Metamorphic fluxes of water and carbon in rivers of the eastern Qinghai-Tibetan Plateau

Geothermal activities are common in active orogenic zones and play an important role in surface-earth geochemical processes. Here we analyzed the water chemical compositions, stable and radioactive carbon isotopes of dissolved inorganic carbon (δ13CDIC and Δ14CDIC) of hot springs and large rivers in the eastern Qinghai-Tibetan Plateau (QTP). The hot springs had major ions’ concentrations varying in a wide range, and most hot springs’ alkalinities resulted from high-temperature silicate alteration. The observed hot springs’ waters were affected by the mixing of metamorphic fluids and surface waters, which altered the water chemistry and Δ14CDIC of the hot springs. Because the observed Δ14CDIC in hot springs was much higher than the theoretical Δ14CDIC in metamorphic fluids (i.e., −1000‰), we used an isotopic mass balance approach (i.e., Δ14CDIC) to quantify the contribution of surface waters to the hot springs. After the ions of metamorphic fluids were calibrated by Δ14CDIC, the metamorphic water fluxes were calculated by the chemical mass balance approach of Ge/Si ratios, using the Monte-Carlo method. We found that the proportion of metamorphic water fluxes to the river discharge ranged from 0.17% for the Yellow River to 0.52% for the Jinsha River, and it would be much higher in some tributaries. The horizontal metamorphic contributions of alkalinity to the large rivers ranged from 0.87% to 3.96% in eastern QTP, and the vertical metamorphic carbon outgassing would be much higher. The metamorphic fluids fluxes estimated in this study were much lower than previous estimations extrapolated from small Himalayan rivers, and thus we thought that the previous estimations of metamorphic carbon fluxes in QTP were likely over-estimated. We propose that systematic studies are needed to be conducted to constrain the metamorphic carbon flux in QTP. This study shed light on the release of metamorphic carbon in the orogenic zone, placing limits on the results of orogenic forcing on chemical weatherability.