Revisiting olivine‐phyric shergottites: Pyroxene crystallization pressure and the role of undercooling

Abstract Shergottites are the most abundant meteorites from Mars and a major source of information on magma reservoirs, transport, and eruption, particularly in the late Hesperian northern hemisphere. Conflicting interpretations have arisen on the crystallization conditions of pyroxene in olivine‐phyric shergottites. The pyroxene Ti:Al barometer suggested deep crustal or upper mantle crystallization, while 1 bar experimental petrology yielded a similar pyroxene Al‐Ti distribution but suggested near surface formation. This barometer was first calibrated for alkali basalt, in which the phases that crystallized changed as a function of pressure, and it has not been demonstrated that its application could be extended to olivine‐phyric shergottites. We have confronted this question by examining the petrogenesis of NWA 6234 and NWA 10170 olivine‐phyric shergottites, which we confirm are paired. Modeling of their crystallization shows that the major‐element composition trend of their composite, complexly zoned pyroxene crystals cannot be reproduced, indicating disequilibrium due to rapid cooling. It also shows that, with an unchanging sequence of crystallizing phases, the partitioning of Al and Ti into pyroxene does not change with pressure. Many olivine‐phyric shergottites experienced strong undercooling, as in many Apollo and terrestrial basalts, consistent with surface eruption during pyroxene crystallization.