Analytical and modelling strategies for thermal histories from in situ (U-Th-Sm) ∕ He data of single apatites

Abstract. (U-Th-Sm) / He is a thermochronometric method used to reconstruct the rates and timing of geological processes. Recent developments in analytical approaches, specifically laser ablation (in situ) measurements, allow quantifying the distribution of parent isotopes (U, Th, and, in apatites, Sm) and decay products (4He) within individual mineral grains. This is particularly important to understand potential date over-dispersion, which can arise from the heterogeneous distribution of parent isotopes, and to develop thermal history modelling for single-grain (U-Th-Sm) / He techniques. We build on previous studies and combine in situ 4He concentration profile measurements with parent nuclide distribution mapping in natural apatites to explore analytical and modelling strategies for single-grain thermal history reconstructions. Specifically, we investigate the effects of laser ablation spot size, the number and location of ablation spots in a grain, and grain size on data resolution and suitability for thermal history modelling. In doing so, we introduce the calculation of Caw, which is the concentration of parent nuclides at each ablation site weighted by alpha-particle stopping distances to account for the redistribution of 4He in the crystal from high-energy alpha decay. We present stacked U, Th, and Sm maps measured at different ablation depths in two apatite grains from South Germany (one with homogeneous and one with zoned parent isotope distribution) and one apatite from the McClure Mountain Syenite age standard. Furthermore, we show in situ 4He profiles of the two South German apatites and inversions for thermal histories. Our results indicate that, for our study and instrument set-up (a RESOchron system (Applied Spectra) consisting of a He-line and an excimer laser), four to six spot measurements at various distances from the grain rim enable measuring an in situ 4He profile. We tested different laser ablation spot sizes (10–30 µm) in grains with a range of 4He concentrations and (U-Th-Sm) / He dates (16 to ∼ 200 Ma) and determined that the optimal spot diameter for in situ 4He profile measurements for apatite grains with (U-Th-Sm) / He dates as young as 16 Ma is 20–30 µm. Additionally, with an ablation spot diameter of 20 µm, a six-spot in situ 4He profile requires a minimum grain diameter (measured perpendicular to the c-axis) of 145 µm. Combined with information from detailed parent nuclide maps, the in situ 4He profiles offer a possibility to reconstruct the thermal histories of single grains, potentially including zoned and irregularly shaped crystals.