Experimental and theoretical constraints on lithium isotope fractionation during brine evaporation and halite precipitation

Abstract

Marine evaporitic halite, precipitating from the advanced stage of extremely evaporated seawater, is the most common and abundant evaporitic mineral. The extent to which lithium (Li) is incorporated into evaporite halite during geological periods of massive halite deposition, and the mechanisms of incorporation, are both unknown. These are each important, however, for the quantification of the isotope effect of the evaporitic sink on δ7Liseawater, and for the potential use of halite as a recorder of seawater δ7Li. Here, new experimental data are presented for the Li fractionation factor for halite from an isothermal evaporation experiment (25∘C for 100 days) using marine-derived brine. The resultant solids were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to confirm the mineralogy and crystallinity of the product. Our results show that Li isotopes are not constant during the evaporation process implying that there must be a process that fractionates Li isotopes during halite precipitation. We suggest that this results from Li ionic substitution for sodium (Na) in crystalline halite, a scenario supported by ab-initio calculations, but we also show that Li within halite is predominantly present within fluid inclusions. Thus, the Li isotopic composition of halite is controlled by a mixture of Li within the fluid inclusions and Li that is incorporated into the halite crystalline structure. Overall, the brine becomes enriched in Li7 during evaporation due to the preferential incorporation of Li6 into chloride precipitates, and the evolution of δ7Librine is controlled by the precipitated mineral assemblage. The theoretical equilibrium fractionation factor 1000lnαmineral−fluid is -7.8▪ for Li-halite (LiNa26Cl27) at 25 C∘ at equilibrium, which is at the lower limit of Li isotope fractionation factors. Rayleigh fractionation models were fitted to the experimental data yielding a fractionation factor of -25▪3▪. The difference in Li isotope fractionation factor between brine and structurally bound Li may result from kinetic fractionation effects. This study presents a scenario highlighting that extensive halite deposits, formed over geological time in environments with relatively high Li concentration, could potentially impact the δ7Li values of seawater during that period, and place constraints on their utility as an archive for δ7Liseawater.