Boron isotope geochemistry of Zigetang Co saline lake sediments, Tibetan Plateau

Abstract

The origin of boron in boron-rich salt lakes in the Tibetan Plateau is highly controversial. In this study, we carried out a detailed study on boron geochemistry and isotope composition of lake sediments collected in Zigetang Co, central Tibet. Evaporites had high boron concentrations of 172.3–418.6 μg/g and δ11B values of −8.2‰ to −3.3‰, suggesting a non-marine origin for the saline lake. The boron isotopic fractionation factor, α, between evaporite and brackish water (αevaporite–brackish) decreased systematically with depth, from 0.9942 at the top of the drill core to 0.9893 at the bottom; the linear variation between αevaporite–brackish and depth reflects boron isotopic fractionation associated with progressive crystallization. The positive correlation between δ11B versus [B] and δ11B versus depth in the evaporite phase reflects pH and boron speciation in the solution control on the adsorption of boron, and B(OH)3 species incorporated preferentially into Mg(OH)2 precipitation at high pH.