Showing papers on "Partial melting published in 2005"
TL;DR: Measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth and temperatures of 700–1,200 °C constrain the recycling rates of key elements in subduction-zone arc volcanism.
Abstract: Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism1,2 The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown3,4,5,6,7 Here we report direct LA-ICPMS measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth of 120–180 km and temperatures of 700–1,200 °C The resultant liquid/mineral partition coefficients constrain the recycling rates of key elements The dichotomy of dehydration versus melting at 120 km depth is expressed through contrasting behaviour of many trace elements (U/Th, Sr, Ba, Be and the light rare-earth elements) At pressures equivalent to 180 km depth, however, a supercritical liquid with melt-like solubilities for the investigated trace elements is observed, even at low temperatures This mobilizes most of the key trace elements (except the heavy rare-earth elements, Y and Sc) and thus limits fluid-phase transfer of geochemical signatures in subduction zones to pressures less than 6 GPa
1,131 citations
TL;DR: Although both high-Al TTG and adakite show strongly fractionated REE and incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites as mentioned in this paper.
599 citations
TL;DR: The calc-alkaline granitoids of the central Sierra Nevada batholith are associated with abundant mafic rocks as discussed by the authors, which commonly have fine-grained and, less commonly, cumulate textures.
518 citations
TL;DR: In this paper, the stability of the rutile/melt partitioning in hydrated basalt was investigated at 1.0-2.5 GPa and 900-1100 °C.
517 citations
TL;DR: In this article, magnetotelluric data from the Tibetan-Himalayan orogen from 77 degrees E to 92 degrees E was used to image subsurface electrical resistivity, interpreted as a partially molten layer.
Abstract: The Cenozoic collision between the Indian and Asian continents formed the Tibetan plateau, beginning about 70 million years ago. Since this time, at least 1,400 km of convergence has been accommodated by a combination of underthrusting of Indian and Asian lithosphere, crustal shortening, horizontal extrusion and lithospheric delamination. Rocks exposed in the Himalaya show evidence of crustal melting and are thought to have been exhumed by rapid erosion and climatically forced crustal flow. Magnetotelluric data can be used to image subsurface electrical resistivity, a parameter sensitive to the presence of interconnected fluids in the host rock matrix, even at low volume fractions. Here we present magnetotelluric data from the Tibetan-Himalayan orogen from 77 degrees E to 92 degrees E, which show that low resistivity, interpreted as a partially molten layer, is present along at least 1,000 km of the southern margin of the Tibetan plateau. The inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in Southern Tibet.
443 citations
TL;DR: In this article, the authors report new chronological, geochemical, and isotopic data for the Miocene (ca. 18-15 Ma) K-rich adakitic volcanic rocks from the Hohxil area of the Songpan-Ganzi block in northern Tibet.
Abstract: It is generally accepted that the Cenozoic potassic volcanic rocks of northern Tibet were derived from a lithospheric mantle source. Here we report new chronological, geochemical, and isotopic data for the Miocene (ca. 18-15 Ma) K-rich adakitic volcanic rocks from the Hohxil area of the Songpan-Ganzi block in northern Tibet. We contend that these rocks were generated by partial melting of the mafic lower crust, in an intracontinental setting unrelated to subduction of oceanic crust. The Hohxil rocks exhibit high Sr/Y and La/Yb ratios, high Sr and La contents, but low Yb and Y concentrations, similar to adakites formed by slab melting associated with subduction. However, their relatively low e N d values (-2.09 to -4.58); high 8 7 Sr/ 8 6 Sr (0.7072-0.7075), Th/U, Th/Ba, and Rb/Ba ratios; and distinctive potassium enrichments (K 2 O > Na 2 O) are very different from the composition of typical adakites. In addition, those K-rich adakitic rocks with the highest SiO 2 contents (>61 wt%) exhibit the lowest 8 7 Sr/ 8 6 Sr ratios and highest e N d values and are the oldest Cenozoic volcanic rocks exposed in the Songpan-Ganzi block, suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma. Taking into account the composition of lower-crustal mafic xenoliths in Tibet, as well as the tectonic and geophysical evidence, we conclude that the Hohxil adakitic magmas were produced by partial melting of amphibole-bearing eclogites with a K-rich mafic bulk composition, in the lower part (≥∼55 km) of the thickened northern Tibetan crust. Partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward-subducted continental lithosphere.
