| 27 Aug 2025
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, four to six spot measurements with various distances from the grain rim enable measuring an in situ 4He profile. We 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, 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.
The submitted manuscript is well written and presents a new methodological approach and algorithm for in situ (U–Th–Sm)/He dating of apatite aimed at reconstructing thermal histories of individual grains. Using apatite samples from southern Germany, the authors demonstrate that the proposed methodology enables the derivation of “He diffusion” profiles. It is shown that profiles corrected for parent nuclide distribution are flat in rapidly cooled grain, whereas in samples with heterogeneous parent nuclide distribution and complex thermochronological history, they exhibit non-flat geometries. It is shown how the obtained analytical data can be mathematically processed to derive information on the thermal history of the grains.
Thus, I consider this study to represent a valuable advancement in the development of in situ (U–Th–Sm)/He thermochronology.
My major concern is that the authors did not succeed in deriving a thermochronological path that adequately fits their observations for the BaF apatite. The obtained 4 profiles are reproducible and, as the authors noted, exhibit a counterintuitive geometry: the central part of the grain is significantly younger than the rims.
When the data contradict the model, it typically indicates either low-quality measurements (which I assume is not the case here) or that the model itself may not be correct. Thus, this issue should be discussed in detail. Why did the classical He-loss model fail to reproduce the results? The authors provide only a brief discussion of this matter. Among the factors considered are the presence of inclusions, uranium-enriched zones, and variations in the degree of crystallinity. However, as it was already mentioned, profiles are reproducible, no inclusions are observed in the analyzed half of the grain, and the effect of apatite crystallinity on helium diffusion is limited. The possible influence of implanted helium is not addressed.
This point is of critical importance—if the model does not accurately describe observed in situ (U-Th-Sm)/He profiles, its validity as a basis for thermal history modeling becomes questionable.
General comments
Additional minor comments can be found within the attached file.