Evaluating the resolving power of apatite 4He/3He thermochronology: Insights from the Fish Canyon Tuff

Apatite 4He/3He thermochronology has the potential to provide high-resolution low-temperature thermal histories that bring valuable insight into near-surface crustal processes. However, this system has yet to be directly evaluated using single-grain 4He/3He analyses from a natural sample with an established thermal history. We present apatite 4He/3He spectra from the widely used Fish Canyon Tuff (FCT) age standard (San Juan volcanic field, southern Colorado, USA), collected at two localities with contrasting thermal histories: (1) a distal locality (FCT-D) where the early Oligocene eruptive age of the FCT is preserved in the apatite (U-Th)/He (AHe) system and the thermal history is well established; and (2) the classic FCT sampling locality (FCT-C) with younger, Early Miocene AHe dates and an unconstrained posteruptive thermal history. FCT-D apatite shows 4He/3He spectra indicative of no diffusive loss, with relative edge depletion of 4He induced by alpha ejection only, corroborating rapid eruptive cooling. In contrast, FCT-C apatite revealed notably diffusive 4He/3He spectra. Thermal-history inversions highlight the resolving power of apatite 4He/3He thermochronology, demonstrating its ability to (1) independently resolve rapid eruptive cooling at the FCT-D site, and (2) improve the resolution of postemplacement reheating and Early Miocene cooling at the FCT-C site. Refined FCT-C thermal histories reveal a distinctive onset of moderate cooling at ca. 20−19 Ma, likely reflecting footwall topographic development coinciding with regional Rio Grande rifting. This collective assessment of 4He/3He systematics further verifies its ability to substantially improve thermal history resolution, which is crucial to elucidating mechanisms driving crustal cooling.