Identification of the deep-seated component in paleo fluids circulated through a potential nuclear waste disposal site: Yucca Mountain, Nevada, USA

摘要

Using a combination of mineralogical, fluid inclusion and stable isotope techniques, chemical data have been obtained that help characterize the hyrogeochemistry of paleogroundwaters that circulated through the vadose zone of Yucca Mountain, Nevada, USA-a prospective site for geological disposal of high-level nuclear waste. The results depict unidirectional evolution of groundwaters from reducing to oxidizing states concomitantly with an overall cooling from approx 85 deg C to < 35-50 deg C. Early calcite has strongly positive delta~13C values (up to + 9 per thousand PDB), which are interpreted as an indication of the partitioning between reduced and oxidized dissolved carbon speciesin anoxic (reducing) environment. Reducing character of fluids is further supported by the chemistry of gases trapped in inclusions (dominant CH_4, CO_2, very little O_2). The maximum homgenization temperatures of fluid inclusions and reconstructed paleoheat flows form a non-uniform field with prominant maximum near the block-bounding Paintbrush fault. The data support the model of invasion(s) of the deep-seated thermal fluids, into the vadose zone, along the permeable fault zone. Chemical properties of early portions of the fluid were acquired during its long residence in the deep-seated part of the Earth's crust. At advanced stages, the fluid evolved in response to the overall cooling and mixing with oxidized waters from shallow aquifers.