Evaluating the classic and distal Fish Canyon Tuff localities with apatite 4He/3He thermochronology

The late Oligocene Fish Canyon Tuff (FCT)—preserved within the San Juan volcanic field of southern Colorado—has long served as a reliable source of multiple accessory minerals for geochronology and thermochronology age standards. Whereas the ‘classic’ FCT sampling locality preserves near consistent ages of ~28–29 Ma across the sanidine ⁴⁰Ar/³⁹Ar, zircon U-Pb, zircon fission track, zircon (U-Th)/He, and apatite fission track systems, the average single-grain apatite (U-Th)/He (AHe) age at this site is notably younger at 20.8 ± 0.4 Ma. Considering that the classic sampling site is positioned at the bottom of a deeply incised valley with ~800 m of local relief, this AHe age has been proposed to record burial of the basal tuff to depths exceeding ~1000 m, with subsequent Early Miocene cooling reflecting valley incision. In contrast, an average single-grain AHe age of 28.5 ± 0.1 Ma was previously recorded from a newly proposed distal FCT locality where the upper-most tuffs are freshly preserved within a quarry. This AHe age is consistent with higher temperature geochronological ages from the same locality, which suggests that the distal FCT experienced no post-emplacement thermal disturbance. The observed, locality-specific difference in AHe ages provides a unique opportunity to calibrate and assess the potential of apatite ⁴He/³He thermochronology to (1) quantify the degree of post-emplacement burial and the rate of subsequent cooling at the classic FCT locality, and (2) record rapid late Oligocene cooling at the distal FCT locality. We respectively test the hypothesis that the classic FCT apatite will yield a comparatively diffusive ⁴He/³He degassing spectra, whereas the distal FCT apatite will preserve a near-uniform ⁴He/³He spectra that is solely affected by alpha-ejection. We consider and discuss newly derived ⁴He/³He results in light of (1) the geological history and landscape evolution of southern Colorado, (2) the potential use of distal FCT apatite as a coupled AHe and ⁴He/³He thermochronology reference material, and (3) the reproducibility of FCT apatite ⁴He/³He spectra using differing proton-irradiation procedures.