Implications Of Halogen Isotopes For Tracking Origin Of Rare Alkaline Metal Resources In Geothermal Springs In Southern Tibet

Halogens are major anions in geological fluids and natural waters,and their isotopes(e.g.?³⁷Cl,?⁸¹Br)together with other geochemical tracers are indicative of fluid sources,water-rock interactions,and magmatic and hydrothermal processes.In an attempt to identify the origin and evolution processes of abundant rare alkali metal?RAM?resources?e.g.,Li,Rb,Cs?in geothermal springs in typical tectonic belts?Tibet Plateau,China?,three geothermal springs?Dagejia,Kawu,Semi?with extraordinarily high bromine abundances in Brahmaputra River-Bangong Lake suture zone are selected as the study areas.A systematic investigation using multiple geochemical indices is carried out in this work.A new ion-exchange procedure has been established for effective separation/purification of chlorine and bromine from complex geological matrices.The challenging problems of chlorine and bromine loss in pretreatment procedure,the matrix effect from soluble silicon and major anions,and the isobaric interferences have been well solved.The high-precision protocols for chlorine and bromine isotope analysis are established.The applications of multiple isotope geochemistry(?¹¹B-?³⁷Cl-?⁸¹Br),in conjunction with other geochemical indices,indicated that the chemical compositions of geothermal spring in the study areas are contributed by following sources:magma chamber,surrounding granite and evaporite-sedimentary rocks.The high Na/Cl ratio?3.41?3.53?and positive?⁸¹Br value?+1.34‰?5.17‰?indicate that the geothermal water components in Dagejia area are mainly controlled by strong water-rock interaction and evaporite dissolution.The negative?⁸¹Br values?-2.9‰?-2.6‰?,positive?³⁷Cl value?+4.6‰?+5.7‰?and Na/Cl ratio close to 1 in Kawu hot spring suggest that evaporite and magama degassing are the main contributions to geothermal spring.The geothermal water in Semi area is characterized by high salinity,positive?⁸¹Br value?-1‰?+5.4‰?and positive?³⁷Cl value?+7.3‰?+8.5‰?,indicating the contribution sources of Semi geothermal spring are evaporite,leaching of bedrocks and magama degassing.All consistent evidence suggests that the abundant RAM resources in Qinghai-Tibet Plateau are accumulated from evaporite dissolution and sedimentary rock leaching,water-rock interaction and magma degassing,and the contribution fraction of each source to RAMs in each geothermal spring are different.Based on the coupling of multiple isotope indices and hydrochemical indices,especially?¹¹B-?⁸¹Br,the contribution sources of chemical components of geothermal water are clearly defined and the accumulation of RAM resources in geothermal water in each study area are fully discussed.The feasibility of using halogen isotope to directly trace the sources of RAM resources is explored in this research,which will benefit to further exploration of RAM resources in Qinghai-Tibet Plateau.