The Fish Canyon Tuff: A new look at an old low-temperature thermochronology standard

Abstract

The Fish Canyon Tuff (FCT) has served as an important source for geochronology standards, particularly for fission track, K–Ar and (40Ar/39Ar) dating, even though efforts to establish precise ages for its constituent minerals have proved to be unexpectedly complex. To evaluate the suitability of FCT apatite as a standard for apatite (U–Th–Sm)/He (AHe) thermochronometry, and to test underlying assumptions about its suitability for apatite fission track (AFT) thermochronometry, we analysed samples from a series of sites throughout the vertical and lateral extent of the host ignimbrite sheet. Samples were collected from the relatively lithic-rich, classic sampling location in the lower part of the thick proximal ignimbrite and a ∼330 m vertical section of FCT immediately above it. Average weighted mean AHe ages from multiple analyses at five sites in this profile range from 20.8±0.4 Ma from the classic site at the base, to an average of 28.4±0.2 Ma (all ±1σ) in the upper part of the section. The AHe age at the classic site is substantially reduced at 20.8±0.4 Ma relative to a reference age for ignimbrite emplacement of 28.2 Ma. Corresponding zircon (U–Th)/He (ZHe) ages for these samples are all concordant at 28.3±0.4 Ma. By contrast, apatite fission track (AFT) ages from the same vertical section are all concordant at 27.4±0.7 Ma with the central age of 28.8±0.8 Ma at the classic site, except for the uppermost sample (23.2±1.7 Ma) for which clear evidence for local, probably fire, disturbance is seen in the track length distribution. The AHe data at the classic site thus provide evidence for substantial post-eruptive Early Miocene cooling of the tuff consistent with its position at the bottom of a deeply incised valley with ∼800 m of local relief and probably >1000 m of removed section above it. The AFT age of the classic sampling site, however, is indistinguishable from ignimbrite emplacement and thus continues to be a useful standard for AFT (but not for AHe) geochronology. Apatites from this site have the highest measured Cl concentrations (0.82 wt%) of any of the FCT apatites analysed, contributing to this suitability. AFT and AHe ages at three distal localities, some 35–45 km to the East of the classic site, where the thickness of the FCT is reduced to <100 m, all yielded concordant ages with a weighted mean of 28.5±0.11 Ma with no evidence for post-emplacement thermal disturbance. One of the distal sites – a quarry in the upper part of the FCT – appears to be an ideal locality for a standard reference material that would be suitable for both AFT and AHe low-temperature thermochronometers. U–Pb ages for zircons and 40Ar/39Ar step heating age spectra for sanidine at this new distal site are essentially identical to those found at the classic site.