Date of Award
Mars, impact processes, igneous differentiation
Asteroid bombardment contributed to extensive melting and resurfacing of ancient (> 3 Ga) Mars, thereby influencing the early evolution of the Martian crust. However, information about how impact melting has altered Mars’ crustal petrology is limited. Evidence from some of the largest impact structures on Earth, such as Sudbury and Manicouagan, suggests that some impact melt sheets experience chemical differentiation. If these processes occur on Mars, we expect to observe differentiated igneous materials in some exhumed rock samples. Some rocks observed in Gale crater are enriched in alkalis (up to 14 wt% Na2O + K2O) and silica (up to 67 wt% SiO2) with low (< 5wt%) MgO. This alkaline differentiation trend has previously been attributed to fractional crystallization of magmas resulting from low-degree mantle melting analogous to ocean island or rift settings on Earth. In this study, we investigate the hypothesis that differentiation of impact-generated melts provides a viable alternative explanation for the petrogenesis of some evolved rocks on Mars. We scaled melt volumes and melt sheet thicknesses for a range of large impact magnitudes and employed the MELTS algorithm to model mineral phase equilibria in those impact-generated melts. Model runs consider a range of possible oxidation states (IW to QFM), crystallization regimes (batch, fractional and liquid segregation), and volatile contents (initially water-saturated and nominally anhydrous). Moderate pressure and water 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 wetter magmas on Mars, consistent with some observed Martian differentiation trends. Low-degree partial melting and/or an alkali-enriched source are required to generate the most alkaline observed compositions, motivating future work to examine partial melting regimes during large impacts.
Koeppel, Ari, "Differentiation in impact melt sheets as a mechanism to produce evolved magmas on Mars" (2018). CUNY Academic Works.