Lithium isotopes in the geothermal waters of the India–Asia continental convergent margin: Source and evolution

Lithium (Li) in geothermal waters along the India-Asia continental convergent margin is a potential Li resource and plays an important role in the Li budget and Li isotopic composition (δ⁷Li) of rivers and oceans. However, its origins and behavior remain unclear. Here, we systematically investigated the δ⁷Li, water (δ¹⁸O and δ²H) and helium (³He/⁴He) isotopes of 21 geothermal water samples as well as a series of shallow groundwater and river water samples from southern Tibet and the Himalayas. The δ⁷Li values of geothermal waters vary from +1.0‰ to +14.3‰ and are negatively correlated with the Li concentration (0.006-35.0 mg/L). For geothermal water with Li concentrations >5 mg/L, Li is sourced mainly from magmatic fluids exsolving from granitic magma chambers in the crust rather than the mantle, with contributions of 49.5% ±3.2% to 85.5% ±1.0%. The δ⁷Li values of these Li-rich geothermal waters are relatively homogeneous and comparable to those of bulk granitic rocks. They are mainly controlled by the Li isotopic compositions of granitic magmatic fluids (-2.6‰ to +5.6‰), and the dissolution of primary minerals and the precipitation of secondary minerals with minimal Li isotopic fractionation during high-temperature (174 ± 5℃ to 315 ± 6℃) water-granite interactions at deep geothermal reservoirs (4.4-7.9 km). For geothermal waters with Li concentrations <5 mg/L, Li originates primarily from water-granitic rock interactions (dominated by biotite dissolution) at 106 ± 3℃ to 207 ± 10℃, contributing approximately 85% ±16% of the total Li. An integrated dissolution-precipitation-mixing model suggests that high δ⁷Li values in Li-depleted samples result from preferential incorporation of ⁶Li into secondary minerals at lower reservoir temperatures within shallower reservoirs (2.7-5.2 km) and mixing of shallow groundwater during the ascent of geothermal waters. This study provides new insights into the fluid geochemistry of crustal granitic magma chambers and highlights that Li-rich geothermal waters in Tibet are controlled by the existence of crustal granitic magma chambers and the scale of faults.