Variation of lithium isotope composition in the marine environment: A preliminary report

Abstract

The Li isotopic compositions of ocean waters, lake waters, hydrothermal solutions, altered and unaltered basalts have been determined using the technique of thermal ionization mass spectrometry of lithium tetraborate. Seawater appears to be homogeneous in Li isotopic composition. The mean δ 6 Li value is −32.3 ± 0.5%., relative to an isotopic standard. Three lakes studied (Lake Tanganyika, Caspian Sea and Dead Sea) yield δ 6 Li similar to the seawater isotopic composition. A fresh basalt glass from 21°N, East Pacific Rise displays a δ 6 Li value of −4.7%,. A hydrothermal solution from a 21°N vent field has δ 6 Li of −10%., indicating incomplete extraction of Li from the igneous minerals or partial retention in secondary phases. The alteration margin of a basalt from the Mid-Atlantic Ridge has δ 6 Li of −8.4%.. The isotope data of submarine basalts suggest preferential removal of 6Li from seawater into alteration minerals during low temperature weathering. The δ 6Li values of two hydrothermal solutions from Guaymas Basin were found to be − 5%.. and −10%.. The results can be interpreted as the net effect of Li addition from basalt and sediments and incorporation in hydrothermal precipitates as the hydrothermal fluids interact with basin sediments. The observed enrichment of 7Li in seawater relative to submarine hydrothermal solutions, its principal Li input, is tentatively attributed to isotopic fractionation associated with low temperature alteration of seafloor basalt and incorporation in authigenic sediments. It appears that the Li isotope system may have characteristics that can resolve the mass balance of Li in the ocean.