Partial Melting Models Research Articles

Petrology‐based modeling of mantle melt electrical conductivity and joint interpretation of electromagnetic and seismic results

AbstractThe presence of melt in the Earth's interior depends on the thermal state, bulk chemistry, and dynamics. Therefore, the investigation of the physical and chemical properties of melt is a probe of the planet's structure, dynamics, and potentially evolution. Here we explore melt properties by interpreting geophysical data sets sensitive to the presence of melt (electromagnetic and seismic) with considerations of petrology and, in particular, peridotite partial melting. We present a petrology‐based model of the electrical conductivity of fertile and depleted peridotites during partial melting. Seismic and magnetotelluric (MT) studies do not necessarily agree on melt fraction estimates, a possible explanation being the assumptions made about melt chemistry as part of MT data interpretation. Melt fraction estimates from electrical anomalies usually assume a basaltic melt phase, whereas petrological knowledge suggests that the first liquids produced have a different chemistry, and thus a different conductivity. Our results show that melts produced by low‐degree peridotite melting (< 15 vol %) are up to 5 times more conductive than basaltic liquids. Such conductive melts significantly affect bulk rock conductivity. Application of our electrical model to magnetotelluric results suggests melt fractions that are in good agreement with seismic estimates. With the aim of a simultaneous interpretation of electrical and seismic data, we combine our electrical results with seismic velocity considerations in a joint model of partial melting. Field electrical and seismic anomalies can be explained by ~1 vol % melt beneath Hawaii and ~1–8 vol % melt beneath the Afar Ridge.

Komsomolskaya diamondiferous eclogites: evidence for oceanic crustal protoliths

The Komsomolskaya kimberlite is one of numerous (>1,000) kimberlite pipes that host eclogite xenoliths on the Siberian craton. Eclogite xenoliths from the adjacent Udachnaya kimberlite pipe have previously been geochemically well characterized; however, data from surrounding diamond-bearing kimberlite pipes from the center of the craton are relatively sparse. Here, we report major- and trace-element data, as well as oxygen isotope systematics, for mineral separates of diamondiferous eclogite xenoliths from the Komsomolskaya kimberlite, suggesting two distinct subgroups of a metamorphosed, subducted oceanic crustal protolith. Using almandine contents, this suite can be divided into two subgroups: group B1, with a high almandine component (>20 mol%) and group B2, with a low almandine component (<20 mol%). Reconstructed REE profiles for B1 eclogites overlap with typical oceanic basalts and lack distinct Eu anomalies. In addition, elevated oxygen isotope values, which are interpreted to reflect isotopic exchange with seawater at low temperatures (<350 °C), are consistent with an upper-oceanic crustal protolith. Reconstructed REE profiles for B2 eclogites are consistent with oceanic gabbros and display distinct Eu anomalies, suggesting a plagioclase-rich cumulate protolith. In contrast to B1, B2 eclogites do not display elevated oxygen isotope values, suggesting an origin deep within the crustal pile, where little-to-no interaction with hydrothermal fluids has occurred. Major-element systematics were reconstructed based on mineral modes; group B1 eclogites have higher MgO wt% and lower SiO2 wt%, with respect to typical oceanic basalts, reflecting a partial melting event during slab subduction. Calculated residues from batch partial melt modeling of a range of Precambrian basalts overlap with group B1 trace-element chemistry. When taken together with the respective partial melt trajectories, these melting events are clearly linked to the formation of Tonalite–Trondhjemite–Granodiorite (TTG) complexes. As a result, we propose that many, if not all, diamondiferous eclogite xenoliths from Komsomolskaya represent mantle ‘restites’ that preserve chemical signatures of Precambrian oceanic crust.

Depletion and refertilization of the Tethyan oceanic upper mantle as revealed by the early Jurassic Refahiye ophiolite, NE Anatolia—Turkey

