Sr, S, and OSO4 Isotopes and the Depositional Environments of the Upper Miocene Evaporites, Spain

Abstract

ABSTRACT Upper Miocene evaporites in the Nijar Basin, southeastern Spain, contain thick, massive and coarse-crystalline selenites of the middle Messinian Yesares Formation. On the basis of their sedimentary fabrics and structures, such as euhedral terminations of crystals, well preserved algal filament inclusions, and standing-growth orientations, these selenites are interpreted to be primary, bottom-growth deposits. Most of these selenites have Sr concentrations of about 1000 ppm (range from 700 to 1800 ppm). 87Sr/86Sr values of Yesares selenites vary between 0.70890 and 0.70896 (n = 18), which are all within the known range of Messinian seawater, suggesting a marine origin. However, quantitative modeling of 87Sr/86Sr vs. Sr/Ca of fluids demonstrates that a minor amount of admixed nonmarine fluid (e.g., 20%) might have contributed to the Yesares marine brines. 34S and 18OSO4 values of Yesares selenites range from 21 to 24‰ CDT and from 10 to 16‰ SMOW, respectively. These ranges are more variable and most values are heavier than those expected for normal Miocene marine gypsum. On the basis of quantitative modeling, these heavy 34S and 18OSO4 values have most likely resulted from reduction and oxidation reactions of sulfur species that occurred in brines rather than from contributions of nonmarine water or from reservoir (crystallization) effects. The use of Sr, S, and O isotopes provides a possible means of distinguishing whether an ancient evaporite was deposited by marine, nonmarine, or hybrid brines. Sr isotopes, however, are insensitive to minor nonmarine contributions on the basis of quantitative modeling, and consequently marine Sr isotopes for evaporites (e.g., gypsum) do not necessarily indicate pure marine depositional environments. Compared to Sr isotopes, 34S and 18OSO4 of marine gypsum are even more insensitive to minor nonmarine contributions because of much higher SO4 concentrations in seawater and seawater-evaporated brines than in most freshwaters. Therefore, Sr, O, and S isotopes should be used with caution to interpret depositional environments and to determine parent fluids of ancient evaporites.