Insights into martian magmatism gained from martian meteorites: Emphasis on spinel, olivine and phase equilibria

Martian meteorites represent the only fragments of Mars available to date. Application of several microbeam techniques, including Electron Microprobe, Scanning Electron Microscopy, Transmission Electron Microscopy, and Secondary Ion Mass Spectrometry to basaltic martian meteorites have resulted in significant insights into martian magmatism, and the nature of the martian crust and mantle. Firstly, the oxygen fugacity of the martian basalts, as determined from analysis of spinel and ilmenite, varies by 2 orders of magnitude. Correlations between oxygen fugacity and geochemical parameters indicate that the source of the basalts, the martian mantle, is reduced and that the variation is the result of oxidation of basaltic magmas by assimilation of material in the martian crust. The oxidizing material may be a hydrous mineral, which would have significant implications for the location of water on Mars. Secondly, an experimental investigation into the EFTA 79001 martian basalt, involving a composition approximating that of one of the lithologies within this rock, indicates that the composition is in need of revision. This composition is thought to be primitive and has been used in experiments to investigate martian magmatic processes. However, comparison of results of experiments with the natural rock indicates that the rock has endured a more complex history than previously realized. The experimental investigation also enabled characterization of V and Cr partitioning between olivine, pyroxene and melt, under varying oxygen fugacity and temperature conditions. Thirdly, the behavior of Co and Ni in olivine from Mars, the Moon and Earth shows a dependence on oxygen fugacity. This is due to an indirect effect of oxygen fugacity on Co and Ni partitioning in olivine, possibly by affecting the nature and modal proportion of co-crystallizing spinel, or by affecting the structure of the melt. Lastly, analyses of Co, Ni, V and Cr in olivine cores in four of the martian meteorites along with experimental and published distribution coefficients allow for back-calculation of parental melt concentrations. These calculations show that the parental melt to two of the meteorites may be able to produce the QUE 94201 martian basalt through fractionation of olivine.