We present new whole-rock major and trace element, Re–Os isotope and mineral chemistry data for the upper mantle peridotites and mafic rocks in the Rehafiye ophiolite in NE Anatolia (Turkey), and discuss their significance for the mantle evolution in Neotethys. The Refahiye peridotites include clinopyroxene (cpx)-rich and cpx-poor harzburgites. Assuming an initial fertile mantle composition, the cpx-rich harzburgites have a limited range of Cr-spl Cr# (33–38), implying 14–18% melt extraction. However, TiO2 contents of the Cr-spl in these rocks are too high (up to 0.24wt.%) to explain their Ti concentrations with a simple melt extraction model in a mid-ocean ridge (MOR) setting, and suggest re-crystallization from an impregnating fertile melt. The interstitial phases of cpx in these samples have high TiO2 (up to 0.40wt.%) and Na2O (up to 0.25wt.%) contents. They also contain interstitial pargasitic amphibole with TiO2 contents varying between 0.69 and 0.88wt.%. These textures and the mineral chemistry data indicate refertilization of the previously depleted, MOR-type peridotites by percolating hydrous melts. The whole-rock REE partial melting modeling is consistent with slightly lower degrees of mantle melting (~12–14%) compared to the melt extraction degrees obtained from the Cr-spl compositions (~15%). Chromian spinel (Cr-spl) phases in the cpx-poor harzburgites show a wider variation of Cr#, ranging between 57 and 75, and reflect 30–40% of partial melting. In contrast, the whole-rock geochemistry of these rocks represents slightly lower degrees of partial melting, varying between 30 and 35%. The enrichment of TiO2 contents of the Cr-spl (up to 0.20wt.%) in some of the cpx-poor harzburgites can be explained by their interaction with Ti-rich hydrous melt formed at supra-subduction (SSZ) tectonic setting. Highly unradiogenic Os isotope composition (0.11956) of a cpx-poor harzburgite sample suggests an ancient melt depletion event experienced by these rocks, whereas its high Re content is a manifestation of subduction enrichment. The mafic units of the Refahiye ophiolite show MORB-like to island arc tholeiite (IAT) geochemical signatures typical of SSZ oceanic crust. A U–Pb zircon age of 183±1Ma obtained from an isotropic gabbro sample suggests that the crystallization of the SSZ-type mafic units in the Refahiye ophiolite is as old as the early Jurassic.

Does partial melting explain geophysical anomalies?

The existence of partial melt is frequently invoked to explain geophysical anomalies such as low seismic wave velocity and high electrical conductivity. I review various experimental and theoretical studies to evaluate the plausibility of this explanation. In order for a partial melt model to work, not only the presence of melt, but also the presence of appropriate amount of melt needs to be explained. Using the mineral physics observations on the influence of melt on physical properties and the physics and chemistry of melt generation and transport, I conclude that partial melt model for the asthenosphere with homogeneous melt distribution does not work. One needs to invoke inhomogeneous distribution of melt if one wishes to explain observed geophysical anomalies by partial melting. However, most of models with inhomogeneous melt distribution are either inconsistent with some geophysical observations or the assumed structures are geodynamically unstable and/or implausible. Therefore partial melt models for the geophysical anomalies of the asthenosphere are unlikely to be valid, and some solid-state mechanisms must be invoked. The situation is different in the deep upper mantle where melt could completely wet grain-boundaries and continuous production of melt is likely by “dehydration melting” at around 410-km. In the ultralow velocity zone in the D″ layer, where continuous production of melt is unlikely, easy separation of melt from solid precludes the partial melt model for low velocities and high electrical conductivity unless the melt density is extremely close to the density of co-existing solid minerals or if there is a strong convective current to support the topography of the ULVZ region. Compositional variation such as Fe-enrichment is an alternative cause for the anomalies in the D″ layer.

The petrogenesis of calc-alkaline granites from the Elat massif, Northern Arabian–Nubian shield

The petrogenesis of calc-alkaline granitic rocks from the northern part of the Arabian–Nubian shield (ANS) was studied by analyzing the mineralogy, petrography, major and trace element compositions and 87Sr/86Sr isotope ratios in suites of late Proterozoic granitoides, diorites and mafic xenoliths from Elat massif. The granites and mafic xenoliths lie on linear correlations in the major element variation diagrams and in the Rb–Sr isotope (isochron) diagram, where they yield an isochron age of 621±12Ma and initial ratio of 87Sr/86Sr=0.7034±2. Our interpretation is that the two groups are cogenetic and that the isochron indicates the formation age of mafic to granitic magmas. Several petrogenetic models were examined to explain this genetic relation. We conclude that the chemical and isotopic data are best satisfied by crustal anatexis – partial melting model. The calc-alkaline granitic magmas (such as those comprising the Elat granitic plutons) are produced by partial melting of hydrous mafic crustal rocks. Non-modal batch melting of an amphibolitic source (formed earlier above a subduction zone) could produce the suite of calc-alkaline felsic magmas, leaving a residual mafic rocks that were later trapped as xenoliths. We assume that the mafic mineral assemblage represented by the most mafic xenolith was in equilibrium with Elat granite. A melting model of a hydrous amphibolitic crustal source predicts REE patterns similar to Elat granite. The initial 87Sr/86Sr ratio of the Elat calc-alkaline granites and mafic xenoliths is consistent with the anticipated value of the juvenile magmas (∼0.7034), indicating that the parental basaltic magmas that formed the amphibolitic source were derived from an enriched-mantle reservoir.

