Lithium isotope ratios in methane-seep carbonates from the Black Sea and the Japan Sea

Abstract

Due to their large fractionation during low temperature fluid–rock interactions, stable isotopes of lithium (Li) are a powerful tool to reveal the origin of pore fluids at cold seeps and hydrothermal vents. The Li isotope ratios in authigenic carbonates precipitated from the seeping fluids are suitable to be used as indicators of the origin of fluids discharged in the past. This study analyzed the Li isotope ratios in methane-seep carbonates collected from the Black Sea and the Japan Sea. Variations in the measured Li isotope ratios and Li/Ca ratios in the carbonates are explained by the mixing between carbonate and clay endmembers, as well as carbonate mineralogy. Using the Al/Ca ratios of the samples, mixing ratios of the clay endmembers and the elemental and isotope composition of the carbonate endmembers were estimated. The Li isotope and Li/Ca ratios of the original fluids were then calculated using the values for the carbonate endmembers and the published isotope fractionation factors and the distribution coefficients. As a result, the calculated values indicated the mixing between seawater and fluids enriched in the light Li isotope.