Subducting Indian Lithosphere Controls the Deep Carbon Emission in Lhasa Terrane, Southern Tibet

The India-Asia orogenic zone is considered to participate significantly in the Earth's geological carbon budget. However, the carbon cycling process at orogenic belts with ongoing continental subduction is poorly constrained. Here, new He-C-N systematics of hydrothermal gases in Lhasa terrane are reported to explore carbon cycling and the role of the subducting Indian slab. Except for samples where the CO2 content and C isotopic signature were modified by calcite precipitation, gas samples in Lhasa terrane display low He-3/He-4 ratios of 0.015-0.895 R-A but with increasing mantle-helium input from south to north. Our modeling results show that the continuous subduction of the Indian continental slab has caused high Th-U contents and time-integrated He-4 production rates in the present mantle wedge beneath the Lhasa terrane. The N-S helium isotopic trend can be interpreted as a northward-decreasing mantle enrichment. Compared to volatiles showing crustal helium but mantle-like delta C-13 ratios, mantle-helium-richer fluids display heavier delta C-13 ratios, higher CO2/He-3 and molar C/N values, indicating a substantial carbonate input. We propose that the heavier carbon isotopes are inherited from the carbonated-enriched mantle (CEM) rather than the carbonate assimilation of the overlying continental crust. Combined with an eastward increase in dip angle of the Indian lithosphere, we suggest that the slab rollback has caused asthenosphere heat flow and consequent decarbonation in CEM beneath northern and eastern Lhasa terrane, leading to enhanced isotopically heavier carbon release. Our study highlights the controlling role of subducting Indian lithosphere on the regional distribution of volatiles in the Lhasa terrane.