Petrologic modeling reveals disequilibrium pyroxene crystallization in paired Martian shergottites, indicating rapid near-surface undercooling rather than deep mantle formation.
Key Points
Resolve conflicting depth estimates for pyroxene crystallization in Martian olivine-phyric shergottites and evaluate the validity of the pyroxene titanium-to-aluminum barometer.
Analyzed the petrogenetic history of paired Martian meteorites NWA 6234 and NWA 10170.
Modeled crystallization sequences and evaluated major-element zoning trends in complex pyroxene crystals under varying cooling rates and pressures.
Crystallization modeling failed to reproduce the major-element zoning of composite pyroxene crystals, identifying severe chemical disequilibrium driven by rapid cooling.
Partitioning of aluminum and titanium into pyroxene remained invariant with pressure when the sequence of crystallizing phases was unchanged, invalidating the alkali basalt Ti:Al barometer for these shergottites.
Strong undercooling observed in the meteorites matched cooling regimes seen in Apollo and terrestrial basalts, supporting near-surface eruption during pyroxene crystallization.