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Nucleosynthetic isotope variations are well documented for refractory elements in meteorites and the Earth, while moderately volatile elements generally display homogeneous compositions. Cadmium is a moderately volatile element with eight stable isotopes generated by a variety of nucleosynthetic processes. To address the extent of the nucleosynthetic variability in moderately volatile elements, new high precision Cd isotope data are presented for bulk samples of six carbonaceous and one enstatite chondrite. In addition, we report the first Cd isotope results of sequential acid leachates for the CM2 chondrite Jbilet Winselwan. Our new Cd data displays nucleosynthetic homogeneity for bulk chondrites and acid leachates within analytical uncertainties, in agreement with results for other moderately volatile elements. This implies that Cd isotopes were efficiently homogenised prior to incorporation into planetary bodies. We propose that Cd never significantly condensed into dust in stellar environments, or alternatively that such Cd-bearing dust was efficiently destroyed and recycled in the interstellar medium. Our leachate data provides evidence for further homogenisation during thermal processing in the protoplanetary disk including parent body processing. The data shows that Cd in carbonaceous chondrites mainly resides in the more easily dissolved phases, most likely sulphides that were affected by aqueous alteration. Less than 1% of the total Cd was recovered in the final leach fractions that employed HF and mainly dissolve silicates and refractory oxides. Cadmium is susceptible to thermal neutron-capture effects due to the large neutron capture cross-section of 113Cd (∼20,000 barns). We report variations of up to −0.6 ± 0.3 for ε113Cd (internally normalised to 116Cd/111Cd) in bulk chondrites, which renders Cd a potential thermal neutron-capture monitor. Most neutron dosimeters, such as Pt, Os and Hf, are sensitive to neutron capture in the epithermal energy range and have applications mainly limited to lunar samples or iron and stony-iron meteorites. The additional use of Cd, susceptible to neutron capture in the thermal energy range, therefore provides a new tool to determine the exposure histories of stony meteorites in more detail. Our study demonstrates that thermal neutron-capture effects in carbonaceous and enstatite chondrites can produce resolvable effects and require attention when assessing nucleosynthetic isotope variations., Geochimica et Cosmochimica Acta, 274, ISSN:0016-7037, ISSN:1872-9533

[EN] Glasses in the LMAS (Li2O, MgO, Al2O3, SiO2) system containing F, CaO, ZrO2, and TiO2weresubjected to crystallization by incorporating P2O5. The glass stability, glass forming ability,and crystallization kinetics have been determined. Crystals of Lithium Aluminum Silicate(LixAlxSi1−xO2), Enstatite (MgSi2O6) and -spodumene (LiAlSi2O6) were detected in the sam-ples containing F, but -spodumene was not observed in the glass-ceramic in its absence.Glasses containing F show a dependence of the activation with the addition of P2O5withenergy values ranging from 426 kJ mol−1to 483 kJ mol−1, being this maximum found for P2O5amounts up to 3 mol%. High concentrations of P2O5and Al2O3improve the glass stability andincrease the crystallization temperature. The most common crystallization mechanism is ofbulk type with a constant number of nuclei. Tw o - and three-dimensional growth of crystalswas observed in glasses containing F but in its absence and at high Al2O3concentration bulkcrystallization with two- and one-dimensional growth is observed., [ES] Vidrios del sistema LMAS (Li2O, MgO, Al2O3, SiO2) y que contienen además F, Ca, ZrO2y TiO2han sido llevados a cristalización mediante la adición de P2O5. La estabilidad vítrea, capaci-dad formadora de vidrio y cinética de cristalización han sido estudiadas. Se ha determinadola presencia de cristales de aluminosilicato de litio (LixAlxSi1−xO2), enstatita (MgSi2O6) y -espodumena (LiAlSi2O6) en los vidrios que contenían F; sin embargo, sin la presenciade este, no se ha podido detectar -espodumena. Los vidrios que contenían F muestranuna dependencia en la energía de activación con el contenido en P2O5, encontrándose val-ores que oscilaban entre 426 kJ/mol y 483 kJ/mol, este último, el máximo encontrado paracontenidos en P2O5de hasta un 3%. A altas concentraciones de P2O5y Al2O3se observó un aumento de la estabilidad vítrea y de la temperatura de cristalización. Durante la cristal-ización, el mecanismo más común observado es una cristalización masiva con un númeroconstante de núcleos. Se observó un crecimiento en dos y tres dimensiones en los cristalesque contenían F, pero en aquellos preparados sin F y a altas concentraciones de Al2O3, lacristalización masiva con crecimiento uni- y bidimensional se apreció preferentemente., The present work was supported by the Algerian Ministry of Higher Education and Scientific Research (Algerian programP. N . E 2016–2017 scholarship fund) and with the collabora-tion of the Institute of Ceramic and Glass ICV-CSIC, Madrid,Spain. This work was developed under the frame of the project MAT2016-78700-R

The nature and distribution of sulfides within 17 porphyritic chondrules of the Sahara 97096 EH3 enstatite chondrite have been studied by backscattered electron microscopy and electron microprobe in order to investigate the role of gas-melt interactions in the chondrule sulfide formation. Troilite (FeS) is systematically present and is the most abundant sulfide within the EH3 chondrite chondrules. It is found either poikilitically enclosed in low-Ca pyroxenes or scattered within the glassy mesostasis. Oldhamite (CaS) and niningerite [(Mg,Fe,Mn)S] are present in about 60% of the chondrules studied. While oldhamite is preferentially present in the mesostasis, niningerite associated with silica is generally observed in contact with troilite and low-Ca pyroxene. The chondrule mesostases contain high abundances of alkali and volatile elements as well as silica. Our data suggest that most of the sulfides found in EH3 chondrite chondrules are magmatic minerals that formed after the dissolution of S from a volatile-rich gaseous environment into the molten chondrules. Troilite formation occurred via sulfur solubility within Fe-poor chondrule melts followed by sulfide saturation, which causes an immiscible iron sulfide liquid to separate from the silicate melt. The FeS saturation started at the same time as or prior to the crystallization of low-Ca pyroxene during the high temperature chondrule forming event(s). Protracted gas-melt interactions under high partial pressures of S and SiO led to the formation of niningerite-silica associations via destabilization of the previously formed FeS and low-Ca pyroxene. We also propose that formation of the oldhamite occurred via the sulfide saturation of Fe-poor chondrule melts at moderate S concentration due to the high degree of polymerization and the high Na-content of the chondrule melts, which allowed the activity of CaO in the melt to be enhanced., Comment: 34 pages, 7 figures, 1 table

