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Ni and Co variations in primary martian magmas exhibit anomalous incompatible behavior, which has remained an unexplained conundrum. Because martian magmas are S-rich, and some trace metals are reported to have enhanced solubility in S-bearing magmas, we have carried out a series of experiments to evaluate the effect of high-S melts on the olivine/melt partitioning of Ni, Co, Mn, V, and Cr. Near-liquidus experiments on a synthetic primary martian mantle melt (Yamato-980459 [Y98]) were completed in a piston-cylinder apparatus at 0.75 GPa. Previous studies in S-free systems illustrate that the partition coefficients for these elements are dependent chiefly on DMg(Ol/melt) (the partition coefficient defined as wt% Mg in olivine/wt% Mg in melt, a proxy for temperature), and were used to calibrate a predictive expression that includes the effects of temperature [i.e., DMg(Ol/melt)], melt composition, and oxygen fugacity. These predictive expressions are then used to isolate any effect in DM olivine/melt due to dissolved sulfur. The results show that S might have a small effect for Co, but not enough to change Co partitioning from compatible to incompatible in our experiments. The addition of a sulfur term to the DCo predictive expressions shows that nearly 8000 ppm of sulfur would be required in the melt (at liquidus temperature of Y98) for DCo to become

This research was supported by the Remote, In Situ, and Synchrotron Studies for Science and Exploration (RIS4 E) node of the NASA SSERVI program and NASA grants NNX16AR18G and NNX17AL07G. XANES spectroscopy data were collected at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation, Earth Sciences (EAR-1128799) and Department of Energy, Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. King was supported by an Australian Research Council Future Fellowship (FT130101524).

The application of Martian meteorite U–Th–Pb isotope systematics to track the geochemical evolution of the Martian mantle has had limited success because of the difficulty in discriminating an indigenous magmatic Pb component from secondary near-surface components that have additionally been overprinted by terrestrial contamination. To mitigate this challenge, a successive acid-leaching experiment was conducted on the Tissint meteorite, the freshest, witnessed fall of a primitive, olivine-bearing Martian basalt. Trace element concentration analyses of acid leachates and residues indicate that secondary terrestrial contaminants were effectively removed by the early steps in the leaching experiments and that the acid residues contain pristine Pb from Tissint. The acid residue, which shows the most depleted REE signature, also has the least radiogenic Pb isotopic composition ( 206 Pb/ 204 Pb = 10.948, 207 Pb/ 204 Pb = 11.187, 208 Pb/ 204 Pb = 30.228). A two-stage mantle evolution model based on this composition indicates that the Tissint mantle has the lowest μ -value ( 238 U/ 204 Pb = 1.62 ± 0.09 ) among the shergottite sources.

Mars Science Laboratory's Sample Analysis at Mars (SAM) investigation has measured all of the stable isotopes of the heavy noble gases krypton and xenon in the martian atmosphere, in situ, from the Curiosity Rover at Gale Crater, Mars. Previous knowledge of martian atmospheric krypton and xenon isotope ratios has been based upon a combination of the Viking mission's krypton and xenon detections and measurements of noble gas isotope ratios in martian meteorites. However, the meteorite measurements reveal an impure mixture of atmospheric, mantle, and spallation contributions. The xenon and krypton isotopic measurements reported here include the complete set of stable isotopes, unmeasured by Viking. The new results generally agree with Mars meteorite measurements but also provide a unique opportunity to identify various non-atmospheric heavy noble gas components in the meteorites. Kr isotopic measurements define a solar-like atmospheric composition, but deviating from the solar wind pattern at 80Kr and 82Kr in a manner consistent with contributions originating from neutron capture in Br. The Xe measurements suggest an intriguing possibility that isotopes lighter than 132Xe have been enriched to varying degrees by spallation and neutron capture products degassed to the atmosphere from the regolith, and a model is constructed to explore this possibility. Such a spallation component, however, is not apparent in atmospheric Xe trapped in the glassy phases of martian meteorites.

This perspective is a very personal account of the history and evolution of experimental trace element partitioning, although I cannot hope to exhaustively cover all aspects of this discipline. Therefore, I emphasise issues with which I am most familiar: (i) partitioning between mafic silicates and silicate melt; (ii) solid metal-liquid metal partitioning – especially the effects of non-metals; and (iii) metal-silicate liquid partitioning. I first entered the field of experimental partitioning as a grad-student in the mid-1970’s and so was able to see some of the growing pains of this discipline up close and personal. Also where appropriate, I will mention applications of experimental partitioning data to geologic and planetary problems.

[Figure][1] Spinel is a very important rock-forming mineral that is found in basalts from Earth, Mars, the Earth’s Moon, and basaltic meteorites. Spinel can be used as a sensitive indicator of petrologic and geochemical processes that occur in its host rock. This paper highlights the role of increasing f O2 (from IW-1 to FMQ+2) in converting a >90% normal spinel to an ~25% magnetite (inverse) spinel, the trajectory of D Vspinel/melt as it relates to the ratio of V3+/V4+ in the melt, and the crystal chemical attributes of the spinel that control the intrinsic compatibility of both V3+ and V4+. This work examines the nuances of the V partitioning and provides a crystal chemical basis for understanding Fe3+, Cr, and V substitution into the octahedral sites of spinel. Understanding this interplay is critical for using spinels as both indicators of planetary parentage and reconstructing the redox history of magmatic systems on the terrestrial planets. Three potential examples for this use are provided. In addition, this work helps explain the ubiquitous miscibility gap between spinels with changing ulvospinel contents. [1]: pending:yes

The apparent lack of plate tectonics on all terrestrial planets other than Earth has been used to support the notion that for most planets, once a primitive crust forms, the crust and mantle evolve geochemically-independent through time. This view has had a particularly large impact on models for the evolution of Mars and its silicate interior. Recent data indicating a greater potential that there may have been exchange between the martian crust and mantle has led to a search for additional geochemical evidence to support the alternative hypothesis, that some mechanism of crustal recycling may have operated early in the history of Mars. In order to study the most juvenile melts available to investigate martian mantle source(s) and melting processes, the trace element compositions of olivine-hosted melt inclusions for two incompatible-element-depleted olivine-phyric shergottites, Yamato 980459 (Y98) and Tissint, and the interstitial glass of Y98, have been measured by Secondary Ionization Mass Spectrometry (SIMS). Chondrite-normalized Rare Earth Element (REE) patterns for both Y98 and Tissint melt inclusions, and the Y98 interstitial glass, are characteristically light-REE depleted and parallel those of their host rock. For Y98, a clear flattening and upward inflection of La and Ce, relative to predictions based on middle and heavier REE, provides evidence for involvement of an enriched component early in their magmatic history; either inherited from a metasomatized mantle or crustal source, early on and prior to extensive host crystallization. Comparing these melt inclusion and interstitial glass analyses to existing melt inclusion and whole-rock data sets for the shergottite meteorite suite, defines mixing relationships between depleted and enriched end members, analogous to mixing relationships between whole rock Sr and Nd isotopic measurements. When considered in light of their petrologic context, the origin of these trace element enriched and isotopically evolved signatures represents either (1) crustal assimilation during the final few km of melt ascent towards the martian surface, or (2) assimilation soon after melt segregation, through melt–rock interaction with a portion of the martian crust recycled back into the mantle.

