Assimilation-Fractional Crystallization Modeling Using Known Martian Compositions

Abstract:

The martian crust was long thought to be primarily composed of tholeiitic basalt, but the Curiosity rover at Gale Crater found surprising crustal diversity. Recent studies have shown that felsic (>55 wt.% SiO₂) Gale targets may form by fractional crystallization, and alkaline targets may form in fractionated magmas derived from metasomatized sources under high pressures. One unexplored possibility is that felsic or alkaline martian rocks may form as magmas undergo assimilation-fractional crystallization (AFC), a process common on Earth that contributes to crustal diversity.

We use the Magma Chamber Simulator software to conduct AFC modeling using known martian compositions to assess the contribution of assimilation to the crustal diversity of Mars. We use the bulk composition of martian meteorite Northwest Africa 7034, a polymict regolith breccia, as the assimilating wallrock composition. We use the primary composition of Gusev Crater target Fastball as the intruding magma composition. In our models, we vary water contents of the initial magma (0.07, 0.5, and 1.0 wt.%) and pressures (1, 2, 4, and 6 kbar) at the Fayalite-Magnetite-Quartz (FMQ) oxygen fugacity buffer. We use varying estimates of ancient martian marsothermal gradients to determine the starting temperature of the wallrock, including a 15°C/km estimate for Gale Crater, and 6°C/km as an estimate for modern Mars.

We find that the limiting factor for crustal assimilation is the starting temperature of the wallrock which varies by depth, location, and with geologic time. Assimilation was likely a more prevalent process on early Mars when crustal temperatures were hot relative to today. We closely approximated Gale Crater alkaline targets at pressures as low as 2 kbar, indicating that magmatic diversity is made possible on Mars through assimilation and fractional crystallization.