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

摘要

The effect of brine-rock interaction on the composition of strontium in evaporitic basins and its impact on the Sr-87/Sr-86 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 (approximate to 0.709). At the same time the sulfur isotope ratios (delta(34)S approximate to 20 parts per thousand) are consistent with deposition from late Cenozoic seawater. This duality, together with the variation of strontium isotopes between the dolomites and halites can he explained by modification of the Sr-87/Sr-86 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 Sr-87/Sr-86 similar to 0.7077). Brine-rock interaction can similarly explain the anomalous Sr-87/Sr-86 ratios in the Messinian and Louann evaporites. It is concluded that this process causes significant changes in the Sr-87/Sr-86 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 10(5) km(2). Examples for such evaporitic sites include the Messinian, Louann and Zechstein basins. Copyright (C) 2000 Elsevier Science Ltd. [References: 44]