Hydrothermal source of radiogenic Sr to Himalayan rivers

Hot-spring waters near the Main Central thrust in the Marsyandi River of ce ntral Nepal have Sr concentrations to 115 muM with Sr-87/Sr-86 to 0.77. Sma ll amounts of hydrothermal water (less than or equal to1% of total river di scharge) have a significant impact on the solute chemistry and the budget o f radiogenic Sr in the Marsyandi. In the upper Marsyandi, river chemistry r eflects carbonate weathering, With Sr-87/Sr-86 less than or equal to 0.72. As the Marsyandi flows across the dominantly silicate High Himalayan Crysta lline terrane, both Sr-87/Sr-86 and [Sr] increase, associated with increase s in the concentration of Na+, K+, and Cl-, all of which are high in the hy drothermal waters. Cation concentrations decrease along the Lesser Himalaya n reach of the river. Hot-spring dissolved CO2 has a delta C-13 value to +5 .9 parts per thousand, indicating that metamorphic decarbonation reactions contribute CO2 to the fluids. Hydrothermal CO2 is partially neutralized in high-temperature weathering reactions, which generate alkalinity and yield abundant radiogenic Sr. Radiogenic hydrothermal carbonate can form from the se solutions and later weather, releasing silicate Sr but imparting carbona te characteristics to the overall water chemistry.