DECONVOLVING LEGACY AND CONTEMPORANEOUS WEATHERING IN A PORPHYRITIC RHYOLITE AND RHYOLITIC TUFF DOMINATED UPLAND CATCHMENT, VALLES CALDERA, NEW MEXICO

The Catalina-Jemez Critical Zone Observatory (CZO) investigations have focused primarily on near surface processes; including remote sensing and direct sampling of vegetation, soils, saprolite and waters. However, water/rock interaction, weathering and solute mobility along flowpaths in the deep (>10 m) CZ that echo near surface CZ processes (i.e. water, energy, and mass fluxes) are not well understood. We postulate a deep groundwater reservoir in fractured rhyolite exerts strong controls on solute discharges in upland catchments of the Jemez CZO within the Valles Caldera National Preserve in northern New Mexico. To investigate weathering processes in the deep CZ, we extracted three continuous cores to 40-50 m depth in summer of 2016 in one of our instrumented forested sub-catchments prior to instrumenting the boreholes as monitoring wells. The research goals were to understand depth- and geologic-dependent trends in the physical, chemical and biological structure, identify mineral transformations that result from hydrologic and biogeochemical dynamics, and describe lateral and vertical groundwater flow and its contribution to the geochemical evolution of the deep CZ. Results show complex weathering profiles at each of the three watershed positions attributable to a combination of legacy hydrothermal alteration, textural controls on weathering, development of preferential flowpaths, and differing hydrologic base levels. We used a multifaceted tool set to deconvolute the extent to which observed variation laterally and with depth resulted from legacy versus contemporary weathering processes. Co-located Ca-zeolites and smectite (up to 50% of bulk mineral composition) coupled with Mg, Fe, and Mn enrichment suggest legacy hydrothermal fluid intrusion contributed to mineral transformations in shallow portions (down to 15 meters) of the catchment east of a north trending fault bisecting the catchment and at depths below 15 meters west of the fault. Conversely, the presence of iron (oxy)hydroxides (up to 4%) coupled with calcite (17%), smectite (up to 49%), illite (up to 22%), absence of zeolites, and depletion of K, Na Si, and Ca in other portions of the catchment suggest contemporaneous glass, plagioclase and mica weathering is occurring at shallow depths and at textural transitions.