399 citations
TL;DR: The effects of water and temperature on the electrical conductivity of the minerals wadsleyite and ringwoodite are determined to infer the water content of the transition zone and significantly exceed the estimated critical water content in the upper mantle, suggesting that partial melting may indeed occur at ∼410 km depth, at least in this region.
Abstract: The distribution of water in the Earth's interior reflects the way in which the Earth has evolved, and has an important influence on its material properties. Minerals in the transition zone of the Earth's mantle (from approximately 410 to approximately 660 km depth) have large water solubility, and hence it is thought that the transition zone might act as a water reservoir. When the water content of the transition zone exceeds a critical value, upwelling flow might result in partial melting at approximately 410 km, which would affect the distribution of certain elements in the Earth. However, the amount of water in the transition zone has remained unknown. Here we determined the effects of water and temperature on the electrical conductivity of the minerals wadsleyite and ringwoodite to infer the water content of the transition zone. We find that the electrical conductivity of these minerals depends strongly on water content but only weakly on temperature. By comparing these results with geophysically inferred conductivity, we infer that the water content in the mantle transition zone varies regionally, but that its value in the Pacific is estimated to be approximately 0.1-0.2 wt%. These values significantly exceed the estimated critical water content in the upper mantle, suggesting that partial melting may indeed occur at approximately 410 km depth, at least in this region.
382 citations
TL;DR: In this paper, a large number of trace elements (Zr, Hf, Nb, Ta, V, Co, Cu, Zn, Sr, REE, Cr, Sb, W, U, Th) were determined for rutile.
346 citations
TL;DR: In this article, the Fe3+/ΣFe ratio of 104 MORB glasses from the Pacific, the Atlantic, the Indian, and the Red Sea spreading centers has been determined using wet chemical Fe2+ analyses and electron microprobe FeOtotal measurements.
312 citations
TL;DR: The storage capacity of the upper mantle is considerably greater than generally appreciated, as recent studies show that H2O uptake in olivine is ∼3 times that originally inferred by Kohlstedt et al. as mentioned in this paper.
296 citations
TL;DR: The most reliable radiometric age dates (zircon U-Pb SHRIMP from an associated layered intrusive body several kilometers in area) indicate generation at ∼259 Ma, consistent with the end-Guadalupian (end Middle Permian) stratigraphic age as mentioned in this paper.
TL;DR: In this article, the authors report elemental and Nd-Sr isotopic data for two coeval postcollisional magmatic suites (∼300 Ma), the Miaogou and Karamay suites, from West Junggar foldbelt (NW China), aiming to determine their source regions and implications for continental growth.
TL;DR: In this paper, the authors investigated the relationship between leucosomes and the growth of garnet poikiloblasts in metapelitic gneiss from Broken Hill.
Abstract: Large garnet poikiloblasts hosted by leucosome in metapelitic gneiss from Broken Hill reflect complex mineral–melt relationships. The spatial relationship between the leucosomes and the garnet poikiloblasts implies that the growth of garnet was strongly linked to the production of melt. The apparent difficulty of garnet to nucleate a large number of grains during the prograde breakdown of coexisting biotite and sillimanite led to the spatial focussing of melting reactions around the few garnet nuclei that formed. Continued reaction of biotite and sillimanite required diffusion of elements from where minerals were reacting to sites of garnet growth. This diffusion was driven by chemical potential gradients between garnet-bearing and garnet-absent parts of the rock. As a consequence, melt and peritectic K-feldspar also preferentially formed around the garnet. The diffusion of elements led to the chemical partitioning of the rock within an overall context in which equilibrium may have been approached. Thus, the garnet-bearing leucosomes record in situ melt formation around garnet porphyroblasts rather than centimetre-scale physical melt migration and segregation. The near complete preservation of the high-grade assemblages in the mesosome and leucosome is consistent with substantial melt loss. Interconnected networks between garnet-rich leucosomes provide the most likely pathway for melt migration. Decimetre-scale, coarse-grained, garnet-poor leucosomes may represent areas of melt flux through a large-scale melt transfer network.