Fluvial landforms on Mars suggest that it was once warm enough to maintain persistent liquid water on its surface. The transition to the present cold and dry Mars is closely linked to the history of surface water, yet the evolution of surficial water is poorly constrained. Based on in situ hydrogen isotope (D/H) analyses of quenched and impact glasses in Martian meteorites, we provide evidence for the existence of a distinct but ubiquitous water/ice reservoir ( D / H = ∼ 2 – 3 times Earth's ocean water) that lasted from at least the time when the meteorites crystallized (173–472 million years ago) to the time they were ejected by impacts (0.7–3.3 million years ago), but possibly much longer. The origin of this reservoir appears to predate the current Martian atmospheric water ( D / H = ∼ 5 – 6 times Earth's ocean water) and is unlikely to be a simple mixture of atmospheric and primordial water retained in the Martian mantle (D/H ≈ Earth's ocean water). This reservoir could represent hydrated crust and/or ground ice interbedded within sediments. Our results corroborate the hypothesis that a buried cryosphere accounts for a large part of the initial water budget of Mars.

In this work we present a series of experiments that examine the relationship between oxygen fugacity and Cr valence ratio in olivine grown from a basaltic liquid. These experiments are specifically targeted for an olivine-rich martian basalt composition that was modeled after the bulk chemistry of the meteorite Yamato 980459 (i.e., Y-98). The chromium valence ratio in the olivine crystals was measured with X-ray absorption near edge spectroscopy (XANES) at the Advanced Photon Source, Argonne National Laboratory. Results from the XANES measurements indicate that the ratio of divalent to trivalent Cr in the olivine is not only systematically correlated with f O 2 , but is also reflective of the molar Cr 3+ /Cr 2+ in the silicate liquid from which it grew. In this way, measurements of Cr valence in olivine phenocrysts can yield important information about the oxygen fugacity and molar Cr 3+ /Cr 2+ of its parental liquid in the absence of a quenched melt phase. Although the results from the experiments presented in this work specifically apply to the Y-98 parental melt, the concepts and XANES analytical techniques discussed within the text present a novel, generalized methodology that may be applicable to any olivine-bearing basalt. Furthermore, the XANES-based measurements are made on a micrometer-scale, thus potential changes of the Cr 3+ /Cr 2+ in the melt during crystallization could be examined with a great deal of spatial detail.

A spiked (with REE, V, Sc) martian basalt Y980459 composition was used to synthesize olivine, spinel, and pyroxene at 1200 C at 5 oxygen fugacities: IW-1, IW, IW+1, IW+2, and QFM. The high spike levels for REE were used for two specific reasons. First, we wanted to be able to analyze REE by both electron microprobe and ion probe. Second, we wanted the most important "Others" components, (i.e., those outside the pyroxene quadrilateral such as Al, Cr3+, Fe3+, REE3+, V3+, V4+, etc.) to be REE3+Mg (Si,Al)2O6. At the doped levels we used, the most important "Others" component is REE3+ in the M2 site coupled with Al in the tetrahedral site. The goal of this paper is to explain the significant increase in the value of D(sub V)(sup pyroxene/melt) with increased Wo content of the pyroxene. We compare augite (Wo approx. 33), pigeonite (Wo approx. 13) and orthopyroxene (Wo approx 3.8). We also show olivine for comparison. The crystal chemical factors which account for this remarkable increase of DV with Wo are twofold. First, with Ca in the M2 site (as in diopside, CaMgSi2O6) the site is large and 8-coordinated while Mg in the M2 site (as in enstatite, Mg2Si2O6) the site is smaller and 6- coordinated. Second, tetrahedral Al in the pyroxene chains provides charge balance and makes the M2 site larger and more compliant for the introduction of REE.

A spiked (with REE, V, Sc) martian basalt Yamato 980459 (Y98) composition was used to synthesize olivine, spinel, and pyroxene at 1200 °C at five oxygen fugacities: IW−1, IW, IW+1, IW+2, and QFM. These run products were analyzed by electron microprobe, ion microprobe, and X-ray absorption near-edge spectroscopy to establish four oxybarometers based on vanadium partitioning behavior between the following pairs of phases: V spinel-melt, V/(Cr+Al) spinel-melt, olivine-melt, and spinel-olivine. The results for the spinel-melt, olivine-melt, and V/(Cr+Al) spinel-melt are applicable for the entire oxygen fugacity range while the spinel-olivine oxybarometer is only applicable between IW−1 and IW+1. The oxybarometer based on V partitioning between spinel-olivine is restricted to basalts that crystallized under low oxygen fugacities, some martian, all lunar, as well as samples from 4 Vesta. The true potential and power of the new spinel-olivine oxybarometer is that it does not require samples representative of a melt composition or samples with some remnant of quenched melt present. It just requires that the spinel-olivine pairs were in equilibrium when the partitioning of V occurred. We have applied the V spinel-olivine oxybarometer to the Y98 meteorite as a test of the method.