The carbonation of serpentinites in the forearc region of the mantle wedge in subduction zones and of serpentinites within the subducting slab by fluids derived from prograde dehydration and decarbonation has important implications for the deep Earth carbon cycle. This study shows that the carbonation of serpentinites under the forearc can establish, over time, a significant reservoir for carbon within a partially hydrated mantle wedge and that carbonation of (ultra-) mafic rocks within the subducting slab contributes to C-transfer to greater depths and might supply carbon for arc volcanism or the deep mantle. We report a new high pressure experimental investigation of the interactions between oxidised C–O–H fluids and serpentinite and model the reaction progress with time series experiments. The CO2, H2O and alkane (CnH 2 n + 2 , n = 1 –6; e.g. methane, ethane) contents in the fluid phase from quenched experimental run products have been analysed by gas chromatography and the results are compared with thermodynamic calculations. With progressive carbonation, the formation of magnesite + chlorite together with quartz, quartz + talc or talc at 1–2 GPa and 500–650 °C was observed. At temperatures above antigorite stability ( T ≳ 700 °C and 2 GPa) magnesite + chlorite is stable together with talc, talc + enstatite, enstatite or enstatite + forsterite for decreasing CO2-content in the fluid. Carbonation of serpentinite is a rapid process where magnesite forms within the first hour of the experiments, filtering CO2 from the fluid effectively and equilibrium is approached within 48 h. The CO2-sequestration and magnesite production are less pronounced at higher temperatures. Therefore, C–O–H fluids released under subarc conditions might migrate through the mantle with only minor changes in their carbon budget whereas significant carbonation likely occurs under the forearc. Furthermore, partition coefficients for Ca, Ba, Sr and Pb between magnesite and a C-poor aqueous fluid have been established to demonstrate the potential of newly formed magnesite to sequester fluid mobile elements.

Chondrites are an important class of meteorites from the perspective of solar system history and evolution and astrobiology. They include carbonaceous (C), ordinary (O), Rumuruti (R), Kakangari (K), and enstatite (E) chondrites. Possible parent bodies of the CCs are the targets of two ongoing asteroid sample return missions, OSIRIS-REx and Hayabusa2, and an OC parent body was the target of the JAXA Hayabusa sample return mission. Reflectance spectroscopy of different classes of chondrites has shown that various spectroscopic properties, such as albedo, overall slope, presence or absence of absorption bands, and absorption band depths and positions can be used to distinguish different types of CCs, as well as to distinguish different classes of OCs, and to recognize R and E chondrites. Laboratory studies of the spectral reflectance properties of chondrites enhance our ability to relate different classes of chondrites to either specific asteroids or a taxonomic class.

Chondrite materials with varying abundances of volatile-bearing phases are expected at the destinations for the asteroid sample-return missions Hayabusa2 and OSIRIS-REx. The targets of the missions are 162173 (1999 JU3) Ryugu and 101955 (1999 RQ36) Bennu. Spectroscopic analyses of these asteroids suggest that their surface materials are related to types 1 and 2 carbonaceous chondrites. Some studies suggest that the parent bodies of these chondrites may have also experienced some thermal and/or shock metamorphism. The physical properties of boulders at asteroid surfaces and fine particles in asteroid regoliths are consequences of the diverse processes that fragmented them, mobilized them, and redeposited them in unique accumulations. Sample-return missions are likely to encounter a broad range of carbonaceous chondrite (CC)-like materials, to which aqueous alteration, thermal, and shock metamorphism imparted changes affecting their sub-micron- to meter-scale physical properties. Consequently, implementation of scale-dependent analytical techniques to the study of the chemical, physical, and geotechnical characteristics of these CC-like materials is fundamental to safe mission operations, sample selection, and return. However, most of the available knowledge for informing and formulating expectations about regolith processes, products, and properties on carbonaceous small bodies comes from missions that studied anhydrous (e.g., Itokawa studied by Hayabusa) and/or much larger asteroids (e.g., Vesta studied by Dawn). No previous mission is likely directly relevant to small ice-free carbonaceous NEOs 162173 Ryugu or 101955 Bennu, although the Rosetta Spaceraft performed a flyby of the large asteroid Lutetia which has variously been classified as M and C type (Patzold et al., 2011). Carbonaceous chondrites carry the best record of the history, distribution, and activity of water in the early solar system. Ordinary and Enstatite chondrites carry only partial records, but these are still critical to understanding the full story. We will describe the records of water-rock interactions on asteroids, as recorded in these meteorites, with particular emphasis on the timing, nature, settings, and fluid compositions. An integral part of this story is the rare, but fortunate, preservation of actual early solar system water as aqueous fluid inclusions.

The non-chondritic neodymium (Nd) 142Nd/144Nd ratio of the silicate Earth potentially provides a key constraint on the accretion and early evolution of the Earth. Yet, it is debated whether this offset is due to the Earth being formed from material enriched in s-process Nd isotopes or results from an early differentiation process such as the segregation of a late sulfide matte during core formation, collisional erosion or a some combination of these processes. Neodymium stable isotopes are potentially sensitive to early sulfide segregation into Earth's core, a process that cannot be resolved using their radiogenic counterparts. This study presents the first comprehensive Nd stable isotope data for chondritic meteorites and terrestrial rocks. Stable Nd measurements were made using a double spike technique coupled with thermal ionisation mass spectrometry. All three of the major classes of chondritic meteorites, carbonaceous, enstatite and ordinary chondrites have broadly similar isotopic compositions allowing calculation of a chondritic mean of δ146/144Nd = −0.025 ± 0.025‰ (±2 s.d.; n=39). Enstatite chondrites yield the most uniform stable isotope composition (Δ146/144Nd = 26 ppm), with considerably more variability observed within ordinary (Δ146/144Nd = 72 ppm) and carbonaceous meteorites (Δ146/144Nd = 143 ppm). Terrestrial weathering, nucleosynthetic variations and parent body thermal metamorphism appear to have little measurable effect on δ146/144Nd in chondrites. The small variations observed between ordinary chondrite groups most likely reflect inherited compositional differences between parent bodies, with the larger variations observed in carbonaceous chondrites being linked to varying modal proportions of calcium–aluminium rich inclusions. The terrestrial samples analysed here include rocks ranging from basaltic to rhyolitic in composition, MORB glasses and residual mantle lithologies. All of these terrestrial rocks possess a broadly similar Nd isotope composition giving an average composition for the bulk silicate Earth of δ146/144Nd = −0.022 ± 0.034‰ (n=30). In the samples here magmatic differentiation appears to only have an effect on stable Nd in highly evolved magmas with heavier δ146/144Nd values observed in samples with >70 wt% SiO2. The average stable Nd isotope composition of chondrites and the bulk silicate Earth are indistinguishable at the 95% confidence level. However, mantle samples do possess variable stable Nd isotope compositions (Δ146/144Nd = 75 ppm) with an average δ146/144Nd value of −0.008‰. If these heavier values represent the true composition of pristine mantle then it is not possible to completely rule out some role for core formation in accounting for some of the offset between the mantle and chondrites. Overall, these results indicate that the mismatch of 142Nd between the Earth and chondrites is best explained by a higher proportion of s-process Nd in the Earth, rather than partitioning into sulfide or S-rich metal in the core.

