C. D. K. Herd

Oxygen fugacity and geochemical variations in the martian basalts: implications for martian basalt petrogenesis and the oxidation state of the upper mantle of Mars

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.

Oxygen fugacity of martian basalts from electron microprobe oxygen and TEM-EELS analyses of Fe-Ti oxides

Abstract The stoichiometry of titanomagnetite spinel in the martian basaltic meteorites is assessed using quantitative analysis of oxygen measured by electron microprobe and electron energy loss spectroscopy in the transmission electron microscope. The spinels are stoichiometric within the errors of the techniques, enabling the calculation of oxygen fugacity with confidence. The oxygen fugacity is calculated using the Ghiorso-Sack and Ca-QUIlF models, which also yield estimates of temperature. The oxygen fugacity of the martian basalts increases from 3 log units below the QFM buffer for QUE 94201 to QFM - 1.8 for EETA 79001 (both lithologies), to QFM - 1.0 for Shergotty, Zagami, and Los Angeles. Dar al Gani 476 spinels contain significant MgAl2O4 and FeCr2O4 components, complicating the use of Fe-Ti oxide models. The oxygen fugacity of Dar al Gani 476 is estimated to be 1.5 log units below QFM, on the basis of the Ghiorso-Sack model. The absolute error on the oxygen fugacity estimates is ± 0.5 log units; however, a consistent electron microprobe analytical routine was applied to all of the basalts, and the relative uncertainty is closer to 0.2 log units. Oxyexsolution has occurred in QUE 94201, but reconstruction of pre-exsolution titanomagnetite compositions permits the calculation of oxygen fugacity. Subsolidus reactions involving oxides and adjacent Fe-rich silicates are documented and the use of the Ca-QUIlF model for calculation of oxygen fugacity from these phases is discussed.

Symplectites derived from metastable phases in martian basaltic meteorites

Abstract The martian basalts Los Angeles, QUE 94201, Shergotty, and Zagami contain several late-stage mineral types including oxides, sulfides, phosphates, and associated silicate assemblages. Symplectites consisting of two- and three-phase assemblages are present in Los Angeles, QUE 94201, and Shergotty. The three-phase symplectite is composed of hedenbergitic pyroxene, fayalitic olivine, and an SiO2 polymorph, and the two-phase symplectite consists of fayalitic olivine and an SiO2 polymorph. These symplectites are commonly found in close association with the Ca-phosphate merrillite [general formula Ca9(Mg,Fe2+)(PO4)7]. Scanning Electron Microscopy was used to examine the distribution and occurrence of the symplectites, and point-count and electron-microprobe data were used to recast symplectites in terms of pyroxene stoichiometry. The reconstructed three-phase symplectite compositions plot close to lunar (metastable) pyroxferroite on the pyroxene quadrilateral diagram and indicate that pyroxene compositions ranged into the “forbidden region” and eventually crystallized pyroxferroite, which subsequently broke down to the three-phase symplectite upon cooling. Twophase symplectites, which occur directly adjacent to merrillite, may be the result of breakdown of metastable ferrosilite. The crystal chemistry of merrillite and its close association with the symplectites indicates that during crystallization of pyroxene, co-crystallizing merrillite contributes to depletion of Ca and Mg and the formation of metastable pyroxferroite and ferrosilite.