Whiticar, M. J.

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.

no abstract available

Abiotic methane in serpentinized peridotites (MSP) has implications for energy resource exploration, planetary geology, subsurface microbiology and astrobiology. Once considered a rare occurrence on Earth, reports of MSP are increasing for numerous localities worldwide in low temperature, land-based springs and seeps. We report the discovery of six methane-rich water springs and two ponds with active gas bubbling in the Ronda peridotite massif, in southern Spain. Water is hyperalkaline with typical hydrochemical features of active serpentinization (pH: 10.7 to 11.7, T: 17.1 to 21.5 C, CaeOH facies). Dissolved CH4 concentrations range from 0.1 to 3.2 mg/L. The methane stable C and H isotope ratios in the natural spring and bubbling sites (d13CCH4: 12.3 to 37‰ VPDB; d2HCH4: 280 to 333‰ VSMOW) indicate a predominant abiotic origin. In contrast, springs with manmade water systems, i.e., pipes or fountains, appear to have mixed biotic-abiotic origin (d13CCH4: 44 to 69‰; d2HCH4: 180 to 319‰). Radiocarbon (14C) analyses show that methane C in a natural spring is older than ca. 50,000 y BP, whereas dissolved inorganic carbon (DIC) analysed in all springs has an apparent 14C age ranging from modern to 2334 y BP. Therefore most, if not all, of the CH4 is allochthonous, i.e., not generated from the carbon in the hyperalkaline water. Methane is also released as bubbles in natural ponds and as diffuse seepages (~101e102 mg CH4 m 2d 1) from the ground up to several tens of metres from the seeps and springs, albeit with no overt visual evidence. These data suggest that the gas follows independent migration pathways, potentially along faults or fracture systems, physically isolated from the hyperalkaline springs. Methane does not seem to be genetically related to the hyperalkaline water, which may only act as a carrier of the gas. Gas-bearing springs, vents and invisible microseepage in land-based peridotites are more common than previously thought. In addition to other geological sources, MSP is potentially a natural source of methane for the troposphere and requires more worldwide flux measurements.

Methane with carbon and hydrogen isotope composition diagnostic of abiotic gas related to serpentinization of peridotites has been detected for the first time in the ophiolitic aquifer in the UAE, along the Wadi Ham fault. This methane is isotopically similar to that previously reported in serpentinization-related springs in Oman, in the same Semail ophiolite nappe. Abiotic gas may be widespread in these ophiolitic rocks. Conventional thermogenic gas fields in the sedimentary basin overthrusted by the ophiolite could be “contaminated” to some extent by this abiotic gas. This may confound the accurate characterization of these thermogenic gases (origin, type of source rock type and maturity), and thus the associated petroleum system model.