Decreasing CO₂partial pressure triggered Mg-Fe-Ca carbonate formation in ancient Martian crust preserved in the ALH84001 Meteorite

Abstract

We retrace hydrogeochemical processes leading to the formation of Mg-Fe-Ca carbonate concretions (first distinct carbonate population, FDCP) in Martian meteorite ALH84001 by generic hydrogeochemical equilibrium and mass transfer modeling. Our simple conceptual models assume isochemical equilibration of orthopyroxenite minerals with pure water at varying water-to-rock ratios, temperatures and CO 2 partial pressures. Modeled scenarios include CO 2 partial pressures ranging from 10.1325 to 0.0001 MPa at water-to-rock ratios between 4380 and 43.8 mol mol )1 and different temperatures (278, 303 and 348 K) and enable the precipitation of Mg-Fe-Ca solid solution carbonate. Modeled range and trend of carbonate compositional variation from magnesio-siderite (core) to magnesite (rim), and the precipitation of amorphous SiO 2 and magnetite coupled to magnesite-rich carbonate are similar to measured compositional variation. The results of this study suggest that the early Martian subsurface had been exposed to a dynamic gas pressure regime with decreasing CO 2 partial pressure at low temperatures (approximately 1.0133 to 0.0001 MPa at 278 K or 6 to 0.0001 MPa at 303 K). Moderate waterto-rock ratios of ca. 438 mol mol )1 and isochemical weathering of orthopyroxenite are additional key prerequisites for the formation of secondary phase assemblages similar to ALH84001's 'FDCP'. Outbursts of water and CO 2(g) from confined ground water in fractured orthopyroxenite rocks below an unstable CO 2 hydrate-containing cryosphere provide adequate environments on the early Martian surface.

Reproducing early Martian atmospheric carbon dioxide partial pressure by modeling the formation of Mg‐Fe‐Ca carbonate identified in the Comanche rock outcrops on Mars

“…The modeled composition of alteration mineral assemblages, their depthdistribution, and carbonate composition (Mg-rich Mg-Fe-Ca solid solution carbonate) were similar to orbital observations of the Martian Nili Fossae region [Ehlmann et al, 2008;Mustard et al, 2008;Ehlmann et al, 2009;Brown et al, 2010]. The modeling results from van Berk and Fu [2011] suggest that rocks in the Nili Fossae region, hosting Mg-rich Mg-Fe-Ca solid solution carbonate and stratigraphically lower phyllosilicate-rich alteration assemblages, may have been exposed to a long-term groundwater alteration process, driven by atmospheric CO 2 partial pressures ranging between ca. 2.0 and 0.01 bar at low temperature (ca.…”

“…2.0 and 0.01 bar at low temperature (ca. 5 C; see van Berk and Fu [2011]). Carbon dioxide partial pressure and temperature are key factors controlling Mg-Fe-Ca solid solution carbonate composition because CO 2 -solubility in water is temperature-dependent [van Berk et al, 2010].…”

“…5 C; see van Berk and Fu [2011]). Carbon dioxide partial pressure and temperature are key factors controlling Mg-Fe-Ca solid solution carbonate composition because CO 2 -solubility in water is temperature-dependent [van Berk et al, 2010]. The Nili Fossae rock's carbonate composition is ill-defined [Ehlmann et al, 2008;Brown et al, 2010]: it could be ironbearing Mg-rich carbonate perhaps plus hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 :4H 2 O), or a Mg-rich ternary solid solution carbonate (Mg-Fe-Ca) of unknown exact composition, alternatively dolomitized MgCO 3 or Mg 5 (CO 3 ) 4 (OH) 2 :4H 2 O.…”

“…[4] In the following, reference is made to van Berk and Fu [2011] unless otherwise indicated. These authors reproduced potential hydrogeochemical conditions for concurrent carbonate and phyllosilicate formation in rock columns by using hydrogeochemical 1DDMT modeling, which was based on chemical equilibrium thermodynamics.…”

“…The Nili Fossae rock's carbonate composition is ill-defined [Ehlmann et al, 2008;Brown et al, 2010]: it could be ironbearing Mg-rich carbonate perhaps plus hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 :4H 2 O), or a Mg-rich ternary solid solution carbonate (Mg-Fe-Ca) of unknown exact composition, alternatively dolomitized MgCO 3 or Mg 5 (CO 3 ) 4 (OH) 2 :4H 2 O. Such a broad compositional range of carbonates meant that van Berk and Fu [2011] could only quantify a corresponding broad range of CO 2 partial pressure that could potentially trigger the precipitation of "Mg-rich ternary solid solution carbonate (Mg-Fe-Ca) of unknown exact composition"; that is, pCO 2 ranging between ca. 2.0 and 0.01 bar at low temperature.…”