Karstbase Bibliography Database

Secondary calcite residing in open cavities in the unsaturated zone of Yucca Mountain has long been interpreted as the result of downward infiltration of meteoric water through open fractures. In order to obtain information on the isotopic composition (delta 18O) of the mineral-forming water we studied fluid inclusions from this calcite. Water was extracted from inclusions by heated crushing and the delta D values were measured using a continuous-low isotope-ratio mass spectrometry method. The delta 18O values were calculated from the delta 18O values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fuid-inclusion microthermometry. The delta D values measured in all samples range between −110 and −90‰, similar to Holocene meteoric water. Coupled delta 18O– delta D values plot significantly, 2 to 8‰, to the right of the meteoric water line. Among the various processes operating at the topographic surface and/or in the unsaturated zone only two processes, evaporation and water–rock exchange, could alter the isotope composition of percolatingwater. Our analysis indicates, however, that none of these processes could produce the observed large positive delta 18O-shifts. The latter require isotopic interaction between mineral-forming fluid and host rock at elevated temperature (N100 °C), which is only possible in the deepseated
hydrothermal environment. The stable isotope data are difficult to reconcile with a meteoric origin of the water from which the secondary minerals at Yucca Mountain precipitated; instead they point to the deep-seated provenance of the mineral-forming waters and their introduction into the unsaturated zone from below, i.e. a hypogene origin.