The giant impact theory is the most widely recognized formation scenario of the Earth's Moon. Giant impact models based on dynamical simulations predict that the Moon acquired a significant amount of impactor (Theia) material, which is challenging to reconcile with geochemical data for O, Si, Cr, Ti and W isotopes in the Earth and Moon. Three new giant impact scenarios have been proposed to account for this discrepancy – hit-and-run impact, impact with a fast-spinning protoEarth and massive impactors – each one reducing the proportion of the impactor in the Moon compared to the original canonical giant impact model. The validity of each theory and their different dynamical varieties are evaluated here using an integrated approach that considers new high-precision Zr isotope measurements of lunar rocks, and quantitative geochemical modelling of the isotopic composition of the impactor Theia. All analysed lunar samples (whole-rock, ilmenite and pyroxene separates) display identical Zr isotope compositions to that of the Earth within the uncertainty of 13 ppm for 96Zr/90Zr (2σ weighted average). This 13 ppm upper limit is used to infer the most extreme isotopic composition that Theia could have possessed, relative to the Earth, for each of the proposed giant impact theories. The calculated Theian composition is compared with the Zr isotope compositions of different solar system materials in order to constrain the source region of the impactor. As a first order approximation, we show that all considered models (including the canonical) are plausible, alleviating the initial requirement for the new giant impact models. Albeit, the canonical and hit-and-run models are the most restrictive, suggesting that the impactor originated from a region close to the Earth. The fast-spinning protoEarth and massive impactor models are more relaxed and increase the allowed impactor distance from the Earth. Similar calculations carried out for O, Cr, Ti and Si isotope data support these conclusions but exclude a CI- and enstatite chondrite-like composition for Theia. Thus, the impactor Theia most likely had a Zr isotope composition close to that of the Earth, and this suggests that a large part of the inner solar system (or accretion region of the Earth, Theia and enstatite chondrites) had a uniform Zr isotope composition. ISSN:0012-821X ISSN:1385-013X

© 2016 Elsevier B.V. Chemical data from the MESSENGER spacecraft revealed that surface rocks on Mercury are unusually enriched in sulfur compared to samples from other terrestrial planets. In order to understand the speciation and distribution of sulfur on Mercury, we performed high temperature (1200–1750 °C), low- to high-pressure (1 bar to 4 GPa) experiments on compositions representative of Mercurian lavas and on the silicate composition of an enstatite chondrite. We equilibrated silicate melts with sulfide and metallic melts under highly reducing conditions (IW-1.5 to IW-9.4; IW = iron-wüstite oxygen fugacity buffer). Under these oxygen fugacity conditions, sulfur dissolves in the silicate melt as S 2− and forms complexes with Fe 2+ , Mg 2+ and Ca 2+ . The sulfur concentration in silicate melts at sulfide saturation (SCSS) increases with increasing reducing conditions (from 10 wt.% S at IW-8) and with increasing temperature. Metallic melts have a low sulfur content which decreases from 3 wt.% at IW-2 to 0 wt.% at IW-9. We developed an empirical parameterization to predict SCSS in Mercurian magmas as a function of oxygen fugacity (fO 2 ), temperature, pressure and silicate melt composition. SCSS being not strictly a redox reaction, our expression is fully valid for magmatic systems containing a metal phase. Using physical constraints of the Mercurian mantle and magmas as well as our experimental results, we suggest that basalts on Mercury were free of sulfide globules when they erupted. The high sulfur contents revealed by MESSENGER result from the high sulfur solubility in silicate melt at reducing conditions. We make the realistic assumption that the oxygen fugacity of mantle rocks was set during equilibration of the magma ocean with the core and/or that the mantle contains a minor metal phase and combine our parameterization of SCSS with chemical data from MESSENGER to constrain the oxygen fugacity of Mercury's interior to IW-5.4±0.4. We also calculate that the mantle of Mercury contains 7–11 wt.% S and that the metallic core of the planet has little sulfur (

In order to estimate the Mo isotope composition and Mo abundance in the Bulk Silicate Earth (BSE), a total of thirty komatiite samples from five localities on three continents were analyzed using an isotope dilution double spike technique. Calculated Mo concentrations of the emplaced komatiite lavas range from 25±325±3 to 66±22 ng/g66±22 ng/g, and the inferred Mo concentrations in the deep mantle sources of the komatiites range between 17±417±4 and 30±12 ng/g30±12 ng/g, with an average value of 23±7 ng/g23±7 ng/g (2SE). This average value represents our best estimate for the Mo concentration in the BSE; it is identical, within the uncertainty, to published previous estimates of 39±16 ng/g39±16 ng/g, but is at least a factor of 2 more precise. The Mo isotope compositions of the komatiite mantle sources overlap within uncertainty and range from View the MathML sourceδMo98=−0.04±0.28 to 0.11±0.10‰0.11±0.10‰, with an average of 0.04±0.06‰0.04±0.06‰ (2SE). This value is analytically indistinguishable from published Mo isotope compositions of ordinary and enstatite chondrites and represents the best estimate for the Mo isotope composition of the BSE. The inferred δ98Mo for the BSE is therefore lighter than the suggested average of the upper continental crust (0.3 to 0.4‰). Thus, from the mass balance standpoint, a reservoir with lighter Mo isotope composition should exist in the Earth's mantle; this reservoir can potentially be found in subducted oceanic crust. The similarity of δ98Mo between chondritic meteorites and estimates for the BSE from this study indicates that during the last major equilibration between Earth's core and mantle, i.e., the one that occurred during the giant impact that produced the Moon, chemical and isotopic equilibrium of Mo between Fe metal of the core and the silicate mantle was largely achieved.

Chondrites are extraordinary mixtures of materials with diverse origins that formed around other stars, in the solar nebula, and in their parent asteroids. Most chondrites were so severely altered by aqueous fluids, thermal metamorphism, and impacts that the original characteristics of their components have been largely erased. But a few pristine chondrites have preserved an exquisite mineralogical, chemical, isotopic, and chronological record of the first few million years of solar system history. The properties of diverse types of carbonaceous, ordinary, and enstatite chondrites focusing on the most pristine samples are reviewed to establish the chemical, isotopic, and mineralogical properties and origins of their components and to elucidate the asteroidal processes that modified them. Refractory inclusions – amoeboid olivine aggregates and Ca–Al-rich inclusions – were the first solids to form in the solar nebula near to the protosun. Chondrules and associated metallic Fe–Ni grains were still forming several million years later when the earliest planetesimals, which melted due to heat from 26 Al decay, were colliding. In the least-altered chondrites, matrix material, which coats chondrules and other components, is largely composed of micrometer-sized silicates and amorphous materials, which formed at high temperatures, plus small amounts (up to 200 ppm) of presolar oxides and silicates.

