Isotopic and chemical composition of Parbati Valley geothermal discharges, north-west Himalaya, India

The isotopic compositions of the waters discharged from Parbati Valley geothermal areas indicate a higher altitude meteoric origin, with discharge temperatures reflecting variations in the depth of penetration of the waters to levels heated by the existence of a ‘normal’ geothermal gradient. On the basis of mixing models involving silica, tritium, discharge temperatures and chloride contents, deep equilibration temperatures of 120–140°C were obtained for Manikaran, possibly reaching 160°C at even greater depth. Geothermometers based on sulfate-water ¹⁸O exchange and gas reactions point to similar temperatures. Exceptionally high helium contents of the discharges correspond to apparent crustal residence times of the waters in the order of 10–100 Ma; relative nitrogen-argon contents support a largely meteoric origin of the waters with a possible fossil brine, but no detectable magmatic component.