Southern limit of mantle-derived geothermal helium emissions in Tibet: implications for lithospheric structure

The isotopic composition of helium emitted from geothermal springs in the s outhern Tibetan plateau, reported as R-c/R-A (R-c = air corrected sample He -3/He-4, R-A = air He-3/He-4), ranges from 0.013 to 0.38, and defines two p rincipal domains. In southernmost central Tibet, helium isotope ratios are typical of radiogenic helium production in the crust (R-c/R-A < 0.05, crust al helium domain). Further north, there is a resolvable He-3 anomaly consis tent with a mantle contribution (R/R-A > 0.1, mantle helium domain). The hi ghest values of 0.27-0.38 R-A occur at the southern end of the Karakoram fa ult. The boundary between the two domains lies 50-100 km north of the Indus -Zangpo suture zone. There seems to be no association between the He-3 anom aly and zones of active normal faulting and litho-tectonic crustal units, s uch as the ultramafic rocks of the Indus-Zangpo suture zone and the Gangdes e intrusive belt. Although scavenging of mantle-derived helium, stored in l arge ultrabasic and basic intrusions in the crust, cannot be ruled out enti rely, we argue that the He-3 anomaly most plausibly reflects degassing of v olatiles from young (Quaternary) mantle-derived melts intruded into the cru st. As such, it defines the southern limit of recent mantle melting and man tle melt extraction beneath the Tibetan plateau. The southern limit of the He-3 anomaly coincides with the junction between the Indian and Asian plate s, in the region where the Indian lithospheric slab steepens and is subduct ed beneath Tibet as suggested by seismic studies. Recent mantle melting and melt extraction is confined to the Asian mantle, but the southern limit of the melt zone may have migrated northwards during the last 10 Ma as the In dian lithosphere has progressively underthrust the Himalayas and Tibet. (C) 2000 Elsevier Science B.V. All rights reserved.