TL;DR: In this article, a non-modal batch melting model was used to estimate the degree of partial melting and the proportion of subduction-derived fluid added to the lithospheric mantle of the Yangtze craton.
Abstract: Potassic volcanism in the western Sichuan and Yunnan Provinces, SE Tibet, forms part of an extensive magmatic province in the eastern Indo-Asian collision zone during the Paleogene (40–24Ma). The dominant rock types are phlogopite-, clinopyroxeneand olivinephyric calc-alkaline (shoshonitic) lamprophyres. They are relatively depleted in Na2O, Fe2O3, and Al2O3 compared with the late Permian–early Triassic Emeishan continental flood basalts in the western part of the Yangtze craton, and have very high and variable abundances of incompatible trace elements. Primitive mantle-normalized incompatible element patterns have marked negative Nb, Ta and Ti anomalies similar to those of K-rich subduction-related magmas, although the geodynamic setting is clearly post-collisional. Spatially, some incompatible trace element abundances, together with inferred depths of melt segregation based on the Mg-15 normalized compositions of the samples, display progressive zonation trends from SW to NE with increasing distance from the western boundary of the Yangtze craton. Systematic variations in major and trace element abundances and Sr–Nd–Pb isotope compositions appear to have petrogenetic significance. The systematic increases in incompatible trace element abundances from the western margin to the interior of the Yangtze craton can be explained by progressively decreasing extents of partial melting, whereas steady changes in some incompatible trace element ratios can be attributed to changes in the amount of subduction-derived fluid added to the lithospheric mantle of the Yangtze craton. The mantle source region of the lamprophyres is considered to be a relatively refractory phlogopite-bearing spinel peridotite, heterogeneously enriched by fluids derived from earlier phases of late Proterozoic and Palaeozoic subduction beneath the western part of the Yangtze craton. Calculations based on a non-modal batch melting model show that the degree of partial melting ranges from 0 6% to 15% and the proportion of subduction-derived fluid added from 0 1% to 0 7% (higher-Ba fluid) or from 5% to 25% (lowerBa fluid) from the interior to the western margin of the Yangtze craton. Some pre-existing lithospheric faults might have been reactivated in the area neighbouring the Ailao Shan–Red River (ASRR) strike-slip belt, accompanying collision-induced extrusion of the Indo-China block and left-lateral strike-slip along the ASRR shear zone. This, in turn, could have triggered decompression melting of the previously enriched mantle lithosphere, resulting in calc-alkaline lamprophyric magmatism in the western part of the Yangtze craton.
TL;DR: In this article, oxygen isotope compositions of 34 adakites, high-Mg andesites, and lavas suspected to contain abundant slab and sediment melts from the Western and Central Aleutians, the Andes, Panama, Fiji, Kamchatka, Setouchi (Japan), and the Cascades are measured and calculated values of olivine phenocrysts in these samples vary between 4.88 and 6.78
TL;DR: In this paper, the authors show that there are significant variations in the iron isotope compositions (δ57/54Fe) of mantle rocks (0.9‰) and minerals (olivines 0.6‰), with spinels showing the greatest total variation of 1.7
TL;DR: In this paper, a method for computing trace-element and isotopic compositions of magmas generated by melting a heterogeneous source in mantle plumes and beneath mid-ocean ridges is presented.
TL;DR: The post-collisional Saraycik granodiorite intruded into a late Paleocene to early Eocene nappe pile that formed during collision of the Pontides in the North and the Anatolide-Tauride platform in the South, leading to the formation of the Izmir-Ankara-Erzincan suture as mentioned in this paper.
