Lithium and strontium isotopic systematics in playas in Nevada, USA: constraints on the origin of lithium

Lithium-rich brine in playas is a major raw material for lithium production. Recently, lithium isotopic ratios (delta Li-7) have been identified as a tool for investigating water-rock interactions. Thus, to constrain the origin of lithium in playas by the use of its isotopes, we conducted leaching experiments on various lacustrine sediment and evaporite deposit samples collected from playas in Nevada, USA. We determined lithium and strontium isotopic ratios and contents and trace element contents of the leachate, estimated the initial delta Li-7 values in the water flowing into the playas, and examined the origin of lithium in playas by comparison with delta Li-7 values of the possible sources. In samples from the playas, delta Li-7 values show some variation, reflecting differences both in isotopic fractionation during mineral formation and in initial delta Li-7 value in water flowing into each playa. However, all delta Li-7 values in this study are much lower than those in river water and groundwater samples from around the world, but they are close to those of volcanic rocks. Considering the temperature dependence of lithium isotopic fractionation between solid and fluid, these results indicate that the lithium concentrated in playas in Nevada was supplied mainly through high-temperature water-rock interaction associated with local hydrothermal activity and not directly by low-temperature weathering of surface materials. This study, which is the first to report lithium isotopic compositions in playas, demonstrates that delta Li-7 may be a useful tracer for determining the origin of lithium and evaluating its accumulation processes in playas.