.

Description: On 1 January 2008 Llaima volcano, a basaltic andesite stratocone in southern Chile, entered a phase of violent Strombolian eruption. Llaima, like many passively degassing systems, has experienced prolonged (decades-long) periods of persistent summit degassing from its open vent. The rapid transition from long-lived passive degassing to violent explosive eruption occurred with limited precursory monitoring signals. This study is motivated by the desire to understand what occurs in these systems when that switch takes place. To this end, we study the products of the 2008 violent Strombolian eruption of Llaima volcano. We present new textural analyses of scoria and geochemical data for five whole-rock samples, troctolite glomerocrysts with and without Cr-spinel, and 182 olivine-hosted melt inclusions from tephra samples. Two populations of scoria (‘brown’ and ‘black’) are distinguished by their variable crystallinity and vesicularity, but are geochemically indistinguishable. Black scoria contains abundant microlites with tabular to acicular morphologies and convolute vesicles up to 1·75 mm in effective diameter. The brown scoria tends to have fewer, acicular microlites, abundant matrix glass, and round vesicles with a narrower size distribution, constrained to 〈 0·4 mm in diameter. Overall, the textures of the black and brown scoria provide evidence for a textural maturation process in which shallow system magma becomes more crystal rich and probably rheologically stiffer as a result of prolonged passive degassing. The Cr-spinel-bearing and Cr-spinel-free troctolite glomerocrysts have plagioclase and olivine compositions of An 65–92 and Fo 81 , respectively. The Cr# in the Cr-spinel ranges from 26 to 37, consistent with magma originating from the deeper plumbing system. Whole-rock compositions for the tephra average 51 wt % SiO 2 , 18 wt % Al 2 O 3 , and ~6 wt % MgO. The major element compositions of olivine-hosted melt inclusions range from 49 to 56 wt % SiO 2 and 3·72 to 7·76 wt % MgO; there is no distinct compositional difference between olivine-hosted melt inclusions sourced from the different scoria. Melt inclusion volatile contents range from below detection to 2·95 wt % H 2 O and 1973 ppm CO 2 (though not in the same melt inclusion). H 2 O and CO 2 concentrations are consistent with open-system degassing and, when compared with differentiation indices (e.g. K 2 O), indicate coupled degassing and crystallization throughout the system. The majority of melt inclusions define a single liquid line of descent indicative of plagioclase and olivine fractionation. Entrapment pressures range from 8 to 342 MPa and fall into two groups: 8–100 MPa (300 m to ~4 km depth) and 〉100 MPa (4–14 km depth), revealing that this eruption tapped a deep plumbing system. We propose here that passive degassing at Llaima is maintained by periodic, small-batch magma injections. Consequently, owing to extensive degassing the upper plumbing system magma crystallized and increased in viscosity. Before the 2008 eruption, some volatiles sourced from the repeatedly injected magmas exsolved from the inferred crystal mush and ascending from deeply sourced degassing magmas, and gradually accumulated within the crystal mush and beneath the stiffened conduit magma. Our results support a model in which eruption triggering occurred when magma injection remobilized the mush and, importantly, unlocked the accumulated gases, which ascended rapidly and generated the observed violent Strombolian explosive activity. Our proposed model contrasts with those models for explosive mafic volcanism that require rapid magma ascent under closed-system degassing conditions. Importantly, our proposed mechanism provides a means for systems with dominantly open-system degassing behavior to switch from passive degassing to explosive eruptions.