Geochronological, geochemical and Sr–Nd–Hf isotopic constraints on the origin of the Cretaceous intraplate volcanism in West Qinling, Central China: Implications for asthenosphere–lithosphere interaction

This paper reports new zircon LA-ICP-MS UPb age and Lu-Hf isotopes, laser-fusion 40Ar/39Ar age, mineral chemistry, bulk-rock major and trace element, and SrNd isotope data from intraplate volcanic rocks in the Duofuntun Area, West Qinling. It was dated at 105.8±0.9Ma (UPb Zircon) and 103±2Ma (matrix glass laser-fusion 40Ar/39Ar dating). The Duofuntun Volcanic Rocks (DVR) are dominantly composed of alkali basalts with minor basanites and basaltic andesites. Some evolved samples of the DVR underwent a variable fractionation of olivine, clinopyroxene, plagioclase, FeTi oxide, Cr-rich spinel and apatite. Chondrite-normalized rare earth element and primitive mantle-normalized trace element patterns of the DVR remarkably resemble those of oceanic island basalts (OIB). Nearly all the alkali basalts show positive Nb, Ta and Ti anomalies and negative Rb, Th and P anomalies, whereas basanites do not display positive Ti anomalies. Nearly all the samples show marked negative Rb and K anomalies relative to their neighbouring elements on a primitive mantle normalized trace element diagram. 87Sr/86Sr(i) and 143Nd/144Nd(i) ratios for all the mafic samples also show an OIB affinity, covering a range of 0.703264–0.705032 and 0.512718–0.512873, respectively. Several lines of evidence indicate that both garnet and amphibole play a key role for the genesis of the DVR. The melt segregation PT conditions in the source region, around 1371°C at 1.98–3.25GPa, combined with trace element collectively suggest a polybaric melting process during the formation of the DVR. The non-modal batch partial melting modelling result indicates the DVR were generated by small-degree polybaric partial melting of amphibole-bearing spinel peridotites (ca. 4%) and amphibole-bearing garnet peridotites (ca. 0.7%). We suggest the DVR were not formed in a plume. Rather, they originated by the interaction between the upwelling asthenosphere, which might provide both materials and heat flux, and hydrous mineral-bearing (amphibole) lithosphere during lithospheric extension.

Petrological modeling of basaltic rocks from Venus: A case for the presence of silicic rocks

AbstractThe presence of highly evolved igneous rocks on Venus is a controversial issue. The formations of highland terranes and pancake domes are the two principal tectonic and volcanic features which argue in favor of the presence of silicic igneous rocks; however, the lack of water on Venus casts doubt on whether or not granites and rhyolites can form. Data returned to Earth from the Venera 13 and 14 landers show that the surface of Venus is composed of basaltic rocks similar in composition to those found on Earth. Here it is shown that anhydrous and hydrous fractional crystallization modeling using the Venera 13 and 14 data as starting materials can produce compositions similar to terrestrial phonolites and rhyolites. It is suggested that at shallow crustal levels (i.e., ≤ 0.1 GPa), mafic magmas can differentiate into silicic magmas resembling phonolites or rhyolites which may or may not erupt. Furthermore, the hydrous equilibrium partial melting models can produce rocks similar to terrestrial andesites and rhyolites, whereas anhydrous models suggest that there may be a uniquely Venusian type of silicic rock. The silicic rocks, if present, could act as “continental nucleation” sites and/or their presence may facilitate preferential sites of shearing and deformation of the Venusian crust.

A U–Pb age for the Late Caledonian Sperrin Mountains minor intrusions suite in the north of Ireland: timing of slab break-off in the Grampian terrane and the significance of deep-seated, crustal lineaments

An intrusion of trachy-andesite, representative of a newly discovered suite of high-K–Ba–Sr, calc-alkaline minor intrusions (termed herein the Sperrin Mountains suite), hosted within the Grampian terrane in the north of Ireland, has been dated by U–Pb zircon at 426.69 ± 0.85 Ma (mid-Silurian; Wenlock–Ludlow boundary). Geochemistry reveals a close association with the Fanad, Ardara and Thorr plutons of the Donegal Batholith and the Argyll and Northern Highlands Suite of Scotland. The deep-seated Omagh Lineament appears to have limited eastward propagation of the Sperrin Mountains suite from beneath the main centre of granitic magmatism in Donegal. A Hf depleted mantle model age (T DM Hf) of c . 800 Ma for trachy-andesite zircons indicates partial melting from a source previously separated from the mantle. Whole-rock geochemistry of the suite is consistent with a model of partial melting, triggered by slab break-off, following thrusting of Ganderia–Avalonia under the Southern Uplands–Down–Longford accretionary prism (i.e. Laurentian margin). The new age constrains the timing of this event in the north of Ireland and is consistent with the petrogenesis of Late Caledonian high-K granites, appinites and minor intrusions across the Caledonides of northern Britain and Ireland. Supplementary material: Geochemical data are available at www.geolsoc.org.uk/SUP18600 .

