CONTROLS ON LITHIUM ISOTOPE FRACTIONATION IN EVAPORITE SYSTEMS

Studies of the hydrogeochemistry of saline lakes in the U.S. Great Basin point to a singular mechanism to produce the bulk of lithium isotope fractionation. The six lakes examined have modern day Li isotope compositions (d⁷Li) that are (a) lower than seawater and major freshwater lakes (+32), and (b) broadly variable (from Great Salt Lake, UT, at +16, to Walker Lake, NV, at +24). We propose that these variations derive primarily from a process in which groundwater evolves to both high salinity and heavy isotopic compositions though progressive fluid-sediment interaction.

The water budget of Mono Lake, CA, is dominated by creeks that drain the Sierra Nevada. However the lake derives substantial Li from spring sources (³50%). Thermal springs have isotopic signatures that trend toward mantle-like (light) values, considered to reflect equilibration with regionally extensive Quaternary volcanic material. Whereas the principal creeks no doubt reflect some degree of isotope fractionation during, for instance, source rock weathering, these waters are both rather minor Li contributions and also isotopically lighter than the lake itself (~+19‰). Hence, substantial isotopically heavy Li (>+19‰) must enter the lake to balance the modern budget. Groundwater springs have d⁷Li=+31, similar to efflorescent salt crusts elsewhere in the basin (+32‰), suggesting a link in the histories of these materials. Analysis of recent Mono Lake carbonates and shoreline salts from Big Soda Lake, NV, indicate that evaporitic crystallization alone is unlikely to drive the scale of Li isotope fractionation present in these and similar systems. Our current data set leads to the conclusion that isotope fractionation takes place in the shallow subsurface as saline groundwater is drawn up during playa formation. Through progressive interaction of groundwater with mineral surfaces, pore fluids are driven to heavier isotopic compositions. Salts precipitating from this brine will carry the heavy isotopic signature. If our subsequent measurements verify this mechanism, and if this process is recorded accurately in the sedimentary record, it could be useful as a climate proxy.