Methane-derived authigenic carbonates of mid-Cretaceous age in southern Tibet: Types of carbonate concretions, carbon sources, and formation processes

Abstract

Methane-derived authigenic carbonates with distinctive structures and morphologies have been documented worldwide, but they are rarely found from ancient strata in the Eastern Tethys Ocean. The methane-derived authigenic carbonates found in southern Tibet are developed in calcareous or silty shales of mid-Cretaceous age in the Xigaze forearc basin and in the Tethyan Himalaya tectonic zone. The morphology, mineralogy, elemental geochemistry and composition of carbon and oxygen isotopes of these carbonates are studied in detail.The carbonates have nodular, tubular, and tabular morphologies. They are primarily composed of carbonate cement that binds and partly replaces host sediment grains; host siliciclastic sediments are composed mainly of quartz and plagioclase feldspar; a few foraminifers; and framboidal or subhedral to euhedral pyrite. Carbonate cements dominantly are micritic calcite, with minor contribution of dolomite.Nodular concretions are characterized by depleted δ13C values, commonly ranging from −30‰ to −5‰. The δ13C values show a gradual decrease from the periphery to the center, and the CaO, SiO2, Fe2O3, Al2O3, K2O, and TiO2 contents generally show a gradual change. These features indicate that the nodular concretions grew from an early-formed center toward the periphery, and that the carbon source of the nodular concretions was derived from a mixture of methane, methanogenic CO2, and seawater-dissolved inorganic carbon.The tubular concretions are characterized by δ13C values of −8.85‰ to −3.47‰ in the Shangba Section, and −27.37‰ to −23.85‰ in the upper Gamba Section. Unlike the nodular concretions, the tubular concretions show central conduits, which are possible pathways of methane-rich fluids, suggesting that the cementation of tubular concretions begins at the periphery and proceeds inward. Moreover, the tubular concretions show morphological similarity with the methane-derived carbonate chimneys, pipes and slabs reported in present-day cold seep settings. We suggest that the carbon source of the tubular concretions was derived from a mixture of seawater-dissolved inorganic carbon and oxidized methane formed by released hydrate.The tabular concretions are characterized by δ13C values of −21.87‰ to −6.67‰ in the Xiege Sections. These depleted δ13C values suggest that the carbon of the tabular concretions was derived at least in part, from AOM. The tabular concretions are characterized by δ13C values of −28.81‰ to −12.99‰ in the Gamba Section. According to the δ13C values and field observation, we infer that their carbon source was more likely to be a mixture of the oxidized methane formed by released hydrate and seawater-dissolved inorganic carbon.