The effect of Cl2 on the phase transformations of the minerals present in a talc (Mg3Si4O10(OH)2) unpurified with clinochlore (Mg5Al2Si3O10(OH)8), magnesite (MgCO3), dolomite (MgCa(CO3)2), hematite (Fe2O3) and pyrite (FeS2) was studied with the purpose of deferricating the mineral and obtaining protoenstatite (MgSiO3), which is the basic component of steatite ceramics. Isothermal and non-isothermal assays in N2 and Cl2–N2 atmospheres were performed using a thermogravimetric device at temperatures between 600 and 980 °C. The reagents and the products were analyzed by differential thermal analysis (DTA), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Results obtained showed that the following phenomena were produced in Cl2: (a) The transformation of vitreous silica (SiO2), from the chlorination reaction of talc, into enstatite (MgSiO3) started at about 700 °C, being dolomite the mineral that favored this reaction. At 800 °C, more enstatite was formed as a result of the reaction between vitreous silica not transformed, MgCl2 and O2 derived from the chlorination of dolomite and magnesite; then, polymorphic transformation of enstatite into protoenstatite was produced. (b) At about 950 °C, CaCl2 produced as a result of dolomite chlorination led to the destruction of the protoenstatite structure, and to the subsequent formation of cristobalite, with release of O2 and gaseous MgCl2. (c) The elimination of iron as FeCl3(g) takes place in all the temperature interval investigated. The most favorable chlorination temperature was 900 °C, since protoenstatite structure was destroyed at higher temperatures and an efficient elimination of iron present in the talc sample was not possible at lower temperatures. Fil: Orosco, Roberto Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Investigaciones en Tecnología Química; Argentina; Fil: Ruiz Salado, Maria del Carmen. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Investigaciones en Tecnología Química; Argentina; Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia; Argentina; Fil: Gonzalez, Jorge Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Investigaciones en Tecnología Química; Argentina; Universidad Nacional de Cuyo; Argentina

The influence of Al2O3 (8 wt.%) on sintering and crystallization features of glass powders based on magnesium silicate (MgSiO3) was experimentally determined. The investigated compositions were Y0.125Mg0.875Si0.875B0.125O3 and Y0.125Mg0.725Ba0.15Si0.875B0.125O3. For the experiments, glasses in bulk and frit forms were produced by melting in Pt-crucible at 1600 degrees C for 1.5 h. Glass-powder compacts were sintered at different temperatures between 900 degrees C and 1100 degrees C. The evolution of crystalline regime was determined by in situ recording of X-ray diffractograms of fine glass powders at elevated temperatures. The results and their discussion showed that addition of 8 wt.% Al2O3 in glass batches affected the thermal properties of the glasses and resulted in MgSiO3-based glass ceramics well sintered between 900 degrees C and 1100 degrees C. In the BaO-free MgSiO3 glass ceramics, clino- and orthoenstatite crystallize while the presence of BaO favours the formation of hexacelsian. (C) 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Ceramics International

Three glass compositions, aiming at producing Mg-metasilicate (MgSiO3) based glass-ceramics, were investigated. The composition with 25 mol% diopside and 75 mol% enstatite (Ca0.125Mg0.875SiO3), considered as the starting point, aimed to reduce the tendency of Mg-metasilicate for polymorphism. In the two derivative compositions, substitutions of yttrium and barium for magnesium as well as boron for silicon were attempted. Glasses in both bulk and frit forms were produced by melting in alumina crucibles at 1600 degrees C for 1-2 h. Sintering and crystallization of glass-powder compacts were carried out at different temperatures in the range of 850-1000 degrees C. The influence of these substitutions on the crystallization process of the glasses and the properties of the resultant glass-ceramics was experimentally determined and discussed in the light of the potential of these compositions in applications of advanced technology. (c) 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Ceramics International

Garnet peridotites containing relict majoritic garnet with abundant lamellae and interstitial blebs of pyroxene were recently discovered within mantle xenoliths and ultra high-pressure metamorphic terranes, but experimental reproduction of such microstructures has not yet been carried out. This chapter presents preliminary experimental results of our studies of the microstructures associated with supersilicic (majoritic) garnet high-temperature decompression (T = 1400°C)from 14 to 13, 12 and 7 GPa, and from 8 to 5 GPa. Experiments were performed in a Walker-style multi-anvil apparatus on a natural mineral mix corresponding to the bulk chemistry of garnet peridotite. We produced exsolution lamellae of Mg2SiO4 and interstitial blebs of diopside in the run products decompressed from 14 to 13 GPa and from 14 to 12 GPa at T = 1400°C. Blebs of both interstitial diopside and enstatite were formed during a slow high-temperature decompression from 14 to 7 GPa. Only blebs of interstitial enstatite were exsolved from majoritic garnet during decompression from 8 to 5 GPa at the same temperature. Mg2SiO4 has not been reported as an exsolution product in natural ultra high-pressure garnet peridotite, nor did we observe it in our experimental charges at pressures consistent with those inferred from studies of mantle xenoliths and orogenic belt garnet peridotites (7–10 GPa). The observation that Mg2SiO4 and diopside may be exsolved during decompression at higher pressures is consistent with expansion of the garnet field at the expense of wadsleyite at P > 13 GPa in Ringwood's diagram. The new experimental results may serve as a template for interpretation of microstructures in natural garnet peridotites, providing a better understanding of the depth from which such rocks have been exhumed.

Some basaltic dikes from the Tapi rift in the Deccan flood basalt province, India, contain abundant clusters of prismatic quench crystals of ferro-enstatite orthopyroxene (Fe39–42Mg53–57Ca2–4). Such profuse orthopyroxene clusters are not yet known from any other flood basalt province in the world. Based on petrography, mineral chemistry and whole-rock chemistry, we conclude that these crystals are formed due to assimilation of shale by Deccan basalt magma. This interpretation is consistent with the known bedrock geology of the area. These quench crystals constitute the first direct, petrographic evidence for crustal contamination in the Deccan province. The liquidus temperature for the dikes is estimated at 1220±10°C, with the crystallization temperature of the orthopyroxene at 1080°C or less., © Elsevier

Liquid appears in equilibrium with phlogopite, forsterite, enstatite, diopside, garnet and vapour (phlogopite–garnet lherzolite) in the system CaO–K2O–MgO–A12O3–SiO2–H2O at 1080 ± 10°C, 30 kbar. This liquid contains 14% olivine, 19% hypersthene and 12% diopside in the C.I.P.W. norm, and contains 3.6% K2O by weight. At 15 kilobars liquid appears in equilibrium with phlogopite, forsterite, enstatite, diopside, spinel and vapour (phlogopite–spinel-lherzolite) at 980 ± 10°C. This liquid contains 1.0% K2O and has 6.5% quartz, 28.5% hypersthene and a mere trace of diopside in the C.I.P.W. norm. Neither of these liquids bears any resemblance to kimberlite fluids. The relatively low K2O and high MgO of these liquids are unambiguously established by bracketing of the relevant compositions. At much higher pressures than 30 kbar it is conceivable that the initial partial melting product of phlogopite–garnet-lherzolite may become critically undersaturated in silica and rich in K2O, when it would resemble kimberlite. The extract required to modify the 30-kbar liquid to become the 15-kbar liquid consists essentially of diopside and phlogopite.

