Methane origin in the Samail ophiolite: Comment on “Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability” [Geochim. Cosmochim. Acta 179 (2016) 217–241]

Abstract

Miller et al. (2016) report a new study of fluids in the peridotites of the Samail ophiolite in Oman related to modern serpentinization (olivine hydration), a process that can provide energy and raw materials for chemosynthetic microbial life. The authors, in particular, report an isotopic composition for methane (CH4) in groundwater near Ibra (up to 1.4 mM) that is unusually 13C-enriched (d13CCH4 +2.4 and +3‰ VPDB), and consider the gas origin to be uncertain, i.e., abiotic or microbial, and to be modulated by significant fractionation due to oxidation or diffusion. The purpose of this comment is to clarify and correct a few points concerning the possible origin of the d13CCH4 values, with the intention to promote a fruitful and constructive debate, considering the interest that there is for serpentinization and the associated formation of various gases. The CH4 data from Miller et al. are re-examined in a global context of gas in serpentinized peridotites and, in particular, by considering published data (isotope composition of CH4 and concentrations of C2+ alkanes) also obtained from the Samail ophiolite, data neglected by the authors. These data significantly impact the interpretation of Miller et al. concerning the possibility that methane can be microbial. Potential isotopic fractionations by oxidation or diffusion, evaluated considering d13CCH4–d2HCH4 correlated variations, the occurrence of significant amounts of ethane and propane in the Oman ophiolite aquifers and Rayleigh fractionation analysis suggest that methane can hardly be considered microbial. Isotopic fractionations, however, are not necessary to explain the unusual d13CCH4 values: an alternative hypothesis is that methane carbon may derive from 13C-enriched carbonates occurring below the Samail ophiolite nappe, a hypothesis not considered by Miller et al.