Abstract: The post-collisional Saraycik granodiorite intruded into a late Paleocene to early Eocene nappe pile that formed during collision of the Pontides in the North and the Anatolide-Tauride platform in the South, leading to the formation of the Izmir-Ankara-Erzincan suture. A relatively shallow pluton intrusion depth (∼5 to 8 km) was estimated from Al-in-hornblende geobarometry and contact metamorphic assemblages. The emplacement age is tightly constrained to ∼52 Ma by two Ar–Ar plateau and total fusion ages on biotite. The main mass of the pluton consists of metaluminous to peraluminous biotite granodiorite and hornblende-biotite granodiorite. In addition, up to 10-m thick dacitic and <25-cm thick aplitic dikes occur. Granodiorites and dacites show many close compositional similarities to high-silica adakites from supra-subduction zone settings, but tend to be slightly more felsic and to have a higher aluminium saturation index. Chondrite-normalized (cn) rare earth element patterns are characterized by high ratios of (La/Yb)cn, concave-upward shapes of the HREE and a lack of significant Eu anomalies. In conjunction with relatively high abundances of Ba and Sr as well as low abundances of Y, HREE and Sc, these patterns suggest a feldspar-poor, garnet ± amphibole-rich fractionating mineral assemblage (residue). All samples have very similar Nd–Sr isotopic characteristics, regardless of rock type. Initial eNd values range from −0.3 to −1.2 and initial 87Sr/86Sr ratios from 0.70491 to 0.70529. It is suggested that the magmas formed by partial melting of mafic lower crust at elevated pressures (∼1 to 2 GPa).
TL;DR: In this article, high-precision major element and Hf isotope data are reported for the Neoproterozoic Suxiong volcanic rocks along the western margin of the Yangtze Block.
Abstract: High-precision major element and Hf isotope data are reported for the Neoproterozoic Suxiong volcanic rocks along the western margin of the Yangtze Block. These volcanic rocks have variable eHf(T) values and Fe/Mn ratios. The relatively primitive basalts have high Fe/Mn ratios and high Hf-Nd isotopic compositions, indicating that they were generated by partial melting of garnet clinopyroxene in mantle plume at high pressure. Thus, the Suxiong basalts are genetically related to the proposed Neoproterozoic superplume. On the contrary, a few differentiated basalts have low Fe/Mn ratios and low Hf-Nd isotopic compositions. They are likely to experience assimilation-fractional crystallization process. The Suxiong rhyolites have consistent Hf and Nd model ages of 1.3–1.4 Ga. They are likely generated by shallow dehydration melting of pre-existing young arc igneous rocks associated with the basaltic underplating/intrusion in a continental rift.
TL;DR: In this paper, the authors propose a model for bulk assimilation of small fragments of crust, driven and masked by reactions during xenolith melting and magma crystallization, which is an important mechanism for crust-mantle mixing.
Abstract: Bulk assimilation of small (millimeters to ∼1 km) fragments of crust—driven and (ultimately) masked by reactions during xenolith melting and magma crystallization—is an important mechanism for crust-mantle mixing. Xenoliths containing mica or amphibole undergo dehydration melting when incorporated into a host magma, yielding mainly plagioclase, pyroxene, Fe-Ti oxides, and hydrous melt. The xenolith is physically compromised by partial melting and begins to disintegrate; xenolithic melt and crystals are mixed into the host magma. Xenocrystic zircon is liberated at this stage. The cryptic character of assimilation is greatly enhanced in any hydrous magma by hydration crystallization reactions (the reverse of dehydration melting). All pyroxenes and oxides (phenocrysts, xenocrysts, or crystals having a hybrid signature) will be subject to these reactions, producing feldspars, amphiboles, and micas that incorporate material from several sources, a particularly effective mixing mechanism. Implicit in the model is a reduced energy penalty for bulk assimilation—much of the assimilant remains in solid form—compared to melt-assimilation models. A large role for bulk assimilation supports stoping as a credible mechanism for the ascent of magmas. While the assimilation of low-density crust and concomitant fractionation provide the isostatic impetus for ascent, the wholesale incorporation and processing of crustal rocks in the magma chamber helps create the room for ascent.
TL;DR: In this article, strong deformed potassium feldspar-rich dikes are widely distributed in the northern part of the Sulu ultrahigh pressure (UHP) metamorphic terrane, eastern China.
Abstract: Strongly deformed potassium feldspar–rich dikes are widely distributed in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The fact that the crystallization ages of these dikes overlap with the age of peak UHP metamorphic conditions implies the presence of melt during metamorphism. Sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic Sulu gneiss. Partial melting may be the key to solving several unusual features of the Sulu and other UHP terranes, such as the almost complete lack of mineralogical evidence for UHP conditions and the limited growth of zircon during UHP conditions in the dominant felsic gneiss. In addition, because partial melting will cause a drastic reduction in the strength of the UHP gneisses, the most likely exhumation mechanism is diapiric rise of a low-viscosity, partially molten mass containing entrained blocks of eclogite, and not a thin sheet as usually proposed.
