Degassing of metamorphic carbon dioxide from the Nepal Himalaya

Matthew J. Evans , Louis A. Derry , Christian France-Lanord (1)

1 CRPG - Centre de Recherches Pétrographiques et Géochimiques

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 CO₂ derived from metamorphic reactions. Hot spring fluids are saturated with CO₂, have [DIC] from 1.3 to >100 mmol kg^-1 and have δ¹³C_DIC values from -13‰ to +13‰_(PDB). Analysis of CO₂ released by decrepitation of fluid inclusions from syn- and postdeformational quartz veins indicate that crustal fluids had δ¹³C from -15‰ to +2‰_(PDB), consistent with production of CO₂ 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 CO₂ in the 32,000 km² Narayani basin of the central Himalaya is >1.3 × 10¹⁰ mol a^-1, exceeding the calculated consumption of CO₂ 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 CO₂ because the weathering sink is substantially offset or even exceeded by the metamorphic source.

Domaines: Planète et Univers [physics]; Sciences de la Terre

Matthew J. Evans, Louis A. Derry, Christian France-Lanord. Degassing of metamorphic carbon dioxide from the Nepal Himalaya. Geochemistry, Geophysics, Geosystems, 2008, 9, ⟨10.1029/2007GC001796⟩. ⟨insu-03619245⟩