Description: The island of Kos at the eastern end of the South Aegean Volcanic Arc hosts numerous late Neogene amphibole- and mica-bearing lamprophyres. The Sr and Nd isotope compositions of these mantle-derived magmatic rocks correlate negatively, extending from the undepleted end of the mantle array towards values typical for continental crust. The crust-like signature is linked to the addition of subducted Mediterranean sediments, which dominate the Pb isotopic composition of all lamprophyres. The mantle endmember of the Sr–Nd isotope mixing array is strongly enriched in incompatible trace elements and shows anorogenic affinities. Whole-rock geochemistry indicates that the lamprophyres originate from depleted lithospheric mantle above the Hellenic subduction zone that was modified by (1) K-rich silicic partial melts or supercritical fluids from subducted metasediments and (2) melts that originate from the asthenosphere below the subducting slab. The first metasomatic component formed phlogopite–orthopyroxene-rich veins, whereas the second component formed amphibole-bearing clinopyroxenites. Subsequent melting preferentially affected these enriched domains. Variable dilution by contributions from ambient peridotite and mixing between the two components caused the large chemical variation observed in the lamprophyres. The asthenosphere-derived component represents an incipient melt with a carbonatite-like trace element signature (e.g. superchondritic Nb/Ta and Zr/Hf; low Ti/Eu). This metasomatic agent probably was introduced into the mantle wedge along ruptures in the slab of the retreating Hellenic subduction zone. Lamprophyres dominated by the sediment signature have isotopic and chemical similarities to basaltic rocks erupted along the South Aegean Volcanic Arc. The much stronger enrichment in incompatible trace elements of the lamprophyres is related to only minor dilution by melts from the ambient mantle. Partial melting in the lithospheric mantle is attributed to extensional tectonics, probably during a stage of rapid slab rollback, with a limited availability of fluid as a fluxing agent. Lamprophyre emplacement occurred along sinistral en echelon structures in a crustal-scale shear zone, which separates Anatolia from the faster extending Aegean back-arc basin.

Description: An exposure of polyphase, Neoarchaean–early Palaeoproterozoic basement rocks has been identified in the northeastern part (10°53·44'N, 78°22·88'E) of the Madurai Province. The dominant migmatitic charnockite contains older enclaves of isoclinally folded syenitic biotite gneiss and boudinaged mafic dykes. A highly evolved (Mg# 10) and high field strength element (HFSE) + rare earth element (REE)-enriched ferroan basic dyke body exhibits a distinct mineralogical zonation produced during intense syn-boudinage infiltration metasomatism, coeval with high-grade metamorphism and anatexis of the host charnockite. Core domains that preserve an anhydrous granulite assemblage (domain A: garnet, clinopyroxene and minor plagioclase) grade through a narrow mineralogical transition zone (domain B) into a broad amphibole-rich rind (domain C: ferropargasite + K-feldspar ± plagioclase, quartz). Pseudosection modelling and thermobarometry yields P – T estimates of ~800 °C, 8 kbar for the granulite-facies metamorphism (M 1 ) and ~730 °C, 7 kbar for the high-grade metasomatism (M 2 ). The changes in fabric, mineral assemblage and whole-rock chemistry across domains A to C reveal near-isovolumetric–isochoric conditions of infiltration-driven metasomatism and an unusual enrichment in large ion lithophile elements (LILE), REE, and HFSE, causing prolific neoblastesis of apatite and zircon, and attest attainment of chemical equilibrium. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analyses for major constituents and accessory phases in the metasomatic domains provide a new set of equilibrium distribution coefficients for the REE. LA-ICP-MS spot analyses of zircon in well-documented microstructural settings identified the following events: ~2·67 Ga: intrusions of foid-bearing syenite and ferroan basic dykes during crustal extension; ~2·6 Ga: high-grade metamorphism M 1 ; ~2·6 Ga: emplacement of voluminous arc-type intrusions (the protolith of charnockite); ~2·48 Ga: high-grade metamorphism and anatexis M 2 of the charnockite protolith and metasomatism of the ferroan metabasite. A distinct Pan-African overprint (M 3 , ~610 Ma, 520 Ma) of the Palaeoproterozoic high-pressure rocks is manifested by hydration related to the exhumation of the deep-seated granulites to mid-crustal levels. This study confirms the continuation of Neoarchaean crust farther south beyond the perceived Palghat Cauvery shear zone system and contradicts the view that this shear zone system represents the Neoproterozoic–Cambrian suture zone along which the Mozambique Ocean was closed.

Description: High-precision electron microprobe analyses were obtained on olivine grains from Klyuchevskoy, Shiveluch and Gorely volcanoes in the Kamchatka Arc; Irazú, Platanar and Barva volcanoes of the Central American Arc; and mid-ocean ridge basalt (MORB) from the Siqueiros Transform. Calcium contents of these subduction zone olivines are lower than those for olivines from modern MORB, Archean komatiite and Hawaii. A role for magmatic H 2 O is likely for subduction zone olivines, and we have explored the suggestion of earlier workers that it has affected the partitioning of CaO between olivine and silicate melt. We provide a provisional calibration of D CaO Ol/L as a function of magmatic MgO and H 2 O, based on nominally anhydrous experiments and minimally degassed H 2 O contents of olivine-hosted melt inclusions. Application of our geohygrometer typically yields 3–4 wt % magmatic H 2 O at the Kamchatka and Central American arcs for olivines having ~1000 ppm Ca, which agrees with H 2 O maxima from melt inclusion studies; Cerro Negro and Shiveluch volcanoes are exceptions, with about 6% H 2 O. High-precision electron microprobe analyses with 10–20 μm spatial resolution on some olivine grains from Klyuchevskoy and Shiveluch show a decrease in Ca content from the core centers to the rim contacts, and a sharp increase in Ca in olivine rims. We suggest that the zoning of Ca in olivine from subduction zone lavas may provide the first petrological record of temporal changes that occur during hydration of the mantle wedge and dehydration during ascent, and we predict olivine H 2 O contents that can be tested by secondary ionization mass spectrometry analysis.