TL;DR: In this article, the authors constructed four high-resolution, finite-element thermal models across the Nicaragua-Cost Rica subduction zone to predict the thermal structure, metamorphic pressure (P)temperature (T) paths followed by subducting lithosphere, and loci and types of slab dehydration reactions.
TL;DR: The Serbian province of Tertiary ultrapotassic volcanism is related to a postcollisional tectonic regime that followed the closure of the Tethyan Vardar Ocean by Late Cretaceous subduction beneath the southern European continental margin this paper.
Abstract: The Serbian province of Tertiary ultrapotassic volcanism is related to a post-collisional tectonic regime that followed the closure of the Tethyan Vardar Ocean by Late Cretaceous subduction beneath the southern European continental margin. Rocks of this province form two ultrapotassic groups; one with affinities to lamproites, which is concentrated mostly in the central parts of the Vardar ophiolitic suture zone, and the other with affinities to kamafugites, which crops out in volcanoes restricted to the western part of Serbia. The lamproitic group is characterized by a wide range of Sr/Sri (0 70735–0 71299) and Nd/Ndi (0 51251–0 51216), whereas the kamafugitic group is isotopically more homogeneous with a limited range of Sr/Sri (0 70599–0 70674) and Nd/Ndi (0 51263–0 51256). The Pb isotope compositions of both groups are very similar (Pb/Pb 18 58–18 83, Pb/Pb 15 62–15 70 and Pb/Pb 38 74–38 99), falling within the pelagic sediment field and resembling Mesozoic flysch sediments from the Vardar suture zone. The Sr and Nd isotopic signatures of the primitive lamproitic rocks correlate with rare earth element fractionation and enrichment of most high field strength elements (HFSE), and can be explained by melting of a heterogeneous mantle source consisting of metasomatic veins with phlogopite, clinopyroxene and F-apatite that are out of isotopic equilibrium with the peridotite wall-rock. Decompression melting, with varying contributions from depleted peridotite and ultramafic veins to the final melt, accounts for consistent HFSE enrichment and isotopic variations in the lamproitic group. Conversely, the most primitive kamafugitic rocks show relatively uniform Sr and Nd isotopic compositions and trace element patterns, and small but regular variations of HFSE, indicating variable degrees of partial melting of a relatively homogeneously metasomatized mantle source. Geochemical modelling supports a role for phlogopite, apatite and Ti-oxide in the source of the kamafugitic rocks. The presence of two contrasting ultrapotassic suites in a restricted geographical area is attributable to the complex geodynamic situation involving recent collision of a number of microcontinents with contrasting histories and metasomatic imprints in their mantle lithosphere. The geochemistry of the Serbian ultrapotassic rocks suggests that the enrichment events that modified the source of both lamproitic and kamafugitic groups were related to Mesozoic subduction events. The postcollisional environment of the northern Balkan region with many extensional episodes is consistent at regional and local levels with the occurrence of ultrapotassic rocks, providing a straightforward relationship between geodynamics and volcanism.
TL;DR: In this paper, the concentrations of Ir, Ru, Pt and Pd have been determined in 29 mid-Oceanic Ridge basaltic (MORB) glasses from the Pacific (N = 7), the Atlantic (n = 10) and the Indian (n= 11) oceanic ridges and the Red Sea (N= 1) spreading centers.
TL;DR: In this paper, the origin and genesis of post-collisional mafic-ultramafic rocks in the Dabie orogen of China are investigated with respect to Mesozoic transport of mass and energy between mantle and crust.
TL;DR: In this article, the authors performed a geochemical and isotopic investigation on granites cored during drilling exploration activity on the Larderello geothermal field. But they did not find any of the typical petrographic features commonly observed in the TMP hybrid granites (enclaves, patchy zoning of plagioclase, amphibole clots).
TL;DR: A suite of dolerite dykes from the Ahlmannryggen region of western Dronning Maud Land (Antarctica) forms part of the much more extensive Karoo igneous province of southern Africa as mentioned in this paper.
