Thermochronology of Secondary Minerals from the Yucca Mountain Unsaturated Zone

摘要

Some fractures and cavities in the welded tuffs of the unsaturated zone at Yucca Mountain contain deposits ofrnsecondary calcite and silica. The restriction of these deposits to the footwalls of fractures and the floors ofrncavities is convincing physical evidence that they formed from meteoric water infiltrating at the surface andrnpercolating through the unsaturated zone along fracture pathways, a conclusion supported by comprehensiverngeochemical and textural studies. As such, the deposits provide an important record of unsaturated zonernhydrology. Nonetheless, fluid inclusions and δ~(18)O values in calcite indicate that depositional temperaturesrnwere, at times, higher than attributable to the modern geothermal gradient. These elevated temperatures havernbeen cited as proof that deposition, instead of from percolating meteoric water descending through thernunsaturated zone, required upwelling of deep-seated hydrothermal fluids. This paper reports the results of fluidrninclusion and δ~(18)O analysis of UZ calcite by the USGS. These studies show that geothermal gradients andrndepositional temperatures were higher in the past. Depositional temperatures appear to have peaked about 10rnMa or more, but had cooled to near modern ambient conditions by 2 to 4 Ma. None of the thermal data,rnhowever, requires hydrothermal inputs and the distribution and morphology of the deposits themselves arerninconsistent with the upwelling hypothesis. Therefore, it is still our conclusion that the secondary minerals havernformed from fluids percolating down through the unsaturated zone along fracture flow paths - under higherrngeothermal gradients in the past but at near modern ambient temperatures during the past 2 to 4 million years.