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 7 and abundant evaporitic mineral. The extent to which lithium (Li) is incorporated into evaporitic halite during geo8 logical periods of massive halite deposition, and the mechanisms of incorporation, are both unknown. These are each 9 important, however, for the quantification of the isotope effect of the evaporitic sink on δ7Liseawater, and for the poten10 tial use of halite as a recorder of seawater δ7Li. Here, new experimental data are presented for the Li fractionation 11 factor for halite from an isothermal evaporation experiment (25◦C for 100 days) using marine-derived brine, compara12 ble in composition to Phanerozoic CaCl2 oceans. The resultant solids were characterized by X-ray diffraction (XRD) 13 and scanning electron microscopy (SEM) to confirm the mineralogy and crystallinity of the product. Our results show 14 that Li isotopes are not constant during the evaporation process implying that there must be a process that fractionates 15 Li isotopes during halite precipitation. We suggest that this results from Li ionic substitution for sodium (Na) in crys16 talline halite, a scenario supported by ab-initio calculations, but we also show that Li within halite is predominantly 17 present within fluid inclusions. Thus, the Li isotopic composition of halite is controlled by a mixture of Li within the 18 fluid inclusions and Li that is incorporated into the halite crystal. Overall, the brine becomes enriched in 7Li during 19 evaporation due to the preferential incorporation of 6Li into chloride precipitates, and the evolution of δ7Librine is con20 trolled by the precipitated mineral assemblage. The theoretical equilibrium fractionation factor 1000lnα(mineral−f luid) 21 is -7.8 ‰ for Li-halite (LiNa26Cl27) at 25 ◦C at equilibrium, which is at the lower limit of Li isotope fractionation 22 factors. Rayleigh fractionation models were fitted to the experimental data yielding a fractionation factor of -25 ± 3 23 ‰. The difference in Li isotope fractionation factor between brine and structurally bound Li may result from kinetic 24 fractionation effects. This study presents a scenario highlighting that extensive halite deposits, formed over geological 25 time, could potentially impact the δ7Li values of seawater during that period, and place constraints on their utility as 26 an archive for δ7Liseawater.