The work submitted is exactly the kind of study that needs to be carried out as the broader community continues to apply in situ beta-decay geochronometers to address geological problems. I find the work to be of high quality, from the experimental setup through to discussion.

As with any study, there are a few changes that could be made that would further improve the work and ensure consistency in interpretation.

1. The authors describe mineral assemblages and textures in detail, yet they are supported by a single mXRF map and 4 ppl images. It would be appreciated if the descriptions could be better illustrated by additional images either in the main text or as supplemental files so the reader can see what is being described. Directly parallel to this is better referencing of all supplied figures within the text.
2. Some of the main points of the paper are based on the interpretation of model Rb-Sr dates; however, detailed information about how the dates were calculated is not provided. The existing information in the Methods section is vague and does not articulate specifically how uncertainties were propagated. Given the central role these dates play in the overall narrative, their calculation needs to be well documented.
3. Finally, in the finalization of the Rb/Sr data, the data from the mica in the study are normalized against a biotite reference material of known age to compensate for a systematic offset between the primary reference material (pressed nanopowder) and actual mica grains. It is specifically noted that no additional uncertainty was added to the unknowns during the final normalization step. It is later noted in the discussion that the biotite reference material returned a date of 452 ± 7 Ma. That uncertainty is significant and, if converted to uncertainty on the slope of the 452 Ma isochron, it would equal ~ 0.777%. Given that all unknown mica are normalized relative to that 452 ± 7 Ma result, it is important to propagate the uncertainty associated with that normalization through to the final dates. Doing so has direct implications for interpretation. For example, an isochron date of 1338.4 ± 6.5 is noted as being older than the expected 1322 Ma date of pluton intrusion (source of head for the contact metamorphism), indicating something within the Rb-Sr systematics. If the uncertainty on the matrix normalization is propagated to the slope on the isochron in quadrature, however, the uncertainty increases to 21.60 Ma, which overlaps the expected 1322 Ma date. A similar interpretation is made about the 1332.2 ± 8.4 Ma date, which, once the uncertainty on the matrix normalization is propagated through, has an uncertainty of 22.16 Ma, which also overlaps with the 1322 Ma expected date. Perhaps there are other, more appropriate ways of propagating the uncertainty from the isochron used to matrix normalize the mica data, but regardless of how it is done, it should be incorporated into the study so that nuanced interpretations can be made.

I want to again emphasize the need for this kind of systematic study for these developing methodologies and my appreciation to the author for taking the time to do it.

Most of the comments that I have are pretty minor. I really like this case study, and I think the partial resetting is very clearly shown. The only, and I mean the only thing that I think makes this not publishable in its current form is the lack of secondary reference materials. This needs to be addressed.

First question – how many secondary reference materials were analyzed, and did their ages overlap with known ages? There is reference to a single in-house biotite material from Scotland, but no information is given about it.

Second question – if the material from Scotland, which has not been shown to be a reference material, since it isn’t reported to have a known age, is the only secondary material analyzed, then how can we trust the ages? It appears that the method of Giuliani et al. (2024) is applied (please cite here) in order to do an isochron-based correction on the 87Rb/86Sr ratios. This is a great approach, but then it has to be applied to at least one (if not more) secondary reference micas to show that the correction works properly. This is not demonstrated. So, there is no traceability for the geochronology. The dates that are reported are absolutely fine relative to one another, and I think the consistency with the SHRIMP dating suggests that there is no significant bias, but without independently showing that the ages of secondary micas can be determined, then the foundation of the geochronology is shaky. I think the details of the age correction need to spelled out (and I agree with the previous comment about propagating uncertainties), and secondary reference biotite grains of known age (not in-house materials) need to be dated to determine the validity of the method.