An experimental technique to measure ultrasonic wave velocities of solids at pressures up to 50 kbar and temperatures up to 600°C using a new sliding-type, cubic-anvil, high-pressure apparatus is described. The sample assembly is designed to make the pressure distribution around a sample as hydrostatic as possible. Good agreement of the present data on a single crystal of MgO at room temperature with Spetzler's [1970] data indicates that the present technique can considerably extend the pressure range of ultrasonic wave velocity measurements. Compressional and shear wave velocities of a polycrystalline enstatite (MgSiO 3 ) were also measured to 40 kbar at room temperature. The results are in good agreement with those for a bronzite single crystal by Frisillo and Barsch [1972], but not with those for a bronzitite or pyroxenite by Christensen [1974] and by Wang [1974]. Shear wave velocities in Twin Sisters dunite have been determined to 50 kbar at room temperature and from 25–600°C at P = 48.5 kbar. These results encourage us to make further refinements of the present technique because of the potential for enabling us to determine seismic velocities of mantle minerals or rocks within a reasonable accuracy under upper mantle P-T conditions without any extrapolation.

Detailed mineralogical and petrological investigations have been carried out on a suite of micaceous kimberlite dykes and sheets of early Mesozoic age intruded into Precambrian gneisses, supracrustal rocks and a granite complex. Forty-two electron microprobe analyses of kimberlite olivines demonstrate a compositional range from Mg92–Mg74, with frequent examples of both normal and reverse zoning. Calcite, iron-rich serpentine and phlogopite are important rock-forming phases with accessory apatite and perovskite. Diopside occurs in certain intrusions (up to 25 % vol. %). Opaques are commonly composite grains of chromite and magnetite with limited ulvospinel solid-solution. Eight microscopic diamonds have been recovered from four samples. Xenolith suites are dominated by lherzolitic peridotites, but include rare crustal rocks. Analyses from six garnet peridotites, three spinel peridotites and five phlogopite peridotites demonstrate significant compositional differences between their respective pyroxenes: the Al in diopside of spinel peridotites is 10 times that of diopside in phlogopite peridotites while Al in diopside of garnet peridotites is intermediate in amount. Diopside from phlogopite peridotites is depleted in Ti. These variations are interpreted as evidence of equilibrium between diopside and coexisting garnet, spinel, or phlogopite. With one exception the diopsides are calcic with limited solid solution towards enstatite; kimberlite diopsides are generally significantly more calcic than varieties in peridotite. Pyroxenes from individual peridotite nodules have similar Fe/Mg ratios although this ratio varies within the xenolith population. Olivine has a restricted compositional range (about Mg91). Large crystals in dunite mylonites are significantly more iron-rich. Garnet and spinel peridotites are interpreted as unmodified representatives of the Upper Mantle. Some phlogopite peridotites probably recrystallized during and after incorporation into the kimberlite magma.

Minerals included in diamonds from several localities in Brazil have been examined. The minerals observed are olivine, enstatite, chrome-pyrope and pyrope-almandine. These minerals have similar compositions to those previously reported for inclusions from other worldwide localities. In addition rutile, ilmenite, zircon, quartz and pyrrhotite occur as inclusions. This is the first record of ilmenite and zircon in natural diamond. The ilmenite is unique compared to most kimberlite ilmenites in that it contains no appreciable content of magnesium. The presence of quartz as an inclusion is unusual.

The system Mg2SiO4–SiO2–H2O–CO2 has been studied at a pressure of 20 kbar as a model for the melting behavior of peridotite in the presence of two volatile components. In the presence of CO2 and H2O, the assemblage forsterite (Fo) + enstatite (En) begins to melt at a higher temperature than in the presence of H2O alone. The liquids are CO2– and H2O–charged, yet show no tendency toward carbonatite immiscibility. Partial melts are generated at temperatures greater than 1400°C when CO2/H2O ratios are greater than 45 mole % CO2 and are silica-undersaturated in the CIPW norm, whereas partial melts produced at lower temperatures and H2O–rich conditions are silica–oversaturated. Liquids in the vapor–absent region of the join Fo–En–H2O which are in equilibrium with Fo + En also exhibit this normative change in character at 1400°C at 20 kbar. The chemical change therefore is not a result of changes in melt structure due to CO2 solution, even though up to about 7 wt.% C02 dissolves in En melt at 20 kbar. Rather, CO2 raises the temperature of the peridotite solidus relative to the beginning of melting with H2O and reduces activity of H2O. Hydrous magmas produced in the mantle in general contain much less H2O than is needed to saturate the melt and hence do not evolve vapor as they rise, until shallow depths are reached. By contrast, magmas containing CO2 and H2O may evolve vapor in the mantle or lower crust. The principal cause is the relatively lower solubility of CO2 in silicate melts, which is less than that of H2O at high pressure and very much less at pressures less than 10 kbar. Vapor rich in CO2 dissolves less total silicate than H2O–vapor and has markedly lower solubility of silica but does dissolve alkalies. The relationship of silica-undersaturated liquid and CO2–rich vapor may explain some of the characteristics of kimberlite or of magmas associated with kimberlite.

Eight garnet peridotite and ten garnet-free peridotite xenoliths from the Lashaine volcano in northern Tanzania, and one garnet peridotite from the Matsoku pipe in Lesotho, were studied petrographically and by electron-microprobe techniques. The primary assemblages are ol + opx + cpx + ga, ol + opx + cpx + chr, ol + opx + ga + chr, ol + opx + ga, ol + opx + chr, and ol + cpx + chr. With the exception of the ol + cpx + chr assemblage, which is more iron-rich, the composition of each of the primary mineral species is essentially the same for all the xenoliths and is independent of the abundances of the minerals. The primary minerals are Ni-rich very low Ca olivine (Fo92), low Ca, Al, Cr, Ti, Mn enstatite (Wo1En93Fs7), low Al, Ti, Mn chrome diopside (WO44En52Fs4), chrome pyrope and Mg, Al-rich chromite. Comparisons with phase equilibria studies suggest that the primary assemblages are stable at approximately 1050°C and 50 kbar, equivalent to a depth of approximately 150 km. The primary xenolith assemblages have been locally altered by three types of secondary processes. (1) Garnet has reacted with the adjacent olivine to produce reaction rims of aluminous orthopyroxene plus aluminous clinopyroxene plus spinel. (2) Some chrome-diopsides show marginal or total alteration that may result from secondary melting. (3) Extremely localized partial melting has occurred at some grain boundaries and the interstitial melt quenched to various combinations of olivine, clinopyroxene, orthopyroxene, spinel, phlogopite and glass. Garnet-bearing and garnet-free assemblages differ in bulk composition but have formed under similar physical conditions. The xenoliths derive from a region of the upper mantle that is homogeneous in terms of mineral compositions but heterogeneous, at least on the scale of the Lashaine xenoliths, in mineral proportions and bulk composition. Phlogopite in one garnet peridotite appears to be primary and several peridotites contain regions that have formed by localized melting of K-rich areas. Either the peridotites are not refractory residues from partial melting in the mantle or K-rich material has been added to them after partial melting but within the mantle, before incorporation into the ankaramite host.