Petrogenesis and U–Pb zircon chronology of adakitic porphyries within the Kop ultramafic massif (Eastern Pontides Orogenic Belt, NE Turkey)

High-silica adakites intrude the Kop ultramafic massif in the eastern Pontide orogenic belt of NE Turkey. These dominantly dacitic to rhyodacitic plagioclase porphyries form clusters of small intrusions. U–Pb dating of zircons from two intrusions yields ages between 52 and 59Ma. The best-constrained crystallization age is 55.83±0.04Ma, indicating that these are the oldest adakitic rocks of the eastern Pontides. The rocks show geochemical signatures typical of adakites. The absence of systematic geochemical variation within the adakite suite indicates a subordinate role for fractional crystallization and binary mixing, and suggests that individual intrusions may represent separate, small batches of magma. Modeling of partial melting suggests that the degree of melting was 5–20%, that the residues after melting likely contained garnet, clinopyroxene, and rutile, and that plagioclase was either absent or totally consumed during partial melting. The most likely source compositions had large-ion lithophile elements >10 times, and heavy rare earth elements <10 times, chondritic concentrations. Modeled bulk distribution coefficients indicate that the adakites were generated as silicic magmas, and are not derived by fractional crystallization from basaltic or andesitic parental melts. Sr, Nd, and Pb isotopic data indicate at least three isotopically distinct sources, one of which is depleted mantle. The other components include upper crust (possibly a mixture of subducted sediment and Hercynian continental basement rocks) and a component that may include both lower crust and sublithospheric mantle. These adakites are unique among the late Cretaceous and Cenozoic igneous rocks of the eastern Pontides in having a large proportion of the depleted mantle component. The prominent role of the depleted mantle in their petrogenesis suggests that the adakites may be related to a slab window in a south-directed subduction zone.

Evolution and origin of the Miocene intraplate basalts on the Aleppo Plateau, NW Syria

The source of intraplate basalts has long been a controversial topic, particularly in continental settings where ambiguity increases because both crustal contamination and crystal fractionation may mask important source characteristics. We present geochemical data to constrain the source and the chemical evolution of the continental intraplate magmas from the Aleppo Plateau and vicinity, NW Syria. New 40Ar/39Ar ages, coupled with published 40Ar/39Ar and K–Ar ages, reveal two discrete Miocene volcanic phases, ~19–18Ma (Phase 1) and ~13.5–12Ma (Phase 2), in the studied area. New chemical and isotopic compositions [87Sr/86Sr=0.7036–0.7051, εNd=+4.5 to +1.1 and (187Os/188Os)t=0.151–0.453] of the lavas reflect the unequivocal influence of crustal assimilation and fractional crystallisation (AFC). Despite the effects of the AFC processes, there still appear to be some differences between the most-primitive, least contaminated magmas of the two volcanic phases, interpreted as a result of source heterogeneity. Whereas the Phase 1 lavas, with relatively high Si, low Ti and trace-element contents, are consistent with partial melting of a largely peridotitic mantle source, the origin of the Phase 2 lavas is more complicated. The latter are characterised by a source component depleted in Si and enriched in Ti, Fe, Ca, P, alkalis, light and middle rare earth elements (REEs) relative to heavy REEs and with sub-chondritic Th–(U)/Nb, Pb/Ce and Zr/Sm. They approach compositions of experimental melts of amphibole-rich metasomatic veins. The compositional variations among the most primitive Phase 2 lavas are difficult to reconcile with varying degrees of partial melting of either the metasomatic veins or peridotite, but could be explained if partial melts of both lithologies were variably mixed, a scenario that could be sensibly envisioned as ascending (peridotitic) plume/asthenosphere derived melts assimilating highly fusible metasomatic veins during their traverse through the lithosphere. This process can be loosely quantified by trace-element forward partial melting modelling that suggests mixing of up to 80% metasomatic melts derived from ~40% melting of amphibole-rich metasomatic veins (which themselves were inevitably compositionally and mineralogically heterogeneous) with 20% plume/asthenospheric melts derived from ~7% melting of a garnet peridotite. Within the compressional framework of northern Arabia, invocation of diapiric material reasonably accounts for the generation of the intraplate basalts in Syria. Derivation of the Phase 2 hybrid melts was probably triggered by lateral flow of this diapiric material beneath the lithosphere subsequent to its arrival, with the migrating flow-front controlling the locus of volcanism. The increase in degree of Si-undersaturation with time for the Phase 1 and Phase 2 lavas is best explained by decreasing temperatures of this flow-front that resulted in less melt contribution from the diapiric mantle while the amphibole-rich veins within the lithosphere continued to be easily fusible, although we cannot totally exclude the possibility that the Phase 2 volcanism tapped a vein-richer domain which formed subsequent to the Phase 1 volcanism.

