Geothermal Fluxes of Heat and CO2 near the MCT Zone, Nepal

Geothermal springs are common in major river valleys near the MCT, with exit temperatures up to 70˚C. δ¹⁸O-δD data indicate the fluids are meteoric; they also have very high Ge/Si ratios. We use the Ge/Si mass balance to constrain the hydrothermal flux to the hydrologic system to the Narayani basin (35,000 km²) near 3 m³/s. The geothermal power delivered to the surface as hot water is 500 MW, which represents a heat flow of 82 mW/m² over the active portion of the basin. Reservoir temperatures are imprecise but probably near 125˚C. Conductive cooling of ascending fluids transfers 960 MW to the shallow crust. Fluid inclusion data constrain the geothermal gradient over the upper 3.7 km to be near 77 ˚C/km. There is no evidence of any magmatic source for the heat flow, but simple calculations show that rapid denudation advects heat into the upper crust at a rate similar to that carried by the geothermal systems, suggesting a rough thermal balance between tectonic/erosive heat transfer and hydrothermal heat loss. Local thermal gradients in the zone of fluid flow are very high, capable of resetting thermochronometers at very shallow levels. The fluids are supersaturated in CO₂ and have δ¹³C(DIC) up to +13‰. The high δ¹³C values are consistent with a metamorphic source followed by extensive degassing in the near sub-surface at 50 - 100 ˚C. The total geothermal CO₂ flux is > 1.3x10¹⁰ mol/y, or more than 4x the rate of CO₂ consumption by silicate weathering in the basin. The net impact of metamorphic degassing and weathering in central Nepal is a strong CO₂ source – not a sink as commonly assumed. The coupling of metamorphism, erosion, heat and fluid flow, and CO₂ fluxes is quantitatively important for crustal deformation processes, interpretation of thermochronometers, and the carbon balance of Himalayan orogenesis.