Showing papers in "Journal of Petrology in 2011"

On Ferroan (A-type) Granitoids: their Compositional Variability and Modes of Origin

Abstract: We recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry.These include alkalic granitoids that may be metaluminous or peralkaline, alkali-calcic granitoids that may be metaluminous, peraluminous or peralkaline, calc-alkalic granitoids that may be metaluminous or peraluminous, and rare calcic ferroan granitoids. These granitoids may form by two distinct end-member processes. Extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali-calcic, or metaluminous alkalic, alkali-calcic, and calc-alkalic, with alkalinity increasing with increasing pressure of differentiation. Partial melting of tonalitic to granodioritic crust produces alkali-calcic to calc-alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures. It is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids. Most granitoids that are referred to as ‘ ferroan. However, the term ‘ has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis. For this reason we recommend that the term ‘ be discontinued and ‘ferroan’ used in its stead.

Identification of Source Lithology in the Hawaiian and Canary Islands: Implications for Origins

Abstract: Results are reported of an exploration of mantle source lithology for intraplate magmas using whole-rock and olivine phenocryst compositions.This analysis includes modern mid-ocean ridge basalts and Archean komatiites as low- and high-temperature reference frames. It is shown that the Ni, Ca, Mn, and Fe/Mn contents of olivine phenocrysts in modern mid-ocean ridge basalts and Archean komatiites are consistent with a normal peridotite source. In contrast, olivine phenocrysts in shield-building lavas on Hawaii are higher in Ni and Fe/ Mn, and lower in Mn and Ca than those expected to crystallize from meltsofa normal peridotite source, and point tothe importance ofpyroxenite as proposed by Sobolev and co-workers. Hawaiian shield stage lavas and their olivine phenocrysts are similar to those expected from partial melts of a 100% stage 2 pyroxenite source. Such a source might form from a variety of melt^rock, melt^melt, and rock^rock reactions. Primary pyroxenite-derived magmas have a range of SiO2 contents that are positively correlated with 187 Os/ 188 Os and negatively correlated with 3 He/ 4 He.These results are consistent with a Hawaiian plume containing recycled crust within a peridotite matrix.Variable amounts of free silica are inferred in Hawaiian pyroxenite sources, which contribute to the production of SiO2-rich magmas. In contrast, peridotite and olivine pyroxenite melting are inferred to produce SiO2-poor pre-shield magmas at Loihi.The interaction of SiO2-rich and -poor magmas in the Hawaiian plume will trigger crystallization, not mixing. Mixing is permitted at low pressures in melt conduits and magma chambers, and work on olivine-hosted melt inclusions will be useful to evaluate its importance. In contrast to Hawaii, many ocean island basalts in localities such as the Canary Islands are deficient in SiO2, and may have been generated by partial melting of olivine pyroxenites that formed by solid-state reaction between recycled crust þ peridotite in the lower mantle. There is likely to be a wide range of whole-rock pyroxenite compositions in the mantle, as well assignificant variability in Mn and Fe/Mn in both peridotite partial melts and their olivine phenocrysts. In general, there are not likely to be welldefined end-member peridotite and pyroxenite sources in the mantle. Nevertheless, taxonomical difficulties encountered in source lithology identification may yield rich rewards, such as a better understanding of the relationship between lithological diversity in the lower mantle and its petrological expression in intraplate magmatism.

A Reappraisal of Redox Melting in the Earth’s Mantle as a Function of Tectonic Setting and Time

Abstract: Redox melting refers to any process by which melt is generated by the contact of a rock with a fluid or melt with a contrasting oxidation state. It was originally applied to melting owing to the oxidation of reduced CH4and H2-bearing fluids in contact with more oxidized blocks in the mantle, particularly recycled crustal blocks.This oxidation mechanism causes an increase in the activity of H2O by the reaction of CH4 with O2, and the increased aH2O causes a rapid drop in the solidus temperature, and is here termed hydrous redox melting (HRM). Recently, a second redox melting mechanism (carbonate redox melting; CRM) has been discovered that operates in more oxidized conditions, and may post-date the first mechanism in the same geographical area, explaining the sequence of igneous rock types from lamproites to ultramafic lamprophyres that occurs during the development of rifts through cratons. The CRM mechanism relies on the oxidation of solid carbon as graphite or diamond that has accumulated in the lithosphere over time. The solidus temperature for rocks with both CO2 and H2O is lower than in conditions with H2O alone; it does not occur at depths less than 65 km, but has recently been confirmed experimentally to depths of at least 200 km. Melts produced by HRM are not SiO2-undersaturated, even at depths of 200 km, and may often resemble lamproites or SiO2-rich picrites, whereas melts produced by CRM are always SiO2-undersaturated and range from carbonatitic to ultramafic lamprophyric or melilititic with increasing degree of melting.The operation of redox melting may be more common than has been recognized because the oxidation state of the upper mantle is not uniform as a function of depth, geodynamic setting or geological time.The general decrease of oxygen fugacity (fO2) of c. 0·7 log units per 1GPa pressure increase dictates that rapidly subducted oceanic lithosphere will be considerably more oxidized than ambient mantle peridotite at depths of 200^300 km. Hydrothermal alteration (serpentinization), addition of continental or carbonate sediments, and dehydration reactions during subduction all contribute to the heterogeneity of oxidation states in the subducted slab, which may vary over 6 log units; this raises the potential for redox reactions on local and regional scales. The oceanic lithosphere has a lower average fO2 than either continental or cratonic mantle lithosphere at a given depth, so that the HRM mechanism dominates in recycled blocks and at the base of the continental lithosphere. The higher thermal gradients dictate that HRM is more common in the modern Earth beneath ocean islands and in upwelling mantle currents than in subduction zones. The oxidation state of the mantle is often described as having been constant since 3·5 Ga, but this overlooks the bias towards continental samples. Redox melting of oxidized recycled blocks (at approximately the fayalite^magnetite^quartz buffer) in the mantle was not important in the Hadean and Archaean, as it had to await the gradual oxidation of the mantle and the establishment of the subduction process, as well as the stabilization of the continents. The lack of CRM explains the lack of carbonatites before 2·7 Ga. However, the lower fO2 of the Archaean asthenosphere and higher volatile contents caused more prevalent HRM in the Hadean and Archaean mantle. Degassing is controlled by solubility of volatile species in melts, which are H2O-rich but C-poor in reducing conditions. Silicate melts under reduced conditions contain much less carbon but more nitrogen than melts in the modern mantle, arguing for a nitrogen-rich, CO2-poor early atmosphere.

