Abstract
AbstractLow‐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.
Highlights
Carbonate minerals are of special interest in the study of the potential for life on Mars, since they are linked to both the water and inorganic carbon cycles (e.g., Niles et al, 2013)
Mineralogy of Bounce Rock with 5 wt% Bulk Fluids Figure 1 shows phase stability diagrams calculated for the composition of Bounce Rock (Table 2) with only divalent iron (Figs. 1a, 1c, and 1e), and 1.55 wt% Fe2O3 representing 10% of FeOtot recalculated to be trivalent (Figs. 1b, 1d, and 1f)
In compositions with 1.55 wt% Fe2O3, prehnite is only present at 200 °C on the 20 °C kmÀ1 geotherm up to 15 vol% and coexists with pumpellyite at low CO2 of ≤0.26 mol kgÀ1 (1.13 wt %) and is not stable >0.44 mol kgÀ1 CO2
Summary
Carbonate minerals are of special interest in the study of the potential for life on Mars, since they are linked to both the water and inorganic carbon cycles (e.g., Niles et al, 2013). Locations where hydrated phyllosilicates and carbonates have been detected, such as in the Nili Fossae region as well as Leighton and McLaughlin craters, could be suitable for a deep biosphere on Mars, as they suggest a prolonged past presence of alkaline water, carbon, and temperatures above freezing, all of which are critical prerequisites for life (Michalski & Niles, 2010; Michalski et al, 2013; Niles et al, 2013). Better constraints on mineral stabilities and reactions in the Martian upper crust as a function of H2O-CO2 fluid variations are critical to assess the habitability potential of the subsurface. Use phase equilibria modeling to study the effect of fluid composition on low-grade metamorphic reactions. While this study is focused on metamorphic subsurface fluids at temperatures above 150 °C, which is higher than the known limit of 122 °C (at elevated hydrostatic pressure) for the survival of terrestrial hyperthermophilic microbes (Takai et al, 2008), these initially hot fluids could cool down over geological timescales and circulate through the subsurface by geothermal gradient-driven groundwater convection (Travis et al, 2003)
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