Low‐temperature dunite hydration: evaluating CH 4 and H 2 production from H 2 O and CO 2

Abiotic methane ( CH 4 ) and hydrogen ( H 2 ) produced after hydration of mafic/ultramafic rocks represent energy sources for microbes that may thrive in the deep subsurface regions of Earth and possibly on other planets. While H 2 is a direct product of serpentinization, CH 4 can form via F ischer– T ropsch Type ( FTT ) reactions (carbon reduction) that, due to potential H 2 migration, can be spatially and temporally detached from serpentinization. We tested an alternative process hypothesized by some scholars, in which CO 2 can be reduced through dunite hydration without initially added H 2 , implying that CH 4 can form in the same serpentinized fluid–rock system. The experiment used natural dunite sand ( F orsterite 92), CO 2 with δ 13 C ~ −25‰ ( VPDB ), and a 1 m m dissolved SiO 2 solution mixed in 30 glass bottles (118 mL) stored for up to 8 months at low temperature (50°C) to simulate land‐based serpentinization systems. In addition, 30 control bottles without olivine were used as blanks. Trivial amounts of CH 4 (orders of 0.2–0.9 ppmv) were detected in both samples and blanks, likely representing analytical noise; essentially, no significant amount of CH 4 formed under the experimental conditions used in this work. Low amounts of H 2 (~2.55 ± 1.39 ppmv) were generated, with production yields that were one order of magnitude lower than in previously published experiments. Moderate concentrations of SiO 2 appeared to hinder low‐temperature H 2 production. Our experiment confirms that the low‐temperature reduction of CO 2 into CH 4 through direct olivine hydration, without initial H 2 , is sluggish and not straightforward, which is consistent with previous studies. The presence of substantial amounts of H 2 , as well as suitable metal catalysts, appears to be essential in the low‐temperature production of abiotic CH 4 , as observed in published FTT experiments.