Volatile elements have influenced the differentiation and eruptive behavior of Martian magmas and played an important role in the evolution of Martian climate and near-surface environments. However, the abundances of volatiles, and in particular the amount of water in the Martian interior, are disputed. A record of volatile reservoirs is contained in primitive Martian basalts (shergottites). Olivine-hosted melt inclusions from a geochemically depleted shergottite (Yamato 980459, representing a very primitive Martian melt) possess undegassed water with a chondritic and Earth-like D/H ratio (δD≤275‰). Based on volatile measurements in these inclusions, the water content of the depleted shergottite mantle is calculated to be 15–47 ppm, which is consistent with the dry mantle hypothesis. In contrast to D/H in the depleted shergottite, melt from an enriched shergottite (Larkman Nunatak 06319), which either formed by melting of an enriched mantle or by assimilation of crust, exhibits an extreme δD of ∼5000‰, indicative of a surface reservoir (e.g., the Martian atmosphere or crustal hydrosphere). These data provide strong evidence that the Martian mantle had retained the primordial low-δD component until at least the time of shergottite formation, and that young Martian basalts assimilated old Martian crust.

article i nfo Article history: Solid metal/liquid metal partitioning experiments were conducted at 9 GPa and 1323-1873 K in the Fe-S system. Analysis of the experimental charges by in situ laser ablation inductively coupled plasma mass spectrometry enabled partitioning results to be obtained for a total of 21 trace elements (Co, Ni, Cu, Ga, Ge, As, Mo, Ru, Rh, Pd, Ag, Sn, Sb, W, Re, Os, Ir, Pt, Au, Pb, and Bi). This new, elevated pressure dataset for the Fe-S system allows a direct comparison to the extensive data available at 0.1 MPa and permits evaluation of the effect of pressure on partitioning in this system. The majority of the elements studied exhibit different solid metal/liquid metal partitioning behaviors at 9 GPa than at 0.1 MPa. Additionally, the nature of these differences varies significantly between the 21 trace elements studied, spanning the range of behaviors of partitioning from more strongly into solid metal, to less strongly into solid metal, and to becoming insensitive to the metallic liquid composition. We conclude that pressure affects solid metal/liquid metal partitioning behavior in the Fe-S system and discuss the implications for fractionations due to Earth's core solidification and for planetary differentiation models.

Brownleeite, ideally stoichiometric MnSi, is a manganese silicide not previously observed in nature that was discovered within an interplanetary dust particle that likely originated from a comet. Three submicrometer brownleeite grains were found, with one of them poikilitically enclosed by Mn-bearing forsterite. Owing to the small size of the brownleeite grains, it was not possible to determine conventional macroscopic properties of this mineral; however, the chemical composition and crystal structure were well constrained by extensive quantitative energy dispersive X-ray analysis and electron diffraction using transmission electron microscopy (TEM). The crystal system for brownleeite is cubic ( a = 4.557 A) with space group P 2 1 3, cell volume = 94.63 A 3 , Z = 4, density (calculated) = 2.913 g/cm 3 , and empirical formula: (Mn 0.77 Fe 0.18 Cr 0.05 )Si. These brownleeite grains likely formed as high-temperature condensates either in the early Solar System or in the outflow of an evolved star or supernova explosion.

In order to quantify possible fractionation of U and Pb into a metallic core, we have performed piston cylinder and multi-anvil press experiments at high pressure (up to 20 GPa) and high temperature (up to 2400 °C) and obtained the distribution coefficient Dmetal–silicate and the exchange partition coefficient Kmetal–silicate for these elements between metal and silicates (mineral or liquid). D Pb metal–silicate and D U metal–silicate depend strongly on the S content of the metallic phase, and also on the oxygen fugacity, in agreement with an effective valence state of 4 for U in silicates and 2 for Pb in silicates. K Pb d metal–silicate and K U d metal–silicate show no discernable pressure and temperature trend. U remains lithophile even at high pressure and high temperature but its lithophile nature decreases at very low oxygen fugacity. From our experimental data, it was possible to calculate the U and Pb contents of the cores of Mars and Earth under core-mantle equilibrium conditions at high pressure and high temperature. From the Dmetal–silicate of the present study, we obtained that: 0.008 ppm Our results suggest that the low concentration of Pb in the terrestrial mantle could not be explained by an early Pb sequestration in the Earth’s core even if S is the dominant light element of the core. If we assume a magma ocean scenario, U might produced a maximum value of 1.5% of the total heat budget of the core with a segregation occurring below ΔIW-3. The values found in the present study for U in the Martian core suggest that the magnetic field activity of Mars before ∼0.5 b.y. after its formation would be difficult to ascribe to the decay of U alone.

International audience; The Highly Siderophile Elements (HSE) are powerful tracers of planetary differentiation. Despite the importance of their partitioning between silicate and metal for the understanding of planetary core formation, especially for the Earth and Mars, there is still a huge discrepancy between conclusions based on different high temperature (HT) experimental studies. These disagreements may be due to the presence of HSE micro and nanonuggets in HT experiments. The formation of these nuggets is still interpreted in different ways. One hypothesis is that these HSE nuggets formed during the quench of the silicate melt, while another hypothesis supposes that these nuggets formed before the quench and represented artefacts of HT experiments. The goal of this work is to clarify whether the presence of HSE nuggets in silicate melts is linked to a quench effect or not. Understanding the formation of these HSE nuggets represents thus a necessary step towards the resolution of the Earth's core formation scenarios. We performed new HT experiments (1275–2000 • C) at different oxygen fugacities (fO 2), between ambient air up to ∼5 log units below the Iron-Wüstite buffer [IW-5], for two different silicate compositions (synthetic martian and terrestrial basalts) mixed with a metallic mixture of Pt–Au– Pd–Ru. Our 1275–1600 • C experiments were contained in either olivine, diopside or graphite crucible; experiments at 2000 • C were performed using a levitation method, so no capsule was necessary. Our samples contained quenched silicate melts, minerals (olivine, pyroxene, spinel depending on the run), a two-phase metallic bead and nano and micro-nuggets of HSE. Our samples underwent fine textural, structural and analytical characterizations. The distribution of the nuggets was not homogeneous throughout the quenched silicate melt. HSE nuggets were present within crystals. Dendritic textures from the quenched silicate melt formed around HSE nuggets, which could be crystallized, showing that the nuggets acted as nucleation sites during the quench. Thus they predated the quench. Finally, these nuggets also had strong heterogeneities suggesting at least a two-stage formation process under reducing conditions. Consequently, our observations clearly show that these HSE nuggets formed before the quench in the silicate melt. Our results agreed with previous studies, which concluded that HSE abundances in the Earth's mantle require the late accretion of chondritic material subsequent to core formation. However, the effects of metallic Si, O, H, or the effect of pressure on the HSE partitioning are still not fully understood. Further work to constrain these effects is to be encouraged to understand the Earth's core formation.

