Subducting Indian Lithosphere Controls the Deep Carbon Emission in Lhasa Terrane, Southern Tibet. Issue 7 (28th June 2022)

• Record Type: Journal Article
• Authors: Zhao, Wenbin; Guo, Zhengfu; Zheng, Guodong; Pinti, Daniele L.; Zhang, Maoliang; Li, Jujing; Ma, Lin
• Is Part Of: Journal of geophysical research. Volume 127:Issue 7(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-28
• Subjects: continental orogenic zone -- He‐C‐N systematics -- carbonated‐enriched mantle -- Indian lithosphere -- Lhasa terrane; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2022JB024250
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: 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 3 He/ 4 He 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 4 He 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 δ 13 C ratios, mantle‐helium‐richer fluids display heavier δ 13 C ratios, higher CO2 / 3 He 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. Plain Language Summary: We report new He‐C‐N data from geothermal areas of Lhasa terrane, the southernmost block of the Asian continent, to explore deep carbon cycling at orogenic belts with ongoing continental subduction. Depending on their 3 He/ 4 He ratios, two groups of volatiles were identified: those dominated by crustal sources ( 3 He/ 4 He < 0.05 R A ) and lighter C isotopes and those with 3 He/ 4 He ratios >0.1 R A and heavier C isotopes. We propose that the continuous subduction of the Indian continental slab caused high time‐integrated 4 He production rates in the mantle source, which can generally explain the extremely low‐ R A volatiles observed in Lhasa terrane and a northward increase of mantle helium. He‐C systematics suggest that heavier carbon isotopic signatures of mantle‐helium‐rich fluids are inherited from a carbonated‐enriched mantle rather than the carbonate assimilation from the overlying continental crust. Combined with geophysical evidence of an eastward increase in dip angle of the Indian slab beneath Lhasa terrane, we stress that increased mantle heat flow and related decarbonation induced by slab rollback can account for the boundary between crustal and mantle‐helium‐rich fluids. Our study highlights how deep processes involving subducting Indian slab play a crucial role in distributing volatiles in Lhasa terrane, southern Tibet. Key Points: The subducting Indian lithosphere plays a crucial role in the regional distribution of volatiles in Lhasa terrane, southern Tibet Helium isotopes in Lhasa terrane can be explained by high time‐integrated 4 He production rates in the mantle wedge Heavier carbon isotopes in fluids are likely inherited from the carbonated‐enriched mantle rather than assimilation of continental crust