Stable Isotope Systematics of Mg, Sr, and Li in Hydrothermal Fluids and Hypersaline Lake Brines in the East African Continental Rift (Djibouti)

Abstract

The Asal Rift is a continental rift segment in which the hydrothermal reactions of hot volcanic rocks and seawater-derived groundwater are comparable to the submarine hydrothermal processes. Formation of recent evaporites in the hypersaline Lake Asal and Lake Abhé is superimposed on this seafloor-type hydrothermal activity. From a geological and planetary science perspective, these continental rift lakes are valuable natural calibration fields for constraining the water–rock interaction and elemental cycles. In this study, we report on the chemical composition and stable isotopic fractionation of Mg, Sr, and Li in the hydrothermal fluids and brines of these two hypersaline lakes. The hydrothermal fluids of both Lake Asal and Lake Abhé were characterized by lower Mg isotopic ratios than seawater. This is explained by the removal of ²⁶Mg from the fluids during the hydrothermal fluid–basalt interaction. In contrast, the brine from Lake Asal is indistinguishable from seawater, and no apparent effect of evaporite minerals on δ²⁶Mg was observed. The ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr values of the hydrothermal fluid vary within the same range as the basalt, indicating that the effect of Sr leaching from the rock is significant. In contrast, the variation in δ⁸⁸Sr is mainly caused by the precipitation of carbonates from the lake brine. The correlation between δ⁷Li and ⁸⁷Sr/⁸⁶Sr is clear regardless of the sample type (hydrothermal fluid, brine, or river water) and is thought to reflect the mixing of solutes. The δ⁷Li values of the hydrothermal end-members can be used as an indicator of the reservoir temperature of fluids.