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Although nucleation of magnetite and/or hematite along dislocations upon oxidation of olivine has been observed by many workers, the effect of oxidation on the magnetic properties of the sample with specific mineralogical alterations has not been studied. Therefore, we investigate this problem using a set of time series 1 bar oxidation experiments at 600 and 900 °C. Results show rapid olivine oxidation and alteration at both 600 and 900 °C, forming magnetite and hematite associated with a change from paramagnetic to ferromagnetic behavior after oxidation. Magnetite and hematite nucleate along dislocations and impurities in the crystal structure, along with surface coatings and within cracks in the crystals.\ud \ud Fresh, unaltered mantle xenoliths containing magnetite have been interpreted as having formed in cold tectonic regimes in the mantle, rather than through oxidation during or after ascent. Mantle xenoliths rapidly ascend through the mantle with estimates of the ascent of up to 90 km/h (3 GPa/h) based on the diffusion profile of water in mantle olivine. The rates correspond to xenoliths ascending through the mantle over hours and not days or weeks. Our results show that olivine oxidation and alteration can occur in days to weeks at 600 °C and within minutes at 900 °C. Therefore, if the xenolithic material is transported to the surface in a cold magma (at temperatures ≤600 °C), then the timescale of ascent is likely not long enough for oxidation to cause magnetite formation or a ferromagnetic signature to occur. However, if the material is transported in a hot oxidized basaltic magma (with temperatures ≥900 °C), then oxidation can cause magnetite formation and a ferromagnetic signature.

Meteorite Northwest Africa (NWA) 6963 was classified as a basaltic shergottite based on mineralogy, but here we show that it is a gabbroic rock with a quartz-alkali feldspar intergrowth that represents a late-stage granitic melt. NWA 6963 contains clinopyroxene and maskelynite grains up to 5 mm in length, with minor ferroan olivine, spinel, ilmenite, merrillite, apatite, Fe-sulfides, and high-Si glass. NWA 6963 also contains areas of quartz and alkali-feldspar intergrowths up to ~1 mm in size. Based on mineral abundances and textural analysis, we suggest that NWA 6963 is an intrusive rock similar to a terrestrial gabbro. Infiltration of the martian crust by young gabbroic bodies would suggest that estimates of crustal composition, density, and thickness based on the surface chemistry alone would be problematic and the martian crust may be even more heterogenous than is seen from orbit alone. Investigations of crater walls, where intrusive crustal rocks would be exposed, are needed to discover the launch sites of the shergottites and the full heterogeneity of the martian crust.

Trace element mineral-magma partitioning models are important in understanding processes by which basaltic magmas are generated. Partitioning models for nickel have been extrapolated from their original applicability for the Earth’s mantle to compositions appropriate for other planets, notably the Moon and Mars. Before partitioning models can be extrapolated to explain nickel concentrations in planetary rocks, these models need to be verified thermodynamically and experimentally using planetary basaltic compositions. Experiments conducted in this study on the Martian Gusev Adirondack-class basalt, Humphrey, with 1 wt% nickel in the magma have shown that Ni affects its liquidus phase relations. By stabilizing olivine to higher temperatures, Ni increases the liquidus temperature. These experiments have shown that the Hart and Davis (1978) model based on iron-free systems cannot be extrapolated to planetary, iron-rich, basaltic systems. This work verifies the independence of the Jones (1984, 1995) and Beattie et al. (1991) models from temperature and pressure effects and suggests extrapolation to planetary compositions is justified but needs further verification. Furthermore, these experiments support the Longhi and Walker (2006) hypothesis that at high temperature nickel may be incompatible.

The apparent dichotomy of Mg/Si and Al/Si ratios between terrestrial rocks and Martian meteorites has been interpreted as indicative of major differences between the terrestrial and Martian magma source regions. We suggest that this apparent dichotomy is not robust when compared with partly cumulate and non-cumulate terrestrial igneous material. Terrestrial cumulate intra-plate nodules with similar mineralogy to the SNC meteorites plot in the SNC field in this compositional space and far removed from the “terrestrial geochemical fractionation” line (Earth’s crust line) of Jagoutz et al. (1979). As is the case for terrestrial partly cumulate igneous rocks, the bulk compositions of cumulate SNC meteorites such Chassigny and Nakhla are dominated by the chemical characteristics of the accumulated minerals, minerals compositionally similar to those found in terrestrial intra-plate magmas. Therefore, the minor amounts of liquid involved play an insignificant role in the Mg/Si and Al/Si signature and no special Martian chemical characteristics can be identified. SNC meteorites considered as possibly representative of liquid compositions are similar in composition and Mg/Si and Al/Si to terrestrial ferropicrites, suggesting that even “liquid” compositions within the SNC space are not uniquely Martian. Therefore, the Mg/Si and Al/Si ratios cannot clearly distinguish Martian from terrestrial rocks.