Abstract:
Fluorite is one of the secondary minerals precipitated in pore spaces at the future nuclear waste repository site at
Yucca Mountain, Nevada. The authors have conducted (U–Th)/He dating of this fluorite in an attempt to constrain
the temperature and timing of paleo-fluid flux into the site. Repeated analysis of colourless fluorite yielded a weighted
average age of 9.7 ± 0.15 Ma (2r), younger than previously determined sanidine 40Ar/39 Ar ages (12.8 Ma) for deposition
of the tuff.
Laboratory He-diffusion experiments conducted on the Yucca fluorite yield a preliminary He closure temperature
(Tc) of 90 ± 10 C (cooling rate of 10 C/Ma) and previous studies have determined that the fluorite precipitated from
warm fluids (65–80 C) at depths of <400 m. However, minerals can experience partial He loss at temperatures well
below the Tc and therefore the (U–Th)/He age of 9.7 Ma is interpreted to be a cooling age. This result implies that
the last period of elevated temperature fluid circulation through the Yucca site was approximately 9.7 Ma ago.
It was observed that the purple coloured outer portion of the fluorite nodule yielded non-reproducible and invariably
older ages than colourless fluorite. Several possible reasons are suggested.
