Boron occurrence in halite and boron isotope geochemistry of halite in the Qarhan Salt Lake, western China

Previous studies have investigated boron (B) isotope composition of salt lake brines in the Qaidam Basin, western China. However, the research on B isotope geochemistry of halite from a sediment core in a typical sedimentary basin has been very limited. In this study, a 102-m-long drill core (ISL1A) was recovered from Qarhan Salt Lake in eastern Qaidam Basin. Forty-three halite samples from upper 44.0m in ISL1A were collected and analyzed for chemical compositions (K+, Na+, Ca2+, Mg2+, Cl−, SO42−, Sr2+ and B3+) and B isotopes in order to investigate their relationships, and B isotope fractionation between halite and brines in the Qarhan Salt Lake. The results show that: (1) more B3+ and Mg2+ values in halite are low concentrations and more concentrated, and low B3+ concentrations have a strong correlation with low Mg2+ in halite, which imply that they might be the same source; (2) low Mg2+ values in halite from ISL1A have a similar trend with mMg2+ (molarity) of fluid inclusion brines from two sediment cores (east to ISL1A about 50km) in the study area, suggesting that low B3+ and Mg2+ concentrations should derive from fluid inclusions in halite based on X-ray diffraction results in ISL1A that no borate and small amount of magnesium salts were deposited; (3) δ11B values of halite in ISL1A range from −0.35 to +5.84‰, which are in the range of those of river water and brines (−1.0 to +9.6‰), and almost overlapped with those of brines (+2.9 to +7.5‰) in the Qarhan Salt Lake. These results suggest that no or minor B isotope fractionation between halite and brine occurred.