An experimental and petrographic investigation of Elephant Moraine 79001 lithology A: Implications for its petrogenesis and the partitioning of chromium and vanadium in a martian basalt

Abstract

A composition approximating the lithology A groundmasss of the Elephant Moraine (EET) 79001 martian bassalt (Eg; McSween and Jarosewich, 1983) has been used to investigate the petrogenesis of the meteorite and the behavior of Cr and V at different oxygen fugacities. Crystallization experiments were carried out over a range of temperatures, and oxygen fugacities of either iron-wustite (IW) or IW + 2 (i.e., 1.5 log units below the quartz-fayalite-magnetite (QFM) buffer). Comparison of trace element concentrations (obtained by secondary ion mass spectrometry (SIMS) analysis) in experimental silicates with those of natural silicates supports the Fe-Ti oxide-derived oxygen fugacity of QFM -1.8 +/- 0.3 for this basalt (Herd et al., 2001). Experimental distribution coefficients, in conjuction with SIMS analyses of rims from the olivine and pyroxene xenocrysts in lithology A, as well as analyses of lithology A groundmass pigeonite cores, are used calculate coexisting liquid concentrations of V and Cr. Liquid compositions derived from pigeonite xenocryst rims and groundmass pigeonite cores are similar, suggesting that the rims of orthopyroxene xenocrysts are overgrowths, which have not previously been accounted for when reconstructing the groundmass composition. This implies that the Eg composition requires modification. A similar exercise for the ferroan rims on olivine xenocrysts yields very different liquid compositions, indicating that these rims are not overgrowths but are part of the xenocryst assemblage. These results are shown to be consistent with the petrography of lithology A xenocrysts.