Calcium isotopic geochemistry of geothermal systems in the tectonically active southeastern Tibetan Plateau

• Published in: The Science of the total environment 2023-04, Vol.867, p.161534-161534, Article 161534
• Main Authors: Chen, Bei-Bei, Li, Si-Liang, Pogge von Strandmann, Philip A.E., Zhong, Jun, Ma, Ting-Ting, Sun, Jian, He, Wen-Yan, Liu, Cong-Qiang
• Format: Article
• Language: English
• Subjects: Calcium cycling; Calcium isotopes; Geothermal system; Groundwater; Tibetan Plateau; Water-rock interaction
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2023.161534

The global Calcium (Ca) cycle is closely coupled to the carbon cycle, and Ca isotopes have potential in tracing it. Even though groundwater is one of the main reservoirs of Ca at the Earth's surface, few data are available for groundwater, and the behavior of Ca and its isotopes in geothermal systems remains unknown. Here we analysed the stable Ca and radiogenic Sr isotope compositions of thermal waters distributed along the Jinsha and Yalong river valleys in the southeastern Tibetan Plateau. The Ca isotopic composition of the thermal water ranges from 0.45 to 2.16 ‰ (δ44/40Ca values relative to SRM 915a). The thermal waters collected from carbonate aquifers have higher δ44/40Ca values than bedrocks, which was attributed to secondary carbonate precipitation accompanied by CO2 degassing. In contrast, δ44/40Ca values in thermal waters collected from clastic and igneous rocks are similar to bedrock. Despite some thermal waters undergoing secondary silicates formation and CaNa ion exchange, such processes maybe not play a significant role in governing the Ca isotopic composition of these thermal waters. This suggests that Ca isotopes can be used to trace secondary carbonate precipitation driven by CO2 degassing (e.g. travertine) in geothermal systems located in tectonically active areas. [Display omitted] •The δ44/40Ca values of thermal waters in the southeastern Tibetan Plateau varies greatly.•Secondary carbonate precipitation governs the δ44/40Ca values of thermal waters collected from carbonate aquifers.•The δ44/40Ca values of thermal waters collected from igneous and clastic rocks are similar to those of bedrock.•Calcium isotopes can be used to trace secondary carbonate precipitation driven by CO2 degassing in geothermal systems.