Description: Trachytes are typically interpreted in terms of extreme fractional crystallization from basaltic magmas. Data from Mauritius suggest otherwise. Here, intrusive, nepheline-bearing trachytes are associated with Older Series basalts (9·0–4·7 Ma), as confirmed by a U–Pb zircon age of 6·8 Ma. Trachyte mineralogy is dominated by low-Ca alkali feldspar with variable Na/(Na + K), lesser high-Na nepheline, aegirine-rich clinopyroxene, titanomagnetite and accessory zircon and apatite. A few samples contain xenocrysts of anorthoclase, Al- and Ti-rich clinopyroxene and kaersutite; these phases show prominent reaction rims that approach typical trachyte mineral compositions. Trachyte major elements cluster at ~63 wt % SiO 2 and Na 2 O + K 2 O ~12 wt %, but reconstructed pre-alteration compositions suggest that they were originally mainly phonolites, and form a prominent Daly Gap when plotted with the basalts. Incompatible trace elements are enriched in all trachytes, except for Ba, Sr and Eu, which show prominent negative anomalies. Rare earth element patterns have variable abundances, prominent negative Eu anomalies and shapes that differ markedly from those of the basalts. Initial Nd values cluster at + 4·03 ± 0·15 ( n = 13), near the lower end of the range for basalts ( Nd = +3·70 to + 5·75), but the initial 87 Sr/ 86 Sr is highly variable (I Sr = 0·70408–0·71034) compared with the relatively constant I Sr of 0·70411 ± 19 for the basalts. Fractional crystallization models, using the PELE and MELTS algorithms, starting with a primitive, nepheline-normative Mauritian basalt parent ( P = 1 kbar, f O 2 = QFM – 3, where QFM is quartz–fayalite–magnetite buffer) fail, because when plagioclase joins olivine in the crystallizing assemblage, successive liquids become depleted in Al 2 O 3 , do not produce nepheline, and do not approach phonolitic or trachytic compositions. Similar results are obtained using more alkaline parent liquids including basanite; however, such compositions are not observed anywhere amongst the Older Series basalts. Plutonic xenoliths from Mauritius do not fill the Daly Gap as in some other occurrences (e.g. Hawaii, Pantelleria, Azores). Fractional crystallization was not the operative process that produced Mauritian trachytes. Likewise, liquid immiscibility is excluded because the compositions do not fall at the ends of known miscibility gaps. What remains as plausible is some type of partial melting process, although the source cannot be Precambrian continental crust, as suggested to exist under Mauritius, because such material should not yield nepheline-bearing melts, and would not account for the Sr–Nd isotopic compositions. Small amounts of contamination with such continental crust, however, can account for the Sr–Nd isotopic compositions of the trachytes. Partial melting of basaltic volcanic rocks or extant gabbroic bodies, either from the oceanic crust or from Réunion plume-related magmas, should yield quartz-saturated melts different from the critically undersaturated Mauritian trachytes. A remaining possibility is that the trachytes represent direct, small-degree partial melts of fertile, perhaps metasomatized mantle; we suggest that the xenocryst assemblage present in some samples might represent fragments of this source. A mantle source is supported by the presence of trachytic and phonolitic glasses in many mantle xenoliths, and experimental results show that low-degree alkaline melts can be produced from mantle peridotites even under anhydrous conditions. If some feldspar is left behind as a residual phase, this would account for the negative Ba, Sr and Eu anomalies observed in Mauritian trachytes. These considerations may also apply to other trachyte and phonolite occurrences worldwide.

