H2 and CH4 outgassing rates in the Samail ophiolite, Oman: implications for low-temperature, continental serpentinization rates

Journal article

Reduced (H₂- and CH₄-rich) and hyperalkaline fluids are products of subsurface reactions accompanying serpentinization of ultramafic rocks. H₂ and CH₄ produced during serpentinization can fuel microorganisms and support habitable subsurface environments. CH₄ is also a potent greenhouse gas and can offset negative greenhouse emissions arising from active CO₂ removal accompanying carbon mineralization in ultramafic rocks. However, the rate at which reduced volatiles are delivered to the surface and the rate of reactions that generate these volatiles at low-temperature conditions are poorly known. In this work, we measured H₂ and CH₄ outgassing rates in several hyperalkaline spring sites in the Samail ophiolite, Oman. H₂ and CH₄ outgassing in these sites are variable and range up to 70,000 and 7,000 mol yr⁻¹, respectively. CH₄ outgassing in spring sites are unlikely to offset negative carbon emissions estimated from active carbon mineralization reactions in the Samail ophiolite. However, diffused CH₄ outgassing from peridotite outcrops remain unconstrained. Compositional and isotopic constraints show that volatiles are likely derived from active serpentinization, fluid inclusion decrepitation, or a combination of both. Calculated active serpentinization rates of up to 8 × 10⁻¹⁴ sec⁻¹ account for measured outgassing rates and these are consistent with slow rates expected at low temperatures. In calculations of serpentinization rates, this work uses reaction-path models to account incorporation of both ferrous and ferric iron in the resulting alteration assemblages, which yields ∼0.3 mol H₂ kg⁻¹ of ultramafic rock altered, lower than simulations based on iron oxidation to magnetite only. Contribution from decrepitation of H₂- and CH₄-bearing fluid inclusions is possible but would require much more mass of ultramafic rocks to account for observed outgassing. Further studies can help quantify extents of each source on active outgassing in Oman. Overall, this work shows that low-temperature serpentinization on geologically short timescales can account for observed flux of reduced volatiles in hyperalkaline environments.

Authors: Leong, JA; Nielsen, M; McQueen, N; Karolytė, R; Hillegonds, DJ

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Elsevier
• Journal: Geochimica et Cosmochimica Acta
• Volume: 347
• Pages: 1-15
• Publication date: 2023-02-17
• Acceptance date: 2023-02-11
• DOI: 10.1016/j.gca.2023.02.008
• EISSN: 1872-9533
• ISSN: 0016-7037
• Language: English
• Keywords: greenhouse emissions; serpentinization; FFR; methane (CH4); gas flux; hydrogen (H2)