Isotopic Insights into the Origins of N₂-H₂-CH₄ emanations in the New Caledonia Ophiolite

Natural emanations consisting of N₂-CH₄-H₂ type gases have been documented across the peridotite nappe of the New Caledonia Ophiolite [1], and the presence of H₂ has been attributed to serpentinization [2, 3]. We present new major and noble gas compositional and isotopic data from low to high H₂ bubbling gas seep sites from both the south (e.g: Lembi River (≤ 20% H₂), Les Pirogues River (≤ 15%), Pourina (≤ 10%)), and north east klippes of the Ophiolite (e.g: Fanama and Nemwegi (≤ 300ppm)), providing new insights into gas origins that can be compared to other serpentinization systems (e.g: Oman, Italy, Turkey, and the Philippines).

Results show that gases are dominated by N₂ (60-95%; d¹⁵N ranging from -0.2 to +0.1‰ vs air), while the H₂ content can reach up to 35% with dD ranging from -740 to -710‰ VSMOW. CH₄ reaches up to 20% with d¹³C ranging from -40‰ to -3.6‰ VPDB. Such major gas composition and isotopic values are characteristic of serpentinization [4, 5]. Additional factors, such as olivine-rich peridotite rocks, precipitation of magnetite, carbonates, and brucite, along with the elevated pH of spring waters (up to 10.5), confirm an active serpentinization system. Hydrogen H₂-CH₄-H₂O isotopic fractionation factors suggest that, despite not being at equilibrium, the hydrogen-bearing fluids are formed at around 50°C, in comparison to 95°C, which was determined using magnetite-dolomite O₂ fractionation [6]. H₂ and CH₄ likely result from low-temperature serpentinization and processes involving inorganic carbon, respectively; potentially catalysed by Ni, Cr, and Chromitite-hosted Ru [7] which are enriched in the peridotite [1, 8]. Microbial activity indicators such as the presence of biogenic methane, when present, aligns with documented microbial communities.

Helium isotopic data (3He/4He) indicate signatures ranging from predominantly radiogenic (0.3 Ra) in the north, where the crust is thick [2], to ASW-like values in the central south (Lembi and La Coulée), to ~25% mantellic contribution in the southernmost coastal Prony region. We argue that the air-like signature is indicative of the degassing of circulating air-saturated groundwater, which aligns well with interpretations that air-like N₂ present in serpentinization systems may originate from aquifers [2, 3, 4].

Seismic and tectonic data reveal multiple deep faults and fractures in the massif du Sud [9], as well as a shallow Moho and 20 km-deep earthquakes that are indicative of active tectonics detected beneath the Prony area [10]. This explains the facilitated migration of mantle fluids to the surface at Prony.

[1] Maurizot et al., 2020(c). Geol. Soc. Lond. Mem. 51(1), 1–12

[2] Deville and Prinzhofer, 2016. Chem. Geol. 440, 139–147

[3] Monnin et al., 2021. JGR Biogeosci. 126, e2021JG006243.

[4] Vacquand et al., 2018. Geochim. Cosmochim. Acta 223, 437–461.

[5] Etiope, 2017. Procedia Earth Planet. Sci. 17, 9–12.

[6] Corre et al., 2023. Sci. Rep. 13(1), 19413.

[7] Molinet-Chinaglia et al., 2024. ChemCatChem 16(24), e202401213.

[8] Maurizot et al., 2020(f). Geol. Soc. Lond. Mem. 51(1), 247–277.

[9] Lagabrielle et al., 2005. Tectonophysics 403(1–4), 1–28.

[10] https://submap.fr