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.
Bibliographical noteFunding Information:
The authors express their thanks to L. Halicz (G.S.I.) for the ICP-MS determination of Rb in the low-Rb samples and to R. Nisan, E. Kasher and A. Agranat who provided technical assistance in Sr separation and chemical analyses of the samples. O. Shamash greatly assisted in sample collection. The paper benefited greatly from the critical reviews and suggestions by J. L. Banner, P. Aharon and an anonymous reviewer. This study was partially supported by ISF grant #694/95 to A. Agnon, BSF grant #96-277 to A. Katz and by the Israel Energy and Infrastructure Ministry grant #94-17-032 to A. Agnon and M. Stein.