Carbon has been proposed as a potential light element in planetary cores, included in models of planetary core formation, and found in meteoritic samples and minerals. To better understand the effect of C on the partitioning behavior of elements, solid/liquid partition coefficients ( D = (solid metal)/(liquid metal)) were determined for 17 elements (As, Au, Co, Cr, Cu, Ga, Ge, Ir, Ni, Os, Pd, Pt, Re, Ru, Sb, Sn, and W) over a range of C contents in the Fe–Ni–C system at 1 atm. The partition coefficients for the majority of the elements increased as the C content of the liquid increased, an effect analogous to that of S for many of the elements. In contrast, three of the elements, Cr, Re, and W, were found to have anthracophile (C-loving) preferences, partitioning more strongly into the metallic liquid as the C content increased, resulting in decreases to their partition coefficients. For half of the elements examined, the prediction that partitioning in the Fe–Ni–S and Fe–Ni–C systems could be parameterized using a single set of variables was not supported. The effects of S and C on elemental partitioning behavior can be quite different; consequently, the presence of different non-metals can result in different fractionation patterns, and that uniqueness offers the opportunity to gain insight into the evolution of planetary bodies.

The redox states of volcanic and impact melts from the Earth, Moon, and Mars have been estimated from the valence state of V in basaltic glasses (Sutton et al. 2005). The V valence has been determined using synchrotron micro X-ray absorption near-edge structure spectroscopy (XANES) (Sutton et al. 2005), which allows for in situ measurements on samples with a micrometer spatial resolution and ~100 ppm elemental sensitivity. Here, we interpret those results for the natural samples and compare them to the literature. The results show that terrestrial melts are dominated by V 4+ , lunar samples by V 3+ , with Martian melts a mixture of both V 3+ and V 4+ . The f O 2 estimates derived from the V valence are consistent with those determined by other proven methods, whereby terrestrial basalts experience f O 2 conditions within 1 or 2 log units of the QFM buffer, lunar basalts equilibrate at 1 to 2 log units below the IW buffer, and Martian basalts fall somewhere between the QFM and IW buffer. The results illustrate the usefulness of this technique; i.e., a robust oxybarometer covering over six orders of magnitude, applicable to samples that record f O 2 conditions from reduced extraterrestrial bodies to the oxidized Earth.

A series of experiments on a synthetic, pigeonite-saturated, basaltic shergottite were carried out to constrain the variation of D (Eu/Gd) pigeonite/melt and D (Eu/Sm) pigeonite/melt with oxygen fugacity ( f O2 ). The experiments have been run under both dry and hydrous conditions. The shergottite was doped with 0.1, 0.5, and 1.0 wt.% Eu, Gd, and Sm oxides in different experiments and was equilibrated at liquidus conditions for 24 hours. D (Eu/Gd) pigeonite/melt in dry melts ranges from 0.156 ± 0.014 ( f O2 = IW − 1) to 0.630 ± 0.102 (IW + 3.5). D (Eu/Sm) pigeonite/melt in dry melts ranges from 0.279 ± 0.021 (IW − 1) to 1.114 ± 0.072 (IW + 3.5). Due to difficulties with low- f O2 experiments, hydrous distribution coefficients were measured, but were not used in the calibration of the Eu-oxybarometers. These two Eu-oxybarometers provide an accurate way to measure f O2 recorded during pigeonite crystallization, thereby yielding a record of f O2 during the earliest period of Martian meteorite parent magma crystallization history. Using this new calibration, Martian meteorite pigeonite cores record f O2 values of IW − 0.6 (±0.3) (QUE94201) to IW + 1.9 (±0.6) (Shergotty). These new values differ in magnitude, but not trend, from previously published data. The pigeonite Eu-oxybarometer yields an f O2 range in the basaltic shergottites of 2 to 3 orders of magnitude. Several processes have been proposed to explain the origin of this f O2 range, the majority of which rely on assimilation of an oxidized source. A potential correlation between this new pigeonite data and recent Fe-Ti oxide data, however, is consistent with intrinsic f O2 differences in the magma source region being responsible for the measured f O2 variations. This implies that the Martian meteorite source region, the mantle or lithosphere, may be heterogeneous in nature. However, the process of assimilation cannot be completely ruled out in that an assimilation event that took place before crystallization commenced would result in the overprinting of the source region f O2 signature.

We have evaluated the highly siderophile element (HSE) signatures of the martian (SNC) meteorites using new and literature data. These Ir and Os concentrations correlate with the Mg# [molar Mg/(Mg+Fe)], Cr and Ni, suggesting that olivine or chromite acts as a host for compatible siderophiles. Our analysis agrees with others who have suggested that the martian mantle has chondritic relative abundances of siderophiles. We also agree that, unlike the Sr and Nd isotopic systems, there is no evidence from Os isotopes for crustal assimilation. Comparisons of the siderophile element ratios of ALH 84001 to younger SNCs give no indication of a change in the martian siderophile element pattern over time. D 2002 Published by Elsevier Science B.V.

The oxygen fugacity of the Dar al Gani 476 martian basalt is determined to be quartz-fayalite-magnetite (QFM) −2.3 ± 0.4 through analysis of olivine, low-Ca pyroxene, and Cr-spinel and is in good agreement with revised results from Fe-Ti oxides that yield QFM −2.5 ± 0.7. This estimate falls within the range of oxygen fugacity for the other martian basalts, QFM −3 to QFM −1. Oxygen fugacity in martian basalts correlates with 87Sr/86Sr, 143Nd/144Nd, and La/Yb ratios, indicating that the mantle source of the basalts is reduced and that assimilation of crust-like material controls the oxygen fugacity. This allows constraints to be placed on the oxidation state of the martian mantle and on the nature of assimilated crustal material. The assimilated material may be the product of early and extensive hydrothermal alteration of the martian crust, or it may be amphibole- or phlogopite-bearing basaltic rock within the crust. In either case, water may play a significant role in the oxidation of basaltic magmas on Mars, although it may be secondary to assimilation of ferric iron-rich material.

Previous investigators have shown that, at high pressure, Re is a good capsule material and that Fe loss to Re under these conditions is minimal (e.g., Herzberg and Zhang 1997). We present here the first systematic low-pressure study of Re loop stability and of Fe loss from silicate melts to Re loops as a function of f O2 . Experiments were performed at 1400 degrees C and one bar pressure over a range of f O2 . For f O2 values as low as QFM-2, Fe loss was found to be negligibly small, even for a charge/loop ratio of only about 2. According to our calculations, for the same conditions, Fe loss to a Pt loop could reach 70% of the initial FeO content. We have also estimated the diffusion coefficient of Fe in Re and found it to be very small ( nearly equal 10-12 cm 2 /s), which is an additional factor in preventing Fe loss. At values of f O2 near QFM, Re metal reacts to form volatile Re oxides. But at f O2 below QFM-1.7, Re loops were found to be stable for any reasonable experimental run duration at 1400 degrees C. At lower temperatures Re may be stable to even higher values of f O2 . These conditions are similar to or slightly more reducing than the accepted redox states for the mantles of the Earth and Mars. Consequently, for many experiments, Re may be a more convenient loop material than Pt.