The Origin of Intra-plate Ocean Island Basalts (OIB): the Lid Effect and its Geodynamic Implications

Abstract: Based on an evaluation of major and trace element data for ocean island basalts (OIB), we demonstrate that oceanic lithosphere thickness variation, which we refer to as the lid effect, exerts the primary control on OIB geochemistry on a global scale. The lid effect caps the final depth (pressure) of melting or melt equilibration. OIB erupted on thick lithosphere have geochemical characteristics consistent with a low extent and high pressure of partial melting, whereas those erupted on thin lithosphere exhibit the reverse; that is, a high extent and low pressure of melting cessation. This observation requires that mantle melting beneath intra-plate volcanic islands takes place in the asthenosphere and results from dynamic upwelling and decompression. Melting beneath all ocean islands begins in the garnet peridotite facies, imparting the familiar 'garnet signature' to all OIB melts (e.g. [Sm/Yb](N) > 1); however, the intensity of this signature decreases with increasing extent of melting beneath thinner lithospheric lids as a result of dilution. The dilution effect is also recorded in the radiogenic isotope composition of OIB, consistent with the notion that their mantle source regions are heterogeneous with an enriched component of lower solidus temperature dispersed in a more refractory matrix. High-quality data on the compositions of olivine phenocrysts from mid-ocean ridge basalt and global OIB sample suites are wholly consistent with the lid effect without the need to invoke olivine-free pyroxenite as a major source component for OIB. Caution is necessary when using basalt-based thermobarometry approaches to estimate mantle potential temperatures and solidus depth because OIB do not unequivocally record such information. For plate ages up to similar to 80 Ma, we demonstrate that the geophysically defined base of the growing oceanic lithosphere corresponds to both an isotherm (similar to 1100 degrees C) and the pargasite (amphibole) dehydration solidus of fertile mantle peridotite. As pargasite in H2O-CO2-bearing mantle peridotite is stable under conditions of T 1100 degrees C and P 3 GPa (similar to 90 km), this solidus is essentially isothermal (i.e. dT/dP similar to 0 in P-T space) with T similar to 1100 degrees C) at depths 90 km, but becomes isobaric (i.e. dP/dT similar to 0 in P-T space) at the similar to 90 km depth. The latter explains why older (> 70 Ma) oceanic lithosphere cannot be thicker than similar to 90 km without the need to invoke physically complex processes such as convective removal.

Internal and External Fluid Sources for Eclogite-facies Veins in the Monviso Meta-ophiolite, Western Alps: Implications for Fluid Flow in Subduction Zones

Abstract: To contribute to our understanding of the mechanisms and pathways of fluid movement through deeply subducted crust, we investigate high-pressure veins cutting eclogite-facies (� 2· 0G Pa and � 6008C) metagabbros of the Monviso Ophiolite, Italian Western Alps. The veins consist mainly of omphacite with minor garnet, rutile, talc and accessory zircon. Most of the vein minerals have major and trace element compositions that are comparable with the host-rock minerals, and vein and host-rock zircons have similar Hf isotopic compositions.These observations support the conclusions of previous studies that these veins largely formed from a locally sourced hydrous fluid during prograde or peak metamorphism. However, the bulk-rock Cr and Ni contents of the veins are significantly higher than those of the surrounding host eclogites. We also document distinct Cr-rich (up to weight per cent levels) zones in omphacite, garnet and rutile in some vein samples. Vein garnet and talc also have relatively high MgO and Ni contents. X-ray maps of vein garnet and rutile grains reveal complex internal zoning features, which are largely defined by micrometre-scale variations in Cr content. Some grains have concentric and oscillatory zoning in Cr, whereas others feature a chaotic fracture-like pattern. These Cr-rich zones are associated with high concentrations of Ni, B, As, Sb, Nb, Zr and high ratios of light rare earth elements (LREE) to middle REE (MREE) compared with low-Cr vein and host-rock minerals. Petrological and mass-balance constraints verify that the Cr-rich zones in the veins were not derived from internally sourced fluids, but represent precipitates from an external fluid.The external source that is consistent with the distinctive trace element characteristics of the vein components is antigorite serpentinite, which forms the structural basement of the high-pressure metagabbros.We propose at least two separate growth mechanisms for the Monviso veins. Most vein infillings were formed during progressive prograde metamorphism from locally derived fluid. Influx of the serpentinitederived or other external fluid was transient and episodic and was probably achieved via brittle fractures, which preferentially formed along the pre-existing vein structures.The dehydration of serpentinite at high pressures in subduction zones may provide crucial volatiles and trace elements for arc magmas. Our results indicate that the movement of these fluids through subducted oceanic crust is likely to be highly channeled and transient so the progressive development of vein systems in mafic rocks may also be crucial for forming channelways for long-distance fluid flow at depth in subduction zones.

The Melting of Carbonated Pelites from 70 to 700 km Depth

Abstract: Phase assemblages, melting relations and melt compositions of a dry carbonated pelite (DG2) and a carbonated pelite with 1·1wt % H2O (AM) have been experimentally investigated at 5·5^ 23·5 GPa and 1070^15508C.The subsolidus mineralogies to 16 GPa contain garnet, clinopyroxene, coesite or stishovite, kyanite or corundum, phengite or potassium feldspar ( 8 GPa with and without H2O, respectively), and then K-hollandite, a Ti phase and ferroan dolomite/Mg-calcite or aragonite þ ferroan magnesite at higher pressures. The breakdown of clinopyroxene at 416 GPa causes Na-rich Ca-carbonate containing up to 11wt % Na2O to replace aragonite and leads to the formation of an Na-rich CO2 fluid. Further pressure increase leads to typical Transition Zone minerals such as the CAS phase and one or two perovskites, which completely substitute garnet at the highest investigated pressure (23·5 GPa). Melting at 5·5^23·5 GPa yields alkali-rich magnesio-dolomitic (DG2) to ferro-dolomitic (AM) carbonate melts at temperatures 200^3508C below the mantle geotherm, lower than for any other studied natural composition. Melting reactions are controlled by carbonates and alkali-hosting phases: to 16 GPa clinopyroxene remains residual, Na is compatible and the magnesioto ferro-dolomitic carbonate melts have extremely high K2O/Na2O ratios. K2O/Na2O weight ratios decrease from 26^41 at 8 GPa to 1·2 at 16 GPa when K-hollandite expands its stability field with increasing pressure. At 416 GPa, Na is repartitioned between several phases, and again becomes incompatible as at53 GPa, leading to Na-rich carbonate melts with K2O/Na2O ratios 1.This leaves the pressure interval of c. 4^15 GPa for ultrapotassic metasomatism. Comparison of the solidus with typical subducting slab-surface temperatures yields two distinct depths of probable carbonated pelite melting: at 6^9 GPa where the solidus has a negative Clapeyron slope between the intersection of the silicate and carbonate melting reactions at 5 GPa, and the phengite or potassium feldspar stability limit at 9 GPa. The second opportunity is related to possible slab deflection along the 660 km discontinuity, leading to thermal relaxation and partial melting of the fertile carbonated pelites, thus recycling sedimentary CO2, alkalis and other lithophile and strongly incompatible elements back into the mantle.

