U/Pb and (U-Th-Sm)/He “double” dating of detrital apatite by laser ablation: A critical evaluation

Paired U/Pb and (U-Th-Sm)/He dating of individual detrital apatite crystals has potential as a valuable tool for constraining the high- and low-temperature thermal evolution of sediment source terrains. Here we present the results of exploratory applications of the laser ablation double-dating (LADD) method – originally developed for detrital zircon research – to the widely available Durango fluorapatite standard in order to evaluate the practicality of the method. Sixty-two laser-ablation analyses of a single large crystal of Durango fluorapatite yielded an inverse-variance weighted mean ²⁰⁶Pb/²³⁸U date of 31.46 ± 0.48 Ma and an inverse-variance weighted mean (U-Th-Sm)/He date of 31.75 ± 0.60 Ma, both of which are in good agreement with previously published conventional dates. While these results are encouraging, several factors suggest that LADD may be less useful for detrital apatites than for detrital zircons given widely available analytical instrumentation. These principally reflect the propensity for apatites to have comparatively lower U + Th concentrations, and thus lower radiogenic He and Pb concentrations, as well as high concentrations of common Pb. These factors contribute to substantially higher analytical imprecision for most LADD U/Pb dates for apatite, occasionally too high for the dates to be geologically useful. Reasonably precise laser ablation (U-Th-Sm)/He dating of detrital apatites requires relatively large crystal sizes (≥ 100 μm in the shortest dimension), with the minimum useful size increasing with decreasing (U-Th-Sm)/He apparent age. In contrast to the geological interpretation of LADD datasets for detrital zircons, the interpretation of datasets for detrital apatites is less straightforward. In particular, researchers should consider carefully the possibility that (U-Th-Sm)/He apparent age distributions are biased by the need to analyze only larger apatites.