Degassing of metamorphic carbon dioxide from the Nepal Himalaya

Geothermal activity is common at the foot of the Higher Himalaya near the Main Central Thrust (MCT), Nepal Himalaya. We have sampled hot springs along a 150 km stretch of the Himalayan front and find that they carry large fluxes of CO 2 derived from metamorphic reactions. Hot spring fluids are saturated with CO 2 , have [DIC] from 1.3 to >100 mmol kg −1 and have δ 13 C DIC values from −13‰ to +13‰ (PDB) . Analysis of CO 2 released by decrepitation of fluid inclusions from syn‐ and postdeformational quartz veins indicate that crustal fluids had δ 13 C from −15‰ to +2‰ (PDB) , consistent with production of CO 2 from both thermal decomposition of organic matter and decarbonation at depth. Modeling of the hot spring fluid compositions indicates that they are strongly degassed. Combining our degassing calculations with estimates of the fraction of hydrothermal alkalinity in local rivers shows that the metamorphic degassing flux of CO 2 in the 32,000 km 2 Narayani basin of the central Himalaya is >1.3 × 10 10 mol a −1 , exceeding the calculated consumption of CO 2 by chemical weathering for the Narayani River basin by a factor of four. Our study implies that the net impact of Himalayan orogenesis on the carbonate‐silicate geochemical cycle is not large‐scale drawdown of CO 2 because the weathering sink is substantially offset or even exceeded by the metamorphic source.