Monte Carlo Simulations of Metasomatic Enrichment in the Lithosphere and Implications for the Source of Alkaline Basalts

Abstract: One hypothesis for the origin of alkaline lavas erupted on oceanic islands and in intracontinental settings is that they represent the melts of amphibole-rich veins in the lithosphere (or melts of their dehydrated equivalents if metasomatized lithosphere is recycled into the convecting mantle). Amphibole-rich veins are interpreted as cumulates produced by crystallization of low-degree melts of the underlying asthenosphere as they ascend through the lithosphere. We present the results of trace-element modelling of the formation and melting of veins formed in this way with the goal of testing this hypothesis and for predicting how variability in the formation and subsequent melting of such cumulates (and adjacent cryptically and modally metasomatized lithospheric peridotite) would be manifested in magmas generated by such a process. Because the high-pressure phase equilibria of hydrous near-solidus melts of garnet lherzolite are poorly constrained and given the likely high variability of the hypothesized accumulation and remelting processes, we used Monte Carlo techniques to estimate how uncertainties in the model parameters (e.g. the compositions of the asthenospheric sources, their trace-element contents, and their degree of melting; the modal proportions of crystallizing phases, including accessory phases, as the asthenospheric partial melts ascend and crystallize in the lithosphere; the amount of metasomatism of the peridotitic country rock; the degree of melting of the cumulates and the amount of melt derived from the metasomatized country rock) propagate through the process and manifest themselves as variability in the trace-element contents and radiogenic isotopic ratios of model vein compositions and erupted alkaline magma compositions. We then compare the results of the models with amphibole observed in lithospheric veins and with oceanic and continental alkaline magmas. While the trace-element patterns of the near-solidus peridotite melts, the initial anhydrous cumulate assemblage (clinopyroxene ± garnet ± olivine ± orthopyroxene), and the modelled coexisting liquids do not match the patterns observed in alkaline lavas, our calculations show that with further crystallization and the appearance of amphibole (and accessory minerals such as rutile, ilmenite, apatite, etc.) the calculated cumulate assemblages have trace-element patterns that closely match those observed in the veins and lavas. These calculated hydrous cumulate assemblages are highly enriched in incompatible trace elements and share many similarities with the trace-element patterns of alkaline basalts observed in oceanic or continental setting such as positive Nb/La, negative Ce/Pb, and similiar slopes of the rare earth elements. By varying the proportions of trapped liquid and thus simulating the cryptic and modal metasomatism observed in peridotite that surrounds these veins, we can model the variations in Ba/Nb, Ce/Pb, and Nb/U ratios that are observed in alkaline basalts. If the isotopic compositions of the initial low-degree peridotite melts are similar to the range observed in mid-ocean ridge basalt, our model calculations produce cumulates that would have isotopic compositions similar to those observed in most alkaline ocean island basalt (OIB) and continental magmas after ~0·15 Gyr. However, to produce alkaline basalts with HIMU isotopic compositions requires much longer residence times (i.e. 1–2 Gyr), consistent with subduction and recycling of metasomatized lithosphere through the mantle. EM magmas cannot readily be explained without appealing to other factors such as a heterogeneous asthenosphere. These modelling results support the interpretation proposed by various researchers that amphibole-bearing veins represent cumulates formed during the differentiation of a volatile-bearing low-degree peridotite melt and that these cumulates are significant components of the sources of alkaline OIB and continental magmas. The results of the forward models provide the potential for detailed tests of this class of hypotheses for the origin of alkaline magmas worldwide and for interpreting major and minor aspects of the geochemical variability of these magmas.

Silicate–Carbonate Liquid Immiscibility and Phase Relations in the System SiO2–Na2O–Al2O3–CaO–CO2 at 0·1–2·5 GPa with Applications to Carbonatite Genesis

Abstract: We present new experiments, combined with a re-evaluation of published data, to characterize the topology of the silicate^carbonate two-liquid solvus in the five-component system SiO2^Na2O^ Al2O3^CaO^CO2 (SNACþCO2). Conjugate liquid compositions have been determined for a wide range of pressures (0·1^2·5 GPa) and temperatures (1225^17008C) as well as variable degrees of CO2 saturation.The expansion of the two-liquid field with increasing pressure and/or decreasing temperature, and the contraction of the two-liquid field for conditions where PCO25Ptotal is accurately presented for the first time.The shape of the two-liquid solvus suggests that alkali-rich carbonatites can have a range of SiO2þAl2O3 contents down to very low values (51wt %), but that low-alkali or alkali-free immiscible carbonatites will always have SiO2þAl2O3 contents greater than 10^15 wt %. The most commonly observed carbonatite rock compositions observed at the Earth’s surface all tend towards low contents of alkalis SiO2 and Al2O3 and would have fractionated silicate phases from the carbonatite parental melts, possibly associated with alkali loss to coexisting fluids. Our results also show that carbonate liquid exsolution can occur from a CO2-undersaturated (PCO25Ptot) silicate melt. Although the expanded high-pressure miscibility gap appears favourable for producing natural silicate melt compositions, a low-pressure (51·0 GPa) magma chamber in the crust or perhaps in the shallow mantle below a rift provides the most likely environment for immiscibility to arise owing to the lower CO2 demand of the silicate magma. Unusual textures in some experiments, suggestive of a deformable liquid state for the CaCO3 phase, are conclusively shown to be characteristic of a non-quenchable, high-temperature polymorph of solid calcite. Similar calcite globules with this rounded appearance, which are also observed in some nephelinite lavas and mantle xenoliths, must be solid calcite and not immiscible liquids. This is consistent with the high SiO2þAl2O3 requirement of low-alkali or alkali-free immiscible carbonate liquids.

High-Temperature Granite Magmatism, Crust–Mantle Interaction and the Mesoproterozoic Intracontinental Evolution of the Musgrave Province, Central Australia

Abstract: 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.

Fe–Mg Partitioning between Olivine and High-magnesian Melts and the Nature of Hawaiian Parental Liquids

Abstract: We conducted 1 atm experiments on a synthetic Hawaiian picrite at fO_2 values ranging from the quartz–fayalite–magnetite (QFM) buffer to air and temperatures ranging from 1302 to 1600°C. Along the QFM buffer, olivine is the liquidus phase at ~1540°C and small amounts of spinel (< 0·2 wt %) are present in experiments conducted at and below 1350°C. The olivine becomes progressively more ferrous with decreasing temperature [Fo_(92·3) to Fo_(87·3), where Fo = 100 × Mg/(Mg + Fe), atomic]; compositions of coexisting liquids reflect the mode and composition of the olivine with concentrations of SiO_2, TiO_2, Al_(2)O_3, and CaO increasing monotonically with decreasing temperature, those of NiO and MgO decreasing, and FeO^* (all Fe as FeO) remaining roughly constant. An empirical relationship based on our data, T(°C) = 19·2 × (MgO in liquid, wt %) + 1048, provides a semi-quantitative geothermometer applicable to a range of Hawaiian magma compositions. The olivine–liquid exchange coefficient, K_(D,Fe^(2+)-Mg) = (FeO/MgO)^(ol)/(FeO/MgO)^(liq), is 0·345 ± 0·009 (1σ ) for our 11 experiments. A literature database of 446 1 atm experiments conducted within 0·25 log units of the QFM buffer (QFM ± 0·25) yields a median K_(D,Fe^(2+)-Mg) of 0·34; K_(D,Fe^(2+)-Mg) values from single experiments range from 0·41 to 0·13 and are correlated with SiO_2 and alkalis in the liquid, as well as the forsterite (Fo) content of the olivine. For 78 experiments with broadly tholeiitic liquid compositions (46–52 wt % SiO_2 and ≤ 3 wt % Na_(2)O + K_(2)O) coexisting with Fo_(92–80) olivines, and run near QFM (QFM ± 0·25), K_(D,Fe^(2+)-Mg) is approximately independent of composition with a median value of 0·340 ± 0·012 (error is the mean absolute deviation of the 78 olivine–glass pairs from the database that meet these compositional criteria), a value close to the mean value of 0·343 ± 0·008 from our QFM experiments. Thus, over the composition range encompassed by Hawaiian tholeiitic lavas and their parental melts, K_(D,Fe^(2+)-Mg) ~ 0·34 and, given the redox conditions and a Fo content for the most magnesian olivine phenocrysts, a parental melt composition can be reconstructed. The calculated compositions of the parental melts are sensitive to the input parameters, decreasing by ~1 wt % MgO for every log unit increase in the selected fO_2, every 0·5 decrease in the Fo-number of the target olivine, and every 0·015 decrease in K_(D,Fe^(2+)-Mg). For plausible ranges in redox conditions and Fo-number of the most MgO-rich olivine phenocrysts, the parental liquids for Hawaiian tholeiites are highly magnesian, in the range of 19–21 wt % MgO for Kilauea, Mauna Loa and Mauna Kea.

