The Shergotty Martian meteorite contains pigeonite and augite with homogeneous magnesian cores, thought to be cumulus crystals. Using elemental maps of thin sections, we estimate the abundance of cumulus pyroxene cores at 13 vol%. This estimate is roughly half that determined from prior melting experiments (28%), and coincides with the observation of a similar discrepancy between the proportions of cumulus pyroxenes estimated from modal observations and melting experiments in the Zagami shergottite. An intercumulus liquid composition calculated using this new cumulus pyroxene estimate is virtually identical to that calculated for Zagami. Our results suggest the possibility that melting experiments may not have been carried out under conditions appropriate for crystallization of the shergottites. We consider two possible explanations: crystallization of pyroxene cores at depth or under different redox conditions. Prior arguments for polybaric crystallization of shergottites, based on pyroxene intergrowths and kaersutite inclusions, are inconclusive. Using the MELTS program, we have explored crystallization at higher pressure. A greater proportion of pyroxene crystallizes at higher pressures, rather than a smaller amount as necessary in this model. Even in calculations at 1 bar, pigeonite crystallizes before augite, and the temperature interval between the appearance of pigeonite and augite expands with pressure. Both pigeonite and augite appear to be in equilibrium with the same liquid composition and thus are thought to have co-crystallized in Shergotty, so it seems likely that MELTS may not correctly predict pyroxene crystallization behavior in this Fe-rich magma. Fe,Ti-oxide compositions in Shergotty were previously used as evidence that the magma crystallized at redox conditions near QFM. However, Ghosal et al. (1998) recently suggested that the oxides record subsolidus, reducing conditions of QFM-3 at 602 degrees C, and our analyses support that conclusion, if a new solution model for oxides is used. They also proposed that Fe3+ in Shergotty pyroxenes indicates magmatic crystallization at approximately QFM-3, and our estimates of Fe3+ calculated from pyroxene stoichiometry in microprobe analyses match their reported values. However, we doubt that stoichiometric calculations provide accurate Fe3+ determinations. Other indicators suggest that magmatic conditions were oxidizing. The FeO* content of Shergotty maskelynite is greater and the negative Eu anomaly in pyroxene cores is shallower than in another basaltic shergottite, QUE94201, which is thought to have crystallized under reducing conditions near QFM-4. Also, the presence of silica, fayalite, and magnetite in mesostasis indicates oxiding conditions, at least at a late magmatic stage. Thus we argue that Shergotty crystallized under redox conditions near QFM. Copyright (C) 1999 Elsevier Science Ltd.
