SOURCES OF CHLORIDE IN HYDROTHERMAL FLUIDS FROM THE VALLES CALDERA, NEW-MEXICO - A CL-36 STUDY

The Valles caldera in New Mexico hosts a high-temperature geothermal s ystem, which is manifested in a number of hot springs discharging in a nd around the caldera. In order to determine the fluid pathways and th e origin of chloride in this system, we measured Cl-36/Cl ratios in wa ters from high-temperature drill holes and from surface springs in thi s region. The waters fall into two general categories: recent meteoric water samples with low Cl- concentrations (<10 mg/L) and relatively h igh Cl-36/Cl ratios [(300-1000) x 10(-15)]; and geothermal brines with high Cl- concentrations (800-9400 mg/L) but low Cl-36/Cl ratios [(11- 26) x 10(-15)]. The Cl-36/Cl ratios for meteoric waters are slightly h igher than expected for this region, suggesting a small addition of an thropogenic Cl-36. Because of low Cl-36/Cl ratios and high Cl- concent rations in the brines, chloride in these waters must be derived from s ubsurface sources. A comparison between the observed Cl-36/Cl ratios i n the brines and those calculated for potential source formations in t his region indicates that the present host formations, mainly volcanic tuffs, cannot be major sources of chloride, and that formations at gr eater depth, such as the Paleozoic and Precambrian formations are more likely to be sources of chloride in the brines. The results suggest t hat brines are meteoric waters which penetrated into the basement wher e they derive chloride from leaching of basement rocks and/or from sal ine pore fluids trapped there, along with likely addition of chloride from Paleozoic strata. Although these fluids have since come to reside in the intracaldera volcanic sequence after convective upwelling, the y do not derive much Cl- from the volcanic strata; and residence times of fluids in the volcanics are < 100,000 years.