Experimental Simulation of Closed-System Degassing in the System Basalt–H2O–CO2–S–Cl

Abstract: Magma degassing processes are commonly elucidated by studies of melt inclusions in erupted phenocrysts and measurements of gas discharge at volcanic vents, allied to experimentally constrained models of volatile solubility. Here we develop an alternative experimental approach aimed at directly simulating decompression-driven, closed-system degassing of basaltic magma in equilibrium with an H^C^O^S^Cl fluid under oxidized conditions (fO2 of 1·0^2· 4l og units above the Ni^NiO buffer). Synthetic experimental starting materials were based on basaltic magmas erupted at the persistently degassing volcanoes of Stromboli (Italy) and Masaya (Nicaragua) with an initial volatile inventory matched to the most undegassed melt inclusions from each volcano. Experiments were run at 25^400 MPa under super-liquidus conditions (11508C). Run product glasses and starting materials were analysed by electron microprobe, secondary ion mass spectrometry, Fourier transform infrared spectroscopy, Karl-Fischer titration, Fe 2þ /Fe 3þ colorimetry and CS analyser. The composition of the exsolved vapour in each run was determined by mass balance. Our results show that H2O/ CO2 ratios increase systematically with decreasing pressure, whereas CO2/S ratios attain a maximum at pressures of 100^300 MPa. S is preferentially released over Cl at low pressures, leading to a sharp increase in vapour S/Cl ratios and a sharp drop in the S/Cl ratios of glasses. This accords with published measurements of volatile concentrations in melt inclusion and groundmass glasses at Stromboli (and Etna). Experiments with different S abundances show that the H2O and CO2 contents of the melt at fluid saturation are not affected. The CO2 solubility in experiments using both sets of starting materials is well matched to calculated solubilities using published models. Models consistently overestimate H2O solubilities for the Stromboli-like composition, leading to calculated vapour compositions that are more CO2-rich and calculated degassing trajectories that are more strongly curved than observed in experiments. The difference is less acute for the Masaya-like composition, emphasizing the important compositional dependence of solubility and melt^ vapour partitioning. Our novel experimental method can be readily extended to other bulk compositions.

Silicate Liquid Immiscibility within the Crystal Mush: Late-stage Magmatic Microstructures in the Skaergaard Intrusion, East Greenland

Abstract: Late-stage microstructures developed during the last stages of solidification of the Skaergaard intrusion comprise a wide array of reactive and non-reactive intergrowths. Reactive microstructures ascribed to open-system behaviour include serrated grain boundaries between pyroxene and plagioclase primocrysts, fish-hook pyroxenes and mafic symplectites. They form by the addition of Fe and Ca and removal of alkalis and silica. Other microstructures include those formed by (internally generated) redox reactions between olivine, Fe–Ti oxides and pyroxene. Non-reactive microstructures include closely spatially associated granophyric intergrowths and previously undescribed ilmenite-rich intergrowths that are interpreted as a consequence of crystallization of separated conjugate immiscible liquids. The open-system reactive microstructures occur predominantly in the cumulates on the chamber floor, appearing in LZb and disappearing (to be replaced by a granophyre–ilmenite-rich intergrowth association) in the Upper Zone. They are not common in the Marginal Border Series. Their distribution mirrors that of efficient expulsion of trapped liquid and can be attributed to the gravitationally driven loss of a Si-rich immiscible component from the interstitial liquid. The loss of the Si-rich component causes the remaining Fe-rich liquid to react with the primocrysts. Bulk-rock major element data are consistent with little or no preferential loss of the Si-rich liquid from the mush, but the resolution of the available data is not sufficient to assess the effect of this relative movement on the liquid line of descent of the bulk magma. The first appearance of the paired conjugate non-reactive intergrowths in MZ* (Marginal Border Series) points to the early onset of immiscibility in the bulk liquid.

Isotopic Evolution of the Idaho Batholith and Challis Intrusive Province, Northern US Cordillera

Abstract: The Idaho batholith and spatially overlapping Challis intrusive province in the North American Cordillera have a history of magmatism spanning some 55 Myr. New isotopic data from the 98Ma to 54Ma Idaho batholith and 51Ma to 43Ma Challis intrusions, coupled with recent geochronological work, provide insights into the evolution of magmatism in the Idaho segment of the Cordillera. Nd and Hf isotopes show clear shifts towards more evolved compositions through the batholith’s history and Pb isotopes define distinct fields correlative with the different age and compositionally defined suites of the batholith, whereas the Sr isotopic compositions of the various suites largely overlap.The subsequent Challis magmatism shows the full range of isotopic compositions seen in the batholith.These data suggest that the early suites of metaluminous magmatism (98^87 Ma) represent crust^mantle hybrids. Subsequent voluminous Atlanta peraluminous suite magmatism (83^67 Ma) results primarily from melting of different crustal components. This can be attributed to crustal thickening, resulting from either subduction processes or an outboard terrane collision. A later, smaller crustal melting episode, in the northern Idaho batholith, resulted in the Bitterroot peraluminous suite (66^54 Ma) and tapped different crustal sources. Subsequent Challis magmatism was derived from both crust and mantle sources and corresponds to extensional collapse of the over-thickened crust.

