Deep-sourced CO₂ emissions from the Eastern Himalaya Syntaxis and the Tengchong Volcanic Field, southeast Tibet

The India-Asia collision zone is believed to play a central role in Earth's geological carbon budget. The processes controlling soil CO₂ flux and mechanism at this continental subduction zone margin remain however poorly constrained. Here, we focus on hydrothermal CO₂ emissions from southeast Tibet, specifically from the Eastern Himalaya Syntaxis (EHS) and the Tengchong Volcanic Field (TVF), with the twin goals of informing models of carbon cycling and ascertaining the source of hydrothermal degassing systems. In-situ measurements of soil CO₂ flux from the Aqiong, Caquka, Nongchaka, Rongduo, and Woka hydrothermal fields of the EHS yield average soil CO₂ fluxes of 72, 5, 13, 66, and 10 g m⁻² day⁻¹, respectively. Regional heat flow, tectonic regimes, and CO₂ content in the spring gas are plausible controlling factors for soil CO₂ flux, which is crucial to quantifying total CO₂ output in continental collision zones. The compositions and He-C-N isotopes of hydrothermal volatiles from two study regions are reported here as well. Analysis of the He-C-N isotopes in terms of distances to the Jiali fault and the nearest Holocene volcano confirms that the Jiali fault and Holocene volcanoes have a significant impact on discharging deep-sourced CO₂ from the EHS and TVF hydrothermal systems to the surface. The ³He/⁴He values of EHS hydrothermal volatiles (92 to 98°E), combined with previous data (82 to 92°E), show a clear increasing trend from south to north. Thus, we propose an updated model of enriched mantle wedge source for the EHS hydrothermal volatiles, based on multi-component mixing calculations of the He-CO₂ systematics. The EHS hydrothermal volatiles exhibit systematics spatial variations from the core location to the bilateral segments, suggesting the highest ³He/⁴He ratio and carbonate-rich characteristics at the core location possibly controlled by the asthenosphere upwelling caused by the deep fragmentation structure.