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

Abstract

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 (<1), Br Cl, Li Cl and B Cl ratios; and (3) Ca-chloridic subsurface brines which are controlled by both salt dissolution and dolomitization processes. Evaporation and salt crystallization of mixtures of the latter two types leads to a “marine-like” brine (e.g., NaMgCl type, Na Cl ratio ⪡ 1) and mineral assemblages similar to that predicted for progressive evaporation of seawater (e.g., Qarhan playa: halite-sylvite-carnallite-bischofite). The δ 11 B-values of the input waters to the Qaidam Basin (range of −0.7 to + 10.9%. vs. NBS- 951 and brines from salt lakes (+0.5 to + 15.0%.) are similar to those of associated granitic rocks ( δ 11B= −2.3 to +3.7%.; n=3) and hence indicate the non-marine origin of these fluids. The highest δ 11B-values are associated with fluids with low B Li ratios, indicating selective removal of elemental boron and 10B by adsorption onto clay minerals. The magnitude of 11B enrichment due to adsorption is ∼15–20%., and thus non-marine brines are well distinguished from marine-derived brines ( δ 11B of +39 to +59%.) preserving the large isotopic difference (∼40%.) of their source waters. It is proposed to use this distinctive isotopic signature for tracing the origin of ancient evaporite environments.