Implications of the Nuvvuagittuq Greenstone Belt for the Formation of Earth's Early Crust

Abstract: of the belt formed at � 4·28 Ga, which would make it the only known remnant of Hadean crust preserved on Earth. The dominant lithology of the belt has a mafic composition that consists of gneisses ranging from cummingtonite amphibolite to garnet^biotite schist composed of variable proportions of cummingtonite þ biotite þ quartz, � plagioclasegarnetanthophyllitecordierite. The composition of this unit ranges from basalt to andesite and it is divided into two distinct geochemical groups that are stratigraphical- ly separated by a banded iron formation (BIF). At the base of the se- quence, the mafic unit is mainly basaltic in composition and generally has relatively low Al2O3 and high TiO2 contents, whereas above the BIF, the unit is characterized by high Al2O3 and low TiO2 contents and exhibits a wider range of compositions from bas- altic to andesitic. The low-Ti unit can be further subdivided into a trace element depleted and a trace element enriched subgroup. The high-Ti unit is characterized by relatively flat REE patterns as opposed to the low-Ti gneisses, which display light REE-enriched profiles with flat heavy REE slopes. The incompatible element depleted low-Ti rocks have U-shaped REE profiles.The geochemical groups have compositional analogues in three types of ultramafic sills that exhibit the same stratigraphic succession. Generally, the mafic gneisses have low Ca, Na and Sr contents, with many samples having CaO contents51wt %. Such low Ca contents are unlikely to represent the original composition of their igneous precursors and are interpreted to reflect intensive alteration of plagioclase. These compositional characteristics along with the presence of cordier- ite þ anthophyllite suggest that the protoliths of the mafic gneisses were mafic volcanic rocks exhibiting variable degrees of hydrothermal alteration. The high-Ti compositional type shares geochemical char- acteristics with tholeiitic volcanic suites with low Al2O3 and high TiO2 contents and is consistent with crystal fractionation at low pressures under dry conditions. In contrast, the low-Ti compositional group is geochemically similar to boninitic and calc-alkaline volcanic suites.The high Al2O3 and lowTiO2 contents in the andesitic com- positions suggest the early crystallization of Fe^Ti oxides and late appearance of plagioclase, and are more consistent with fractionation at elevated water pressures. The succession from 'tholeitic' to 'calc-alkaline' magmatism seen in the Nuvvuagittuq greenstone belt is typical of the volcanic successions of many younger Archean green- stone belts. Regardless of the exact tectonic setting, this volcanic suc- cession suggests that the geological processes responsible for the formation and evolution of Archean greenstone belts were active at 3·8 Ga and perhaps as early as 4· 3G a.

The Petrology and Geochemistry of St. Helena Alkali Basalts: Evaluation of the Oceanic Crust-recycling Model for HIMU OIB

Abstract: We present major element, trace element, and petrographic data on alkali basalts from St. Helena, and examine the geochemical characteristics of a recycled component involved in the source of HIMU (Pb/ Pb420·5) ocean island basalts. Petrographic and compositional variations in the St. Helena basalts are best explained by the combined effect of fractional crystallization and accumulation of phenocrysts. Primary melt compositions are estimated by correcting for the effects of crystal^liquid differentiation by reconstructing the order of crystallization and the relative amount of fractionated phases.This calculation indicates that the St. Helena alkali basalts are derived from a common primary magma with 14^20 wt % MgO. Simple partial melting of fertile mantle peridotite, depleted mid-ocean ridge basalt (MORB)-source mantle, or garnet pyroxenite fails to produce the St. Helena primary melt. Instead, this primary melt can be reproduced if there are contributions from ancient recycled oceanic crust and depleted peridotite [(Rb/ Nb)PM1⁄4 0·38^0·80]. Subducted sediment can be excluded to explain the low (Rb, Ba, U)/Nb and Ce/Pb of St. Helena basalts. Geochemical modeling using major and trace element abundances, together with Sr, Nd, Pb, and Hf isotope ratios, indicates that the St. Helena primary melt can be formed by 1^2% melting of a peridotitic source that was refertilized by a small amount (8^18%) of melt derived from recycled oceanic crust. This source has a similar trace element pattern to modern normal (N)-MORB, but element abundances are 0·1^0·2 times N-MORB values. The calculated recycled crust has a wide range of present-day Pb isotopic ratios (Pb/ Pb of 21·7^79·3 and Pb/ Pb of 40·8^89·3), Sr/ Sr of 0·7018^0·7028, Nd/Nd of 0·51274^0·51285, and Hf/Hf of 0·28262^0·28293 after a residence time of 1·2^2·8 Gyr. Rb, Ba, Pb, Sr, and light rare earth element abundances in the recycled crust are depleted compared with modern N-MORB, whereasTh, U, Sm, and Nd abundances fall within the range of compositional variations in modern N-MORB. The trace element compositions of the recycled oceanic crust can be explained by element behavior during seafloor alteration and subduction zone dehydration of oceanic crust. Therefore, recycling of ancient subducted oceanic crust is a potential process for producing the St. Helena HIMU basalts.

Multistage Evolution of Dolerites in the Karoo Large Igneous Province, Central South Africa

Abstract: EarlyJurassic sheet-like intrusions (sills and dykes) are abundant in the Karoo Basin in South Africa, and were emplaced as a part of the Karoo Large Igneous Province. Here we discuss the evolutionary history of dolerite sills and dykes in different parts of the basin on the basis of new major and trace element analyses of dolerite samples collected from drill-cores (five sites spanning 1700 m of basin stratigraphy) and previously published data on sills and dykes in the Golden Valley Sill Complex (GVSC). In addition, we present Sr^ Nd isotope data for selected samples.The dolerites are subalkaline tholeiitic basalts and basaltic andesites characterized by enriched trace element patterns, variable degrees of depletion in Nb^Ta relative to light rare earth elements, negative to positive Pb anomalies, and mild to moderate enrichment in initial Sr^Nd isotopic ratios. The aim of this study is to unravel the evolutionary history of the melts that gave rise to the dolerites. We propose that the primary melts were derived from sub-lithospheric mid-ocean ridge basalt (or ocean island basalt) source mantle and had acquired a weak subduction signature (relative depletion in Nb^Ta, mildly enriched Sr^Nd isotopic ratios) through interaction with metasomatized lithospheric mantle. In the deep crust the magmas underwent assimilation and fractional crystallization (AFC) processes involving up to 10% assimilation of granulites with strong arc-type geochemical signatures. The AFC processes may alternatively have taken place in the uppermost mantle. Distinct geochemical characteristics among the GVSC and drill-core units reflect different amounts of AFC. During and/or after intrusion into the sedimentary rocks in the Karoo Basin the magmas underwent a second stage of fractional crystallization (50^60%) and local contamination by their sedimentary wall-rocks. High U concentrations and U/Th ratios in some dolerites in the southwestern part of the Karoo Basin were probably caused by fluids released from shales rich in organic material (e.g. Ecca Group shales) during devolatilization and contact metamorphism. Contamination in a GVSC unit may reflect interaction withTa^Th^U-rich minerals of the type found in stratiform uranium ore bodies in the Karoo Basin, or fluids that have interacted with such rocks. Considering that continental flood basalts are emplaced through continental crust and sedimentary basins, it is likely that other LIPs have similar evolutionary histories to that proposed for the Karoo Basin.