The partitioning behavior of Cr into olivine in basaltic systems has been parameterized and can now be modeled over a wide range of redox conditions and liquid compositions. The Cr (super 2+) /Cr (super 3+) in spinel-saturated experimental systems can be estimated based on a simple model of Cr solubility in basalt. Fe (super 3+) appears to suppress the presence of Cr (super 2+) in basaltic systems. We predict that, in Fe-free systems, all Cr is trivalent at log f O2 = -3 (i.e., QFM+3 to QFM-4), whereas all Cr is trivalent at approximately Ni-NiO(QFM+1) in Fe-bearing systems. Cr (super 2+) predominates under redox conditions or =QFM, appropriate for most terrestrial or martian basalts) strongly covaries with the ratio of non-bridging oxygens to tetrahedrally coordinated cations (NBO/T) (Mysen 1983) and can be estimated using the equation D (super (ol/liq)) (sub Cr3+) = -0.39.NBO/T+1.29. This relationship appears to be valid over the entire pressure range of olivine stability, from 1 atm to 15 GPa. D (sub Cr2+) (i.e., < or =IW-1, appropriate for lunar and some asteroidal basalts) is sensitive to liquid composition and temperature and can be estimated using either D (super (ol/liq)) (sub Cr2+) = 0.24.D (super (ol/liq)) Mg -0.07 or D (super (ol/liq)) (sub Cr2+) = 0.66.[10.000/T(K)]-4.48. The 1/T equation is probably only valid at 1 atm pressure, but the D Mg equation may be useful at higher pressures as well. The Cr content of spinel-saturated liquids is a function of temperature, composition, and f O2 . However, the Cr content of spinel-saturated liquids is buffered by spinel and is insensitive to the bulk Cr content of the system (e.g., Roeder and Reynolds 1991). Therefore, the Cr content of a crystallizing, spinel-saturated basalt cannot be modeled using Raleigh fractionation models.

Experimental studies of the metal/silicate partitioning of Ir have produced two sets of results differing by a factor of 10 6 ( Jones and Drake, 1986 ; O'Neill et al., 1995 ). In an attempt to understand this discrepancy, we have performed instrumental neutron activation analyses of multiple glass chips from these siderophile element partitioning experiments. From seven to ten individual chips (~ 10–100 μ g each) were analyzed from each experimental glass. The major target was Ir, which was detected at concentrations ranging from 690 ppb (ng/g) to less than 1 ppb. Detection limits are less than 10 −13 grams Ir. Almost all of the run products appear to be homogeneous with respect to lithophile elements, but most vary considerably in their Ir concentrations. It appears that most of the experiments analyzed so far contain small amounts of metal in the glass separates. One experiment from Jones and Drake (1983) appears to be homogeneous, but in fact may not be. More recent experiments are definitely not homogeneous, and require the existence of a metallic phase distinctly different from the bulk solid metal or liquid metal/sulfide phases also present in the charges. Iridium contents of the glass in the stirred crucible experiment studied ( O'Neill et al., 1996 ) decreased continuously over a period of several months, even as the oxygen fugacity was increasing, suggesting that the experiment never achieved equilibrium. Iridium contents in multiple samples of the glasses varied over factors of 2 to 3x. Further subdivision of one low-Ir glass did not result in a reduction in the variation of Ir between aliquots. Thus, the variation in Ir concentration appears to remain fairly constant, irrespective of the scale of the sampling.

The melt portion of a parental magma, undergoing equilibrium olivine crystallization, passes through a succession of compositions unique to its initial composition. Because the succession of compositions follows uniquely from the parental magma's composition, a liquid line of descent model fit to several of the derivative-melt compositions can recover an estimate of the parental magma's composition. We have developed an algorithm to accomplish the model fitting and thus far equipped it to treat olivine fractionation including departures from perfect equilibrium crystallization. Here we report validation tests on the method, which we call “Derivative-Magma Analysis” (DMA), applied to computer-synthesized data and two instances of laboratory-synthesized data (one as a single-blind test). For the computer- and laboratory-synthesized data, DMA recovered the parental-magma compositions within common analytical accuracy. For the most difficult oxide to estimate, MgO, the recovered values deviated from the measured values by less than 0.5 wt% for the two laboratory-synthesized datasets. For other major-element oxides, recovered values typically deviate from nominal values by less than 0.2 wt%. DMA also reliably estimated the compositions of the olivines that coexisted with each derivative melt. Applied to the Kilauean summit eruption of 1959, DMA estimates a parental magma with Mg# = 73 (MgO ≈ 16 wt%). This value, while significantly higher than estimates from some authors who favor low-Mg parental magmas, rules out the extremely high-MgO parental magmas favored by other authors.

Reports of percent level concentrations of normally lithophile elements such as Zr, Nb, and Ta in some refractory noble metal nuggets in CAIs raise important questions about the redox state of the early solar nebula. Consequently, we have determined the metal/silicate liquid partition coefficients for Zr, Nb, and Ti at 1548 K and solar oxygen fugacity (fo 2 ∼ 10 −16 bar). In most experiments, these elements formed intermetallic compounds with Pt-metal. Even though our partition coefficients for Zr (2.06–4.55) were substantially greater than previously determined values, they are still too small to account for percent levels of Zr in a refractory metal nugget from an average CAI. In addition, partition coefficient values obtained for Nb and Ti show an extremely strong affinity for the metal phase. Yet, Nb is rarely observed in refractory metal nuggets and Ti has never been observed. Given the abundance of Ti in CAIs, substantial concentrations of Ti should be present in refractory metal nuggets. This discrepancy is attributed to either: (1) the solarfo 2 was too oxidizing during condensation to form the predicted intermetallic compounds; or (2) the lithophile elements originally formed intermetallic compounds with Pt-metals during condensation in the solar nebula but were subsequently oxidized in a latter heating event. Whereas our experiments suggest that conditions were never reducing enough for Ti to alloy with Pt metal, other redox determinations (e.g.Ti 3+ /Ti 4+ ratios in CAI) imply approximately solar oxygen fugacities. Our experiments serve to emphasize the range in redox states implied by metal and silicate assemblages in CAI.

