Boron geochemistry from some typical Tibetan hydrothermal systems: Origin and isotopic fractionation |
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| Institution: | 1. Geochemistry & Hydrogeology, Department of Geosciences, University of Bremen, Germany;2. Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan, ROC;1. State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, PR China;2. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, PR China;3. School of Earth Sciences and Engineering, Hohai University, Nanjing 210098, PR China;4. Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, PR China;1. Department of Earth Sciences, National Cheng-Kung University, No. 1 University Rd., Tainan 701, Taiwan;2. Institute of Oceanography, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan;3. School of Geography, Earth and Environmental Sciences, University of Birmingham, B15 2TT, UK;4. Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, Athens, GR 157 84, Greece |
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| Abstract: | The Tibetan plateau is characterized by intense hydrothermal activity and abnormal enrichment of trace elements in geothermal waters. Hydrochemistry and B isotope samples from geothermal waters in Tibet were systematically measured to describe the fractionation mechanisms and provide constraints on potential B reservoirs. B concentrations range from 0.35 to 171.90 mg/L, and isotopic values vary between ?16.57 ‰ and +0.52 ‰. Geothermal fields along the Indus-Yarlung Zangbo suture zone and N–S rifts are observed with high B concentrations and temperatures. The similar hydrochemical compositions of high-B geothermal waters with magmatic fluid and consistent modeling of B isotopic compositions with present δ11B values imply that the B in high-B geothermal waters is mainly contributed by magmatic sources, probably through magma degassing. In contrast, geothermal fields in other regions of the Lhasa block have relatively low B concentrations and temperatures. After considering the small fractionation factor and representative indicators of Na/Ca, Cl/HCO3, Na + K and Si, the conformity between modeling results and the isotopic compositions of host rocks suggests that the B in low-temperature geothermal fields is mainly sourced from host rocks. According to simulated results, the B in some shallow geothermal waters not only originated from mixing of cold groundwater with deep thermal waters, but it was also contributed by equilibration with marine sedimentary rocks with an estimated proportion of 10%. It was anticipated that this study would provide useful insight into the sources and fractionation of B as well as further understanding of the relationships between B-rich salt lakes and geothermal activities in the Tibetan plateau. |
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| Keywords: | The Tibetan plateau Geothermal fields Boron isotopes Isotopic fractionation Boron source |
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