Mineralogy and composition of the oceanic mantle

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Volatile-rich metasomatism in montferrier xenoliths (Southern France) : implications for the abundances of chalcophile and highly siderophile elements in the subcontinental mantle

Abstract: Despite a relatively 'uniform' fertile composition (Al2O3 = 2 center dot 19-4 center dot 47 wt %; Fo% = 89 center dot 2 +/- 0 center dot 3%; Cr#(Spl) = 8 center dot 9 +/- 1 center dot 5%), the Montferrier peridotite xenoliths show a wide range of S contents (22-590 ppm). Most sulphides are interstitial and show peculiar pyrrhotite-pentlandite intergrowths and low abundances of Cu-rich phases. Sulphide-rich samples are characterized by strong enrichment in the light rare earth elements and large ion lithophile elements without concomitant enrichment of the high field strength elements. Such trace-element fractionation is commonly ascribed to metasomatism by volatile-rich melts and/or carbonatitic melts. S and Se (11-67 ppb), as well as S/Se (up to approximate to 12 000), are correlated with La/Sm. Cu, however, remains broadly constant (30 +/- 5 ppm). These features strongly suggest that the percolation-reaction of such volatile-rich fluids has led to sulphide enrichment with an atypical signature marked by strong fractionation of the chalcophile elements (i.e. S vs Se and Cu). S-rich xenoliths are also characterized by high (Pd/Ir)(N) (1 center dot 2-1 center dot 9; where subscript N indicates normalized to chondrite), (Pd/Pt)(N) between 1 center dot 5 and 2 center dot 2, and (Os/Ir)(N) up to 1 center dot 85. Despite the relative uniform fertile composition of the xenoliths, Re/Os ranges between 0 center dot 02 and 0 center dot 18. Os-187/Os-188 is extremely variable even within a single sample and can be as high as 0 center dot 1756 for the most S-rich samples. Sulphides show highly fractionated and variable abundances of the highly siderophile elements (HSE) [0 center dot 03 (Pd/Ir)(N) 0 center dot 17) would imply that such fluids are derived from an uncommon type of mantle source possibly related to carbonatite melts.

Thermodynamic Model for the Effect of Post-entrapment Crystallization on the H2O–CO2 Systematics of Vapor-saturated, Silicate Melt Inclusions

Abstract: Melt inclusions (MI) represent the best source of information concerning the pre-eruptive volatile contents of magmas. If the trapped melt is enriched in volatile species, following trapping the MI may generate a vapor bubble containing volatiles that have exsolved from the melt.Thermodynamic modeling of vapor-saturated albitic composition (NaAlSi3O8) MI shows that the CO2 content of the melt phase in the MI is sensitive to small amounts of post-entrapment crystallization (PEC), whereas the H2O content of the melt is less sensitive to PEC. During PEC, CO2 is transferred from the melt to the vapor phase and the vapor bubble may contain a significant amount, if not most, of the CO2 in the MI.The contrasting behaviors of H2O and CO2 during PEC lead to H2O^CO2 trends that are similar to those predicted for open-system degassing during magma ascent and decompression.Thus, similar H2O^CO2 trends may be produced if (1) vapor-saturated MI are trapped at various depths along a magmatic ascent path, or (2) MI having the same volatile content are all trapped at the same depth, but undergo different amounts of PEC following trapping. It is not possible to distinguish between these two contrasting interpretations based on MI volatile data alone. However, by examining the volatile trends within the context of other geochemical monitors of crystallization or magma evolution progress, it may be possible to determine whether the volatile trends were generated along a degassing path or if they reflect various amounts of PEC in an originally homogeneous melt inclusion assemblage. The volatile trends resulting from PEC of MI described in this study are directly applicable to silica-rich (granitic) MI trapped in non-ferromagnesian host phases, and are only qualitatively applicable to more mafic melt compositions and/or host phases owing to modifications resulting from Fe exchange with the host and to post-entrapment re-equilibration processes.

Rift-Related Transition from Andesite to Rhyolite Volcanism in the Taupo Volcanic Zone (New Zealand) Controlled by Crystal–melt Dynamics in Mush Zones with Variable Mineral Assemblages

Abstract: 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 SiO2 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 ( 1wt % H2O) 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 1wt % H2O (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 after450% crystallization from this upper crustal reservoir produces the large volumes of rhyolitic magma erupted over the past 0·7 Myr (44000 km from ignimbrite-forming eruptions).

Differentiation of Tholeiitic Basalt to A-Type Granite in the Sept Iles Layered Intrusion, Canada

Abstract: The undeformed 564 Ma Sept Iles layered intrusion (Quebec, Canada) is a large igneous body of c. 20 000 km 3 . From the base to the top, it consists of a Layered Series dominated by troctolite and gabbro, an anorthositic Upper Border Series and a dominantly granitic Upper Series. The parent magma of the Layered Series is inferred to be an iron-rich tholeiitic basalt (48 wt % SiO 2 ; 15 wt % FeO t ). Whole-rock compositions from the chilled margin, dykes cross-cutting the Layered Series and silicic rocks from the Upper Series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous A-type granites (77 wt % SiO 2 ). Initial 143 Nd/ 144 Nd (0·51201–0·51207) and 87 Sr/ 86 Sr (0·70353–0·70548) indicate a juvenile-mantle source and minimal contamination by old crust (1–2%) during crystallization. Geochemical modeling, using the MELTS thermodynamic calculator combined with equations predicting mineral–melt equilibria from experiments on tholeiitic basalts, indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the Layered Series. Fe–Ti oxides saturate early in the Layered Series, after 14% fractionation of plagioclase–olivine cumulates. Further fractionation of Fe–Ti oxide-bearing gabbros drives the residual liquids toward silica enrichment and iron depletion. Major and trace element modeling indicates that the A-type granites from the Upper Series were produced by protracted fractional crystallization of an iron-rich basaltic parent magma, at a fraction of residual liquid of only 8%. The observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations. Most of the intermediate compositions correspond to magmatic mafic enclave-bearing granitoids and display geochemical evidence of hybridization. Intermediate compositions produced by fractional crystallization are scarce and a Daly gap occurs from 57 to 67 wt % SiO 2 . This gap could result either from the fractional crystallization process or from silicate–liquid immiscibility during that compositional interval.

The Feeder System of the Deccan Traps (India): Insights from Dike Geochemistry

Abstract: Three large dike systems are exposed in the DeccanTraps flood basalt province of India: the dominantly north^south-trending west coast swarm, the east^west-trending Narmada^Tapi swarm in the north^ central Deccan, and the Nasik^Pune swarm in the central western Deccan. Combined major and trace element and Sr^Nd^Pb isotope data reveal that probable feeder dikes for the three main lava formations in the upper part of the lava pile (Poladpur, Ambenali and Mahabaleshwar Formations) are abundantly represented in the Nasik^Pune and coastal swarms. As a group, these dikes have no clear preferred trend. Among the highly oriented dikes of the Narmada^Tapi and west coastal areas, some have affinities with the lower part of the lava pile (Jawhar, Igatpuri, Thakurvadi and Bushe Formations) and these appear to have been intruded under the influence of regional north^south and east^west extension, respectively. Other dikes in the Narmada^Tapi swarm have the high- 206 Pb/ 204 Pb characteristic of flows in the far northeastern Deccan. These data suggest that Deccan lava flows could have reached as much as 700 km in length. Directed lithospheric extension appears to have been an important control on the emplacement of feeder dikes for the lower and middle formations. In contrast, emplacement of the voluminous upper formations, which span the Cretaceous^Tertiary boundary and 29R^N magnetic reversal and are estimated to make up � 50% of Deccan lava volume, was not controlled by directed regional extension. This conclusion contradicts predictions of rifting-based models for Deccan volcanism. Finally, isotopically distinct, north^south-trending dikes cut upper-formation flows and dikes along the coast; these dikes represent minor magmatism linked to early Paleocene east^west extension following the main phase of volcanism, in association with rifting of the Seychelles Bank from India.