Liquid carbonate/ liquid silicate partition coefficients (D) for a suite of incompatible elements (Nb, Mo, Ba, Ce, Pb, Th, U) have been determined experimentally. For one experiment, D's for Ra and Pa were also determined. The partitioning behavior is shown to be a regular function of the ionic field strength (zr) of the partitioned elements. At constant temperature, increasing pressure from 7 to 20 kbar caused little change in the measured D. At constant pressure, increasing temperature causes all D's to approach unity, as the temperature nears the critical temperature and the two liquids become compositionally similar. Addition of P or F to the system enlarges the stability field of carbonate but does not affect the partitioning behavior of our suite of elements. However, both P and F are strongly partitioned into the carbonate liquid. Omphacite/carbonate liquid D's at 50–60 kbar have also been determined. Our results for DU and DTH are similar to those found previously for augite/silicate liquid. Thorium is more compatible than U in pyroxene, indicating that excesses of 230Th/238U found in MORB cannot be explained by fractionation between pyroxene and melt. The Soret effect in carbonate liquids is quite small, in contrast to the effect in silicate liquids. In general, we see no evidence for Soret separation in our experiments, implying that endmember Ca-, Na-, and Mg-carbonate liquids have simple speciations and mix nearly ideally. This observation suggests that the results of our liquid/liquid partitioning studies do not strongly depend on the exact composition of the carbonate. Our experimentally determined liquid-liquid partition coefficients can explain recent fractionations of 210T/238U and 226Ra/238U in carbonatitec lavas from the Oldoinyo Lengai volcano, but cannot explain similar fractionations in 230Th/238U. 210Pb has the shortest half-life of these nuclides, and is, therefore, the most sensitive to the exact timing and mechanism of fractionation. Because we can explain 210Pb/238U in terms of recent, instantaneous liquid immiscibility, we favor this model for the origin of the deviations of 210Pb/238U and 226Ra/238U from those expected under secular equilibrium. If our interpretation is correct, some other fractionation process(es) must have fractionated Th (and perhaps Pa) from U, Ra, and Pb in the Oldoinyo Lengai carbonatites.

We have performed melting experiments between 1170–1325°C and at an oxygen fugacity of IW-1 on the ordinary chondrites Lost City (H) and St. Severin (LL) in order to investigate the origin of diogenites and their possible relationship to eucrites. Low-Ca pyroxene disappears from the Lost City residuum just above 1220°C at a Mg# near 66. Accordingly, Lost City does not have a sufficiently large stability field for orthopyroxene to be a likely parent composition for melts which could crystallize diogenites. In contrast, orthopyroxene in St. Severin persists up to about 1325°C, at which point it has a Mg# similar to that of typical diogenites (∼75). This is only marginally compatible with diogenite origins, as the most magnesian pyroxenes from diogenites (Mg# 82) and howardites (Mg# 85) are more MgO-rich than any pyroxene formed from a St. Severin melt. Accordingly, if diogenites formed from a source having the bulk composition of an ordinary chondrite, than either the fO2 had to be lower than that inferred for eucrite formation or, more likely, the parent body had a lower FeSi ratio than that of an LL chondrite. Also, low temperature melts of St. Severin are depleted in elements compatible with pyroxene and so, while broadly eucritic, do not closely match the compositions of eucrites that are thought to be primary partial melts. Hence, it doesn't appear possible to produce both diogenites and eucrites from the same source region composition, if eucrites are primary partial melts. Reconnaiss:mce experiments at higher oxygen fugacity (IW + 2) produce broadly angritic melts for Lost City, as was observed for Murchison and Allende. In contrast, our Na2O-poor St. Severin charge produced a broadly eucritic melt at this higher fO2 This difference in melting behavior is probably because LL chondrites have a much lower bulk FeSi ratio than the H, CM, or CV chondrites.

The purpose of the study is to provide an alternative perspective on both the mass balance issue and on the issue of secular change in the Rb-Cs ratio in the earth mantle. In particular, it is argued that the apparent secular change in the Rb/Cs ratio may be simply an artifact of alteration of older rocks through weathering and/or metamorphism subsequent to formation. It is also shown that there is not necessarily a need for a hidden, undepleted reservoir, and the consistency of this result with other mass calculations is explored. New methods for estimating the Rb/Cs ratio of the bulk silicate earth are suggested.

The compositions of experimental partial melts from the Allende (CV) and anhydrous Murchison (CM) chondrites depend strongly on ambient oxygen fugacity ( fo 2 ). At low fo 2 (IW-1) partial melts are tholeiitic and resemble the compositions of eucrites, whereas at high fo 2 ( IW +2) melts are silica undersaturated and resemble the compositions of angrites. In particular, the major element composition of our (1170°C, IW-1) melt of Murchison is strikingly similar to natural eucrites. Allende partial melts, while broadly eucritic, do not match most eucrites in detail. Conversely, IW+2 Allende partial melts are more like angrite compositions than are partial melts of Murchison. Chromium contents of eucrites and of our spinel-saturated Murchison and Allende experiments are extremely similar, suggesting that eucrites are also spinel saturated. This observation favors a partial melting origin for eucrites such as Sioux County, as opposed to an origin by fractional crystallization. However, most of the main group eucrites have slightly lower Mg#'s and slightly higher CaO/A1 2 O 3 ratios than Sioux County, suggesting that, relative to Sioux County, the majority of the main group eucrites either have experienced small amounts of fractional crystallization or were produced by slightly smaller amounts of partial melting. If the eucrites were produced by simple partial melting, the MnO content of the eucrite parent body must be rather high compared to either Allende or Murchison. In contrast to comparisons between our IW—1 experiments and eucrites, none of our IW+2 experiments have exactly reproduced a natural angrite composition. There are also minor but significant discrepancies between our partial melting experiments and experiments on angrite compositions. While these differences may, in part, be due to our poor knowledge of the angrite parent body, they may be explicable if at least some of the angrites are not primitive partial melts but, rather, impact melts.

Low-temperature metal-silicate partition coefficients are extrapolated to magma ocean temperatures. If the low-temperature chemistry data is found to be applicable at high temperatures, an important assumption, then the results indicate that high temperature alone cannot account for the excess siderophile element problem of the upper mantle. For most elements, a rise in temperature will result in a modest increase in siderophile behavior if an iron-wuestite redox buffer is paralleled. However, long-range extrapolation of experimental data is hazardous when the data contains even modest experimental errors. For a given element, extrapolated high-temperature partition coefficients can differ by orders of magnitude, even when data from independent studies is consistent within quoted errors. In order to accurately assess siderophile element behavior in a magma ocean, it will be necessary to obtain direct experimental measurements for at least some of the siderophile elements.

