Characterization of groundwater chemistry beneath Gale Crater on early Mars by hydrothermal experiments

Hydrothermal groundwater may have played many roles in hydrogeochemical cycles on early Mars, including being a source of cations and reductants, a sink of CO2, and a causative fluid of post-depositional diagenesis. However, few laboratory experiments have been investigated hydrothermal reactions within the Martian crust. Here we describe hydrothermal experiments simulating water-rock reactions in the Martian subsurface, with the aim of characterizing the fluid chemistry. Experiments at 120 ?C and 200 ?C show that the hydrothermal fluids have i) high Si concentrations (-1-10 mmol kg(-1)), ii) low Fe and Mg concentrations (<-10-3 mmol kg(-1)), iii) low H-2 concentrations (< 10-2 mmol kg(-1)), and iv) alkaline pH (in-situ pH-8). Effective drawdown of CO2 (4-5 mmol kg(-1)) occurs through the formation of calcite. Concentrations of Si in the fluids are buffered by the dissolution equilibrium of quartz at 200 ?C and its metastable phase (cristobalite) at 120 ?C. These characteristics would not explain the observed secondary mineral assemblages of Vera Rubin Ridge (VRR), in particular high abundance of Fe (hydro)oxides and low levels of silica and clay minerals, in the Murray formation of Gale Crater. This implies that upwelling hydrothermal groundwater can be precluded as a source of post-depositional diagenetic fluids. Our results under reaction temperatures of <= 200 ?C indicate that upwelling hydrothermal groundwater provided only limited amounts of reductants and greenhouse effect gases (Fe2+ and H-2) to the surface of early Mars.

Automated summary

Hydrothermal experiments simulating water-rock reactions in the Martian subsurface were conducted to characterize the fluid chemistry. The results suggest that the characteristics of Martian hydrothermal fluids cannot explain the observed secondary mineral assemblages in Gale Crater, indicating that upwelling hydrothermal groundwater is not the source of post-depositional diagenetic fluids.