HEAT FLOW AND DEGASSING OF METAMORPHIC CO2 NEAR THE MAIN CENTRAL THRUST

Hot springs located in major incised valleys near the Main Central Thrust have surface temperatures 40 – 70 °C.  High Ge/Si ratios in the hydrothermal fluids relative to stream waters enable the hot spring flux to be calculated on a watershed basis.  As rivers cross the zone of geothermal activity, they develop anomalies in Ge/Si, K⁺, Cl^-, and other solutes.  10 to 20% of the silicate derived alkalinity on the streams is derived from hydrothermal reactions.  Observed spring temperatures and calculated fluxes define an overall hydrothermal heat loss in the Narayani basin of 500 (+500/-250) MW.  Fluid inclusions from hydrothermal veins that crosscut the metamorphic deformation fabric constrain  temperatures near 300 °C and pressures near 1100 bars (Darling et al, 2002), consistent with steep geothermal gradients.  The observed heat flow and geothermal gradients are consistent with a simple steady state model of crustal advection and rapid incision.  Tectonically advected heat is removed quantitatively by meteoric hydrothermal circulation.  A preliminary 2-D model constrains the depth and time scale of meteoric fluid circulation.

MCT hot spring fluids often have high δ¹³C_DIC, with many springs in the range of +6 to +12 per mil.  The springs are supersaturated with CO₂ and actively precipitate travertine.  The high δ¹³C_DIC can be modeled by a two stage process.  Metamorphic decarbonation produces a ¹³C enriched fluid phase from carbonate decomposition near 400 °C, up to about 4 per mil.  Degassing in the near subsurface, between ca. 100 and 50 °C evolves a 13C-depleted gas phase, with Rayleigh enrichment of the residual DIC.  Degassing of 80±20% of the original CO₂ is necessary to drive δ¹³C_DIC to +8 to +12 per mil, and these values are consistent with reaction path modeling of the fluids during ascent.  Comparison with the silicate alkalinity flux in local rivers implies that the magnitude of CO₂ degassing is similar to the rate of CO₂ uptake by silicate weathering.  The net effect of high Himalayan processes on the carbon cycle may be close to zero.