Technical Note: Reducing age dispersion in in-situ (U-Th)/He dating: How to optimize ablation pit geometries with OptiPit

Hoemann, Hagen; Hueck, Mathias; Dunkl, István

doi:10.5194/egusphere-2026-2590

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https://doi.org/10.5194/egusphere-2026-2590

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27 May 2026

| 27 May 2026

Technical Note: Reducing age dispersion in in-situ (U-Th)/He dating: How to optimize ablation pit geometries with OptiPit

Hagen Hoemann, Mathias Hueck, and István Dunkl

Abstract. Radiometric dating requires the measurement of related parent and daughter isotopes to produce meaningful ages. The alpha ejection during the decay of uranium and thorium spatially separates the parent and daughter nuclides analysed in (U-Th)/He dating by a few tens of microns within the crystals. In-situ (U-Th)/He dating applies two superimposed laser ablation spots to analyse the liberated parent and daughter nuclides sequentially, assuming that the analysed He is derived from the analysed U and Th. However, computer simulation considering alpha ejection visualizes the helium-source volume around the helium-pit and reveals that in-situ (U-Th)/He dating undermines that assumption. Only a fraction of the parent nuclei that produced the measured volume of helium are probed during analysis. Furthermore, while those parent nuclei contribute equally to the analysed radionuclide budget, their contribution to the helium budget is a complex function depending on the relative location of the alpha emitter to the laser ablation spots, as well as the geometry of the latter. This geometry controls the overdispersion of in-situ (U-Th)/He ages produced by radionuclide zonation, which can be reduced to a minimum by optimizing the geometries. This contribution serves as a guideline on how to minimize age overdispersion produced by the interplay of radionuclide zonation and the geometry of the laser ablation spots and demonstrates how to assess the reliability of in-situ (U-Th)/He ages by optimizing the laser pit geometry.

Received: 05 May 2026 – Discussion started: 27 May 2026

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Hagen Hoemann, Mathias Hueck, and István Dunkl

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-2590', Cody L. Colleps, 07 Jul 2026
This contribution provides an in-depth assessment of a proposed approach to reduce apparent (U-Th)/He date discrepancy induced by a known, yet under evaluated, shortcoming in laser ablation (U-Th)/He dating—i.e., the requirement to ablate and analyze parent and daughter contributions in two separate analytical sessions. The authors provide a well-written, thorough summary explaining how not only the simple mismatch in helium and U-Th laser ablation pits may impact date disparity, but also how the measured helium from a laser ablation pit reflects a more complex total budget of internally produced (within pit) and externally sourced helium. This paper quantitatively demonstrates (with useful illustrations) how differing double-pit geometries can result in highly variable discrepancies in this parent-daughter measurement imbalance. The authors accompany this manuscript with a moderately user-friendly, Window-based software, OptiPit, that allows users to explore the impact of, and optimize, double-pit geometries for in-situ (U-Th)/He analyses that may be unique for each analyzed grain considering varying analytical limitations controlled by age, parent concentrations, grain size, zonation, thermal history, etc.
I believe the authors’ excellent theoretical demonstration of this phenomenon alone warrants this manuscript worthy of publication in Geochronology. Whereas the OptiPit software is fully functional, decently easy to use, and provides a nice 2D/3D visual aid, I do partly question its overall future utility in light of a few potential issues discussed below. Regardless of its prospective utility, I believe this manuscript and OptiPit will be valuable resources to the community, as they effectively communicate some underappreciated complexities inherit to in-situ (U-Th)/He dating. I believe this manuscript is suitable for publication following minor-to-moderate modifications, and below I provide a few comments and suggestions. I thank the authors for their interesting, meticulous, and well-thought-out manuscript.
MAIN COMMENTS:
The use of OptiPit largely centers on the approach to place a second ICP laser pit either within the Helium pit (Geometry A) or engulfing the Helium pit (Geometry B). Alternatively, Maier et al. (2026) recently proposed an approach where whole-grain parent zonation maps were obtained after multiple helium ablation pit transects were analyzed across a single grain. Instead of obtaining the Helium budget from a single ablation pit, the Helium budget was determined from the apparent mapped zonation. A benefit of the Maier et al. (2026) approach is that parent zonation within and around the helium pit can be evaluated. The use of OptiPit anchors on the simpler double-ablation approach; perhaps these contrasting approaches warrant some discussion in the manuscript? Are there times when the double-ablation pit may be preferred with OptiPit optimization (perhaps with detrital analyses?), or are there times where the Maier et al. (2026) approach may be preferred? In the latter case, is there a way for OptiPit to be incorporated to properly determine the helium budget from the parent zonation maps?

