Geochemical constraints on origin of hydrothermal volatiles from southern Tibet and the Himalayas: Understanding the degassing systems in the India-Asia continental subduction zone

Abstract

Hydrothermal activities (e.g., hot springs and geysers) are extensively distributed in southern Tibet and the Himalayas, forming an east-west trending, ~2000-km-long, hydrothermal belt that represents the ongoing degassing systems in the India-Asia continental subduction zone (IACSZ). In this study, we report new data of chemical compositions and He-C-N isotopes for gas samples from representative hot springs in the hydrothermal belt, aimed at understanding volatile origin of the hydrothermal degassing systems in the IACSZ and their tectonic implications. According to spatial location, the samples are divided into western, central and eastern subgroups, which display spatially discernible geochemical characteristics, such as 3He/4He and CO2/3He ratios. The essential components required for volatile evolution include silicate rocks, carbonate rocks, sedimentary organic matter and a mantle-derived end-member, which are all expected in the IACSZ following a tectonic model incorporating an accretionary wedge and a magmatic front. On the basis of He-C-N isotope mixing calculations, together with constraints from petrogeochemical and geophysical studies, an enriched mantle wedge (EMW) is proposed as a potential candidate for the source of the mantle-derived components, which highlights the importance of recycled Indian continental materials compared to previous models that do not take this possibility for mantle source enrichment in account. The regional crustal rock assemblages composed of silicate rocks, carbonate rocks and sedimentary organic matter are interpreted as contaminants of the EMW-derived volatiles (as exemplified by degassing systems in the magmatic front) or as source materials of hydrothermal volatiles from degassing systems in the accretionary wedge. Following tectonic framework of the IACSZ, we suggest that spatial variations in volatile geochemistry (e.g., 3He/4He) are predominantly controlled by tectonic affinities (i.e., the accretionary wedge and magmatic front) of the hydrothermal degassing systems in southern Tibet and the Himalayas. The crustal-like 3He/4He ratios and high N2 and 4He contents agree well with the high sedimentary contributions to degassing systems in the accretionary wedge (including the Himalayas and fore-arc basins), whereas the high mantle-derived helium emissions in southern Tibet exhibit close affinities with contributions from the EMW-derived melts beneath the magmatic front. Moreover, regional fault systems with variable scales and depths of penetration would act as an extra factor that may perturb the across-IACSZ profile of 3He/4He ratios controlled by tectonic settings. Our interpretation on degassing systems in southern Tibet and the Himalayas may have the potential to provide constraints from volatile geochemistry for understanding material recycling mechanism and tectonic settings of the IACSZ.