A suite of Jurassic dykes occur near Orroroo, South Australia. The petrography, mineralogy and geochemistry of fresh core from one of the dykes is similar to other kimberlites, particularly from the type area near Kimberley, South Africa. This dyke is classified as a hypabyssal, calcite, phlogopite kimberlite. Typical kimberlite indicator minerals have been recovered from concentrates but ultramafic xenoliths are rare. The dykes can be geographically sub-divided into three groups which correlate with variations in abundance and chemistry of the heavy minerals and in diamond content. Diamonds have been recovered and examined. Enstatite and magnesio-wustite occur as syngenetic inclusions in the diamonds. The results suggest that many of the indicator minerals are xenocrystic and that the different groups of dykes have had a different mantle petrogenesis and pre-intrusion history.

From hydrostatic, high-pressure, X-ray diffraction studies of forsterite, Mg2SiO4, and enstatite, MgSiO3, the following zero pressure isothermal bulk moduli and their pressure derivatives were determined. ρo Kto K′to forsterite 3.234 g/cm3 1.20 Mbar 5.6 enstatite 3.227 g/cm3 1.25 Mbar 5 (assumed) Full-size table Table options View in workspace Download as CSV The cell edge (linear) and volume compressions are compared with those calculated from elastic constants [Graham and Barsch, 1969; Frisillo and Barsh, 1972] and with shock compression data [Ahrens et al., 1971; Ahrens and Gaffney, 1971].

Experiments at pressures from 40 to 60 kbar and temperatures from 1100°C to 1500°C were carried out on the join enstatite-diopside. The influence of pressure on the width of the solvus can be determined more accurately with this set of experimental data covering a wide P, T range. The thermodynamic models of Lindsley et al. (1981), Davidson et al. (1982) and Holland et al. (1979) all tend to overemphasize the influence of pressure. When used to estimate temperatures at and above 40 kbar (which is not intended by the above authors) gross overestimations are the consequences esp. at and below 1300°C. The assumption of no influence of pressure (e.g. Wells, 1977) on the solvus leads to good temperature estimates at 1000 and 1100°C. Temperatures above 1100° C are underestimated by up to 100°C.

Compositions of coexisting garnet, orthopyroxene and clinopyroxene in the system CMAS were determined with a multi-anvil split-sphere type ultrahigh-pressure apparatus. Crystallization experiments were made using a hydrous glass starting material EWC(enstatite 72.5 mol %, wollastonite 20.0 %, corundum 7.5 %) in the pressure range 50 - 100 kbar at temperatures from 1200° to 1500°C. Reversal experiments were made at the same pressure and temperature conditions using a crystalline mixture of identical compositions to EWC and comprised of pure enstatite, diopside and pyrope (EDP). The pressure effect on the pyroxene solvus is confirmed to be present to at least 100 kbar; at a constant temperature the diopside-enstatite solvus widens with increasing pressure. Alumina solubility in orthopyroxene is consistent with reversed experimental data by piston-cylinder at pressures less than 40 kbar, whereas dT/dP slopes of the alumina isoplethal lines for clinopyroxene by the piston-cylinder experiments are considered to be overestimated. The composition of garnet coexisting with two pyroxenes varies systematically as a function of pressure and temperature; as temperature increases the ratio grossular to pyrope decreases and as pressure increases the amount of pyroxene component dissolved in the garnet solid-solution increases. From their garnet compositions, sheared garnet lherzolite xenoliths from Lesotho kimberlites are considered to have equilibrated at pressures 50 to 70 kbar, whereas the granular lherzolites from the same area may have originated at pressures below about 50 kbar.

Abyssal peridotites dredged from the ocean ridges range from diopside-poor harzburgite to lherzolite, but all contain enstatite saturated with diopside, indicating that melting of the abyssal mantle was constrained by the pseudo-invariant point Ol+En+Di+Sp+Melt. We find systematic regional differences, which suggest that a range of primary melt compositions and large variations in the apparent degree of melting exist in the mantle beneath ocean ridges. We note in particular that North Atlantic peridotites are highly depleted relative to the average abyssal peridotite. Alpine-type peridotites overlap the range for abyssal peridotites, but extend to far more depleted and enriched compositions. Many contain enstatite undersaturated with respect to diopside. Frequently alpine-type peridotites contain highly magnesian Al-poor and Cr-rich minerals lying outside the abyssal range. Melting of many alpine peridotites, therefore, has occurred well into the three phase field Ol+En+Sp+Melt under different conditions than for abyssal peridotites. Alpine-type peridotites evidently represent parageneses extending from relatively undepleted sub-continental upper mantle to periodtites melted in a volcanic-arc or near-arc oceanic environment.

The recently discovered Lake Ellen kimberlite, in northern Michigan, indicates that bedrock sources of diamonds found in glacial deposits in the Great Lakes area could lie within the northern United States. Magnetic surveys show a main kimberlite 200 m in diameter and an adjacent body 25 × 90 m(?). Xenocrysts and megacrysts are ilmenite (abundant, 12.5-19% MgO), pyrope-almandine and Cr-pyrope (up to 9.3% Cr 2 O 3 ), Cr-diopside (up to 4.5% Cr 2 O 3 ), olivine (Fo 91), enstatite and phlogopite. The kimberlite contains fragments of crustal inclusions as well as xenoliths from a heterogeneous upper mantle. Eclogites, as large as 3 cm, show granoblastic equant or tabular textures and consist of jadeitic cpx (up to 8.4% Na 2 O, 15.3% Al 2 O 3 ), pyrope-almandine, ± rutile ± kyanite ± sanidine ± sulfide. Garnet pyroxenite contains pyrope (0.44% Cr 2 O 3 ) + cpx (0.85% Na 2 O, 0.53% Cr 2 O 3 ) + M9Al spinel. Calculated temperatures of equilibration are 920°-1060°C for the eclogites and 820°-910°C for the garnet pyroxenite using the Ellis-Green method. Five peridotite garnet-clinopyroxene composite xenocrysts have calculated temperatures of 980°-1120 °C using the Lindsley-Dixon 20 kb solvus. Spinel pyroxenite and clinopyroxene-orthopyroxene composites have lower calculated temperatures of 735°C and 820°-900°C, respectively.

