Helium and carbon isotopic compositions of hot spring gases in the Tibetan Plateau

Abstract

The Tibetan Plateau is one of the most active geothermal areas in the Himalayan Geothermal Belt. However, the heat source of the geothermal manifestation is unknown because of a lack of Quaternary volcanoes in this area, except in western Yunnan and northern Tibet. In order to unravel the heat source and the origin of volatile components in hot spring gases in the southern Tibetan plateau, we have measured helium and carbon isotope ratios of hot spring gases from Yangbajain district and some northern sites. The observed helium compositions in the hot spring gases are explained by the mixing of an air component and underground components whose mean end-member 3 He/ 4 He ratio is 0.12 R atm for Yangbajain and 0.22 R atm for the northern sites, respectively. Despite the lack of Quaternary volcanism, such 3 He/ 4 He ratios are an order of magnitude higher than that of stable continental crust (0.02 R atm). These results may imply the existence of intrusive magmas that contain mantle helium in the thick Tibetan crust. The mantle helium with high 3 He/ 4 He ratio may have been diluted to the observed values by radiogenic helium produced in the crust. The observed high CO 2/ 3 He ratios of the samples (10 10–10 12) relative to that of MORB (2×10 9) indicate that most carbon dioxide in the samples is derived from crustal materials, such as organic matter and marine carbonate minerals. Regional differences are observed not only in the mean end-member 3 He/ 4 He ratios of the underground components, but also in the δ 13C(CO 2) values of the hot spring gases between Yangbajain (−9.1 to −6.3‰) and the northern sites (−4.3 to −1.0‰). These features suggest heterogeneity among basement rocks. The lower 3 He/ 4 He ratio in Yangbajain may be due to a larger addition of radiogenic 4 He from basement rocks that are enriched with uranium and thorium. The higher δ 13 C (CO 2) value of the hot spring gases from the northern sites may be due to the addition of carbon dioxide from underlying sedimentary rocks containing more marine carbonate minerals.