Work

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

Year: 2026

Type: article

Abstract: 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 N 2 (60–95%; δ 15 N = −0.2 to + 0.1‰ vs Air), and contain CH 4 (up to 23%) whose δ 13 C generally range from −40‰ to −20‰ VPDB in the Ophiolitic Springs and is lighter in the Basement Springs (δ 13 C(CH 4 ) = −70 to −35‰ VPDB). Ophiolitic Sites also show elevated H 2 content reaching up to 35%, with δD values ranging from −740 to −710‰ VSMOW, whereas Basement Sites display very low levels of H 2 (<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 H 2 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 CH 4 and CO 2 indicate that CH 4 in the Ophiolitic Sites likely originates from both biogenic and abiotic processes, while microbial methanogenesis is the major source of CH 4 in the Basement Springs and at the Tiébaghi Tunnel. The H 2 -CH 4 -H 2 O 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 ( 3 He/ 4 He) 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 3 He/ 4 He, 4 He/ 20 Ne, 36 Ar/N 2 and 84 Kr/N 2 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 N 2 in the gas emissions.

Source: Geochimica et Cosmochimica Acta

DOI: 10.1016/j.gca.2026.03.045