One thing that may differ significantly in practice between the end-member Geometries A and B is the laser ablation down-hole fractionation of U and Th during parent ICP analyses. Geometry A results in a notably easier downhole-fractionation to correct for compared to Geometry B. May there be a case that an increased uncertainty/complexity in the Geometry B laser ablation fractionation correction actually further amplifies in-situ date disparities? I question whether this increased complexity in downhole fractionation may actually result in a preference for Geometry A, regardless if the OptiPit results favor Geometry B?

Whereas I appreciate the powerful demonstration of potential zonation impacts in section 3.4, I do not entirely understand the utility/application of this function beyond purely demonstrative purposes. Of course, with the double ablation approach, the lateral parent zonation gets averaged out, so it can only be assumed that x% of the shells are enriched/depleted by a factor of y. Although the authors suggest the use of other zonation proxies, this may still be largely indirect. For apatite as an example, it has been suggested that U and Th zonation do not always correspond with BSE/CL zonation patterns (Ault and Flowers, 2012), nor have many assessed Raman mapping on apatite (at least to my knowledge), and so it’s perhaps impossible to properly assess the impact of zonation in practice without true measurements (perhaps prior Ce mapping in apatite could help as a parent zonation reference). Without the direct measurement of zonation, is the utility of OptiPit zonation calculations aimed at simply providing a range of permissible dates based on assumed end-member zonation patterns within the ICP pit?

The authors state a couple times in the manuscript that radionuclide zonation and variable double-pit geometries controls age bias and dispersion inherit to the in-situ method. While in part true, these statements partly undermine the impact that thermal history mediated diffusion has on observed in-situ date dispersion. How do we truly distinguish between analytically induced in-situ (U-Th)/He date dispersion versus expected date dispersion in natural samples considering all the natural complexities involved in the (U-Th)/He system? Do measured data exist testing observed analytical disparities from contrasting pit geometries on large standard xenocrysts like Durango apatite, where we may expect spatially uniform in-situ (U-Th)/He dates?

On the topic of in-situ date variability: How much should inter and intragranular age dispersion should actually be embraced with in-situ dating, given that date variability has potential to provide even more detailed thermal-history information considering the complex kinetics that we partly understand (Glotzbach and Ehlers, 2024). Only in rare cases where a grain was rapidly quenched and unaltered after may we actually expect an in-situ, single spot (U-Th)/He date from the center of a grain to be interpreted as an age and match the grain’s bulk (U-Th)/He date. To my understanding, in its current state, OptiPit only assumes this comparatively simple scenario with no diffusion. In nearly all other thermal history scenarios, spatial variability in in-situ dates should be expected, as well as discrepancies between observed in-situ dates and the bulk grains (U-Th)/He dates. The authors are clearly aware of the effects of diffusion—they note that OptiPit does not currently implement diffusional loss and adequately discuss such complications in Section 4. Although I acknowledge it may be beyond the scope of this manuscript, it would be helpful to evaluate at first order the sensitivity of the pit geometries to an ablation pit with identical helium concentrations but varying spatial distribution within the helium pit controlled by (1) a rapidly cooled quenched thermal history, and (2) a steady-state, purely diffusive end member. Would we actually expect the parent-daughter imbalance mismatch to increase or decrease if we factor in the mobility of helium after its initial ejection emplacement?

Consider providing a simple ‘layman’ definition/explanation for the term ‘mismatch’ introduced in LINE 341 that may help improve the reader’s understanding of the figures.

OptiPit incorporates simple perfect cylindrical geometries for assessment. I’m curious if the authors have attempted to evaluate the impact of the Parent-Daughter imbalance with true, imperfect laser ablation pits measured from optical profilers?

