Noble gases constraints on the origin and evolution of geothermal fluids from the Yangbajain geothermal field, Tibet.
西藏羊八井热田地热流体成因及演化的惰性气体制约

The relative abundances and isotopic composition of noble gases in selected wells were determined to identify heat source, the relationship between shallow and deep thermal fluids, and fluid migration. The thermal fluids contain large 4He excesses with 3He/ 4He ratios of 0.087~0.259 times the ratio in air. Heat source in the region is inferred to be partial melting of the crust at depth. The dense cold groundwater pulled by gravity moves down through the pediment fault of the Nyainquentanglha Mts. Along the flow path, the water absorbs heat from wallrock and gradually loses its density. Differential densities between the thermal and cold waters drive the thermal water to shallow depths. The deep thermal fluid found only in the northern part of the field has the highest 4He enrichment F( 4He)~7500] and the highest 3He/ 4He ratio ( 0.259Ra ).When the thermal water approaches the surface, the pressure in the fracture zone decreases so that the thermal water can boil CO 2 and H 2S rich steam escaping from the boiling water along its ascent channel. Production wells ZK354 and ZK357, located in the upwelling zone, have slightly higher 3He/ 4He ratios and F( 4He) values than the others nearby. When the upflow of the deep thermal water is blocked, the pressure drives it laterally toward the southeast where it enters the shallow reservoir after mixing with the cold groundwater. The shallow reservoir consists of Quaternary alluvium and altered granite, opal and calcite precipitate from the thermal water to form a self sealed alluvium cap. As the shallow thermal fluid migrates towards the southeast, helium will escape from the reservoir through a thin and loose alluvium cap while at the same time radiogenic helium produced in the underlying granite, will be added to the migrating fluid. These cause the 3He/ 4He ratios and F( 4He) values decrease from northwest to southeast. The relationship between F( 84 Kr) and F( 132 Xe) values implies that krypton and xenon components are of a meteoric origin for both the shallow and deep thermal fluids.