Constraints on the formation of carbonates and low-grade metamorphic phases in the Martian crust as a function of H2O-CO2 fluids

Abstract

Low-grade metamorphic hydrous minerals and carbonates occur in various settings on Mars and in Martian meteorites. We present constraints on the stability of prehnite, zeolites, serpentine, and carbonates by modeling the influence of H2O-CO2 fluids during low-grade metamorphism in the Martian crust using compositions of a Martian basalt and an ultramafic cumulate. In basaltic compositions with 5 wt% fluid, our models predict prehnite in less oxidized, CO2-poor conditions (≤0.44 mol kg−1 CO2) on warmer geotherms of 20 °C km−1. At fluid-saturated conditions, epidote and laumontite are replaced by quartz, calcite, chlorite, and muscovite. In ultramafic compositions with 5 wt% fluid, antigorite (serpentine) is stable at CO2-poor conditions of ≤0.33 mol kg−1, while talc forms at 0.05–0.56 mol kg−1 CO2. At fluid-saturated conditions, antigorite is replaced by talc and chlorite, and at higher X(CO2) by magnesite and quartz. Our models therefore suggest that prehnite, zeolites, and serpentine have formed in a CO2-poor environment on Mars implying that fluids during their formation either did not contain high amounts of CO2 or had degassed CO2. Carbonates and potentially talc would have formed in the presence of a CO2-bearing fluid and therefore at different alteration stages than for prehnite, zeolites, and serpentine either in the same hydrothermal event during which the fluid composition changed gradually due to cooling and precipitation or by separate and successive alteration events with fluids of different compositions.