Abstract: A suite of dolerite dykes from the Ahlmannryggen region of western Dronning Maud Land (Antarctica) forms part of the much more extensive Karoo igneous province of southern Africa. The dyke compositions include both low- and high-Ti magma types, including picrites and ferropicrites. New Ar-40/Ar-39 age determinations for the Ahlmannryggen intrusions indicate two ages of emplacement at similar to 178 and similar to 190 Ma. Four geochemical groups of dykes have been identified in the Ahlmannrygggen region based on analyses of similar to 60 dykes. ne groups are defined on the basis of whole-rock TiO2 and Zr contents, and reinforced by rare earth element (REE), Sr-87/Sr-86 and Nd-143/Nd-144 isotope data. Group I were intruded at similar to 190Ma and have low TiO2 and Zr contents and a significant Archaean crustal component, but also evidence of hydrothermal alteration. Group 2 dykes were intruded at similar to 178Ma; they have low to moderate TiO2 and Zr contents and are interpreted to be the result of mixing of melts derived from an isotopically depleted source with small melt fractions of an enriched lithospheric mantle source. Group 3 dyke were intruded at similar to 190Ma and form the most distinct magma group; these are largely picritic with superficially mid-ocean ridge basalt (MORB)-like chemistry (flat REE patterns, Sr-87/Sr-86, similar to 0.7035, is an element of Nd-i similar to 9). However, they have very high TiO2 (similar to 4 wt %) and Zr (similar to 500ppm) contents, which is not consistent with melting of MORB-source mantle. The Group 3 magmas are inferred to be derived by partial melting of a strongly depleted mantle source in the garnet stability field. This group includes several high Mg Fe dykes (ferropicrites), which are interpreted as high- temperature melts. Some Group 3 dykes also show evidence of contamination by continental crust. Group 4 dykes are low-K picrites intruded at similar to 178Ma; they have very high TiO2-Zr contents and are the most enriched magma group of the Karoo-Antarctic province, with ocean-island basalt (OIB)-like chemistry. Dykes of Group I and Group 3 are sub-parallel (ENE-WSW) and both groups were emplaced at similar to 190Ma in response to the same regional stress field, which had changed by similar to 178Ma, when Group 2 and Group 4 dykes were intruded along a dominantly NNE-SSW strike.
TL;DR: In this article, a back-arc extensional setting related to subduction of the Palaeo-Pacific plate is suggested to explain the petrogenesis of the Xiangshan volcanic complex and quenched enclaves.
Abstract: A late Mesozoic belt of volcanic–intrusive complexes occurs in SE China. Volcanic activity at Xiangshan in the NW of the belt took place mainly in the Late Jurassic (158–135Ma). The volcanic rocks from the Xiangshan volcanic complex include rhyolitic crystal tuffs, welded tuffs, rhyolite lavas, porphyritic lavas, and associated subvolcanic rocks. Mineral assemblages in these magmatic rocks include K-feldspar, plagioclase, quartz, Fe-rich biotite and minor amphibole, orthopyroxene and almandine. Mineral geothermometry indicates a high crystallization temperature (>850 C) for the Xiangshan magmas. The volcanic rocks are generally peraluminous; SiO2 contents are between 65 4% and 76 8% and the samples have high alkalis, rare earth elements (REE), high field strength elements and Ga contents and high Ga/Al ratios, but are depleted in Ba, Sr and transition metals. Trace element geochemistry and Sr–Nd–O isotope systematics imply that the Xiangshan magmas were probably derived from partial melting of Middle Proterozoic metamorphic lower-crustal rocks that had been dehydrated during an earlier thermal event. These features suggest an A-type affinity. Quenched mafic enclaves, hosted by the subvolcanic rocks, consist mainly of alkali feldspar, plagioclase, clinopyroxene, phlogopite and amphibole. Geothermometry calculations indicate that the primary magmas that chilled to form the quenched enclaves had anomalously high temperatures (>1200 C). The quenched enclaves have boninitic affinities; for example, intermediate SiO2 contents, high MgO and low TiO2 contents, high Mg-numbers and high concentrations of Sc, Ni, Co and V. However, they also have shoshonitic characteristics, e.g. enrichment in alkalis, high K2O contents with high K2O/Na2O ratios, high light REE and large ion lithophile element contents, low initial eNd values ( 4 2) and high initial Sr/Sr ratios (0 7081). We suggest a phlogopite-bearing spinel harzburgitic lithospheric mantle source for these high-Mg potassic magmas. Underplating of such anomalously high-temperature magmas could have induced granulite-facies lower-crustal rocks to partially melt and generate the Xiangshan A-type volcanic suite. A back-arc extensional setting, related to subduction of the Palaeo-Pacific plate, is favoured to explain the petrogenesis of the Xiangshan volcanic complex and quenched enclaves.
