Geochemical processes and origin of boron isotopes in geothermal water in the Yunnan-Tibet geothermal zone

Abstract

Boron concentrations and isotope compositions have been measured for 93 water samples from the hot springs and drill-holes in the geothermal system in the Yunnan-Tibet Geothermal Belt (YTGB), China. Boron concentrations range from 0.036-472.4 ppm, and the δ 11B values range from -16.0‰ to 13.1‰, indicating the non-marine origin for each geothermal system. We observed a clear binary mixing relationship between the B concentrations and B isotope compositions in Tibet geothermal area. This relationship can be well explained by two sources, i.e., marine carbonate rocks and magmatic rocks, for the Tibet geothermal water. No evidence supports a mantle contribution to B. In addition, we found that the precipitation only plays a dilution role for B of geothermal waters. δ 11B values for the precipitation across the southern Tibetan Plateau area range from -6.0‰ to -6.8‰ at least. Due to data scarcity in Yunnan geothermal area, we observed possible different boron sources from the Tibet geothermal system. Comparing it with other geothermal systems in the world, we found that the samples from YTGB have the lowest δ 11B values and the largest range of B concentration, which might be related to their special geological background. On the whole, the world geothermal δ 11B-Cl/B relation suggests a mixing process between marine and non-marine sources. Additionally, we suggest that B source of B-enriched geothermal waters is mainly from B-enriched crustal country-rocks, instead of mantle.