The Mineralogical Diversity of Alkaline Igneous Rocks: Critical Factors for the Transition from Miaskitic to Agpaitic Phase Assemblages

Abstract: Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na 2 O + K 2 O) < Al 2 O 3 ] and peralkaline [(Na 2 O + K 2 O) > Al 2 O 3 ] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe–Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (± ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite + aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na–K–Ca–(Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na–K–Ca–(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (µNa 2 O, µCaO, µK 2 O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution. High-Ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase. In contrast, agpaites showing Ca-depletion originate by extensive fractionation of plagioclase from basaltic parental melts. In some peralkaline granites evolutionary trends are observed that culminate in agpaite-like HFSE-mineral associations in the most evolved rocks.

Crystallization of the Skaergaard Intrusion from an Emulsion of Immiscible Iron- and Silica-rich Liquids: Evidence from Melt Inclusions in Plagioclase

Abstract: The presence of Fe- and Si-rich liquids found as melt inclusions in apatite and olivine in the Upper Zone of the Skaergaard intrusion, East Greenland, demonstrates the occurrence of liquid immiscibility in the late-stage evolution of tholeiitic magmas in a plutonic setting. However, it remains unclear at which stage of crystallization unmixing began. To constrain the onset and the petrological importance of liquid immiscibility in the Skaergaard and tholeiitic magmas in general, we have studied crystallized melt inclusions entrapped in early primocryst plagioclase. Such melt inclusions become abundant from the top of the Lower Zone and upwards in the Layered Series, in primocryst plagioclase of composition An54^26. The daughter phase assemblage is the same in all the inclusions, although the modal proportions of the daughter phases are highly variable: plagioclase (42^59%), clinopyroxene (28^41%), ilmenite (4^9%), magnetite (3^10%), apatite (1^9%) and accessory phases (51%). Accordingly, the bulk compositions of reheated and homogenized melt inclusions show large variations in SiO2 (40^54 wt %), FeO t (7^23 wt %), P 2O5 (0^1 ·9 wt %) and K2 O( 0^2·8 wt %), and have variable CaO/Al2O3 ratios.These variations are best explained by trapping of varying proportions of immiscible iron- and silica-rich melts and demonstrate that liquid immiscibility started in the upper part of the Lower Zone. We conclude that a significant part of the Skaergaard intrusion crystallized from an emulsion of Fe- and Si-rich immiscible melts.The heterogeneous trapping of a mixture of Fe- and Si-rich immiscible liquids in primocryst plagioclase indicates that the dispersed droplets in the Lower and Middle Zones were smaller than the size of the inclusion (5500mm). In the Upper Zone, most of the inclusions in apatite are composed of the conjugate end-member liquids, indicating a larger size for the dispersed droplets. Metre-sized pods and layers of melanogranophyre in the upper part of the intrusion are believed to represent pooled bodies of the immiscible Si-rich liquid. Differentiation of an emulsified magma must be considered in petrogenetic models for the Skaergaard intrusion.

Perspective on the Genesis of E-MORB from Chemical and Isotopic Heterogeneity at 9–10°N East Pacific Rise

Abstract: The discovery of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin, time scales, and length scales of mantle heterogeneity. However, the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the petrogenetic, temporal, and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships and to explore how melting and melt transport processes contribute to or modify enriched mantle source compositions and generate E-MORB melts beneath mid-ocean ridges, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U-Th-Ra isotopes for a suite of lavas that were collected off-axis, including several E-MORB, at 9-10 degrees N along the East Pacific Rise (EPR). These data show coherent mixing trends among long-lived radiogenic isotopes, U-series nuclides, and incompatible trace elements, implying that mixing of melts from different sources occurs at different depths. Our results are consistent with previous studies that show that melting occurs in a two-porosity melting regime, with high-porosity channels forming deeply in the presence of garnet and transporting enriched melts with large (230)Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large (226)Ra excesses at the expense of diminished (230)Th excesses. Forward modeling of the trace element data also is consistent with mixing of melts in a two-porosity melting regime. U-series age constraints suggest that E-MORB neither erupt at systematically different times from N-MORB, nor necessarily through different pathways. Previous studies of E-MORB at 9-10 degrees N have suggested that E-MORB compositions could be explained by off-axis eruption. However, when considered in light of previously published magnetic paleointensity and U-series age constraints, recent geological studies, and the most widely accepted contemporary understanding of volcanic construction at 9-10 degrees N EPR, the asymmetric, off-axis distribution of E-MORB at 9-10 degrees N EPR is consistent with, and more simply explained by, a model in which E-MORB erupted within the axial summit trough (AST) and flowed down the ridge flanks (similar to 0-3 km). These E-MORB subsequently spread away from the AST, and, finally, were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of ages of E-MORB at 9-10 degrees N EPR and the geochemical and isotopic mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce.

Enriched Basaltic Andesites from Mid-crustal Fractional Crystallization, Recharge, and Assimilation (Pilavo Volcano, Western Cordillera of Ecuador)

Abstract: The origin of andesite is an important issue in petrology because andesite is the main eruptive product at convergent margins, corresponds to the average crustal composition and is often associated with major Cu^Au mineralization. In this study we present petro- graphic, mineralogical, geochemical and isotopic data for basaltic andesites of the latest Pleistocene Pilavo volcano, one of the most frontal volcanoes of the Ecuadorian Quaternary arc, situated upon thick (30^50km) mafic crust composed of accreted Cretaceous oceanic plateau rocks and overlying mafic to intermediate Late Cretaceous^Late Tertiary magmatic arcs. The Pilavo rocks are bas- altic andesites (54^57·5 wt % SiO2) with a tholeiitic affinity as opposed to the typical calc-alkaline high-silica andesites and dacites (SiO2 59^66wt %) of other frontal arc volcanoes of Ecuador (e.g. Pichincha, Pululahua). They have much higher incompatible element contents (e.g. Sr 650^1350 ppm, Ba 650^1800 ppm, Zr 100^225 ppm, Th 5^25 ppm, La 15^65 ppm) and Th/La ratios (0·28^0·36) than Pichincha and Pululahua, and more primitive Sr (87Sr/86Sr 􏰃0·7038^0·7039) and Nd (eNd 􏰃 þ5·5 to þ6·1) isotopic signatures. Pilavo andesites have geochemical affinities with modern and recent high-MgO andesites (e.g. low-silica ada- kites, Setouchi sanukites) and, especially, with Archean sanukitoids, for both of which incompatible element enrichments are believed to result from interactions of slab melts with peridotitic mantle. Petrographic, mineral chemistry, bulk-rock geochemical and isotopic data indicate that the Pilavo magmatic rocks have evolved through three main stages: (1) generation of a basaltic magma in the mantle wedge region by flux melting induced by slab-derived fluids (aque- ous, supercritical or melts); (2) high-pressure differentiation of the basaltic melt (at the mantle^crust boundary or at lower crustal levels) through sustained fractionation of olivine and clinopyroxene, leading to hydrous, high-alumina basaltic andesite melts with a tholeiitic affinity, enriched in incompatible elements and strongly impoverished in Ni and Cr; (3) establishment of one or more mid-crustal magma storage reservoirs in which the magmas evolved through dominant amphibole and clinopyroxene (but no plagioclase) fractionation accompanied by assimilation of the modified plutonic roots of the arc and recharge by incoming batches of more primitive magma from depth. The latter process has resulted in strongly increasing incompatible element concentrations in the Pilavo basaltic andesites, coupled with slightly increasing crustal isotopic signatures and a shift towards a more calc-alkaline affinity. Our data show that, although ultimately originating from the slab, incompatible element abundances in arc andesites with primitive isotopic signa- tures can be significantly enhanced by intra-crustal processes within a thick juvenile mafic crust, thus providing an additional process for the generation of enriched andesites.

