Decoding cycling and enrichment of boron and associated lithium in waters of the Tibetan Plateau with boron isotopes

Abstract

The Tibetan Plateau (TP), which has various water bodies exhibiting extreme enrichment of boron (B), lithium (Li) and other elements, is a unique and cost-effective supplier of B and Li globally. However, the sources and enrichment mechanisms of these elements remain disputed. Herein, big data on B isotopes and geochemistry were compiled to reveal cycling and enrichment of B and associated Li in the complex mountain-basin systems of the TP. Various water bodies on the TP show remarkably high B and Li concentrations compared with the global average levels. B and Li are supernormally enriched or even mineralized in some salt lakes, oilfield brines, and geothermal systems. Water with high B concentrations exhibits simultaneous ¹¹B depletion. The spatial patterns of B-Li concentrations and δ¹¹B values are characterized by a westward increase and a southward decrease, respectively. The mechanisms of B and Li cycling and enrichment, from source to sink, on the TP can be summarized as follows: 1) Slab subduction and following continental collisions induced partial melting of the mid-lower crust, resulting in late-stage highly differentiated crust-derived magma upwelling and invasion into the mid-upper crust. Hot springs thereby maintain an efficient release of abnormally abundant B and Li from magmatic fluids to the surface through deep geothermal circulation. 2) In the eastern and southeastern TP, Massive degassing of mantle fractions prevails along large strike-slip faults. B and Li enriched from mantle- or crust-derived fluids can reach the surface via local volcanism. 3) Water-rock interaction and weathering trigger further activation, mobilization, and enrichment of B and Li in water. The aforementioned geological processes supply constantly deep-source B and Li to rivers and groundwater as cold-hot springs. Eventually, B and Li are fed into the lake basin jointly with supergene processes; after prolonged evaporation-concentration, excellent B-Li-rich salt lake resources are formed.