Paul V. Burger

Valence state partitioning of Cr and V between pyroxene-melt: Estimates of oxygen fugacity for martian basalt QUE 94201

Abstract Based on the partitioning of Cr and V between pigeonite cores and bulk composition, we estimate that martian basalt QUE 94201 crystallized at an fO₂ between IW+0.2 and IW+0.9. These estimates are based on calibration curves for DCr, DV, and DCr/DV (pyroxene/melt) derived from experimental charges that were synthesized at fO₂ conditions of IW-1, IW, and IW+1. We believe our fO₂ estimate is robust because (1) the fO₂ is measured in the earliest crystallizing pyroxenes; (2) the calibration curves are based on the same bulk composition as the natural sample; and (3) that bulk composition represents a melt from the martian mantle, so an accurate DCr and DV are measured. Presently, the two best candidates for martian melts, Y 980459 and QUE 94201, indicate an fO₂ of IW to IW+1 for the upper martian mantle.

Volatile abundances of coexisting merrillite and apatite in the martian meteorite Shergotty: Implications for merrillite in hydrous magmas

Abstract Whitlockite and merrillite are two Ca-phosphate minerals found in terrestrial and planetary igneous rocks, sometimes coexisting with apatite. Whitlockite has essential structural hydrogen, and merrillite is devoid of hydrogen. Whitlockite components have yet to be discovered in samples of extraterrestrial merrillite, despite evidence for whitlockite-merrillite solid solution in terrestrial systems. The observation of merrillite in meteoritic and lunar samples has led many to conclude that the magmas from which the merrillite formed were “very dry.” However, the Shergotty martian meteorite has been reported to contain both apatite and merrillite, and recently the apatite has been shown to contain substantial OH abundances, up to the equivalent of 8600 ppm H2O. In the present study, we determined the abundances of F, Cl, H2O, and S in merrillite from Shergotty using secondary ion mass spectrometry (SIMS). We determined that the merrillite in Shergotty was properly identified (i.e., no discernible whitlockite component), and it coexists with OH-rich apatite. The absence of a whitlockite component in Shergotty merrillite and other planetary merrillites may be a consequence of the limited thermal stability of H in whitlockite (stable only at T <1050 °C), which would prohibit merrillite-whitlockite solid-solution at high temperatures. Consequently, the presence of merrillite should not be used as evidence of dry magmatism without a corresponding estimate of the T of crystallization. In fact, if a whitlockite component in extraterrestrial merrillite is discovered, it may indicate formation by or equilibration with hydrothermal or aqueous fluids.

Direct determination of europium valence state by XANES in extraterrestrial merrillite: Implications for REE crystal chemistry and martian magmatism

Abstract The relative proportion of divalent and trivalent Eu has proven to be a useful tool for estimating fo2 in various magmatic systems. However, in most cases, direct determination of the Eu valence state has not been made. In this study, direct determination of Eu valence by XANES and REE abundance in merrillite provide insights into the crystal chemistry of these phosphates and their ability to record conditions of magmatism. Merrillite strongly prefers Eu3+ to Eu2+, with the average valence state of Eu ranging between 2.9 and 3 over approximately six orders of magnitude in fo2. The dramatic shift in the REE patterns of merrillite in martian basaltic magmas, from highly LREE-depleted to LREE-enriched, parallels many other trace element and isotopic variations and reflects the sources for these magmas. The behavior of REE in the merrillite directly reflects the relationship between the eightfold-coordinated Ca1 site and adjacent sixfold Na and tetrahedral P sites that enables charge balancing through coupled substitutions.

A unique glimpse into asteroidal melting processes in the early solar system from the Graves Nunatak 06128/06129 achondrites

Abstract The recently recovered Antarctic achondrites Graves Nunatak 06128 and 06129 are unique meteorites that represent high-temperature asteroidal processes in the early solar system never before identified in any other meteorite. They represent products of early planetesimal melting (4564.25 ± 0.21 Ma) and subsequent metamorphism of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. This melting event is unlike those predicted by previous experimental or geochemical studies, and indicates either disequilibrium melting of chondritic material or melting of chondritic material under volatile-rich conditions.

