Mineralogical evidence for Quaternary serpentinization in the New-Caledonian ophiolite: Implication for the low-temperature genesis of H2- and CH4-bearing alkaline fluids

H₂-bearing fluids (pH 10 – 12) issued in alkaline springs found in several ophiolitic complexes worldwide are believed to result from the alteration of ultramafic rocks by infiltration of meteoric waters. The mineralogical fingerprint of the reactive percolation of such an alkaline fluid is revealed by veinlet mineralization occurring in the New Caledonian ophiolite (Massif du Sud). In two localities separated by ~ 15 km (Georges Pile and GR2H mines), late veins in a partially serpentinized peridotite contain magnetite crystals younger than 2 Ma as inferred from (U-Th)/He geochronometry. While the serpentinite host at Georges Pile is largely overprinted by lateritic weathering, primary parageneses are preserved at GR2H. There, magnetite occurs along with dolomite and Fe-poor lizardite as filling in millimeter sized veins cross-cutting the mesh texture of the partially serpentinized dunite. Temperature of the aqueous fluid from which the vein material precipitated is estimated to be ~95°C from in situ δ¹⁸O data on the magnetite-dolomite pair, indicating a low-temperature alteration process. Thermochemical calculation shows that this aqueous fluid was alkaline and most likely H₂-bearing. Chemically, it strongly resembles waters that are issued today in H₂ and CH₄ – bearing (hyper)alkaline springs of the Massif du Sud. δ¹³C isotopic composition of dolomite is exceptionally high, between 7.1 and up to 17.3 ‰ and is interpreted as evidence for low-temperature methanogenesis.