The composition of partial melt of carbonated peridotite in the pressure interval 25-30 kb changes rapidly from a silicate melt (alkalic near the solidus) at low pressure to a carbonatitic melt where dolomite is introduced as a solidus phase and back to a silicate melt (haplokimberlitic) at still higher pressure. The details of these transitions are important to the genesis of alkalic magmas, carbonatites and kimberlites, but are obscured by the small pressure interval over which large changes in melt composition occur, by several subsolidus reactions which alter mineral assemblages (and thus melting reactions and melt compositions), and by the inability to quench and/or analyze partial melts generated by small degrees of partial melting. Melts in equilibrium with Fo+Di+En+Potassic phase+CO 2 show increasing CaO and MgO, at constant CaO/MgO=0.7 (wt), and decreasing K 2 2 AlO 3 and SiO 2 with increasing pressure to about 25 kb; kalsilite-component is stabilized while the diopside stability volume is depressed. Above 25 kb, the CaO/MgO of partial melts increases to a value greater than one when dolomite becomes a solidus phase and the enstatite stability volume expands. Partial melts are never more MgO-rich than the kalsilite-dolomite-enstatite-CO 2 join at pressures below about 30 kb. At P > 30 kb, K 2 O contents are low ( 2 O, kalsilite is the stable and refractory potassic phase. Polybaric melt composition trends at P

A reexamination of Hugoniot equation of state data and three new release adiabat points indicates that results for enstatite-bronzite composition pyroxene are compatible with its transforming to a perovskite phase at high pressure (For En_(90): ρ_0 = 4.20 g/cm^3, K_0 ~ 2.6 ± 0.35 Mbar, K'_0 ~ 3.5 ± .65). The release adiabat data, as well as results from porous samples, imply that the shock-wave data do not define an equilibrium, high-pressure phase Hugoniot below about 1.00 Mbar. These also suggest a further transformation to a phase (or assemblage) with density about 5% (or more) greater than that of or thorhombic perovskite. The data would allow such a transformation to occur at pressures as low as 0.60 Mbar under shock, representing an upper bound for the equilibrium transition pressure. Hugoniot data on magnesian olivines also appear to represent states of thermodynamic disequilibrium or a mixed-phase region below about 0.80-1.00 Mbar. However, Hugoniot points for Mgpyroxene and Mg-olivine coincide at pressures above 0.70 Mbar, suggesting that these minerals transform to high-pressure phases (or phase assemblages) of comparable density. Since MgO (presumably as periclase or in a closely related structure) attains relatively low densities at these pressures, the shock- wave data are in strong disagreement with the disproportionation of Mg_2SiO_4 to a MgSiO_3 (perovskite) + MgO assemblage above 0.80-1.00 Mbar. Conversely, the shock- wave data do not preclude a transformation of the type Mg_2Si_2O_6 → Mg_2SiO_4 ("post- perovskite" phase) + SiO_2 (rutile or fluorite structure). Again, these results would imply polymorphism to very dense "post- perovskite" phases. Based on the arguments made for pyroxene, such a transformation could occur at pressures as low as 0.60 Mbar under equilibrium conditions. We note that the combined results of high-pressure experiments allow Mg-pyroxene compositions to be as likely candidates for the lower mantle as olivine.

A grospydite xenolith from the Roberts Victor kimberlite pipe in South Africa presents an unusual phase assemblage of clinopyroxene, garnet, kyanite, coesite, and sanidine. The rock as previously described (Smyth and Hatton, 1977) consists of 50% omphacitic clinopyroxene, 28% garnet (Gr 50 Py 28 Alm 22 ), 9% kyanite, 6% coesite, and 1% sanidine (Or 99 ). Assuming the addition of three additional compatible phases (phlogopite, enstatite, and H 2 O vapor) and a simplified chemistry of the phases present, a Schreinemaker's thermodynamic analysis was attempted in order to estimate the pressure and temperature of equilibrium of the rock. Four reactions involving six components are likely to have determined an invariant point for the assemblage. (1) 3 Kyn + 6 Cpx + Phl = San + 3 Gt + 6 En + H 2 O (2) 6 Cos + 3 Gt + Phl = San + 3 Kyn + 6 Cpx + H 2 O (3) Kyn + 2 Cpx = Cos + Gt + En (4) 3 Cos + Phl = San + 3 En + H 2 O Using tabulated as well as estimated thermodynamic data for the phases, the calculated values for equilibrium temperatures and pressures for the reactions yield an invariant point for the assemblage at a depth of about 160 km (49 kbars) and a temperature of about 1060°C. This point likely represents a subsolidus recrystallization stage of origin.

Twenty-nine inclusion-bearing diamonds, mostly from Africa, were studied in order to determine the mineral assemblages and compositions of the inclusions. Those interpreted as syngenetic with diamond (e.g. garnet, pyroxene, olivine) frequently have crystal faces related to the diamond rather than to their own structure; they are termed xenohedral. Ten diamonds contain minerals comparable to those in the garnet lherzolite assemblage; these are pyrope-knorringite garnet (0.7-9.2% Cr2O3), diopside-orthopyroxeneintergrowths (interpreted as subcalcic diopside exsolved into diopside and orthopyroxene,(En86-94), enstatite (En92-93), olivine (FO92-93; 0.03-0.15% Cr2O3) and chromite. Fifteen diamonds have minerals comparable to those in eclogites enclosed in kimberlites; these are pyrope-grossular-almandine garnet, omphacite (2.7-10.7% A1203, 1.4-5.8% Na2O, 0.04-0.87% K2O), kyanite, rutile, magnetite (probably intergrown with sulfide) and phlogopite. Minerals of uncertain origin, associated with the eclogitic inclusion assemblages, are sanidine, a polymorph of silica, and filamentous material rich in SiO2, A12O3 and K2O. Epigenetic minerals are muscovite, amphibole (hornblende, actinolite), sulfide (pentlandite, chalcopyrite) and altered kyanite. In the garnet lherzolite suite subcalcic diopside assemblages may have formed in the sheared lherzolite zone at 1200-1400°C, and reequilibrated in the granular lherzolite zone at 1000°C, of the upper mantle. The eclogite suite assemblage has a compositional range similar to that found in diamondidferous and non-diamondiferous eclogite xenoliths from kimberlites. K-bearing omphacite requires experimental study. The presence of kyanite is the first reported occurrence of this eclogite mineral in diamond. Magnetite, intergrown with sulfide, is part of the eclogite assemblage. No mixing of the two suites has been found in a single diamond, with one possible exception.

The Igwisi Hills are a group of volcanic hills in Tanzania where a unique series of eruptions has produced volcanic rocks that have many of the features of kimberlites. The rocks are “igneous conglomerates” that are characterized by the presence of abundant ellipsoids of olivine in a fine grained matrix. The matrix contains major amounts of carbonate, serpentine, and complexly zoned Mg–Al–Cr spinels, minor perovskite, apatite and limonite. Large magnesian olivines are highly rounded and some have partially recrystallized to more Fe-rich and Ni-poor subhedral to euhedral grains. Several olivine ellipsoids are partly or wholly rimmed by a black coating consisting of perovskite, and Mg–Al spinel. Mineral inclusions within the olivines include chrome pyrope, similar in composition to garnets in kimberlites, low Al enstatite, low Al magnesian chrome diopside, Mg–Al chromite, and a highly magnesian phlogopite (Mg/Mg + Fe ~ .94). All of these are apparently primary phases in equilibrium with olivine. The total assemblage is similar in many respects to that found in garnet peridotite xenoliths such as those from the Lashaine volcano in northern Tanzania. The Igwisi irruptives apparently contain material derived from phlogopite-bearing, garnet peridotites with a primary mineral assemblage (assuming that all these phases coexist at depth) indicative of equilibrium at upper mantle temperatures and pressures. This primary assemblage was disrupted and brought rapidly to the surface in a gas-charged, carbonate-rich fluid. Rapid upward transport, extrusion, and rapid cooling have preserved inclusions of upper mantle ultramafics in a carbonate-rich matrix and have tended to prevent reaction between inclusions and matrix that might otherwise have yielded a more typical kimberlite.

