Changzheng Li1, 2, Guodong Zheng1, Yanxin Wang1, 2, *, Daniele L. Pinti3
1 School of Environmental Studies, China University of Geosciences, Wuhan 430074, China.
2 State Key Laboratory of Deep Geothermal Resources, China University of Geosciences, Wuhan 430074, China.
3 GEOTOP & Département des sciences de la Terre et de l’atmosphère, Université du Québec à Montréal, H3X 3Y7 QC, Canada.
Finding signatures to identifypotential magmatic heat/volatile sources of geothermal systems is fundamental for the development and exploitation of high-enthalpy geothermal resources. Noble gases, particularly the helium isotopic ratio 3He/4He, are effective for discerning mantle-derived heat sources in geothermal systems. However, conventional noble gas tracing methods relying solely on 3He/4He ratios struggle to identify magmatic signatures in high-enthalpy geothermal systems with low 3He/4He ratios. Such systems may be dominated by crustal magmatism or modified by magma aging, as exemplified by our work on the volcanically quiescent Yadong-Gulu Rift (YGR), one of the world’s largest high-temperature geothermal fields on the Tibetan Plateau.
In this study, we first systematically investigated the hydrochemistry and noble gas geochemistry of geothermal fluids from Gulu, and expanded our work to the full extent of the YGR, and then compared our results with those of other major geothermal areas such as Yellowstone (US) and Rungwe (Tanzania). For bubbling gas samples exsolved from hot springs, we reconstructed the composition of the undegassed parent geothermal fluid prior to surficial hydrothermal degassing using the20Ne/36Ar ratios. The radiogenic 4He/40Ar* ratios of the parent geothermal fluid at Gulu range from 0.49 to 9.42 and exhibit significant magmatic degassing fractionation from crustal and mantle values, providing robust evidence for the existence of actively degassing magma beneath YGR. In addition, the 4He/40Ar* and Cs/Rb mass ratios in the geothermal systems show a decreasing trend from south to north along YGR, which is attributed to a progressive south-to-north transition in heat sources from crustal-derived partial melts to mantle-derived partial melts. Our findings revealed an enriched mantle endmember beneath the Tibetan Plateau, where magma aging leading to underestimated mantle volatile contributions by conventional approaches. These results challenge the longstanding paradigm that crustal magmatism dominates the heat/volatile supply to high-temperature geothermal reservoirs across the Tibetan Plateau and provide a novel theoretical and methodological framework for heat/volatile-source identification in geothermal systems with low 3He/4He ratios.
Keywords: geothermal resources; magmatic heat sources; noble gases; hydrothermal degassing; Yadong-Gulu Rift; Tibetan Plateau
Author Profile
Changzheng Li, Male, Doctor student, mainly engaged in geothermal and noble gas research. E-mail address: lczcugofficial@163.com
