HR: 1340h
AN: T23C-0581    [Abstracts]
TI: Hydrothermal Flux of Metamorphic Carbon Dioxide From the Central Nepal Himalaya
AU: * Evans, M J
EM: mjevan@wm.edu
AF: The College of William and Mary, Department of Geology PO Box 8795, Williamsburg, VA 23187 United States
AU: Derry, L A
EM: lad9@cornell.edu
AF: Cornell University, Dept. of Earth and Atmospheric Sciences Snee Hall, Ithaca, NY 14853 United States
AU: France-Lanord, C
EM: cfl@crpg.cnrs-nancy.fr
AF: CRPG, 15 Rue N.D. des Pauvres, Nancy, 54501 France
AB: 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.
DE: 1030 Geochemical cycles (0330)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1830 Groundwater/surface water interaction
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
SC: Tectonophysics [T]
MN: Fall Meeting 2005