Maoliang Zhang^1*, Wei Liu², Haoying Zhang¹, Yi Liu¹, Sheng Xu¹

(¹Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin 300072, China; ²College of Resources and Environmental Engineering, Inner Mongolia University of Technology, Hohhot 010051, China)

Abstract: Degassing of deeply-derived fluids (e.g., He and CO₂) is widely observed in seismically active regions associated with volcanic unrests and tectonic movements, providing potential insights into the seismogenic processes from a perspective of deep fluids. Here, we present geochemical studies on He-CO₂ degassing from the Tengchong volcanic field (TVF) and the Xianshuihe fault (XSF) in the southeastern Tibetan Plateau, aiming to reveal the role of deep fluids in triggering regional seismicity. Higher fluxes of mantle He [(1.08 ± 0.84) × 10¹⁰ atoms m^–2 s^–1] and CO₂ [(1.29 ± 1.00) × 10⁴ mol km^–2 yr^–1] are observed near active volcanoes in the TVF, which are ~4 times the background values [mantle He and CO₂flux = (0.26 ± 0.16) × 10¹⁰atoms m^–2s^–1and (0.32 ± 0.19) × 10⁴mol km^–2yr^–1]. Spatially, the high He-CO₂fluxes of magmatic origins correspond well with several earthquake swarms and deep low-frequency earthquakes that are likely driven by magmatic fluids [1]. On the other hand,vigorous release of deeply-derived CO₂-rich fluids (³He/⁴He = 0.94−2.73 Ra; δ¹³C_CO2 = −8.9 to −2.6‰) dominates bend section of the XSF, where localized mantle melting and metamorphic decarbonization may have produced the CO₂-rich fluids and fed the hot spot of high CO₂ flux (9.66 × 10⁹ mol/yr) along the XSF. The high CO₂ flux hot spot could explain the high risk of earthquake hazard in the XSF bend section, such as the 2022 Ms 6.8 Luding earthquake [2]. Taking the results of fluid origin and transport, CO₂ fluxes, and regional seismicity together, we suggest that deep CO₂-rich fluids may have played a crucial role in generating overpressure conditions involved in seismogenic processes of the TVF and XSF. The spatial relationships between hydrothermal He-CO₂ degassing and regional seismicity could present a snapshot for how deep fluids of various origins may contribute to seismic dynamics and thus should be considered in earthquake prediction based on gas/fluid geochemistry.

Keywords: Helium isotopes; Outgassing fluxes; Earthquakes; Active volcanoes; Active faults

*Author Profile: Maoliang Zhang, Male, Professor, PhD, mainly engaged in gas geochemistry research. E-mail: mzhang@tju.edu.cn (* presenting author)