Hydrothermal Flux of Metamorphic Carbon Dioxide From the Central Nepal Himalaya

Hot springs are common along the Himalayan front at or near the Main Central Thrust. The combination of sharply incised south flowing rivers, rapid uplift, and steep relief bring meteoric waters and hot rocks together to form localized but numerous hydrothermal systems flanking the riverbeds. The hot springs have temperatures ranging from 20-70°C and high solute loads (up to 10 g/L). Germanium/silicon ratios in the hot springs are among the highest reported (50-800 μmol/mol), and are 10-200x those of the rivers. Ge/Si mass balance calculations show that hydrothermal waters account for 0.1 to 4% of total discharge for individual tributaries in central Nepal, and ca. 0.5% of the total discharge for the Narayani. The hot springs derive up to 100% of their total alkalinity from silicate mineral alteration, and have been shown to provide up to 25% of the silicate alkalinity in the Narayani River. Many of the springs along the Himalayan front are actively degassing carbon dioxide and dissolved CO2 concentrations are high. Bicarbonate is the dominant anion in the hot spring waters with concentrations up to 55000 μmol/kg. Carbon isotope values for dissolved inorganic CO2 (DIC) in the springs average -0.5‰ but range from -13‰ to as high as +13‰ with springs in the Marsyandi, Seti, Trisuli, and Modi systems showing positive values. These enriched δ13C values are rare and suggest a complex history. We propose a three-step process, involving the release of CO2 from underlying lesser Himalayan sediments during metamorphism, mixing of these metamorphic fluids with meteoric waters, and subsequent degassing of this mixture in the near-surface as a means to produce heavy DIC. Simple modeling suggests that nearly all (99%) of the DIC must be degassed from the fluid at the near-surface in order for such significant δ13C enrichment to occur. The significant degassing required to produce enriched DIC combined with the high [HCO3-] indicate a large CO2 flux from the central Nepal Himalaya, on scale with the drawdown from silicate alteration.