Chemical and boron isotope compositions of non-marine brines from the Qaidam Basin, Qinghai, China

The general chemistry and boron isotope composition were investigated in fresh waters and hypersaline brines from the Qaidam Basin, northern Qinghai-Xizang (Tibet) Plateau, China. This basin is a large, tectonically active system, isolated from the ocean and composed of thick clastic and evaporite sediments. The modern playas are subject to intense evaporation and are characterized by hypersaline brines as well as potash and borate evaporites. The chemical composition of the dissolved solutes in the modern brines and waters reveals three main sources: (1) inflow of hot springs enriched in sodium, sulphate and boron. Evaporation of these waters leads to a high Na/Cl ratio (>1), a NaClSO 4 brine and an evaporite mineral assemblage of halite-mirabilite-borate (Lakes Daqaidam and Xiaoqaidam); (2) inflow surface river waters which are modified by preferential dissolution of halite and potassium and magnesium salts characterized by a Na(Mg)Cl solute type with low Na/Cl (