Multiple origins of methane in fluids circulating in the Othrys ophiolite, central Greece

Abstract

The complex geology of Greece includes two important parallel running ophiolitic belts. The Othrys Massif in central Greece belongs to westernmost of them. In and around this wide ophiolite outcrop, some cold hyperalkaline and some hypothermal (T < 40°C) alkaline springs are present. Thirty water samples were collected at 17 different sites and both bubbling and dissolved gases were analysed for their chemical (He, Ne, H2, O2, N2, CH4, C2H6, CO2 and H2S) and isotope (He, δ13C-CO2, δ 13C-CH4, δ 2H-CH4) composition. All samples except one have H2S contents below detection limit (10 μmol/mol), whilst H2 (from <2 to 2500 μmol/mol), CO2 (up to 26,000 but generally below 1000 μmol/mol) and O2 (up to 16,000 but generally below 3000 μmol/mol) present low concentrations. Gases in alkaline waters (pH <10) are generally dominated by CH4 (from 128,000 to 915,000 μmol/mol), while hyperalkaline (pH > 11) waters are N2 dominated (from 727,000 to 977,000 μmol/mol). Generally, He isotope composition excludes contributions from a mantle source, showing a mostly pure crustal contribution for the alkaline waters and a prevailing atmospheric contribution for the hyperalkaline ones. Methane may have different origins, which can be subdivided in biogenic (either directly produced by microbial activity or deriving by decay of organic matter at T > 150°C) and abiogenic (from pure inorganic reactions). Among the latter, one of the most debated origins comes from serpentinization processes of ultramafic rocks in ophiolitic sequences at low temperatures (T < 80 °C). Furthermore, secondary processes (diffusion, inorganic or microbial oxidation, etc.) may mask the original chemical and/or isotope composition. Primary and secondary processes acting on CH4 can be recognised mainly through its isotope (δ13C and δ2H) composition and the ratio between CH4 and C2+C3 hydrocarbons (Bernard ratio). Samples collected in the Othrys Massif display a wide range of both isotope compositions of CH4 (δ13C-CH4 from -74.5 to -14.5 ‰ and δ2H-CH4 from -343 to -62 ‰) and Bernard ratio (from 220 to 15,800). The relatively high values of the ratio seem to exclude great contributions from thermogenic CH4. Alkaline waters present the most negative isotope values for CH4, evidencing a biogenic (microbial) origin, whereas many of the hyperalkaline waters have CH4 isotope values compatible with an abiogenic origin through serpentinization processes but also at points, very negative values are present, indicating sometimes a clear biogenic contribution. Finally, few samples both from alkaline and hyperalkaline waters show some evidence of secondary oxidation processes.