Stable isotope investigations of an active geothermal system in Valles Caldera, Jemez Mountains, New Mexico

Abstract

15O/ 16O, 13C/ 12C and D/H measurements have been made on rocks and minerals from drill cuttings (0 to 1700 m depth) from Valles Caldera, New Mexico, a post-Pliocene collapsed volcano containing an active hydrothermal system. δ 18O values of calcite reflect isotopic equilibrium with large amounts of a meteoric-hydrothermal fluid (δ 18O = −12% 0) at temperatures between 60 and 280°C. Calcites appear to have retained their original 13C/ 12C ratios, typical of either marine or fresh-water origin. The mineralogical changes which have accompanied the hydrothermal activity resulted in decreases in whole-rock δ 18O values of as much as 7% 0 as compared to the original δ 18O of the volcanically-derived rocks. The isotopic composition of hydrothermal quartz in altered rocks is entirely unrelated to the most recent episode of alteration. D/H ratios of kaolinite formed in equilibrium with the meteoric-hydrothermal fluid have allowed the determination of approximate kaolinite-water hydrogen isotope fractionation factors between 60 and 280°C. These data are compatible with previous determinations at 400 and 300°C. The In α D (kao-H,O) relationship with 1/ T 2 is not linear over the temperature range 400 to 25°C.