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 (α_{evaporite–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 α_{evaporite–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.