Hydrated sulfate minerals (bloedite and polyhalite): formation and paleoenvironmental implications

Abstract

Salt minerals that are used to reconstruct paleoenvironments should be either primary or samples that provide primary-level information. Using hydrated sulfate minerals that are commonly found in saline lacustrine sediments (i.e., bloedite and polyhalite), the paleoenvironment of the Qaidam Basin, in the northeastern Tibetan Plateau was reconstructed. In this study, we determined the primary and secondary mineral formations based on their S, Mg, H, and O isotopic compositions. While polyhalite is a secondary mineral, bloedite precipitated out from the brine at 0.39 Ma, and ultimately became a secondary mineral at 0.36 Ma. The bloedite and polyhalite Mg isotopes did not record primary signals, but they still provide valuable insights into the paleoenvironments in which they formed. The climate in our study area is very dry; based on the temperature of the brine, this region experienced high temperatures at 0.39 Ma, 0.36 Ma, and 0.12 Ma. We identified one major chemical inconsistency: the bloedite 18O-hydrated water and 18O-SO4 values had basically achieved equilibrium, while the polyhalite and gypsum exhibited no oxygen exchange between their SO4 and hydrated water components. The possible reason for the inconsistency was the differences in mineral crystal structures. We hope that future studies will reconcile this conflicting information.