Dizhi lixue xuebao (Feb 2024)
Variation patterns of boron and lithium isotopes in salt lakes on the Qinghai–Tibetan Plateau and their application in evaluating resources in the Damxung Co salt lake
Abstract
Objective The Qinghai–Tibet Plateau is rich in salt lake resources, known particularly for the concentration of elements such as boron and lithium, forming many distinctive resource-type salt lakes. Compared with ordinary salt lakes, a notable characteristic of resource-type salt lakes is the abundant supply of elements such as boron and lithium. Consequently, these elements' sources and accumulation patterns are key scientific issues for understanding the genesis and mineralization patterns of resource-type salt lakes. Boron and lithium isotopes, characterized by significant mass differences and variations in natural isotope ratios, serve as effective tracers for studying the material sources of boron and lithium in salt lakes. However, the application of boron and lithium isotopes in salt lake systems faces the following three challenges: (1) There is insufficient understanding of how boron and lithium isotopes respond to the fundamental geochemical processes of salt lakes. The salt dissolution process that occurs when supply water flows into lake basins is the main reason for drastic changes in geochemical parameters. Inadequate recognition of salt dissolution processes can lead to an overinterpretation of boron and lithium isotope fractionation changes, weakening their tracking capabilities. (2) Isotope fractionation degree is conflated with changes in isotope composition. In salt lake research, discussions of the solid phase's influence on boron and brine's lithium isotopes are often based solely on fractionation factors between the solid and liquid phases, without considering the ratios of boron and lithium amounts involved in the fractionation process. (3) Discrepancies still exist in understanding the fractionation patterns of boron and lithium isotopes during salt crystallization. Methods In light of these problems, our study systematically reviews and analyzes the mechanisms of boron and lithium isotopic fractionation in salt lake systems and summarizes some essential understandings. Conclusion (1) Only salt crystallizations have specific impacts on B and Li isotopes in salt lakes. Since there is a genetic association between salt assemblages and specific salt lake hydrochemical types, the salt lakes with the same hydrochemical type exhibit consistent patterns of B and Li isotope changes during their evolutionary processes. Until halite precipitation, the B and Li isotopic compositions in sulfate- and chloride-type salt lakes are in accord with δ11B and δ7Li values of their sources instead of being controlled by their salt deposits. In contrast, the paths of B and Li isotopic changes of carbonate-type salt lakes are complex and are divided into two branches: calcite subtype and hydromagnesite subtype. After calcite precipitation, the δ11B value of the salt lake increases, and its δ7Li value is marginally above source characteristics (less than 2‰). After hydromagnesite precipitation, the δ11B value of the salt lake is also marginally above source characteristics (less than 2‰). After the stage of halite precipitation, the B and Li isotopic compositions of salt lakes in all types show an increasing trend. (2) Based on the evolutionary processes of B, Li, and K during seawater evaporation, the amounts of B, Li, and K in the current salt lake represent most of the corresponding resources in the lake if the salt lake never experienced complete dryness such as playa. For the salt-dissolving lake, most of the B, Li, and K resources are preserved in salt deposits and interstitial brine at the bottom of the lake. It is optimal for the resource potential of a carbonate-type salt lake in the salt-dissolving lake. (3) The B sources of the current Damxung Co salt lake located in the Tibetan Plateau are from clay carbonates exposed to the lake shore and highly soluble salts and interstitial brine at the bottom of the lake. The geothermal waters produced during early hydrothermal activity are the original B source of the Damxung Co salt lake. Based on mass balance equations, it is estimated that the B resource at the bottom of the Damxung Co salt lake is at least 9.1×106t (B2O3), and the lithium resource is at least 8.6 ×106t (LiCl).
Keywords
