Combining methane clumped and bulk isotopes, temporal variations in molecular and isotopic composition, and hydrochemical and geological proxies to understand methane's origin in the Ronda peridotite massifs (Spain)

In serpentinised peridotite and ultramafic rock systems, methane (CH₄) origin is frequently considered abiotic, but variable microbial and thermogenic components can also exist. Typically, the origin of CH₄ is studied using bulk, ¹³C/¹²C and ²H/H isotopic composition, molecular gas composition, occasionally radiocarbon (¹⁴C), microbiology and geological context. Recent advances in CH₄-clumped isotope methods have yielded novel insights into the formation of CH₄: nonetheless, their interpretation in natural gas samples is often uncertain and requires additional research. Here, we study the origin of the gas released in hyperalkaline (pH > 10) springs in the Ronda Peridotite Massifs (southern Spain), combining bulk and clumped CH₄ isotopes with molecular gas composition, hydrochemical (Total Organic Carbon and Platinum Group Elements in water), geothermal and geo-structural data. Five springs analysed in 2014 have been re-examined for changes in gas chemistry over time, and three newly discovered gas-bearing springs are analysed for the first time. Regardless of whether springs have microbial or abiotic isotopic fingerprints, we find that bulk CH₄ isotopes are fairly stable over a seven-year period. This suggests that the CH₄ source(s) or postgenetic processes (such as oxidation and diffusion) have not undergone significant temporal changes. Major variations in H₂ and CH₄ concentrations in certain springs may be the result of changes in gas pressure and migration intensity. Paired CH₄ clumped isotopes (Δ¹²CH₂D₂ - Δ¹³CH₃D) were analysed in two bubbling springs, where the presence of CH₄ can be interpreted as non-microbial based on ¹³C enrichment, absence of ¹⁴C, and the presence of ethane and propane. However, these isotopes are in disequilibrium, which prevents the quantification of the gas formation temperature. Within the Δ¹²CH₂D₂ - Δ¹³CH₃D diagram, the data lie within both the microbialgenic zone, suggested by previous authors, and the abiotic zone that results combining data from laboratory gas synthesis and other natural gas samples. Therefore, attributing a microbial origin to CH₄ based only on clumped isotopes is less definite than previously assumed. The amount of Total Organic Carbon appears to be correlated with the origin of CH₄, as it is higher in ¹³C-depleted CH₄ samples and lower in ¹³C-enriched samples. Palladium (Pd) and Rhodium (Rh) dissolved in water (the more soluble Platinum Group Elements) can be a proxy for the chromitite ore deposits contained in plagioclase tectonite layers throughout the investigated area, which may act as catalysts for abiotic CO₂ hydrogenation. Clumped isotope disequilibrium and the reported absence of diffuse CH₄-bearing fluid inclusions in the peridotites appear to rule out high temperature gas genesis in post-magmatic inclusions. These observations, along with the moderate temperatures at the base of the peridotite massifs and the consistent occurrence of gas along tectonic contacts between serpentinised (H₂-bearing) peridotite and carbon-bearing rocks, are compatible with the theory of low-temperature CO₂ hydrogenation.