Abiotic methane in continental ultramafic rock systems: Towards a genetic model

Abstract

Methane (CH4), often in substantial quantities, is reported for numerous surface manifestations (seeps, springs) and aquifers in continental serpentinized ultramafic rocks, in ophiolites, peridotite massifs and intrusions. Frequently, this gas is considered to have a dominant abiotic origin, with variable, though generally minor, components of biotic gas (thermogenic or microbial). Abiotic CH4 production through low temperature Fischer- Tropsch Type reactions (FTT) between a carbon (C) compound and H2 is endorsed by most of scholars, but direct derivation from olivine hydration (serpentinization), magmatic sources and fluid inclusions have also been suggested. Here, we review the application of FTT in geology, discussing the appropriateness of several C compounds (CO2, CO, formic acid, formate or elemental C) as CH4 precursors and of aqueous versus gas phase reactions. We examine published gas geochemical and flux data that provide clear constraints on the methane origins. In the analysis we add new isotopic and gas seepage data acquired in surface gas manifestations at Acquasanta, in the Voltri ophiolite (Genova, Italy). Multiple lines of evidence including (a) stable isotope compositions of CH4, CO2 and helium, (b) radiocarbon-free CH4, (c) isotopic disequilibrium between CH4 and H2O, (d) low temperatures of CH4 production based on clumped isotopes and heat flow data, and (e) methane seepage forms and intensities, suggest that CH4 is not formed directly in water or from magmatic sources or fluid inclusions. Rather, all the lines of evidence taken together are compatible with the hypothesis of low temperature (< 140 °C) Sabatier reactions (CO2 hydrogenation) in gas-phase and within metal-rich (catalyst) ultramafic rocks. A similar abiotic origin could occur for methane observed in Precambrian shields. Chromitites can support considerable rates of gas generation, potentially higher than those in some shales. In particular, the gas flux intensity and seepage distribution suggest that gas-bearing ultramafic rock systems may be considered analogous to conventional, biotic natural gas systems where, after production in source rocks, CH4 could migrate, accumulate in reservoir rocks and seep to the surface. Microbial CH4 generated in water at lower temperatures, generally as minor secondary contributions, may also commingle in these environments.