Insights into the redox history of the NWA 1068/1110 martian basalt from mineral equilibria and vanadium oxybarometry
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Christopher D.K. Herd
Abstract
The NWA 1068/1110 martian meteorite contains megacrystic olivine and associated chromite set in a basaltic groundmass of pyroxene, maskelynite (shock-isotropized plagioclase), phosphates, ilmenite, and ulvöspinel. Spinel is compositionally zoned from chromian cores to titanian rims, and lacks the chromite-ulvöspinel miscibility gap. Co-crystallizing olivine + pyroxene + spinel and ulv öspinel + ilmenite assemblages were identified on the basis of compositional and textural criteria, and were used to determine the oxygen fugacity from the earliest to the latest stages of crystallization. Megacrystic olivine with earliest chromite and low-Ca pyroxene yield an oxygen fugacity 2.5 log units below the Quartz-Fayalite-Magnetite (QFM) buffer, whereas titanian-spinel rims, olivine rims, and co-crystallizing pyroxene yield QFM + 0.3 and groundmass ulvöspinel + ilmenite assemblages yield QFM + 0.5. Vanadium in spinel was examined to determine whether it would be sensitive to the changing redox conditions of the NWA 1068/1110 melt. Only small changes in V concentration were observed between spinel cores and rims; however, the relationship between V and Ti in spinel is consistent with an increase in the valence state of V from V4+ to V5+ in accordance with the increase in oxygen fugacity. Application of a quantitative version of the “V-in-chromite„ oxybarometer results in an oxygen fugacity of QFM -3, consistent with mineral equilibria results, although assumptions and uncertainties limit its application to circumstances where the parental melt V concentration is known independently of the redox conditions. Oxygen fugacity results and textural observations are strongly indicative of a xenocrystic origin for the olivine megacrysts and associated chromite, and lend support to a model in which martian basalt oxygen fugacity is determined by mantle-source redox conditions as opposed to assimilation of oxidized crustal material.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Orbicular oxides in carbonatitic kimberlites
- Fluid-mediated polymetamorphism related to Proterozoic collision of Archean Wyoming and Superior provinces in the Black Hills, South Dakota
- Lithium isotopic systematics of granites and pegmatites from the Black Hills, South Dakota
- Identifying the effects of petrologic processes in a closed basaltic system using trace- element concentrations in olivines and glasses: Implications for comparative planetology
- Tectonic implications of diverse igneous blocks in Franciscan mélange, Northern California and southwestern Oregon
- The petrogenesis of the Apollo 14 high-Al mare basalts
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- The atomic arrangement of merrillite from the Fra Mauro Formation, Apollo 14 lunar mission: The first structure of merrillite from the Moon
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- α-PbO2-type nanophase of TiO2 from coesite-bearing eclogite in the Dabie Mountains, China–Reply
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