Thermodynamic constraints on the geochemistry of low-temperature, continental, serpentinization-generated fluids

Abstract

The hydrous alteration of ultramafic rocks, known as serpentinization, generates fluids that can fuel microbial communities and enable the synthesis of simple organic compounds. Serpentinization reactions can proceed even at the ambient, low-temperature conditions present in continental aquifers raising questions about the limits of life deep in the Earth's subsurface. Through thermodynamic calculations, we investigate various reactions that facilitate the transformation of oxic, slightly acidic rainwater into reduced, hyperalkaline fluids during low-temperature serpentinization. We explore a suite of factors (variabilities in temperature, host-rock compositions, fluid salinity, and the buffering capacity of various serpentinization-relevant minerals) that offer broad insights into the chemical environments formed through low-temperature serpentinization. Results of calculations show that alteration of olivine-rich lithologies will lead to fluids constrained by the chrysotile-brucite-diopside equilibrium assemblage, close in pH to those measured from the most alkaline fluids hosted in ultramafic rocks. Variabilities in the compositions of fluids hosted by continental serpentinizing systems can be attributed to a shift from being in equilibrium with diopside to calcite, among other reactions. Results of calculations also show that it would be difficult to distinguish fluids reacting with either fresh or altered ultramafic rocks based solely on their pH, and total dissolved Ca, Mg and Si content. Our models also account for Fe incorporation into solid solutions of serpentine and brucite and show that the global H₂ flux from continental serpentinization could be considerably lower than estimates based on iron oxidation to magnetite only. Lastly, we present the energetic landscape available to subsurface microorganisms by focusing on two microbial process using H₂: methanogenesis and hydrogen oxidation. Limited but available energy (0.2–1.7 calories/kg fluid) can be exploited by methanogens, permitting the possibility of deep communities in serpentinizing aquifers. More energy is available for methanogenesis (0.2–6 calories/kg fluid) and hydrogen oxidation (0–17 calories/kg fluid) when upwelling, deep-seated, serpentinization-generated fluids mix with shallow groundwater. Ultimately, predictions set forth in this study provide a framework for testing ideas that can explain the compositions of fluids and microbial communities sampled at ultramafic environments here on Earth and perhaps in the near future, on ocean worlds in our solar system.