Petrology of Lasail plutonic complex, northern Oman ophiolite, Oman: An example of arc-like magmatism associated with ophiolite detachment

Lasail plutonic complex (4.7×3.8km), as a typical example of late stage intrusive rocks, is located to the south of Wadi Jizi, and intrudes into the base of V1 volcanic rocks and sheeted dike complex. The Lasail plutonic complex consists of various rock types ranging from ultramafic cumulates to tonalite, and is associated with minor amounts of axis stage gabbro to quartz diorite. These rocks are classified into the following eleven rock types: massive gabbro 1, quartz diorite 1 (axis stage intrusive rocks), olivine websterite, olivine gabbronorite, gabbronorite, hornblende gabbronorite, leucogabbronorite (layered gabbros), massive gabbro 2, diorite, quartz diorite 2, and tonalite (late stage intrusive rocks). The layered gabbros are intruded by the massive gabbro 2, and often occur as large irregular blocks in the massive gabbro 2. The massive gabbro 2 intrudes into the layered gabbros, and sometimes grades into hornblende gabbronorite layer of the layered gabbros. In some places, anastomosing veins of the hornblende quartz gabbronorite injected into the gabbronorite, which continue to the layer of leucogabbronorite. These gabbroic rocks are intruded by small intrusions of diorite, quartz diorite 2, and tonalite. The quartz diorite 2 forms rather larger intrusive bodies that intrude into the gabbroic rocks in the higher level. The tonalite occurs as thin dike and sheet mainly in the layered gabbros. N-MORB normalized trace element patterns for the massive gabbro 2 are characterized by enrichment of LILEs relative to REEs, and resemble to ‘subduction component’ from the island arc tholeiite except a weak enrichment for middle to light REE and P. In contrast, the massive gabbro 1 (axis stage gabbro) shows rather flat pattern similar to MORB with no remarkable ‘subduction component’. From the examination of Sr and Nd isotopic signature, the Lasail plutonic rocks are characterized by higher ε-Sr values than those of MORB. This suggests that the primitive magma of Lasail complex was influenced by the rocks with higher ε-Sr values than those of MORB, e.g., the axis stage rocks interacted with seawater. These lines of evidence suggests that the massive gabbro 2 was formed by the partial melting of residual MORB mantle which is contaminated with slab melt derived from the axis stage rocks interacted with seawater. In addition, petrogenesis of felsic rocks in the Lasail complex can be explained by the partial melting model of pre-existing layered gabbro. U–Pb zircon ages analyzed by LA-ICPMS are 100±2 and 99±2Ma for late stage tonalite and 100±1Ma for axis stage quartz diorite. These ages are slightly older than the ages reported for felsic rocks in the Oman ophiolite (ca., 95Ma), and suggest that the conversion from ridge stage to detachment stage took place rapidly.

Comparative Sr–Nd–Hf–Os–Pb isotope systematics of xenolithic peridotites from Yangyuan, North China Craton: Additional evidence for a Paleoproterozoic age

Concentrations of trace elements in orthopyroxenes and clinopyroxenes, and the Sr, Nd, Hf and Pb isotopic compositions of clinopyroxenes from 11 well-characterized spinel peridotite xenoliths carried in the ~30Ma Yangyuan alkali basalts, North China Craton, reveal a complex history of chemical modification of the lithospheric mantle. The clinopyroxenes were separated from a suite of peridotites for which whole-rock Re–Os isotopic systematics have previously suggested a melt depletion event at ~1.8Ga, and were chosen to span a range of rare earth element (REE) patterns with (La/Yb)N=0.13 to 8.2. Present-day isotopic compositions are highly variable: 87Sr/86Sr=0.70229 to 0.70443, εNd=−0.6 to +24, 206Pb/204Pb=15.74 to 19.08, and εHf=+13.5 to +167. Partial melting models indicate that the Yangyuan clinopyroxenes have light REE (LREE) abundances that are too high to be pristine residues of fractional melting, reflecting subsequent enrichment of the LREE, e.g., via melt addition. Ancient melt addition accounts for the decoupling observed between Nd and Hf isotopic data for most samples, although a few samples may also have been modified by recent melt addition, shortly before the entrainment of the xenoliths into the host basalt.Strontium, Nd and Pb isotopic systems provide no useful age constraints for this suite, although some samples have Paleoproterozoic model ages. By contrast, Yangyuan clinopyroxenes yield a Lu–Hf correlation trend corresponding to an apparent age of 1.66±0.10Ga (2σ) that is within uncertainty of their whole-rock Os model age (1.8±0.2Ga). Given that the crust in this area originally formed during the Archean, these results suggest mantle lithosphere removal and replacement during the Paleoproterozoic. Our data demonstrate that, in some instances, the Lu–Hf isotope system may be used as a complement to the whole‐rock Re–Os isotope system in dating melt depletion of peridotites.

