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

Abstract

Asteroid bombardment contributed to extensive melting and resurfacing of ancient (>3.5 Ga) Mars. Evidence from the largest impact structures on Earth and the Moon suggests that vertically thick impact melt sheets experience chemical differentiation. Recently observed materials in Gale crater are enriched in alkalis (up to 14 wt% Na2O + K2O) and silica (up to 67 wt% SiO2) with low MgO (<5 wt%). This differentiation trend has previously been attributed to fractional crystallization of mantle-derived magmas. The influence of impact melting on Mars' crustal petrology, however, has not been explored in depth. In this study, we investigate differentiation of impact-generated melts as a mechanism for petrogenesis of evolved rocks on Mars. We scale melt volumes for multiple impact magnitudes and employ the MELTS algorithm to model mineral phase equilibria in those melts as they crystallize. We consider a range of possible melt geometries, oxidation states (IW to QFM), crystallization regimes (equilibrium and fractional), and water contents (anhydrous to water-saturated). Moderate pressure and dissolved volatiles are required to produce alkaline differentiation trends in mafic melts. Large impacts that melt a water-bearing early basaltic crust provide a mechanism to generate wet magmas on Mars, consistent with some modeled Martian differentiation trends. Low-degree partial melting and/or an alkali-enriched source are needed to generate observed highly alkaline compositions, motivating future work to further assess whether these conditions can be achieved in impact-generated melts.