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 thr ough low temperatures (50-200 degreesC). 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 c rystal size such that larger crystals retain a larger fraction of radiogeni c 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-exist ing apatites from the Bighorn Mountains, Wyoming that range from 100 to 350 Ma. These correlations are a sensitive indicator of the rock's thermal his tory in a temperature range below the system's nominal closure temperature (T(c)similar to 70 degreesC for apatite), and are consistent with a thermal history involving residence in the upper 2-3 km of crust since the Precamb rian, with maximum temperatures of 65-80 degreesC just prior to Laramide or ogenic exhumation. The influence of crystal size on He ages will be most ap parent in rocks where temperatures have been in the range of partial He ret ention for long periods of time (similar to 30-70 degreesC for > 10(7) year s). In such cases, accurate interpretation of (U-Th)/He ages must incorpora te the effect of crystal size, and this method may provide insights to ther mal histories of rocks in previously inaccessible low-temperature ranges. ( C) 2001 Elsevier Science B.V. All rights reserved.