Yunkai Lei¹, Shuai Wang², Xuelian Huang², Xiaocheng Zhou^3,4

(1 School of Environmental Studies, China University of Geosciences, Wuhan 430078, China; 2 State Key Laboratory of Deep Geothermal Resources, School of Sustainable Energy, China University of Geosciences, Wuhan 430078, China; 3 Institute of Earthquake Forecasting, China Earthquake Administration, Beijing 100036, China; 4 School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China)

Abstract: The Cathaysia Block lacks Quaternary surface volcanism but hosts widespread low- to medium-temperature geothermal systems and records mantle-derived volatile inputs. This study integrates regional data on He isotope compositions and fluxes in fluids, geothermal-water hydrochemistry, and carbon isotopes to investigate deep volatile release, fault-controlled transport, and subsequent modification within shallow geothermal systems. Air-corrected ³He/⁴He ratios range from 0.10 to 6.41 Ra, and mantle-derived He fluxes reach 0.11–33.41 × 10¹⁰ atoms m⁻² s⁻¹. Mantle-derived He release in inland areas is mainly associated with seismicity-enhanced fault permeability, whereas the higher He fluxes in coastal areas, together with regional thermal anomalies and geophysical evidence, indicate partial melting of the deep mantle and associated degassing. At comparable mean circulation depths, the higher reservoir temperatures and heat-flow values of active-fault-controlled geothermal systems relative to stable-fault-controlled geothermal systems indicate better deep connectivity of active faults, which enhances the upward transfer of mantle-derived heat and volatiles into shallow geothermal systems. Stronger heat input increases reservoir temperatures and enhances the potential for mineral self-sealing. CO₂/³He and δ¹³C–CO₂ compositions further record the widespread modification of deep-sourced CO₂ through carbonate mineral precipitation. Mineral precipitation can progressively fill fractures and reduce effective permeability, whereas recurrent fault activity can disrupt mineral infill and restore fluid pathways. Therefore, gas geochemistry provides an effective bridge between regional-scale partial mantle melting and the evolution of crustal geothermal systems.

Keywords: Helium isotopes; Mantle melting; Mantle-derived volatiles; Cathaysia Block; Active fault

Author Profile (first author): Yunkai Lei, Male, Graduate Student, M.Eng, mainly engaged in geothermal geochemistry, noble gas isotope tracing, and deep fluid migration research. E-mail: LYKcug@163.com