Abiotic Methane in Land-Based Serpentinized Peridotites: New Discoveries and Isotope Surprises

Abstract:

Until 2008, abiotic methane in land-based serpentinized ultramafic rocks was documented (including gas and C- and H- isotope compositions) only at sites in Oman, Philippines, New Zealand and Turkey. Methane emanates from seeps and/or hyperalkaline water springs along faults and is associated with molecular hydrogen. These were considered to be very unusual and rare occurrences of gas. Now, methane is documented for peridotite-based springs or seeps (in ophiolites, orogenic massifs or intrusions) in US, Canada, Costa Rica, Greece, Italy, Japan, New Caledonia, Portugal, Spain and United Arab Emirates. Gas flux measurements are indicating that methane can also flux as invisible microseepages from the ground, through fractured peridotites, even far removed from seeps and springs. Methane C-isotope ratios range from -6 to -37 permil (VPDB) for dominantly abiotic methane. The more 13-C depleted values, e.g., California, are likely mixed with biotic gas (microbial and thermogenic gas). Methane H-isotope ratios cover a wide range from -118 to -333 permil (VSMOW). The combination of C- and H-isotopes clearly distinguish biotic from abiotic methane. Radiocarbon (14-C) analysis from bubbling seeps in Italian peridotites indicate that the methane is fossil (pMC <0.2), i.e., older than 50,000 years, similar to the Chimaera seeps in Turkey. So, methane observed in the hyperalkaline waters cannot be produced in the same waters because these are only a few thousands years old. The low temperatures of land-based peridotites (generally <100 °C at depths of 3-4 km) also constrain methane production. We discuss some hypotheses concerning gas generation in water vs. dry systems, i.e., in deeper, older, waters or in unsaturated rocks). We also discuss low vs. high temperatures, i.e., at the present-day low T conditions or at higher temperatures eventually occurring in the early stages of peridotite emplacement on land.