Discovery and genesis of helium-rich geothermal fluids along the India–Asia continental convergent margin

• Published in: Geochimica et cosmochimica acta Vol. 360; pp. 175 - 191
• Main Authors: Hao, Yinlei, Kuang, Xingxing, Feng, Yuqing, Wang, Yingchun, Zhou, Hui, Zheng, Chunmiao
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.11.2023
• Subjects: Gas geochemistry; Geothermal volatiles; Helium accumulation; India–Asia continental subduction zone; Isotopes; Gas geochemistry; Geothermal volatiles; India–Asia continental subduction zone; Helium accumulation; Isotopes
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2023.09.011

Large-scale helium accumulations are rarely reported in tectonically active continental subduction zones. This study shows geochemical variations in helium and other volatiles (N2, CO2, Ne and Ar) in hot springs from the India–Asia continental convergent margin (Indus–Yarlung suture, IYS) to the intracontinental extensional region in southern Tibet and identifies a 1200-km-long helium enrichment zone (He > 5000 ppm) within 50 km north of the IYS. In total, 144 gas samples were categorized into four groups: Group A, with dominant N2 species and He concentrations (0.07–2.31 vol%) 1.4–23 times higher than industrial-grade He resources (0.05–0.1 vol%); Group B, with dominant CO2 species and high He concentrations (0.05–1.57 vol%); and Groups C and D, depleted in He (<0.05 vol%) and dominated by CO2 and N2, respectively. The gas mixtures exhibit systematic variations from the extensional zone to the continental convergent margin, changing from deep-origin, He-depleted, CO2-dominated fluids (Group C) to relatively shallow crust-derived, He-rich, N2-dominated gases (Group A). The former exhibits small but detectable amounts of mantle-derived He (2.3 ± 2.6%) and CO2 (<3.2%), primarily atmosphere-derived N2 (δ15N = 0.0 ± 0.6‰) and Ar (40Ar/36Ar = 307 ± 25), and crustal CO2 (82 ± 9% from carbonate and 17 ± 9% from sediments/metasediments) from the subducted Indian slab-carbonate and Tibetan crust. The low He flux of Group C matches the steady release of He from the 70-km-thick crust into the geothermal waters during 0.02–43.5 ka. Shallow calcite precipitation and dissolution reactions cause CO2 loss and slight He enrichment in the extensional zone. Group A samples at the convergent margin show pure crustal He, more radiogenic 40Ar (40Ar/36Ar = 330 ± 67), and higher sedimentary contributions of 28 ± 23% for N2 and 37 ± 11% for CO2. A high He flux near the IYS requires an external source with high 4He/N2 (∼0.05) and 4He/20Ne (103–105) to provide >62% of the total He flux. This finding is best explained by He production in the south-dipping Gangdese granitic belt and deep crust, accumulation below the accretionary wedge, and episodic liberation along Great Counter thrust structures since 0.7 Ma by Tibetan Plateau uplifts and intense hydrothermal activity. The He-rich fluids mix with the sedimentary and atmosphere-derived N2 from the accretionary wedge, forming He-rich N2-type gases. This study systematically reports large-scale He enrichment along the continental convergent margin for the first time and demonstrates that complex crustal-scale structures in the convergent margin significantly controlling gas geochemistry. These results have profound implications for potential helium resource exploration and He-CO2-N2 recycling in continental subduction zones.