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Hydrogen and Oxygen Isotopic Studies of Thermal Waters in Xizang

Zheng Shuhui, Zhang Zhifei, Ni Baolin, Hou Fagao, Shen Minzi Department of Geology, Department of Chemistry  
Deuterium and oxygen-18 measurements have been made of thermal waters from 34 geothermal areas in Xizang. According to δD value of thermal water, the region is divided into four parts. The first district is located in the south of the High Himalayas and in the south-east end of Xizang. δD is approximately-100. The other districts are located within Xizang plateau. The δD value increases from-160 (south) to-80 (north), while it must decrease under the effects of altitude and latitude. This can be accounted for the evaporation effect due to the arid climate. The ratio of annual evaporation to annual precipitation in the north of Xizang is larger than 15.0. It causes the concentration of heavy isotope in surface water.Deuterium and oxygen-18 values and the chemical composition of thermal waters from drillholes and natural springs in the Yangbajain geothermal field have been investigated. The thermal waters are moderately enriched in the heavy isotope along a line,whose slope significantly deviates from local meteoric values. The δ18O values of the waters in hot springs are roughly proportional to their Cl content. These facts suggest that the waters are predominantly meteoric in origin but contain a small amount of NaCl rich deep fluid of a high heavy isotopic ratio differing from the meteoric water. The isotopic compositions of recharging ground water are obtained by extrapolation of the mixing lines. It seems that the recharging water is not the surface water from the vicinity of Yangbajain but might be a ice-snow water with lower values of δD and δ18O coming from distant placesThe isotopic and chemical investigations of thermal water in Gobdu geothermal field are similar to the Yangbajain geothermal field.
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