The sources of volatile and fluid‐mobile elements in the Sunda arc: A melt inclusion study from Kawah Ijen and Tambora volcanoes, Indonesia

Subduction zone recycling of volatiles (H2O, Cl, S, F) is controlled by the nature of subducted materials and the temperature‐pressure profile of the downgoing slab. We investigate the variability in volatile and fluid‐mobile trace element enrichment in the Sunda arc using melt inclusion data from Kawah Ijen and Tambora volcanoes, together with published data from Galunggung, Indonesia. Combining our results with data from other arcs, we investigate the mobility of these elements during slab dehydration and melting. We observe correlations between Sr, H2O and Cl contents, indicating coupling of these elements during subduction zone recycling. Sulfur is more variable, and fluorine contents generally remain at background mantle values, suggesting decoupling of these elements from H2O and Cl. Partial melting and dehydration models constrain the source of Sr and the volatiles and suggest that the altered oceanic crust (AOC) is the main source of the hydrous component that fluxes into the mantle wedge, in agreement with thermo‐mechanical models. Sediment melt remains an important component for other elements such as Ba, Pb, Th and the LREE. The Indonesian volcanoes have variable concentrations of volatile and fluid‐mobile elements, with Kawah Ijen recording higher AOC‐derived fluid fluxes (Sr/Nd and H2O/Nd) compared to Galunggung and Tambora. Kawah Ijen has H2O/Ce ratios that are comparable to some of the most volatile‐rich magmas from other cold slab subduction zones worldwide, and the highest yet measured in the Sunda arc.

Iron isotopes in ancient and modern komatiites: Evidence in support of an oxidised mantle from Archean to present

The mantle of the modern Earth is relatively oxidised compared to the initially reducing conditions inferred for core formation. The timing of the oxidation of the mantle is not conclusively resolved but has important implications for the timing of the development of the hydrosphere and atmosphere. In order to examine the timing of this oxidation event, we present iron isotope data from three exceptionally well preserved komatiite localities, Belingwe (2.7Ga), Vetreny (2.4Ga) and Gorgona (0.089Ga). Measurements of Fe isotope compositions of whole-rock samples are complemented by the analysis of olivine, spinel and pyroxene separates. Bulk-rock and olivine Fe isotope compositions (δ57Fe) define clear linear correlations with indicators of magmatic differentiation (Mg#, Cr#). The mean Fe isotope compositions of the 2.7–2.4Ga and 0.089Ga samples are statistically distinct and this difference can be explained by greater extent of partial melting represented by the older samples and higher mantle ambient temperatures in the Archean and early Proterozoic relative to the present day. Significantly, samples of all ages define continuous positive linear correlations between bulk rock δ57Fe and V/Sc and δ57Fe and V, and between V/Sc and V with TiO2, providing evidence for the incompatible behaviour of V (relative to Sc) and of isotopically heavy Fe. Partial melting models calculated using partition coefficients for V at oxygen fugacities (fO2s) of 0 and +1 relative to the fayalite-magnetite-quartz buffer (FMQ) best match the data arrays, which are defined by all samples, from late Archean to Tertiary. These data, therefore, provide evidence for komatiite generation under moderately oxidising conditions since the late Archean, and argue against a change in mantle fO2 concomitant with atmospheric oxygenation at ~2.4Ga.

Melting of crustal rocks as a possible origin for Middle Miocene to Quaternary rhyolites of northeast Hokkaido, Japan: Constraints from Sr and Nd isotopes and major- and trace-element chemistry

Felsic volcanic rocks (mainly rhyolites) and basalts found in northeast Hokkido, Japan, result from intense volcanism during the Middle Miocene (14–9Ma), Late Miocene (8–6Ma) and Pliocene to Quaternary (5–2Ma). Rhyolites were examined to determine any genetic relationship to coeval basalts, high magnesian andesite (HMA), lower crustal rocks and mantle peridotite.Rhyolites have initial Sr and Nd isotopic ratios (SrI and NdI) which overlap with coeval north Hokkaido basaltic rocks and HMA. Previously published Sr and Nd isotopic data show that most Middle Miocene to Quaternary (14–0Ma) basaltic rocks from north Hokkaido have a relatively narrow SrI- and NdI- range (SrI 0.70299 to 0.70400, and NdI 0.51281 to 0.51311), with no temporal variation in either SrI or NdI. Basalt and HMA from three locations are, however, more undepleted in terms of both SrI and NdI than other north Hokkaido basaltic rocks. Some of rhyolites (termed undepleted rhyolites here) have similar SrI and NdI to some north Hokkaido basalts and HMA. Rhyolites with similar SrI and NdI to other north Hokkaido basaltic rocks are termed depleted rhyolites.Although the similarity of SrI and NdI between rhyolites and coeval basalts and HMA can be accounted for by fractional crystallization, this process is inconsistent with the REE chemistry of basalts, HMA and rhyolites, and with the results of fractional crystallization modeling. However, a few rhyolites may result from the fractional crystallization of basaltic and HMA magmas with assimilation of some metasedimentary rocks. Small degrees of partial melting of a metazomatized mantle peridotite is an unlikely mechanism to explain the genesis of rhyolites according to REE chemistry and partial melt modeling of an amphibole bearing spinel lherzolite source.Gabbros of the Hidaka metamorphic belt are a possible source for isotopically depleted rhyolites, as both the rhyolites and gabbros have similar SrI and NdI. I-type tonalite and some gabbros in the Hidaka metamorphic belt are possible source rocks for isotopically undepleted rhyolites based on similarity in SrI and NdI. These hypotheses are supported by partial melt modeling of olivine gabbro and I-type tonalite respectively.A possible tectono-magmatic model for the production of post-Middle Miocene rhyolites from NE Hokkaido involves upwelling of the asthenosphere during the Middle Miocene, associated with the spreading of the Kurile back-arc basin and Japan Sea back-arc basin. This would have resulted in thinning of the overlying lithosphere beneath north Hokkaido, and the production of asthenosphere-derived basaltic rocks with low SrI and high NdI throughout north Hokkaido since the Middle Miocene, but less common production of lithosphere-derived basaltic rocks and HMA (with high SrI and low NdI). Basaltic magmas formed since the Middle Miocene either erupted, or caused melting of the crust, resulting in the generation of rhyolitic magma.

