Deep carbon degassing from strike-slip fault and rift system in India-Asia collision zone: Insights from fluid geochemistry in hydrothermal systems

Abstract

Degassing of carbon-rich fluids through hydrothermal activities in continental fault systems represents one of the dominant ways within global carbon cycle. Complex phase transitions and geochemical transformations occur during their ascent to the surface, leading to chemical and isotopic composition alterations of fluids. Here, we present a combined dataset of chemical and isotopic data of spring water and bubbling gas samples collected from strike-slip Karakoram fault (KKF) and northern Xaniza rift (NXR) region in southern Tibetan Plateau, aiming to provide comprehensive constrain on the provenance and inventory of carbon-rich fluids in hydrothermal systems. The carbon isotopic compositions of dissolved inorganic carbon (DIC) positively correlate with dissolved values of CO₂, suggesting significant hydrothermal degassing in the system. The initial concentrations and carbon isotopic compositions of pre-degassing DIC, as well as the degrees of degassing, are determined based on the fractionation modeling between gaseous CO₂ and residual DIC. In order to provide a quantitative constraint on the pristine DIC inventory in hydrothermal waters, we conducted an integrated modeling method involving calcite precipitation, hydrothermal degassing, and mixing between deep-sourced carbon and organic matter. The modeling suggests that hydrothermal fluids from the NXR region exhibit slightly higher proportions of endogenic carbon (91.0%–95.9%) but lower contribution of subsurface organic matter (1.5%–6.8%) compared to the KKF region (75.1%–88.1% and 4.6%–24.9%). Our study emphasizes the substantial role of extensional continental rifting in deep carbon degassing through hydrothermal activities within the India-Asia collision zones.