EXPLORING THE ROLE OF APATITE CHEMISTRY ON APATITE (U-TH)/HE DATES

Apatite (U-Th)/He dating is a well-accepted tool used to constrain thermal histories. The accumulation and annealing of radiation damage is an important control on apatite He diffusivity, and recent apatite He diffusion kinetic models include this effect. It is well-known that apatite chemistry (e.g., wt % Cl) influences radiation damage annealing, such that apatite chemistry could in turn influence apatite He diffusivity and therefore the interpretation of apatite (U-Th)/He data. Our study tested this hypothesis by comparing (U-Th)/He dates for pairs of samples consisting of fluorapatites and suspected chlorapatites from nearby locations in the Gore Range, Colorado and the southern Canadian Shield, Ontario. Samples were screened for high-grade apatites, apatite chemistry was determined by electron microprobe and LA-ICP-MS, and (U-Th)/He dates were acquired for a subset of grains. Dates obtained from the Gore Range samples were bimodal, suggesting multiple apatite populations. Both samples were characterized by low wt% Cl despite previous work that reported high values for one of these samples. The Ontario samples yielded contrasting (U-Th)/He data distributions, but the difference in Cl content between them was not substantial (0.9 wt% versus 0.5 wt%). Given the poor correlation between wt% Cl and the (U-Th)/He dates in both sample pairs, chemical data were used to compute rmro values, an annealibility proxy that accounts for multiple chemical variables. The rmro values for all analyzed apatites indicate greater resistance to radiation damage annealing than typical fluorapatites. Although our initial investigation focused on wt % Cl, it is clear that Cl cannot be solely responsible for the distribution of He data in either sample set. Additional work is required to determine if other lattice substitutions that influence the annealability of apatite can explain the (U-Th)/He results.