Geothermal gas geochemistry in Southeast Tibetan Plateau margin influenced by magma chambers and fractures
Journal
Journal of Volcanology and Geothermal Research
Journal Volume
462
Start Page
108323
ISSN
0377-0273
Date Issued
2025-06
Author(s)
Wu, Jia-Yi
Cao, Hai Gang
Zhao, Peng Cheng
Guo, Yu Ping
Chen, Xue-Gang
Li, Xiaohu
Tsai, Jui-Fen
Abstract
Hydrothermal gases play a pivotal role in elucidating the cycling of deep materials and subsurface processes. Here, we analyzed the chemical and isotopic compositions of 40 gas samples and compiled geochemical data on >280 samples discharged from the Tengchong hydrothermal field, located on the Southeast Tibetan Plateau margin. Helium isotopes span from 0.13 Ra (the 3He/4He ratio of air) to 5.91 Ra, indicating a mixing between mantle-derived helium and crustal components. Spatial distributions of helium isotopes are closely associated with the locations of magma chambers beneath the Tengchong field. N2/Ar ratios and nitrogen isotopic compositions suggest that N2 and Ar were primarily contributed by atmospheric sources and groundwater, with minor N2 contributions from mantle-derived materials. CO2 was mostly originated from the thermal decomposition of limestone and magma degassing, with a few samples affected by secondary processes like carbonate precipitation. The chemical and carbon‑hydrogen isotopic compositions of alkanes indicate that methane was mainly sourced from thermogenic processes. The central magma chamber and its adjacent faults are characterized by the most intensive magmatic activity and the closest connection to deep materials. A gas migration and diffusion model using a crustal 3He endmember of 6 × 10−9–9 × 10−8 (volume ratio of 3He to total gases) well explains the decreasing 3He concentrations with the distance from magma chambers. Hence, the highly penetrable helium in quiescent regions like Tengchong could migrate through overlying rocks without the need of gas channeling along faults. Similarly, our model can also explain the relationships between Ar concentrations and the distance from springs to faults and magma chambers. In contrast, the geochemical characteristics of other gas components (e.g., CO2, N2) are closely associated with the location of major faults. This study offers valuable insights into the spatial relationships between gas geochemistry and subsurface magmatism and fault distribution, highlighting the influence of hydrothermal processes on diverse patterns of gas migration and source in hydrothermal fields.
Subjects
Hydrothermal gas
Magma chamber
Noble gas
Spatial analysis
Tibetan Plateau
SDGs
Publisher
Elsevier BV
Type
journal article