Hydrogeochemical fluxes and processes contributing to the formation of lithium-enriched brines in a hyper-arid continental basin

Abstract

The accumulation of solutes delivered via shallow groundwater and surface water in arid to hyper-arid closed basins, and the complex hydrogeochemical processes associated with their transport, are key to deciphering mechanisms responsible for the formation of brines and salts in these environments. A rigorous investigation of the hydrogeochemical fluxes and the formation of the Li-enriched brine in the hyper-arid basin of the Salar de Atacama, Chile is presented. Water fluxes within the Salar de Atacama basin range from 0.003 m3/s to 1.50 m3/s with a poor correlation between discharge and drainage basin area. The inflowing waters have major elemental and Li concentrations that are within the same order of magnitude over a period of several decades. The element fluxes indicate, for example, that 58% of the Li in the halite nucleus and brines is derived from the south and southeast parts of the basin. The average δ18Ovsmow and δDvsmow of inflow waters indicate that there are varied sources of recharge corresponding to the defined sub watershed regions. The average water flux-weighted 87Sr/86Sr of inflow waters allows further definition of where in the regional watershed the solutes are derived. The predicted water flux weighted 87Sr/86Sr of the brines (0.70813) compared to the measured average 87Sr/86Sr (0.70801), indicates that there may be unaccounted sources of water and solutes to the basin brines. Elemental accumulation time scales for the volume of halite and brine in the nucleus balance on time frames of 0.5 Ma (Mg), 1.0 Ma (Ca), 1.9 Ma (Li) and 2.2 Ma (K) as compared to 47 Ma (Na) and 53 Ma (Cl), further evidence that Na and Cl require additional sources not accounted for in the modern inflows to the topographic watershed. Dissolved noble gas concentrations of marginal and nucleus brines specify that the brines formed in an atmospheric equilibrated state and have since been isolated below the salt crust. A process-based model is presented to explain the extremely high concentrations of Li in brines which incorporates the δ7Li isotope signatures of waters in the basin to further decipher, on a first order, the contributions of processes such as low temperature weathering and secondary phase formation.