The origin of Miocene evaporites in the Tuz Golu basin (Central Anatolia, Turkey): Implications from strontium, sulfur and oxygen isotopic compositions of the Ca-Sulfate minerals

Abstract

Sulfur, oxygen and strontium isotope compositions of the sulfate minerals (gypsum and anhydride) of the Miocene evaporites in the Tuz Golu basin which is the largest, closed and inner basin of Turkey located in the Central Anatolia, were investigated to evaluate the origin of the sulfates and interpret the precipitation paleo-environment. The isotopic, mineralogical and chemical properties and composition of the samples collected from drillings were investigated. The delta S-34(CDT) and delta O-18(SMOW) values of the sulfate minerals range from 18.78 to 35.90 parts per thousand and 7.55-22.57 parts per thousand, respectively. Most of the delta S-34 values are similar to marine (Messinian) water. In addition, the Sr-87/Sr-86 ratios of the minerals vary from 0.707803 to 0.708209. Most of the delta S-34 and delta O-18 values and Sr-87/Sr-86 ratios of the investigated samples in the Tuz Golu basin are similar to those of the isotopic values of the Messinian marine sulfates indicating that the minerals were precipitated mostly from marine water. The S-34 and O-18 values of some samples are higher than those of the evaporites precipitated from the Miocene seawater and indicate that the evolution of the sufates in the basin was also controlled by the recycling-dissolution of the predeposited marine sulfates (Oligocene-Eocene), arid conditions and bacterial sulfate reduction (BSR) processes. Additionally, the low delta O-18 values and Sr-87/Sr-86 ratios with high Sr content of some samples other than those of the Miocene marine sulfates may have been caused by the influence of volcanic emanations, hydrothermal fluid, re-oxidation of sulfur and partially the contribution of the external or internal nonmarine water to the basin (e.g., meteoric water or groundwater/surface water). The Ca-sulfate minerals with high Sr contents and low Sr-87/Sr-86 ratios may have precipitated under the seawater-dominated hydrothermal fluid mixing conditions.