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

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.