DID (SOME) CARBONATE & SERPENTINE ALTERATION OF ULTRAMAFIC ROCKS ON MARS OCCUR VIA AMBIENT WEATHERING OVER BILLIONS OF YEARS?

How much of the observed, relatively common carbonate and rarer serpentine alteration on Mars might have formed during slow weathering at ambient surface conditions over several billion years, rather than via “hydrothermal alteration” involving thermally convecting aqueous fluids at relatively high temperature and humidity on early Mars?

Thermodynamic calculations for Mg-end-member olivine (forsterite), brucite, serpentine (chrysotile), magnesite, and quartz reacting with the Martian atmosphere at a range of CO2 and H2O partial pressures, and a range of temperatures, show that chrysotile + brucite are marginally stable or unstable with respect to forsterite, depending on temperature (latitude, season, time of day), while magnesite + quartz are strongly supersaturated with respect to forsterite [1].

New thermodynamic calculations for Fe-end-member olivine (fayalite), Fe-brucite, ferrihydrite (Fe(OH)3), serpentine (greenalite, hisingerite, cronstedtite, oxy-greenalite), siderite, magnetite and quartz, at a range of PCO2, PH2O, T and oxygen fugacity, indicate that both siderite + quartz and greenalite + ferrihydrite are significantly supersaturated relative to fayalite at current Martian surface conditions. Only minor proportions of more oxidized serpentine species are likely to form.

At likely (uncertain) rates over ~ 3 Gyrs, such weathering could extend a few cm below surfaces exposed to the atmosphere. Assuming similar reaction rates for carbonation and hydration, at a given extent of reaction progress (“gas/rock ratio”), more carbonate will form from reaction with abundant atmospheric CO2, while much less serpentine will form via reaction with trace H2O.

Thus, in keeping with one idea in [2], the high proportion of carbonate versus serpentine inferred from orbital and surface observations [3] might arise via ambient weathering of ultramafic rocks at conditions similar to the present, without requiring aqueous fluids or elevated temperatures.

Ongoing work will refine these results, and move toward consideration of Fe-Mg solid solutions in reactants and products.

[1] PB Kelemen et al (2020) 51^st Lunar and Planetary Science Conference 1213

[2] BL Ehlmann et al (2008) Science 322, 1828-1832

[3] Please see references in [1]