Lithium isotope systematics of methane-seep carbonates as an archive of fluid origins and flow rates

Abstract

Fluid sources and flow rates at methane seeps control gas hydrate formation, and are potentially archived in authigenic carbonates. Lithium (Li) in seeping fluids exhibits a broad range of isotopic fractionation during its ascent to the seafloor, providing information on temperatures and flow rates of the fluids. To test the applicability of Li isotopes as a fluid tracer in authigenic carbonates, this study investigates the element and Li isotope composition of methane-seep carbonates collected from the Black Sea and the Japan Sea. The Li/Ca ratios and δ7Li values of the aragonite-rich and calcite-rich samples (10–60 μmol mol−1 and 3.5–15.6‰; 21–142 μmol mol−1 and 8.3–24.4‰, respectively) are distinct from those in carbonates precipitated from seawater, and indicate an input of deep-seated fluids enriched in 6Li. After correcting for a minor admixture of clay-bound Li, the Li/Ca ratios and δ7Li values of the parent fluids were estimated to be 2400–11 000 μmol mol−1 and 17.2–34.5‰, respectively. Variations in the estimated values cannot be explained by fluid mixing alone, but rather reflect the isotopic fractionation caused by interaction of the deep fluids with secondary minerals during fluid ascent. The δ7Lifluid values as low as 17.2‰ for the Japan Sea carbonates indicate a deep fluid source with estimated temperatures of >26 °C derived from >250 m below the seafloor. Reaction-transport modeling shows that the deep fluid was transported to the zone of carbonate precipitation with advection rates of ≥10−2 cm yr−1. Higher advection rates were indicated for the Japan Sea fluids than for Black Sea fluids, probably due to a larger contribution of dissolved methane to hydrate formation. Spatiotemporal variations in fluid sources and advection rates were indicated by distinct (Li/Ca)fluid and δ7Lifluid values between the carbonates and pore fluids. The Li isotope systematics of seep carbonates are a potent indicator of the origin and flow rate of deep-seated fluids and their spatiotemporal variability, which is explored herein in detail for the first time.