Cold-seep carbonate as an archive of lithium isotopic composition of pore fluids

Abstract

Lithium and its stable isotopes are key tracers of geofluids. This study tested if a cold-seep carbonate can be used as an archive of the Li isotopic composition of pore fluids, using a carbonate collected from a gas hydrate area of the Japan Sea. Calcium carbonate was sequentially leached with a weak acid, and Li was then purified from the leachates through ion chromatography. The Li concentrations and isotopic compositions measured for the leachates were interpreted to result from the mixing between Li in inorganic calcite and in secondary minerals. Endmember values of calcite were estimated and converted to the Li concentrations and isotopic compositions of the source fluids, using the partition coefficient and isotope fractionation factor. The resulting values were close to those measured for pore fluids in hydrate-bearing sediments of the same area.