Influence of crystal size on apatite (U–Th)/He thermochronology: an example from the Bighorn Mountains, Wyoming

Near-surface tectonic and geomorphic processes involve cooling of rocks through low temperatures (50–200°C). Because rates of helium diffusion in apatite, titanite, and zircon are sensitive to temperature variations in this range, uranium–thorium/helium thermochronometry ((U–Th)/He dating) is well-suited to establishing the timing and rates of these processes in the geologic record. However, because fractional loss of He is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic He, (U–Th)/He ages must vary not only with thermal history but also with crystal size. Here we present crystal size-correlated He ages from co-existing apatites from the Bighorn Mountains, Wyoming that range from 100 to 350 Ma. These correlations are a sensitive indicator of the rock’s thermal history in a temperature range below the system’s nominal closure temperature (Tc∼70°C for apatite), and are consistent with a thermal history involving residence in the upper 2–3 km of crust since the Precambrian, with maximum temperatures of 65–80°C just prior to Laramide orogenic exhumation. The influence of crystal size on He ages will be most apparent in rocks where temperatures have been in the range of partial He retention for long periods of time (∼30–70°C for >107 years). In such cases, accurate interpretation of (U–Th)/He ages must incorporate the effect of crystal size, and this method may provide insights to thermal histories of rocks in previously inaccessible low-temperature ranges.