Progressive metamorphism of serpentinite and allied rocks causes systematic changes in the composition of the chrome-spinel phase, when the latter is in equilibrium with chlorite and two magnesium silicate minerals. The stable spinel in antigorite-serpentinites is Al-poor magnetite, Cr-magnetite, or ferrit-chromite, depending on the local Cr/Fe 3+ ratio in the rock. With increasing metamorphic grade up to middle amphibolite facies conditions (talc + olivine, or Ca-poor amphibole + olivine stable) more chromiferous spinels (chromites) are encountered, containing modest amounts of Al. Further increase in grade (enstatite + olivine stable) extends the range of possible spinel compositions to green MgAl 2 O 4 -rich spinel. The Al-content is governed by P -, T -, and f H 2 O-sensitive equilibria involving chlorite. Mg/Fe 2+ ratios in the spinel are a function of Cr, Al and Fe 3+ in the spinel, and the ratio Mg/Fe 2+ in coexisting silicates. Solid solution in natural chromites on the magnetite-chromite join is complete at ≈500°C, and close to the join chromite-Al spinel (with variable Fe/Mg) it is complete at ≈700°C. The temperature dependence of K D , the olivine-spinel Fe-Mg partition coefficient, is greater than implied by the JACKSON (1969) geothermometer. A tentative, revised, graphical calibration is offered, based on microprobe-analyzed high-grade metamorphic pairs and pairs from basaltic pumice. This new plot gives results which are broadly consistent with relative temperatures of equilibration inferred from other geothermometers, for alpine peridotites, peridotite nodules and meteorites.

The solidi of two dry bimineralic eclogites (probable high-pressure cumulates) are slightly lower but their liquidi are substantially higher in temperature than the solidus of garnet–lherzolite. Olivine does not appear at the liquidus of either rock and enstatite appears only transiently in the solidus–liquidus interval of one rock at 20 kbar. The two dry eclogites cannot, therefore, represent primary liquids produced from a peridotite source nor can they be partially melted to yield peridotite residua. One eclogite shows total solution of garnet in clinopyroxene at the solidus close to 20 kbar, otherwise garnet is retained as a solidus phase in both rocks between 20 and 30 kbar. A garnet–olivine websterite, thought to have formed originally as an olivine + two aluminous pyroxenes cumulate, develops this mineralogy only close to the dry solidus at 30 kbar, 1575°C. Spinel appears at lower pressures, garnet at higher pressures. An iron- and aluminium-rich garnet–lherzolite, possibly representing a frozen high-pressure primary magma in equilibrium with harzburgite, does not show the required replacement of olivine by orthopyroxene as liquidus phase until pressures in excess of 45 kbar, implying a depth of origin greater than 140 km.

The concentrations of gold and iridium have been determined in various meteoritic and terrestrial materials by the technique of neutron activation. The concentrations of gold and iridium are uniform among the carbonaceous chondrite subgroups, and the Ir/Au weight ratio is nearly constant at 2.1. Cosmic atomic abundances relative to Si = 106 based on analyses of type I carbonaceous chondrites are 0.23 for gold and 0.42 for iridium. Iridium is found to be strongly depleted with respect to gold among the enstatite chondrites, and in all cases the Ir/Au weight ratio is less than unity. The observed concentrations of the two elements in separated magnetic and nonmagnetic fractions derived from various chondrite groups, reflect the relative oxidation states of the chondrite subgroups, as well as the relative siderophilic character of the two elements as predicted by thermodynamics. The achondrites are seen to be strongly depleted in both gold and iridium. The results of analyses of separated chondrules, meteoritic troilite, as well as metallic spherules of meteoritic origin are discussed. Data are also presented for the concentrations of the two elements in some igneous rocks of possible upper mantle origin.

Fractionation of the lithophile element ratios, Si/Mg, Si/Ca, Mg/Ca, Si/Al and Si/Ti in the principal chondrite types has been examined. Most of the analytical data are from H. B. Wiik. For Si/Mg in particular and Si/Ca and Mg/Ca to a lesser extent, fractionation between different chondrite types is distinct whereas that of Si/Al and Si/Ti is generally not so well developed. Ti appears to be strongly depleted in the enstatite chondrites. The degree of fractionation for each ratio differs from that of the other ratios. Aside from Si and Mg, there is an urgent need for improved analytical data because analytical error tends to blur or even obliterate the natural variation of a ratio, whether within a given chondrite type or between different types.

Results are considered of the determination of thirteen trace elements in sixty-seven iron meteorites, and of eight trace elements in the separated metal phases of eight bronzite, nine hypersthene and four enstatite chondrites, using neutron activation methods followed by radiochemical separations. It is found that the level of the trace elements Ge, Ga, Sb, As, Pd, Au and Re is distinctly different in the metal phases of the different chondrites. Thus, whereas the mean Ge content of the metal phase of bronzite, hypersthene and enstatite (Types I and II) chondrites is 64, 125 and 157 ppm respectively, the Ga content of both bronzite and hypersthene metal phases is 11 ppm and of the metal phase of enstatites, 62 ppm. Sb, As and Au, however, are highest in the hypersthene metal phase and lowest in the bronzites. Pd and Re are again highest in the hypersthene metal phase, but now lowest in the enstatites. The trace element distribution in the metal phases of these three classes of chondrites is generally confined to a narrower range than is found for these elements in iron meteorites. However, it is also found that some structurally similar iron meteorites have closely similar trace element composition. In particular, five groups of iron meteorites can be specified, namely: seven hexahedrites; three coarsest octahedrites; seven coarse octahedrites; twelve medium octahedrites; and six fine octahedrites. These five groups are also examined in relation to the classification of iron meteorites into specific “Ga-Ge groups”. A comparison is then made for trace elements in the metal phases of the three chondrite groups and the five structural and compositional] groups of iron meteorites. The most important observation is that the metal phases of the bronzites and hypersthenes have the same, or closely similar, trace element/Ni ratios for Ge, Ga, Sb, As, Au, Pd and Re. The enstatite metal phase, however, shows little similarity to the ordinary chondrite metal phases. The significance of these observations is noted in relation to the two proposed modes of origin of the metal phases of chondrites.