“…There is active microbial methanogenesis occurring in the peridotite-hosted aquifer in the Samail ophiolite, including in well NSHQ-14 10 m away from Hole BA3A (Fones et al, 2019(Fones et al, , 2020Kraus et al, 2020). Nothaft et al (2020) have used a combination of geochemical, isotopic, and microbial observations to show that microbial methanogenesis, as well as an abiotic source of CH 4 -some of which could be derived from pre-existing fluid inclusions with serpentinized rims in olivine (Klein et al, 2019)contribute important amounts of CH 4 . The extremely low fO 2 in the deeper waters in boreholes BA1B, BA1A, NSHQ-14, BA3A and BA4A (Kelemen, Bach, et al, 2020;Kelemen, Matter, et al, 2020) close to the dissociation limit for H 2 O, the presence of widespread, nanoscale awaruite inclusions in serpentine documented in this study, and the regional observation of streams of H 2 -rich gas emanating from peridotite-hosted alkaline springs in the Samail ophiolite (e.g., Neal & Stanger, 1985;Vacquand et al, 2018) all raise the possibility that abiotic CH 4 is forming via hydrogenation of CO 2 or dissolved carbonate species.…”