Figure 1. Variations in the concentrations of dissolved gas species.

Results of H₂, N₂, CH₄, ΣCO₂ (=CO_2(aq)+CO₃²⁻+HCO₃⁻) and ΣNH₃ (=NH_3(aq)+NH₄⁺) during the experiments of (a) the opx experiment at 300 °C, and (b) the olivine experiment at 300 °C. Dissolved H₂ was generated through the oxidation of Fe(II) in olivine to magnetite and serpentine, which were observed in the rocks after the experiments. The decreasing ΣCO₂ is due to the formation of carbonate in the solid phase. The ΣNH₃ concentrations are high and almost constant during the experiments. Our results provide no evidence for CH₄ or N₂ production from CO₂ or NH₃, respectively (CH₄ production <5 μmol kg⁻¹; N₂ production <50 μmol kg⁻¹) (see Supplementary Note 1). The experimental data are given in the Supplementary Table 2.