Description: The 1·3 Ma Mesa Falls Tuff (MFT), the second and volumetrically smallest of the Yellowstone caldera-forming eruptions, was examined using a joint zircon petrochronological and sanidine 40 Ar/ 39 Ar approach to constrain the thermal and chemical evolution, autocrystic growth, antecrystic recycling, and eruptive age of the host magma. A total of 451 laser ablation inductively coupled plasma mass spectrometry in situ spot analyses collected from 323 zircon crystals from five pumice blocks and two welded ash-flow tuff samples provide trace element and Ti-in-zircon thermometry data, which are in turn complemented by high-precision 206 Pb/ 238 U dates from over 50 of those grains. Sanidine grains from two of the pumices were analyzed by incremental step-heating or total fusion 40 Ar/ 39 Ar dating techniques performed on single crystals using a multi-collector mass spectrometer, yielding an eruption age of 1·300 ± 0·001 Ma. Zircon dates range from 1·57 to 1·30 Ma. Rare grains older than 1·37 Ma may contain inherited cores recycled from the Huckleberry Ridge Tuff (HRT) or other associated smaller volume, effusive Yellowstone magmas; however, the bulk of the Mesa Falls Tuff crystal load cannot be attributed to a long-lived, residual Huckleberry Ridge Tuff magma body. Zircon compositions define trends of strengthening negative europium anomaly and increasing incompatible trace element concentrations over ~150 °C of cooling. Crystals defining this full compositional spectrum range in age from 1·33 to 1·30 Ma; the dominant mode of 19 grains yields a mean crystallization age of 1·303 ± 0·002 Ma, within uncertainty of the sanidine 40 Ar/ 39 Ar age, attesting to the rapidity of magma accumulation, differentiation and crystallization prior to eruption. A subset of composite grains composed of extremely differentiated core compositions overgrown by Mesa Fall Tuff-like rims probably represents earlier solidified sidewall or roof accumulations later remobilized within the main Mesa Falls Tuff magma. Fractional crystallization modeling utilizing temperature-dependent zircon–melt partition coefficients is successful in reproducing the trends in incompatible trace element enrichment within zircon grains as a function of decreasing temperature and increasing europium anomaly. Zircon geochemistry thus provides a robust proxy for magma evolution, from the time of zircon saturation through differentiation and eruption. Integrated sanidine and zircon dates coupled with the thermochemical trends indicate that the Mesa Falls Tuff magmatic system differentiated over a period of 〈 30 kyr, with the bulk of zircon crystal nucleation and growth occurring within 10 kyr of eruption. These petrochronological studies of the MFT and HRT clearly illustrate that the long-term volumetric extrusive rate at Yellowstone (~2 x 10 –3 km 3 a –1 ) is punctuated by episodes of much higher magmatic flux (~2 x 10 –2 to ~2 x 10 –1 km 3 a –1 ).

Description: As the major component, Archean granitoids provide us with an insight into the formation of the early continental crust. We report the study of a series of Neoarchean granitoids, including tonalite–trondhjemite–granodiorite (TTG) and potassic granitoids, in the Xingcheng region of the eastern North China Craton. Zircon U–Pb dating shows that the TTG granitoids were emplaced in the Neoarchean within a 75 Myr period (2595–2520 Ma), with coeval mafic magmatic enclaves, followed by intrusion of potassic granitoids. The geochemistry of the TTG granitoids is consistent with partial melting of Mesoarchean enriched mafic crustal sources at different depths (up to 10–12 kbar equivalent pressure) during a continental collision event. The potassic granitoids are derived from either low-degree melting of Mesoarchean enriched mafic crustal sources or remelting of Mesoarchean TTGs in response to post-collisional extension, and were hybridized with Neoarchean mantle-derived mafic melts to various degrees. The TTG and potassic granitoids in the Xingcheng region record the evolution from collision of micro-continental blocks to post-collisional extension, consistent with other studies, suggesting that the amalgamation of micro-continental blocks is what gave rise to the cratonization of the North China Craton at the end of the Archean. The rock assemblage of these granitoids resembles those of syn- and post-collisional magmatism in Phanerozoic orogenic belts, and the estimated average composition is similar to that of the present-day upper continental crust, suggesting that a prototype upper continental crust might have been developed at the end of the Archean from a mixture of TTG and potassic granitoids. Together with concurrent high-grade metamorphism in the North China Craton, we conclude that collisional orogenesis is responsible for continental cratonization at the end of the Archean in the North China Craton.