d26Mg values and 87-Sr/86-Sr ratios are used as tracers of calcite and dolomite formation in the late Miocene Lake Bira. Mg and Sr isotope ratios were analyzed in freshwaters and brines that currently feed the Sea of Galilee (the modern remnant of Lake Bira) and in limestones and dolostones comprising the Bira Formation. 26Mg and 87Sr/86Sr ratios of the Sea of Galilee waters (~0.89‰, ~0.7075) are consistent with the mixing of mainly carbonate and basaltic waters with subsurface Ca-chloride brines (e.g., Tiberias Spa). The 26Mg values in the limestones and dolostones of the Bira Formation range from ~ -1.0 to ~ -3.5‰. and -2.8 to -1.8 ‰, respectively. The 26Mg values in Lake Bira waters at that time were between ~-2 ‰ to ~1 ‰, as calculated from the fractionation factors between water and either calcite or dolomite (-2 ‰ and -0.75 ‰, respectively). Isotope mixing calculations suggest that waters with positive 26Mg values (estimated as ~1.2 ‰) were added to the lake. We suggest that these waters were Ca-chloride brines formed in the late Miocene Jordan Valley by interaction between evaporated seawater and the local limestones. These brines deposited the contemporaneous thick sequences of salt (halite) and gypsum in the Jordan Valley east of the lake. Dolomitization of the limestones increased the 26Mg of the brines during their re-circulation through the surrounding aquifers due to Rayleigh fractionation while forming the Ca-chloride solution.
Limestone formation required enhanced freshwater input; a process accompanied by increasing hydrological head that induced an enhanced inflow of the Ca-chloride brine with high 26Mg to the lake. Dolomite formation was associated with the weakening of the hydrological head, and diminishing flow of the brine to the lake.
This dolomitization scenario provides a model for dolomitization in other marine margin environments of the Paratethys and Proto-Mediterranean.