Fluorite (U–Th)/He thermochronology: Constraints on the low temperature history of Yucca Mountain, Nevada

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 (2σ), 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.