The link provided to download OptiPit was dead, but I was able to easily search the database for the software. The software was relatively easy to open and test with the README file, though the experience could be improved with a user guide with screenshots and walk-through example.

LINE-BY-LINE COMMENTS:
LINE 19: Consider rephrasing to “IN PART controls the overdispersion”
LINES 67–68: Unclear what is meant by “production of meaningful He ages impossible in some samples” First, are the authors referring to ‘conventional’ dates, or in situ dates here? Second, I believe ‘impossible’ is bit strong here—datasets with complex, damage induced date-disparity can still provide meaningful information.
LINES 69–77: To me, a huge benefit of in-situ dating missing from this list is the ability to directly measure thermal history-dependent helium diffusive profiles in a grain at varying resolutions.
LINE 277: Replace “chapter” with “section?”
LINE 353: I don’t believe ‘inevitable’ is the proper word here? We should not ever expect a zero mismatch, correct? If so, consider rephrasing to state that a zero mismatch is inherently not possible with the double-pit in situ approach.
REFERENCES:
Ault, A. K., and Flowers, R. M., 2012, Is apatite U–Th zonation information necessary for accurate interpretation of apatite (U–Th)/He thermochronometry data?: Geochimica et Cosmochimica Acta, v. 79, p. 60-78.
Glotzbach, C., and Ehlers, T. A., 2024, Interpreting cooling dates and histories from laser ablation in situ (U–Th–Sm) ∕ He thermochronometry: a modelling perspective: Geochronology, v. 6, no. 4, p. 697-717. 10.5194/gchron-6-697-2024
Maier, A. K., Glotzbach, C., and Falkowski, S., 2026, Analytical and modelling strategies for thermal histories from in situ (U-Th-Sm) ∕ He data of single apatites: Geochronology, v. 8, no. 1, p. 165-189. 10.5194/gchron-8-165-2026
Citation: https://doi.org/10.5194/egusphere-2026-2590-RC1
RC2: 'Comment on egusphere-2026-2590', Christoph Glotzbach, 05 Aug 2026

This manuscript addresses an important methodological issue that arises when applying in-situ (U-Th)/He thermochronology. The authors investigate the complex geometric relationship between the helium extraction pit and the parent radionuclide analysis pit, and demonstrate how the geometry of the in-situ He and LA-ICP-MS pits influences the representativeness of the measured age. This is a relevant contribution that will be of interest to researchers applying in-situ thermochronology methods.
Overall, the manuscript is well structured and clearly written. The introduction provides an excellent overview of the motivation for the study and convincingly explains the need for the proposed approach. The modelling framework appears comprehensive and the study has the potential to become a useful reference for future analytical workflows.

My main concern relates to the presentation of the methods. I found the methodology section difficult to follow because it effectively requires the reader to reproduce the modelling step-by-step while reading the manuscript. I recommend moving much of this procedural information to the Supplementary Material and instead focusing the main manuscript on the concepts, consequences, and visualization of the modelling results. A detailed user guide for OptiPit, together with instructions for using the accompanying Excel spreadsheet, would be more appropriate as supplementary material.
I attempted to run OptiPit on a workstation PC, but the estimated computation time exceeded 1000 hours. It is possible that I selected inappropriate settings; however, this highlights the need for a detailed "How-To" guide and example project files that users can load directly into OptiPit. Such documentation would greatly improve the accessibility and reproducibility of the workflow.

The manuscript should also clarify whether the proposed optimization procedure needs to be performed for every individual grain or only once for a given mineral system and analytical setup. Performing these calculations for every grain would be highly time consuming and may not be practical, especially because LA-ICP-MS analyses are typically performed using consistent pit sizes throughout an analytical session.

In the discussion, I believe the interpretation of the modelling results is somewhat overstated. The authors explicitly acknowledge that their modelling does not account for kinetic effects, such as those associated with radiation damage, nor for helium diffusion. Therefore, the statement that symmetric zoning does not affect age determination provided that the He and ICP pits are appropriately selected is not fully supported. Indeed, the subsequent discussion acknowledges these limitations. I recommend revising these conclusions to better reflect the assumptions and limitations of the model.