Magma and volatile supply to post-collapse volcanism and block resurgence in Siwi Caldera (Tanna Island, Vanuatu Arc)

Abstract: V24C-04. Allen, S. R. (2005). Complex spatterand pumice-rich pyroclastic deposits from an andesitic caldera forming eruption: The Siwi pyroclastic sequence, Tanna, Vanuatu. Bulletin of Volcanology 67,

Large-volume Rhyolite Genesis in Caldera Complexes of the Snake River Plain: Insights from the Kilgore Tuff of the Heise Volcanic Field, Idaho, with Comparison to Yellowstone and Bruneau–Jarbidge Rhyolites

Abstract: Generation of large-volume rhyolites in the shallow crust is an important, yet enigmatic, process in the Snake River Plain and worldwide. Here, we present data for voluminous rhyolites from the 6·6–4·5 Ma Heise volcanic field in eastern Idaho. Heise is arguably the best site to evaluate shallow rhyolite genesis in the Snake River Plain; it is the youngest complete record of caldera cluster volcanism along the Yellowstone hotspot track and it culminated with the eruption of the most voluminous low-δ 18 O rhyolite known on Earth: the 1800 km 3 Kilgore Tuff (δ 18 O = 3·4‰). Such low-δ 18 O values fingerprint meteoric waters, and thus the shallow crust. New oxygen isotope data for phenocrysts, obtained by laser fluorination, correspond to a low-δ 18 O magma value of 3·4 ± 0·1‰ (2 standard error) for Kilgore Tuff samples erupted >100 km apart; however, ion microprobe data for single zircon crystals show significant diversity, with δ 18 O values that range from –1·3‰ to 6·1‰. U–Pb zircon ages, mineral chemistry, whole-rock major and trace element geochemistry, Sr and Nd isotope data, and magmatic (liquidus) temperatures are similar and/or overlapping for all studied samples of the Kilgore Tuff. Normal-δ 18 O Heise tuff units that preceded the Kilgore Tuff define a temporal compositional trend in trace element concentrations, trace element ratios, and Sr and Nd isotope ratios that is consistent with fractional crystallization from a common reservoir, whereas low-δ 18 O Kilgore cycle units have compositions that define a sharp reversal in the temporal trend back towards the composition of the first normal-δ 18 O Heise tuff (6·62 Ma Blacktail Creek Tuff). The data support derivation of the voluminous low-δ 18 O Kilgore Tuff from remelting of hydrothermally altered ( 18 O depleted) intracaldera and subvolcanic portions of the Blacktail Creek Tuff. Single pockets of melt with variable low-δ 18 O values were assembled and homogenized on a caldera-wide scale prior to the climactic Kilgore Tuff eruption, and the best record of this process is provided by the δ 18 O diversity in Kilgore Tuff zircons. Temporal trends of oxygen isotopic depletion and recovery in rhyolite eruptions of the Heise volcanic field are clearly linked to caldera collapse events, and remarkably consistent with trends in the Yellowstone Plateau volcanic field. At Heise and Yellowstone, magmatic δ 18 O values can be predicted on the basis of cumulative eruptive volumes, with a decrease in δ 18 O by ∼1‰ for every ∼1000 km 3 of erupted rhyolite. The Kilgore Tuff of the Heise volcanic field has the same timing, magnitude of δ 18 O depletion, and cumulative eruptive volume as the youngest phase of voluminous rhyolitic eruptions in the Yellowstone Plateau volcanic field, indicating that the Kilgore Tuff may serve as a useful analog for these and perhaps other large-volume low-δ 18 O rhyolites on Earth.

Late Neoproterozoic P-T evolution of HP-UHT Granulites from the Palni Hills (South India): New Constraints from Phase Diagram Modelling, LA-ICP-MS Zircon Dating and in-situ EMP Monazite Dating

Abstract: Late Neoproterozoic (c. 555 Ma) high-pressure-ultrahigh-temperature (HP-UHT) metamorphism has been documented for MgAl-rich migmatitic granulites from the Palni Hills in the Southern Granulite Terrane (South India). Conspicuous reaction textures indicate a clockwise P-T evolution, which is constrained through P-T pseudosection modelling and thermobarometry. The transformation of sillimanite to kyanite, which coexisted with orthopyroxene and/or garnet, records an early stage of loading. During subsequent heating to UHT conditions at deep-crustal levels (c. 1000 degrees C, 13 center dot 0 kbar) kyanite was transformed to sillimanite, and distinct peak-temperature assemblages (orthopyroxene + sillimanite + mesoperthite + rutile +/- garnet +/- quartz +/- sapphirine, garnet + biotite + sillimanite + spinel + corundum + rutile + plagioclase and garnet + orthopyroxene + rutile + plagioclase +/- quartz) formed in specific bulk compositions through biotite-dehydration-melting reactions. A sequence of corona and sapphirine-bearing symplectite textures records subsequent isothermal decompression of the order of c. 6 kbar at persistent extreme temperatures (1010-920 degrees C). UHT decompression is consistent with the uniformly high Al contents of porphyroblastic, coronitic and symplectitic orthopyroxene (up to 10 center dot 4 wt % Al(2)O(3)). Regrowth of garnet and biotite documents post-decompressional cooling to subsolidus conditions of < 800 degrees C at mid-crustal levels (c. 6 kbar). HP-UHT metamorphism and the clockwise P-T path of the Palni Hills granulites is attributed to a single late Neoproterozoic tectono-metamorphic event, which has been consistently dated at c. 555 Ma through laser ablation inductively coupled plasma mass spectrometry U-Pb analyses of zircon and in situ electron microprobe U-Th-total Pb analyses of monazite. The MgAl-rich granulites occur as enclaves in enderbitic orthogneiss. The intrusion of the orthogneiss in the late Archean (2534 +/- 28 Ma) marks the beginning of voluminous granitoid emplacement in the Southern Granulite Terrane between 2530 and 2440 Ma, which presumably caused a first high-grade metamorphic event in the early Paleoproterozoic (2469 +/- 13 Ma), recorded by zircon cores in the MgAl-rich granulites. The clockwise P-T-t evolution indicates that HP-UHT metamorphism in the central part of the Southern Granulite Terrane is related to collisional tectonics during the final assembly of Gondwana in the late Neoproterozoic. Extreme heating is ascribed to upwelling of the asthenosphere during delamination of the thickened lithospheric mantle. Fast uplift of the rocks followed by mid-crustal isobaric cooling reflects extension of the hot overthickened crust and its subsequent cooling to a normal geotherm.