TL;DR: Experimental measurements of helium solubility in olivine at atmospheric pressure indicate that helium might be more compatible than uranium and thorium during mantle melting and that high 3He/4He ratios can be preserved in depleted residues of melting.
Abstract: High 3He/4He ratios found in ocean island basalts are the main evidence for the existence of an undegassed mantle reservoir. However, models of helium isotope evolution depend critically on the chemical behaviour of helium during mantle melting. It is generally assumed that helium is strongly enriched in mantle melts relative to uranium and thorium, yet estimates of helium partitioning in mantle minerals have produced conflicting results. Here we present experimental measurements of helium solubility in olivine at atmospheric pressure. Natural and synthetic olivines were equilibrated with a 50% helium atmosphere and analysed by crushing in vacuo followed by melting, and yield a minimum olivine-melt partition coefficient of 0.0025 +/- 0.0005 (s.d.) and a maximum of 0.0060 +/- 0.0007 (s.d.). The results indicate that helium might be more compatible than uranium and thorium during mantle melting and that high 3He/4He ratios can be preserved in depleted residues of melting. A depleted source for high 3He/4He ocean island basalts would resolve the apparent discrepancy in the relative helium concentrations of ocean island and mid-ocean-ridge basalts.
TL;DR: In this article, the chemistry of chromite ore is very variable, ranging from high Al (Cr#=0.43-0.55) to high Cr (Cr # = 0.60 -0.83) compositions.
Abstract: The Mayari-Baracoa ophiolitic belt in eastern Cuba hosts abundant chromite deposits of historical economic importance. Among these deposits, the chemistry of chromite ore is very variable, ranging from high Al (Cr#=0.43–0.55) to high Cr (Cr#=0.60–0.83) compositions. Platinum-group element (PGE) contents are also variable (from 33 ppb to 1.88 ppm) and correlate positively with the Cr# of the ore. Bulk PGE abundances correlate negatively with the Pd/Ir ratio showing that chromite concentrates mainly Os, Ir and Ru which gives rise to the characteristic negatively sloped, chrondrite-normalized PGE patterns in many chromitites. This is consistent with the mineralogy of PGEs, which is dominated by members of the laurite–erlichmanite solid solution series (RuS2–OsS2), with minor amounts of irarsite (IrAsS), Os–Ir alloys, Ru–Os–Ir–Fe–Ni alloys, Ni–Rh–As, and sulfides of Ir, Os, Rh, Cu, Ni, and/or Pd. Measured 187Os/188Os ratios (from 0.1304 to 0.1230) are among the lower values reported for podiform chromitites. The 187Os/188Os ratios decrease with increasing whole-rock PGE contents and Cr# of chromite. Furthermore, γOs values of all but one of the chromitite samples are negative indicating a subchondiritc mantle source. γOs decrease with increasing bulk Os content and decreasing 187Re/188Os ratios. These mineralogical and geochemical features are interpreted in terms of chromite crystallization from melts varying in composition from back-arc basalts (Al-rich chromite) to boninites (Cr-rich chromite) in a suprasubduction zone setting. Chromite crystallization occurs as a consequence of magma mixing and assimilation of preexisting gabbro sills at the mantle–crust transition zone. Cr#, PGE abundances, and bulk Os isotopic composition of chromitites are determined by the combined effects of mantle source heterogeneity, the degree of partial melting, the extent of melt-rock interactions, and the local sulfur fugacity. Small-scale (μm to cm) chemical and isotopic heterogeneities in the platinum-group minerals are controlled by the mechanism(s) of chromite crystallization in a heterogeneous environment created by the turbulent regime generated by successive inputs of different batches of melt.