XANES measurements of Cr valence in olivine and their applications to planetary basalts

Abstract 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 fO2, but is also reflective of the molar Cr3+/Cr2+ 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 Cr3+/Cr2+ 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 Cr3+/Cr2+ in the melt during crystallization could be examined with a great deal of spatial detail.

Valence state partitioning of Cr between pyroxene-melt: Effects of pyroxene and melt composition and direct determination of Cr valence states by XANES. Application to Martian basalt QUE 94201 composition

Abstract DCr augite/melt is approximately double that of DCr pigeonite/melt in synthetic Martian basaltic samples equilibrated at the same fO₂. This increase is not related to changing fO₂ and the valence of Cr, but rather to the increased availability of elements for coupled substitution with the Cr3+ ion, namely Na and Al. The availability of Al and Na to partition into pyroxene is due to delayed nucleation of plagioclase for the QUE 94201 Martian basalt composition. Direct valence state determination by XANES shows that Cr3+ is the dominant valence state in pyroxene at IW-1, IW, and IW+1. Trivalent Cr is apparently much more compatible in the pyroxene structure than divalent Cr, and thus an increasing DCr for both augite/melt and pigeonite/melt with increasing fO₂ is a function of the increased activity of Cr3+ in the crystallizing melt.

Terrestrial analogs of martian jarosites: Major, minor element systematics and Na-K zoning in selected samples

Abstract Natural jarosites selected for study have mixed domains of jarosite, KFe33+ (SO4)2(OH)6, and natrojarosite, NaFe33+(SO4)2(OH)6. Minor elements include Al in the octahedral B-site, and P, As, Mo, and V in the tetrahedral T-site. High abundances of As are detected in some samples. Oscillatory zoning of Na and K in a subset of these samples has been explored using BSE images, X-ray maps, and a 1 μm EDS beam. Our work shows that zoning is composed of less than micrometer-sized bands of near end-member compositions. This agrees with the XRD work of Desborough et al. (2006), where 32 natural hypogene and supergene jarosites were found to be mixtures of near end-member compositions and showed <5% solid solution. This indicates a wide solvus (miscibility gap) between jarosite and natrojarosite. It also suggests that special crystallization effects are active in solid-solution and aqueoussolution interactions. Here, the optimal conditions for the crystallization of end-member compositions are at low temperatures (<100 °C), and when the two end-members of a binary solid-solution series have different solubilities, as do jarosite and natrojarosite (Glynn 2000). These conditions are commonly found in supergene environments, and are best illustrated by spectacular oscillatory zoning of jarosite and natrojarosite in samples from the Apex Mine, Arizona and Gold Hill, Utah.

Partitioning of Eu between augite and a highly spiked martian basalt composition as a function of oxygen fugacity (IW-1 to QFM): Determination of Eu2+/Eu3+ ratios by XANES

Abstract We have determined DEu between augite and melt in samples that crystallized from a highly spiked martian basalt composition at four fO₂ conditions. DEu augite/melt shows a steady increase with fO₂ from 0.086 at IW-1 to 0.274 at IW+3.5. This increase is because Eu3+ is more compatible than Eu2+ in the pyroxene structure; thus increasing fO₂ leads to greater Eu3+/Eu2+ in the melt and more Eu (total) can partition into the crystallizing pyroxene. This interpretation is supported by direct determinations of Eu valence state by XANES, which show a steady increase of Eu3+/Eu2+ with increasing fO₂ in both pyroxene (0.38 to 14.6) and glass (0.20 to 12.6) in the samples. Also, pyroxene Eu3+/Eu2+ is higher than that of adjacent glass in all the samples, which verifies that Eu3+ is more compatible than Eu2+ in the pyroxene structure. Combining partitioning data with XANES data allows for the calculation of specific valence state D-values for augite/melt where DEu3+ = 0.28 and DEu2+ = 0.07.

Developing vanadium valence state oxybarometers (spinel-melt, olivine-melt, spinel-olivine) and V/(Cr+Al) partitioning (spinel-melt) for martian olivine-phyric basalts

Abstract 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 nearedge 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.

Normal to inverse transition in martian spinel: Understanding the interplay between chromium, vanadium, and iron valence state partitioning through a crystal-chemical lens

Abstract 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 fO₂ (from IW-1 to FMQ+2) in converting a >90% normal spinel to an ~25% magnetite (inverse) spinel, the trajectory of DVspinel/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 ülvospinel contents.