An important practical aspect that should also be discussed is the difficulty of accurately measuring the volume of laser pits with low surface-area-to-volume ratios. Depending on the technique used for pit-volume determination, deep and narrow pits may not be measured accurately, introducing additional uncertainties that are difficult to quantify. Another methodological consideration that is currently not addressed concerns the effect of differing ICP pit geometries between unknown samples (two overlapping cylinders) and reference materials (simple cylinder). Variations in pit geometry could potentially influence LA-ICP-MS data reduction and calibration. Although I am not aware of previous studies that have explicitly investigated this issue, it has been one of the main reasons why I prefer a nested analytical approach (corresponding to the authors' Geometry A), in which a relatively wide and shallow He pit is followed by a slightly narrower ICP pit drilled within it. Besides minimizing potential calibration differences, this approach also allows radionuclide variations with depth to be quantified. In contrast, the proposed Geometry B would more likely produce a mixed signal when compositional zoning is present.

Overall, I find this to be a valuable methodological contribution that is suitable for publication after revision. Addressing the points above, particularly improving the accessibility of the methods and moderating some of the interpretations, will significantly strengthen the manuscript and increase its usefulness for the broader thermochronology community.
Please find a few additional minor comments…

Detailed comments:

Line 234-238: I had a quick look into the spreadsheet, looks complicated and I am somewhat lost in this part of the manuscript. Please explain in more detail what the user should do to make sure of OptiPit.
Line 243: You mentioned this earlier in the manuscript. It is quite obvious that your LA-ICP-MS pit must be the size of the HePit plus the maximum stopping distance to measure all area that contribute to the measured helium. Maybe you simply say somewhere that the maximum LA-ICP-MS pit size is constained by this assumption and you have tested within that constrain.
Line 277: I would not call it ‘schools of thought’ since there are not yet so many papers published with one or the other approach.
Line 349: I thought that the radius of the He pit in geometry A is 30 microns, and that is limiting the radius of the ICP pit to <30 microns. Same for geometry B with 25 microns He pit, while the best ICP pit is also 25 microns. Maybe you say in the sentence before that you also test cases where geometry A and B will be switching to B and A.
Line 350-351: For the geometry B both the He and ICP pit are essentially the same, therefore it is not sampling around the He pit.
Line 370: Use ‘larger contribution’ instead of ‘bigger share’ if that makes sense here.
Line 462: Have you done additional modelling that proves your statement? I would guess that the age offsets that you mentioned previously will increase with increase radionuclide ratios of >5. In addition in the sentence before you mentioned that you have not considered diffusion and kinetic effects and therefore you cannot strictly saying that accurately dating is possible. This also
Line 501-505: Mention also that the He content is also defining the size of the He pit. Grains with low radionuclide and/or age would require larger pit sizes and going deeper is also limited by the ability to measure precisely the volume of laser pits (which is more accurate in cases the laser pits are wide and flat).

Figures
Figure 5: This figure is partly difficult to understand and does require (at least me) to read the text several times. Part a) and d) are easier to get, but b) and c) are difficult and I wonder if this is important to show and can be shown in an easier way.
Figure 8: It is not clear what exactly is shown in the left panel. My guess it is the top view of the modelling domain. I do not really understand the lower x-axis labelling and what are the half circles? Please at least add more explanation in the figure caption.

Citation: https://doi.org/10.5194/egusphere-2026-2590-RC2

Hagen Hoemann, Mathias Hueck, and István Dunkl

Supplement

https://doi.org/10.5194/egusphere-2026-2590-supplement

Model code and software

OptiPit Software Version 1.1 and source code Hagen Hoemann https://rdms.rd.ruhr-uni-bochum.de/concern/datasets/rb68xh68k?locale=en

Hagen Hoemann, Mathias Hueck, and István Dunkl

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Short summary

Information about the evolution of the upper crust can be gained from the (U-Th)/He thermochronometer, of which the in-situ technique probes sub-volumes of entire grains. We demonstrate that date dispersion using this technique due to He displacement during radioactive decay can be attributed to the geometry of the analysed sub-volumes and its interplay with radionuclide zonation. OptiPit provides a tool to model and evaluate this interplay and to reduce dispersion of in-situ (U-Th)/He datasets.


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