Methane (CH \({}_{4}\) ) is common in fluids sourced from low-temperature hydrating (serpentinizing) peridotites, but the carbon sources, rates, and mechanisms of CH \({}_{4}\) formation are uncertain. In CH \({}_{4}\) dissolved in groundwaters pumped from four wells of up to 400 m depth in the Samail Ophiolite, Oman, we observed \({}^{14}\) C contents ranging from radiocarbon-dead to \(0.3038\pm 0.0015\) fraction modern. Chemical and isotopic analyses of groundwaters and hydrocarbon gases align with microbiological data indicating that methanogens inhabiting H \({}_{2}\) -rich \((>100 \mu \text{mol }{\text{L}}^{-1}\) ), \({\text{pH}}>11\) fluids produce the \({}^{14}\) C-rich CH \({}_{4}\) . This “young” microbial CH \({}_{4}\) constitutes a portion of the light hydrocarbons dissolved in the subsurface fluids, which also contain a distinct pool of relatively 13C- and 2H-enriched CH \({}_{4}\) and C2+ alkanes that are likely abiotic and older. Our study of groundwaters accessed via wells complements prior studies, which have mostly found 14C-free, gas-phase CH4 from natural seeps in ophiolites and interpreted an abiotic source from unsaturated rocks. Most importantly, our radiocarbon data show that transport and localized conversion of atmospheric CO \({}_{2}\) to CH \({}_{4}\) in peridotites reacting with water at temperatures < 60 ˚C occurs at surprisingly fast rates, within the last \(10\hspace{0.17em}000\) years.