A self-consistent terminology for partitioning data is presented. Ratios of the concentration of a component in two phases are termed partition coefficients and given the symbol D. Ratios of partition coefficients are termed exchange coefficients and given the symbol KD. The prefix “bulk” implies that these coefficients are weighted according to the proportions of coexisting phases. Bulk partition and bulk exchange coefficients are denoted by D and KD, respectively.

The partitioning of Pu and Sm between diopside/liquid and whitlockite/liquid has been investigated experimentally to evaluate the geochemical coherence of Pu and the light REEs. ^(Pu)D/^(Sm)D is 2 ~ for both diopsidic pyroxene and whitlockite. This small amount of fractionation would be decreased further if Pu were compared to Ce or Nd. Our experimental results thus validate the suggestion that Pu behaves as a LREE during igneous processes in reducing environments. Our data and the data of Ray et al. (1983) indicate that temperature rather than melt composition is the most important control on elemental partitioning. This is true even though we demonstrate that additions of only 1–2 wt.% of P2_O_5 to the diopside-anorthite-albite system change ^(Pu)D_(cpx) by a factor of two. Our data suggest that P_2O_5 in aluminosilicate melts serves as a complexing agent for the actinides and lanthanides.

Although the moon is depleted in volatile elements compared to the earth, these depletions are not in accord with simple volatility. For example, the Cs/Rb ratios of the earth and moon inferred from basalt are approximately one seventh and one half of the CI ratio, respectively. Volatility considerations alone predict that the lunar Cs/Rb ratio should be equal to or lower than the terrestrial ratio if the moon was derived entirely from earth mantle material. Thus hypotheses such as rotational fission which invoke derivation of lunar material entirely from the earth's mantle may be excluded. The collisional ejection hypothesis of lunar origin requires at least 18 percent of lunar material to be derived from a projectile with dehydrated CI composition to match the lunar Cs/Rb ratio, and 25-50 percent to match both the lunar Cs/Rb ratio and absolute concentrations of Cs and Rb. It remains to be demonstrated that this relatively large contribution of projectile material is consistent with other elemental abundances and element ratios in the moon.

The minor-element/major-element trends among the ureilites were investigated using electron microprobe data on olivine and pigeonite cores in eight low-shock ureilites. The results show well-defined correlations between Fe/X (where X is one of the minor elements Mn, Cr, Ca, Al, Ti, P, or Ni) and Fe/Mg ratios. For the lithophile minor elements, these trends are linear, with positive slope, and pass through or near the origin, indicating various degrees of FeO reduction of the parent magmas. The trends shown by P and Ni are consistent with this interpretation and require, in addition, equilibrium crystallization of 20-27 mole pct metal. A model is proposed for generation and crystallization of ureilite parent magmas, which predicts that the ureilite parent body had a differentiated crust, did not have a core, and was at least 235 km in radius.

Molar olivine/liquid partition coefficients for Mg, Fe, Mn and Ni are shown to be linearly correlated. These correlations appear to be independent of temperature, pressure and, to some extent, bulk composition and are best explained by the dominance of change-of-state thermodynamics during partitioning. The correlations presented here, when coupled with considerations of olivine stoichiometry, allow (olivine/basalt) partition coeficients of Mg, Fe, Mn and Ni to be predicted if the composition of the basalt is known. Since the correlations hold for major, minor and trace elements and for both “compatible” and “incompatible” elements, the inferences drawn from this study appear to have general relevance.

Previous estimates of the composition of the EPB have tacitly assumed chondritic abundances of refractory LIL elements. A model is presented here which utilizes refractory element ratios instead. Consideration of these ratios and of known olivine-liquid partition coefficients for Sc, Mg and Si imply that the EPB mantle may be accurately modeled as a simple mixture of 25 percent eucrite + 75 percent olivine. The model also implies that eucrites such as Juvinas and Sioux County were derived by large degrees of partial melting (20-30 percent). Comparison of the model EPB mantle to known meteorite classes indicates a similarity between the element abundances of the EPB and C3 chondrites.

Theoretical lunar mantle seismic velocity and density profiles were calculated for a series of specific bulk composition models, thermal models, and evolutionary models. It was found that most of the calculated seismic profiles are in qualitative agreement with the Nakamura (1983) observational mantle seismic velocity model. With respect to the density modeling results, the most interesting finding is that none of the considered models yield mantle density increases that are sufficient to match the adapted upper bound on the lunar moment-of-inertia factor (0.3928). The results suggest that the moon possesses a dense metallic core with a mass in the range of 1 to 4 percent of the lunar mass and that the alumina-rich models of Morgan et al. (1978) and Taylor (1982) are best able to match the middle mantle velocity increase that characterizes the Nakamura model.

The chronologies of the Shergotty, Zagami, ALHA 77005, and EETA 79001 meteorites were reexamined on the basis of shergottites' petrography and mineral chemistry data. Among the various isochrons, the concordant Rb-Sr (about 180 Myr) and U-Th-Pb (about 190 Myr) internal isochrons are interpreted as representing the time of igneous crystallization, while the Rb-Sr, Sm-Nd, and Pb-Pb whole-rock isochrons are interpreted as mixing lines, and are reasonably attributed to igneous processes such as wall-rock assimilation and magma mixing. If the approximated age of less than 200 Myr is correct, the shergottites represent the youngest known extraterrestrial basalts. This conclusion supports the hypothesis that the SNC meteorites are samples of Mars.

