Differentiation in impact melt sheets as a mechanism to produce evolved magmas on Mars

Asteroid bombardment contributed to extensive melting and resurfacing of pre-Noachian and Noachian Mars, thereby influencing the early evolution of the Martian crust, however information about the petrologic evolution of Martian impact melts is limited. Evidence from the largest impact structures on Earth, such as the Sudbury Complex, suggests that some impact melt pools may undergo differentiation. Mars Science Laboratory (MSL) ChemCam measurements reveal that some rocks in Gale crater are enriched in alkalis (up to 14 wt% Na2O + K2O) and SiO­2 (up to 67 wt%). Some of these rocks contain large feldspar phenocrysts and have been interpreted to be products of fractional crystallization in ancient magma chambers. In this study, we investigate the hypothesis that differentiation in impact-generated melt pools provides a viable alternative explanation for the petrogenesis of some evolved rocks on Mars. We employed the rhyolite-MELTS algorithm to model crystallization in impact melt pools. During crystallization, liquid-crystalline segregation points were calculated from physical properties, adopting compaction equations used previously for thick terrestrial flood basalt flows that contain segregation sheets and isolated high-silica veins. Initial model runs cover a range of oxidation states (IW to QFM+1), melt sheet depths (0.1 to 100 km), and initial melt compositions (crust-like, mantle-like and mixed). Our results demonstrate that under some conditions, crystallization in impact melt sheets can reproduce compositions that overlap with those observed among rocks from Gale crater.