Article Dans Une Revue Geochimica et Cosmochimica Acta Année : 2026

Isotopic insights into the origins of N2–H2–CH4 gases in continental serpentinizing systems: evidence from the youngest large, well-preserved ophiolite

Résumé

We present data of major and noble gas composition from mainly newly reported, natural springs – related gas seeps across Grande Terre, the main island of New Caledonia (NC). Two types of springs are investigated: (1) Ophiolitic Springs which emerge from the NC Ophiolite, recognised as the youngest large and well-preserved ophiolite, including seven sites in the Massif du Sud and one site in a chromite mine tunnel located in the Tiébaghi Massif in the north of Grande Terre; (2) Four Basement Springs which emerge from the pre-Late Cretaceous basement in the eastern central part of Grande Terre. Our results reveal that both seep types are dominated by air-like N2 (60–95%; δ15N = −0.2 to + 0.1‰ vs Air), and contain CH4 (up to 23%) whose δ13C generally range from −40‰ to −20‰ VPDB in the Ophiolitic Springs and is lighter in the Basement Springs (δ13C(CH4) = −70 to −35‰ VPDB). Ophiolitic Sites also show elevated H2 content reaching 35%, with δD values ranging from −740 to −710‰ VSMOW, whereas Basement Sites display very low levels of H2 (<300 ppm). The gas composition and isotopic signatures at Ophiolitic Sites are characteristic of serpentinization environments, suggesting that this process is the primary pathway for H2 generation; consistent with previous studies. This is confirmed by the elevated dissolved OH− which results in elevated pH (up to 11.83), among other factors. Isotopic data for CH4 and CO2 indicate that CH4 in the Ophiolitic Sites likely originates from both biogenic and abiotic processes, while microbial methanogenesis is the major source of CH4 in the Basement Springs and at the Tiébaghi Tunnel. The H2-CH4-H2O isotopic fractionation factors suggest equilibration at approximately 50–100°C, indicating a low-temperature serpentinization system within the New Caledonia Ophiolite, which aligns with previous studies. Consistent with the regional geology and tectonics, helium isotopic data (3He/4He) reveal a predominantly radiogenic signature (∼0.3 Ra) at the Basement Sites where the crust is thicker, and a significant mantle-derived component (∼13–37% mantle helium) in the southernmost coastal Prony region. For the central part of the Massif du Sud peridotite nappe where crustal and mantle-derived contributions are absent, we propose that the air-like signatures revealed by 3He/4He, 4He/20Ne, 36Ar/N2 and 84Kr/N2 ratios reflect atmospheric entrainment and the degassing of air-saturated groundwater during water table fluctuation, which also accounts for the predominance of the air-like N2 in the gas emissions.

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Cite 10.5281/zenodo.19197133 Ouvrage izerumugaba-jdlp. (2026). izerumugaba-jdlp/New-Caledonia: Isotopic insights into the origins of N2–H2–CH4 gases in continental serpentinizing systems: evidence from the youngest large, well-preserved ophiolite (Version V.F). Zenodo. https://doi.org/10.5281/ZENODO.19197133

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hal-05585680 , version 1 (28-04-2026)

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Jean de la Paix Izerumugaba, Anne Battani, Eric Deville, Camille Mazière, Julie Jeanpert, et al.. Isotopic insights into the origins of N2–H2–CH4 gases in continental serpentinizing systems: evidence from the youngest large, well-preserved ophiolite. Geochimica et Cosmochimica Acta, 2026, 420, pp.260-279. ⟨10.1016/j.gca.2026.03.045⟩. ⟨hal-05585680⟩
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