Fission and alpha track radiography techniques have been used to measure partition coefficients (D) at trace (ppm) concentration levels for the actinide elements Th, U, and Pu between synthetic whitlockite and coexisting “haplobasaltic” silicate liquid at 1 bar pressure and 1250°C at oxygen fugacities from 10^(−8.5) and 10^(−0.7) bars. Pu is much more readily incorporated into crystalline phases than is U or Th under reducing conditions (fO_2 = 10^(−8.5)), because Pu is primarily trivalent, whereas U and Th are tetravalent. Definitive valence state assignments cannot be made, but our best estimates of corrected partition coefficients for Pu^(+3), Pu^(+4), Th^(+4), U^(+4), and U^(+6) are, for whitlockite 3.6/

The partitioning of Ir, Ge, Ga, W, Cr, Au, P, and Ni between solid metal and metallic liquid is investigated as a function of temperature and the S-concentration of the metallic liquid. The partition coefficients for siderophile elements, such as Ir, W, Ga, and Ge, are found to increase by factors of 10-100 as the S-concentration of the metallic liquid increases from 0-30 wt percent. Partition coefficients for other siderophile elements, such as Ni, Au, and P, increase by factors of only 2-3. In contrast, a decrease is seen in the partition coefficients for the more chalcophile element Cr. These experimentally determined coefficients are used in conjunction with a fractional crystallization model to reproduce the geochemical behavior of Ni, P, Au, and Ir during the magmatic evolution of groups IIAB, IIIAB, IVA and IVB iron meteorites. The mean S-concentration for each group increases in the order IVB, IVA, IIIAB, IIAB, which is in accordance with cosmochemical prediction. However, the geochemical behavior of Ge, Ga, W, and Cr cannot be reproduced in an internally consistent way. It is concluded that the magmatic histories of these iron meteorite groups are more complex than has been generally assumed.

Measurements of solid metal/liquid metal trace element partition coefficients, which are used to interpret the crystallization history of magmatic iron meteorite groups differ greatly between different research groups, using different experimental techniques. Specifically, partition coefficients measured utilizing 'static' experiments which approach equilibrium cannot be reconciled with the results of 'dynamic' experiments which mimic fractional crystallization. We report new tests of our 'static' experimental technique and demonstrate that our methodology yields reliable equilibrium values for Ni, P and Ge partition coefficients. Partition coefficients in the Fe-Ni-S-P system are well matched by interpolation between the Fe-Ni-S and Fe-Ni-P subsystems. In contrast, the predictions of 'dynamic' experiments do not agree with our measurements and, consequently, the ability of 'dynamic' experiments to reproduce iron meteorite Ge vs. Ni fractionation trends successfully must be regarded as fortuitous.

Two very different sets of Pu-244/U-238 ratios have been reported for early solar system materials, one high and the other low, and this paper examines how one of these groups may yield an incorrect ratio. Recently measured partition coefficients for Pu and Sm (Jones, 1981) are used to evaluate Pu-Nd fractionation during achondrite genesis, and Pu-244/U-238 ratios of chondrites, achondrites, and Ca-Al-rich inclusions are compared and discussed. A low Pu-244/U-238 ratio is favored, and some implications of this low ratio for galactic nucleosynthesis and meteorite age dating are briefly discussed.

Abundances of elements in shergottite, nakhlite, and Chassigny meteorites which originated on a single planet, the shergottite parent body (SPB), were examined with the aim of elucidating the chemical conditions of metal separation and core formation in the SPB and of testing present models of planetary core formation. Using partition coefficients and the SPB mantle composition determined in earlier studies, the abundances of Ag, Au, Co, Ga, Mo, Ni, P, Re, S, and W were modeled, with free parameters being oxygen fugacity, proportion of solid metal formed, proportion of metallic liquid formed, and proportion of silicate that is molten. It is shown that the abundances of all elements (except Mo) could be reproduced using models with these four free parameters. In contrast to the SPB, an equivalent model used to predict element abundances in the earth's mantle was shown by Jones and Drake (1986) to be inadequate; there is at present no hypothesis capable of quantitatively reproducing the elemental abundances of the earth's mantle. The contrast suggests that these two terrestrial planets (assuming that the SPB is Mars) may have accreted or differentiated differently.

A three-component model for the moon's bulk composition was developed on the basis of the results of Delano (1986) on lunar pristine glasses. This suite of models is based on the following two assumptions: (1) that the early moon differentiated into two primary reservoirs, a FeO-rich magma ocean and an olivine-rich residuum of about Fo(90); and (2) that this magma ocean then differentiated into an olivine-dominated cumulate, about equal to or greater than Fo(80), and a primitive liquid composition. Because serious uncertainties remain concerning the moon's complex differentiation history, this three-component model does not yield a single unique composition for the bulk silicate moon; instead, it yields a range of restricted compositions that are permitted within the conceptual framework of the model.

A detailed study of the U distribution of the St. Severin chondrite has been made by fission track radiography in order to clarify the interpretation of fission Xe thermal release data in terms of the mineralogical location of the fission Xe within the meteorite. This is of importance because the ^(244)Pu/^(238)U ratio for St. Severin has been widely adopted as the average solar system value. The U contents of the constituent minerals cannot account for the total rock U which, instead, appears to be primarily localized on grain boundaries. The greatest localizations of U are in olivine-poor, orthopyroxene-rich ‘clasts’. Our data coupled with those of Podosek (1970a) show that ^(244)Pu in St. Severin was also located on grain boundaries and that the bulk of Pu and U are unfractionated within this meteorite. Due to recoil, the ^(244)Pu fission Xe is found in 10 micron surface layers on major phases. Assuming that the grain boundaries (on which the Pu was located) was formed during metamorphism, the ^(244)Pu/^(238)U ratio for St. Severin applies to a time subsequent to the textural recrystallization of the meteorite. Our data support the interpretation of Podosek and our best estimate of the solar system ^(244)Pu/^(238)U is 0.015.

The method of measuring the derivatives of the earth’s magnetic field which is proposed depends on the fact that a small steady magnetic field will modify the e.m.f. induced in the secondary coil of a detector with a core of magnetizable material possessing high initial permeability and a high rate of increase of the permeability with the strength of the magnetizing field. It is shown that a detector of this type is equivalent to an inductor coil rotating with angular speed equal to the periodicity of the alternating magnetizing field.

The ureilite meteorites are carbonaceous olivine-pyroxene achondrites. They typically contain up to 4 wt.% carbon (carbonaceous matrix) as graphite, diamond, and lonsdaleite. Shock degradation has effectively obliterated primary textures in the carbonaceous matrix of previously described ureilites, a factor that has hampered efforts to explain the origin of this material. In contrast, the Antarctic ureilite ALHA78019 displays perfectly preserved primary textures in the carbonaceous matrix characterized by euhedral graphite blades intergrown with Fe-Ni metal and sulfide (diamonds are absent). This petrographic feature suggests that most graphite in ureilites originated by crystallization from a C-rich metallic phase. Assuming that fO2 is controlled by C-CO-CO2 reactions, the compositions of silicates and metals in ureilites imply a two-stage redox history. The noble gases and rare earths of ureilites are discussed in light of this model.