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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).

[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

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.

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.

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.

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.