Evidence for a hypogene paleohydrogeological event at the prospective nuclear waste disposal site Yucca Mountain, Nevada, USA, revealed by the isotope composition of fluid-inclusion water

摘要

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 D and delta O-18) 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-flow isotope-ratio mass spectrometry method. The delta O-18 values were calculated from the delta O-18 values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fluid-inclusion microthermometry. The delta D values measured in all samples range between -110 and -90 parts per thousand, similar to Holocene meteoric water. Coupled delta O-18-delta D) values plot significantly, 2 to 8 parts per thousand, to the tight 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 percolating water. Our analysis indicates, however, that none of these processes could produce the observed large positive delta O-18-shifts. The latter require isotopic interaction between mineral-forming fluid and host rock at elevated temperature (>100 degrees C), which is only possible in the deep-seated 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.