| 02 Jul 2025
Cosmogenic 3He exposure dating in mafic rocks by 'Virtual mineral separation' of pyroxene
Abstract. We describe a ‘virtual mineral separation’ method for measuring the cosmogenic 3He concentration in pyroxene in mafic rocks that consist mainly of plagioclase and pyroxene, without physically separating the minerals. This approach is significantly faster and more cost-effective than the conventional method, which requires physical separation and purification of pyroxene grains by time-consuming and labor-intensive crushing, acid cleaning, magnetic separation, HF etching, and handpicking under a microscope. The premise of the method is that helium diffusivity is much higher in plagioclase than in pyroxene, so controlled preheating of a mixed whole-rock sample can degas 3He from plagioclase while effectively retaining all 3He in pyroxene. A second heating step releases all 3He from pyroxene for measurement. To then obtain a 3He concentration in pyroxene rather than the whole rock, we determine the pyroxene weight fraction in the sample using X-ray computed tomography (CT). A comparison of 3He concentrations in pyroxene measured using virtual mineral separation with those measured in the same samples by physical mineral separation in previous work shows no evidence of systematic bias between the methods. Virtual mineral separation greatly simplifies the workflow for 3He exposure-dating of mafic rocks, reduces time, effort, and cost, and permits measurements on very small samples. This enables new emerging applications of exposure dating, such as quantifying stochastic surface processes, ecosystem studies, and potential subglacial bedrock exposure dating.
Competing interests: Greg Balco is a member of the editorial board of Geochronology
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Review Geochronology
Title: Cosmogenic 3He exposure dating in mafic rocks by ‘Virtual mineral separation’ of pyroxene
Author(s): Marie Bergelin, Greg Balco, and Richard A. Ketcham
MS No.: egusphere-2025-3033
MS type: Research article
Summary short
The manuscript presents a new approach to the preparation and measurement of 3He in pyroxenes from whole rock, that significantly reduces labour time and enables higher sample throughput. A combination of heating steps, based on the different diffusivity of 3He in other minerals (here mafic rock and plagioclase) and the analysis and calculation of the total pyroxene content using X-ray computer tomography is presented to obtain this goal. The obtained data is compared to the standard preparation technique and evaluated to be ‘indistinguishable from those derived from physical mineral separation, with no evidence of systematic bias’.
General Comments
The manuscript presents a solid methodology and a well-introduced approach. There are no concerns regarding the methods applied or the results obtained. The only issues identified are minor formal aspects that should be addressed. The manuscript is well written, and the reference list is complete. These small changes do not impact the overall quality or validity of the work, and the manuscript can therefore be considered acceptable with minor revisions.
Please make clear if the diffusion experiments and especially the model for gas release was conducted in this project or are largely based on Bergelin et al. (2025).
I understand that you incorporated the results of Method 1 directly in the same paragraph (the same goes for Method 2). Maybe think of changing the subheadings to Method and Results or something similar where you can directly incorporate your results. Otherwise differentiate between the method section and the result section (see Method 3).
Please find in the following some section/paragraph comments, as well as some line to line comments. In addition, I have made some comments, which I included in the attached PDF.
Introduction
Line Section 52-60: Avoid reporting from the we perspective if you refer to your previous paper, as it might be confusing for the reader if you are now writing about the recent project or referring to the published project.
Method Section
2.1. Diffusion kinetics
Avoid writing from the me/we perspective. In the Method Section (2.1 Diffusion kinetics..) it sounds like you are already discussing data from this new study. Rather write for example: In a recent study, Bergelin et al. (2025) found the diffusion….. This section sounds already like you are showing new results from this manuscript. Differentiate more between what was done compared what is new and the result of this new project.
Line 101: Please add the oven and with which device you captured the fractional release. You cited Bergelin et al. 2025, but please just add this information here.
Line 109 on: Are these results of the fractional releases from the previous work Bergelin et al. 2025 or from this work? If these are new results, they should be placed in the results section.
Fig. 1: Please add a and b to the Figure as mentioned in the figure caption. Maybe it is useful if you also add a cumulative plot as you are referring to them in the text ( At 260°C plag…released 97%..).
Please clarify if the modelling is new or done already in Bergelin et al. 2025, or if you model the fractional gas release of plag and pyrx based on results of and presented in Bergelin et al. 2025. Checking Bergelin et al. 2025, it looks like most of this is done/based in Bergelin et al. and serves here as an extra verification for the step-heating separation.
Line 144 on: Also sounds like results from your modelling and should be placed in the result section.
2.2 Validation
Table 1: Results should be placed in the result section or rename the subheadings. Add 3He to calculated conc. (atoms/gr). Source = Reference. See also paragraph Line 176 to 182.
I understand why you integrated directly the results here, as being the basis for the next step, the virtual separation. Maybe change subheadings.
3.1. Sample description and preparation
Line 200: Why did you choose this grain size fraction, compared to the previously introduce pyroxene separates?
3.2 CT scan
CT Scan duration 1sample /1hr?
The frequency distribution was calculated by the Zeiss software or Blob3D? How long does it take for the software to analyse one sample (so 1200 16-bit TIFF images). You might want to add, that Blob3D is a free software (easier access for other scientist if they want to use your virtual approach).
If I understand right, this segmentation strategy was done on each of 1200 16Bit TIFF images per sample, the attenuation groups where then cumulative summed up per sample and % calculated for high attenuation (i.e. other oxides), pyroxene and plagioclase?
Can you incorporate one whole-rock analysis as image (with pyxr, plag and oxides marked with colour) and include this into Fig. 3 for comparison?
3.3 Pyroxene weight percent estimation
Rephrase Fig. 4 caption and text. Fig. 4 shows the mineral composition of your pyroxene separates differentiated by XRF analysis by Bergelin et al. (2024).
3.4 Validation of weight percent estimation
Results in Method Section.
4 Method 3
Cosmogenic 3He measurements
Some initial results (for the CRONUS-P standard) are presented, which might be better placed in the results section. Moreover, is there any reason or explanation for the difference in the standard measurements compared to the CRONUS-P value?
These are the results from method 3 basically. Either integrate them into the method 3 part (similar to the other methods) and rename the section or include all results in here.
I would also mention the high difference between the whole rock samples as listed in Table 3. Maybe mark them in the plot of Fig. 5A, B.
6.1 Reproducibility
Capitalize aliquots A B C.
Can you explain or give an interpretation for the much lower CRONUS-P standard deviation in Fig. 6?
6.2 Inclusion of ilmenite as a trace mineral
Are there any other oxide minerals, which should be considered which retain helium? 3He concentrations are slightly lower in the magnetic separates, I would question the distinguishably.
Can you add the data with the correction for oxides in the supplementary dataset?
6.3 Sources of scatter in data set
Line 436-443: Maybe it’s worth to conduct XRD (in the future) on a few samples (maybe with highest differences) to get information about the mineralogy of an entire sample?
Maybe add to a modified ‘fast’ virtual separation method a second or prolonged mixing to ensure that any heterogeneity is as good as it gets avoided or mixed? It will not affect the sample preparation significantly.
6.4 Advantages and disadvantages
Can you estimate the sample processing time for a pyroxene-separated vs virtual separated sample?
You discuss the whole-rock method, but do not discuss the big differences in 3He between the virtual and the pyroxene separated sample, see Table 2 sample 16-ROB-52-BAS. Is the extra gas cleanup and helium purification the main issue?
6.5 Implications of virtual mineral separation
Line 532: Maybe you want to add,”if the specific requirements for whole-rock chip He analysis are met, then sampling of numerous single rock chips….”