Generation and emplacement of shear-related highly mobile crustal melts: the synkinematic leucogranites from the Variscan Tormes Dome, Western Spain

The Tormes dome consists of S-type granites that intruded into Ordovician augen gneisses and Neoproterozoic–Lower Cambrian metapelites/metagreywackes at different extents of migmatization. S-type granites are mainly equigranular two-mica granites, occurring as: (1) enclave-laden subvertical feeder dykes, (2) small external sill-like bodies with size and shape relations indicative for self-similar pluton growth, and (3) as large pluton bodies, emplaced at higher levels than the external ones. These magmas were highly mobile as it is inferred from the high contents of fluxing components, the disintegration and alignment of pelitic xenoliths in feeder dykes and at the bottom of some sill-like bodies. Field relations relate this 311 Ma magmatism (U–Pb monazite) to the regional shearing of the D3 Variscan event. Partial melting modeling and the relatively high estimated liquidus temperatures indicate biotite-dehydration partial melting (800–840°C and 400–650 MPa) rather than water-fluxed melting, implying that there was no partial melting triggered by externally derived fluids in the shear zones. Instead, the subvertical shear zones favored extraction of melts that formed during the regional migmatization event around 320 Ma. Nd isotope variation among the granites might reflect disequilibrium partial melting or different protoliths. Mass-balance and trace element partial melting modeling strongly suggest two kinds of fertile crustal protoliths: augen gneisses and metapelites. Slight compositional variation among the leucogranites does not reflect different extent of protolith melting but is related to a small amount of fractional crystallization (<13% for the equigranular granites), which is generally more pronounced in shallower batholitic leucogranites than in the small and homogeneous sill-like bodies. The lower extent of fractional crystallization and the higher-pressure emplacement conditions of the sill-like bodies support a more restricted movement through the crust than for batholitic leucogranites.

Petrogenesis of the Kaikomagatake granitoid pluton in the Izu Collision Zone, central Japan: implications for transformation of juvenile oceanic arc into mature continental crust

The Miocene Kaikomagatake pluton is one of the Neogene granitoid plutons exposed in the Izu Collision Zone, which is where the juvenile Izu-Bonin oceanic arc is colliding against the mature Honshu arc. The pluton intrudes into the Cretaceous to Paleogene Shimanto accretionary complex of the Honshu arc along the Itoigawa-Shizuoka Tectonic Line, which is the collisional boundary between the two arcs. The pluton consists of hornblende–biotite granodiorite and biotite monzogranite, and has SiO2 contents of 68–75 wt%. It has high-K series compositions, and its incompatible element abundances are comparable to the average upper continental crust. Major and trace element compositions of the pluton show well-defined chemical trends. The trends can be interpreted with a crystal fractionation model involving the removal of plagioclase, biotite, hornblende, quartz, apatite, and zircon from a potential parent magma with a composition of ~68 wt% SiO2. The Sr isotopic compositions, together with the partial melting modeling results, suggest that the parent magma is derived by ~53% melting of a hybrid lower crustal source comprising ~30% Shimanto metasedimentary rocks of the Honshu arc and ~70% K-enriched basaltic rocks of the Izu-Bonin rear-arc region. Together with previous studies on the Izu Collision Zone granitoid plutons, the results of this study suggest that the chemical diversity within the parental magmas of the granitoid plutons reflects the chemical variation of basaltic sources (i.e., across-arc chemical variation in the Izu-Bonin arc), as well as a variable contribution of the metasedimentary component in the lower crustal source regions. In addition, the petrogenetic models of the Izu Collision Zone granitoid plutons collectively suggest that the contribution of the metasedimentary component is required to produce granitoid magma with compositions comparable to the average upper continental crust. The Izu Collision Zone plutons provide an exceptional example of the transformation of a juvenile oceanic arc into mature continental crust.

