Gas geochemistry of hot springs at the Tengchong field, Southwest China: Controlled by the spatial distribution of magmatic chamber
Journal
Journal of Volcanology and Geothermal Research
Journal Volume
402
Pages
-
Date Issued
2020
Author(s)
Abstract
The Tengchong volcanic field, located at the collision zone between the Indian and Eurasian plates, is well known for its large-scale Holocene volcanic eruptions and extensive hydrothermal activities. Bubbling gases from eight geothermal springs at the Tengchong field were collected and analyzed for their chemical and isotopic compositions (3He/4He, 20Ne, N2, Ar, δ13CCH4 and δ13CCO2). The results show that the gas samples could be categorized into a group I dominated by N2 and a group II with CO2 as major component. The air-corrected 3He/4He ratios ranged from 0.17 Ra to 4.76 Ra and the calculated mantle contributions dramatically varied from 1.9% to 77.9% for group II. By contrast, an invariant air-corrected 3He/4He ratio of about 0.30 Ra and a relatively constant mantle contribution of 4.0% to 5.0% was obtained for group I. The sampling sites just on top of the magma chamber received significant mantle-derived helium and magmatic gas components (at the proximal site of the magma chamber: >95% magmatic gas contributions). The calculated N2/Ar ratio of the hydrothermal endmember for group II (sample sites close to the magma chamber) was 61.7–66.2, suggesting that the reaction zone temperature is between 223 °C and 252 °C. The influence of the magma chamber decreases with increasing distance of the sampling sites from the chamber apex. The sampling sites (group I) at the marginal area discharge gases with relatively low 3He/4He ratios and magmatic contributions (19–24%). The temperature of the reaction zone for this group was estimated to be 74 °C based on the N2/Ar ratio of 42. This study provided important constraints to relate hydrothermal gas geochemistry directly with the spatial distribution of the underlying magma chamber. Plain language summary: Geochemical characteristics of gases discharged from hydrothermal systems are usually affected by deep magmatic processes, shallow hot fluids circulation, and water-rock interactions in the reaction zone. In this study, we analyzed the gas abundances and isotopic compositions of hydrothermal gas discharged from the Yunnan-Tibet geothermal belt. We used a gas mixing model to quantify the contributions of mantle-derived/magmatic endmember and hydrothermal endmember. The 3He/4He ratios, magmatic contribution, and temperature of the reaction zone decrease significantly with increasing distance from the magma chamber underneath the Tengchong field, highlighting the intimate relationships between gas transport, hydrothermal circulation and the extent of the magma chamber.
SDGs
Type
journal article