Mg isotopes and chemical composition of waters and Late Miocene carbonate rocks

Description

The data provided here are part of a research article entitled "Mg isotopes as monitors of water sources and dolomitization process in late Miocene lacustrine bodies", submitted to Geochimica et Cosmichimica Acta. The Mg isotope compositions are applied as tracers of fresh and saline water sources and monitors of groundwater circulation in the hydrological-limnological system of the modern Lake Kinneret (the Sea of Galilee) and its late Miocene predecessors, lakes Bira and Gesher. The Mg and Sr isotope ratios and major elements compositions were analyzed in freshwaters and brines in the watershed of Lake Kinneret and limestones and dolostones that comprise the late Miocene Bira and Gesher Formations, which were deposited in the ancient water-bodies. A box model shows that dolomitization of limestone aquifers by seawater evaporated up to the halite facies, can produce a brine with exceptionally high d26Mg. This brine formed the hypersaline Lake Zemah, located in the tectonic Kinnarot Basin east of Lake Bira. The existence of this hypersaline lake is revealed by the kilometers thick of evaporite (halite) sequences that were deposited in Lake Zemah during the late Miocene. During wet periods, the Zemah brine penetrated the freshwater Lake Bira and the freshwater-brine mixture accommodated the deposition of primary calcites comprising the lacustrine limestones of Bira Formation. These limestones have rather high d26Mg. During arid periods, Lake Bira contracted, the brine discharge from Lake Zemah stopped and low d26Mg Mediterranean Sea waters penetrated and mixed with Lake Bira’s brackish waters leading to dolomitization of the calcite muds. These lacustrine dolomites have lower d26Mg than their underlying limestones. Later, during the Messinian the area was dominated by swamps of the Gesher Fm. Dolomitization proceeded by bacterial activity, as evident from the low d13C values in the Gesher Fm. dolomites. The temporal decrease in the d26Mg values of the Ca-chloride brines from ~1.2 ‰ (in the Tortonian Lake Bira) to ~0.5 ‰ (in the Messinian Gesher swamps) to 0.15 ‰ (in the modern Tiberias spa) indicates a continuous dilution of the original brine with seawater (until the Zanclean transgression ~5.3 Ma) and later with freshwaters. The dolomitization process described in the manuscript for the late Miocene lacustrine carbonates, may be relevant to other environments where the exchange of Mg occurs between hypersaline brines and less-saline solutions, e.g., marine lagoons that characterized the late Neogene circum-Mediterranean. Moreover, the mixing processes between solutions of distinct salinities in the marginal lagoonal-lacustrine environments of late Miocene circum-Mediterranean may illustrate the conditions of dolomitization in the shallow marine environments of the Mesozoic seas.

Steps to reproduce

Dolostone and limestone samples were collected from three exposed sections of the Bira and Gesher Formations at Hurbat Ze’ev and Nahal Hamud, located at the Lower Galilee in Northern Israel and Kokhav-Ha’Yarden at the flank of the Jordan Valley (Fig. 1); a detailed description of the sections is given in Rozenbaum et al. (Sed. Geol., 2019, 383, 268-292). A few samples of primary calcite were collected by a sediment trap deployed at station A of Lake Kinneret at a water depth of 38 m (Fig. 1). Waters were sampled in several springs and runoff in the areas of the Galilee, Mt. Hermon, the Jordan River, and Judea Hills (locations are presented in Fig. 1). Procedures of water sampling and treatment follow Fruchter et al. (2017, Geochim. Cosmochim. Acta, 215,17-32). Magnesium isotope ratios were measured in the carbonate fraction of the samples (mainly pure dolostones and limestone) that were dissolved in diluted acetic acid. Isotope analysis was done at the Biological and Chemical Research Centre of the University of Warsaw using a Multi Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) “Plasma II” (Nu Instruments, Wrexham, UK) equipped with 16 Faraday cups; three of them were used to measure the isotopes of magnesium, 24, 25 and 26, respectively. On-line magnesium isotope ratio analyses were done using Ion Chromatography (IC) coupled to MC-ICPMS. Strontium isotope ratios and chemical compositions (e.g., Sr, Mg and Ca concentrations) were analyzed in the carbonate fraction dissolved by diluted acetic acid. 87Sr/86Sr ratios were measured at the Geological Survey of Israel (GSI) using Nu Instruments MC-ICPMS. The SRM 987 Sr standard was measured every 10 samples, yielding an average ratio of 87Sr/86Sr = 0.71024 ±3 (2 sigma of the mean, n = 20). Sr isotopic ratios were corrected using the NIST SRM 987 standard for Sr (87Sr/86Sr = 0.71025). Sr, Mg and Ca concentrations were determined by ICP-OES (Perkin-Elmer, Optima3000) at the GSI. Analytical errors are estimated to be 5%.

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