The origin of Vesta’s crust: Insights from spectroscopy of the Vestoids

High quality VNIR spectra of 15 Vestoids, small asteroids that are believed to originate from Vesta, were collected and compared to laboratory spectra and compositional data for selected HED meteorites. A combination of spectral parameters such as band centers, and factors derived from Modified Gaussian Model fits (band centers, band strengths, calculation of the low to high-Ca pyroxene ratio) were used to establish if each Vestoid appeared most like eucrite or diogenite material, or a mixture of the two (howardite). This resulted in the identification of the first asteroid with a ferroan diogenite composition, 2511 Patterson. This asteroid can be used to constrain the size of diogenite magma chambers within the crust of Vesta. The Vestoids indicate that both large-scale homogeneous units (>5 km) and smaller-scale heterogeneity (<1 km) exist on the surface of Vesta, as both monomineralogic (eucrite or diogenite material alone) and mixed (both eucrite and diogenite) spectra are observed. The small-scale of the variation observed within the Vestoid population is predicted by the partial melting model, which has multiple intrusions penetrating into the crust of Vesta. It is much more difficult to reconcile the observations here with the magma ocean model, which would predict much more homogeneous layers on a large-scale both at the surface and with depth.

Age and geochemistry of the oceanic Manihiki Plateau, SW Pacific: New evidence for a plume origin

We present 40Ar/ 39Ar age and geochemical (major and trace element and Sr–Nd–Hf–Pb isotope) data from submarine samples recovered from the basement of the Manihiki Plateau during the R/V Sonne research expedition SO193. The samples, predominately tholeiites, with minor occurrences of basaltic andesites and hawaiites, give a mean age of 124.6 ± 1.6 Ma from four different localities on the plateau. Based on TiO 2 content, we define two groups of volcanic rocks that differ in trace element and isotopic compositions. Partial melting modeling suggests that the low-Ti group lavas were derived through large degrees of melting (c. 30%) of a peridotitic source at mantle potential melting temperatures of c. T p = 1510 °C, more than 100 °C above the ambient mantle potential melting temperature. Since the primary water contents of both groups of lavas are low (0.1–0.3g wt.%) and the source is peridotitic, excess temperature is most likely the reason for the large degrees of melting producing the large volume of plateau basalts, consistent with the involvement of a mantle plume. The incompatible element contents of the low-Ti group lavas show a multistage history with enrichment in the most incompatible elements of a previously highly depleted source. They have isotopic compositions similar to enriched mid-ocean-ridge basalt (EMORB) and similar to the common focal zone (FOZO) component. The high-Ti group lavas have more enriched incompatible element compositions overall. Their isotopic compositions tend towards an enriched mantle (EMI)-type endmember, similar, although less extreme, than lavas from the Pitcairn Islands. The geochemistry of the Manihiki Plateau can best be explained by a plume containing three components: 1) a dominant peridotitic FOZO-type component, 2) delaminated EMI-type subcontinental lithospheric mantle (SCLM), and 3) a HIMU (recycled oceanic crustal)-type component possibly in the form of eclogite/pyroxenite. The similarity in age and geochemical composition of Manihiki, Hikurangi and Ontong Java basement lavas, including volcanism in some adjacent basins, suggests that the Greater Ontong Java Volcanic Event covered c. 1% of the Earth's surface with volcanism.

Relative Chronology in High-Grade Crystalline Terrain of the Eastern Ghats, India: New Insights

The two major lithology or gneiss components in the polycyclic granulite terrain of the Eastern Ghats, India, are the supracrustal rocks, commonly described as khondalites, and the charnockite-gneiss. Northern Eastern Ghats belt, north of the Godavari rift has been defined as the Eastern Ghats Province, while that to the south has been defined as the Ongole domain; and although, these distinct crustal domains also record different ages of granulite metamorphism, both of these domains are dominated by the two lithologies. Many of the workers considered the khondalites as the oldest component with unknown basement and the charnockite- protoliths as intrusive into the khondalites. However, published geochronological data do not corroborate the aforesaid relations. Onset of khondalite sedimentation in the Proterozoic Eastern Ghats Province, constrained by detrital zircon data, as around 1.3 Ga and the charnockite-protolith emplacement between 1.9 and 2.9 Ga, argue against intrusion of felsic magma (tonalite, now enderbite!) in to the khondalites. The field relations of the hornblende-mafic granulite with the two gneiss components together with Sm-Nd isotopic data of the hornblende-mafic granulites (both the xenoliths within charnockites and those interbanded with the khondalites) indicate that khondalite sediments were deposited on older mafic crustal rocks. Mafic basement and supracrustal rocks were subsequently deformed and metamorphosed together during collisional orogeny at high to ultra-high temperatures – partial melting of mafic rocks producing the charnockitic melt; and partial melting of pelitic sediments producing the peraluminous granitoids. This is compatible with all the geochronological data as well as the petrogenetic model of partial melting for the charnockitic rocks in the Eastern Ghats Belt. The Ongole domain, south of the Godavari rift, though, is distinct in terms of the age of first/ earliest UHT metamorphism, but here too the charnockite-protoliths are older mafic rocks evidently not intrusive in to the khondalites.