The impact of brine-rock interaction during marine evaporite formation on the isotopic Sr record in the oceans: Evidence from Mt. Sedom, Israel

The effect of brine-rock interaction on the composition of strontium in evaporitic basins and its impact on the ⁸⁷Sr/⁸⁶Sr ratios in contemporaneous seawater are examined for the Sedom (Dead Sea Rift Valley, or DSR), the Messinian (Mediterranean) and the Louann (Gulf of Mexico) evaporites. For that purpose, mineralogical, chemical and isotopic (Sr, S) analyses were performed on the Sedom Fm. evaporites (halite, anhydrite and dolomite). ⁸⁷Sr/⁸⁶Sr ratios are distinctively lower in the Sedom evaporites (dolomites: 0.7082-0.7083; halites: 0.7083-0.7087) than in the contemporaneous late Pliocene seawater (~0.709). At the same time the sulfur isotope ratios (δ³⁴S ~ 20‰) are consistent with deposition from late Cenozoic seawater. This duality, together with the variation of strontium isotopes between the dolomites and halites can be explained by modification of the ⁸⁷Sr/⁸⁶Sr ratio in the lagoon water by influx of Ca-Chloride brines. The brines were formed by dolomitization of marine carbonates of the DSR Cretaceous wall rocks (where ⁸⁷Sr/⁸⁶Sr ~ 0.7077). Brine-rock interaction can similarly explain the anomalous ⁸⁷Sr/⁸⁶Sr ratios in the Messinian and Louann evaporites. It is concluded that this process causes significant changes in the ⁸⁷Sr/⁸⁶Sr ratios of evaporitic lagoons. A water and strontium mass balance of the Sedom data is used to show the impact on the strontium oceanic budget. Extrapolation to larger evaporitic basins indicates that the combined global riverine and hydrothermal influx of strontium can be matched by halite or gypsum precipitating lagoon of 2-3.5 X 10⁵ km². Examples for such evaporitic sites include the Messinian, Louann and Zechstein basins. Copyright (C) 2000 Elsevier Science Ltd.