Origin of Indus ophiolite-hosted ophicarbonate veins: Isotopic evidence of mixing between seawater and continental crust-derived fluid during Neo-Tethys closure

Abstract Ophicarbonates are found in ophiolite-associated peridotites and bear signature of fluid-rock interaction either on ocean floor or during ophiolite emplacement. The occurrence of ophicarbonate veins hosted by ultramafic rocks of the Indus Ophiolite, Nidar Valley, Ladakh Himalaya, is interpreted within the context of serpentinization of the allochthonous Cretaceous upper mantle section during uplift of the Himalaya orogen. Mixed dolomite-calcite obtained from two veins show carbon isotope ratios consistent with an origin from bicarbonate-dominated fluid and inconsistent with incorporation of atmospheric carbon dioxide in subaerial contact with high pH waters of serpentinization. Strontium isotope ratio of 0.711452–0.712041 are consistent with serpentinization subsequently to emplacement of the ophiolite and by interaction with Cretaceous seawater mixed with continental crust-derived fluid during Neo-Tethys closure. The predominance of lizardite and chrysotile in ophicarbonate veins as documented by Raman spectra also supports a low temperature serpentinization during ophiolite emplacement. The presence of hydrocarbon peaks in fluid inclusions in serpentinized olivine suggests that some of the dissolved inorganic carbon reacted with hydrogen released through oxidation of iron. The ophicarbonates and serpentines in peridotite of Indus ophiolite bear signature of a paleo-environment for Fischer-Tropsch reactions during the last phase of Neo-Tethys closure.