The other thing that I think would really help strengthen this study overall would be to include some of the trace elements in the analysis. In my lab we have shown without a doubt that Rb does not react with SF6 – the author also points this out. You can save counting time by not measuring 104RbF and instead count for 5 ms on Li, Cs, Ba, and a number of other elements. Figure 9 is good, but it would also be interesting to see what these other trace elements are doing during this. We have observed some really interesting patterns between TE and partial resetting of micas in the Alps, and Cs and Ba are particularly telling. Obviously this can’t be added after the fact, but it might be useful to note for the future. See work by Kunz and others on TE in micas.

Other comments:

Italicize in situ throughout, as is the standard for Latin words

Line 10 and throughout: I suggest that LA-ICP-MS/MS is the accepted abbreviation for this type of tandem mass spectrometry. That is what we have tried to establish in GGR. It allows for the addition of MC on the beginning, if needed, and refers to any MS/MS machine. Also, the ICP is always before the MS/MS anyway, so I would argue that it is more physically correct as well.

In the abstract, the author goes back and forth between past and present tense. I suggest keeping things present tense except for methods that were performed. For example, line 16 should be retain, not retained, as these biotites display in the previous sentence.

Introduction – might be helpful to add a little bit of the debate about the experimental constraints here too (Hofmann and Giletti 1970 vs Hammouda and Cherniak 2000), and the work of Villa 2016. I know that the experimental constraints are a mess, but it’s worth noting that as an important part of why the natural laboratory provides better constraints for real-world diffusion in micas

Line 51 – this might be a good place to mention that high precision is achieved by LA-MC-ICP-MS/MS (Bevan et al. 2021, Craig et al. 2021, Dauphas et al. 2022, Cruz-Uribe et al. 2023, Huang et al. 2025). So the precision on an individual spot by quadrupole is lacking, but the in situ Rb-Sr method has the potential for extremely high precision. I think it is also worth citing a little bit more of the in situ Rb-Sr literature here, particularly studies such at Kutzschbach et al. 2024 (mapping), Giuliani et al 2024 (data processing), Stijn Glorie, etc. Also, since this manuscript is about single spot model dates, it is important to cite Rösel and Zack (2022) and Cruz-Uribe et al. (2023) in this context.

Line 60 – I think this is a great test case. We’ve been hunting for years for the best way to isolate the thermal component, and this seems to be as good as it gets

Figure 3 – all of the minerals are labeled except the biotite

Line 201 – question about P/A factors: I think we’ve all been having this discussion in the 8900 community for a while. As far as I can tell in talking with folks in the factory in Japan, it is not actually possible to tune the P/A factors in mass-shift mode. After years I still haven’t been able to force my machine to do this. I think we all have different ways of dealing with it, and I just wondered if you had a magical trick to make it work! I’ve found that the precision of the P/A tune (which is done without gas, despite trying to trick it into using gas) is not good enough to have linearity in the isotope ratios. I don’t think it’s an issue with anything you’re doing, I’m really just curious if you’ve found a way around the factory problems.

Lines 205-220 please refer to these as reference materials as opposed to standards. Thank you from the IAG and ISO.  

Line 215 – we have also noticed a pretty big difference in ablation between plagioclase and glass, particularly NIST glass, using a ns laser. Have you thought about using BCR-2G or BHVO-2G for the Sr isotopes in plag?

Additional methods comments – I assume you are using 85 to calculate 87Rb, as the reaction is very efficient but not complete. Please give these details, and cite Meija for the IUPAC 85/87 ratio. Please also give ICP-MS dwell times for each isotope

Section 5.1 – these are great. I think the approach is very similar to Cruz-Uribe et al. (2023) who used KDE plots to examine the distribution of single integration dates across multiple spots in sample. I think it is important to cite that method here. I realize these are single spots instead of single integrations, but I think it is a similar enough approach that it warrants a citation. I like the statistical approach that you are taking though, and I am glad to see these types of approaches being used to interrogate geochronologic data. This is really important for the Rb-Sr system.

Figure 6 – it would be helpful to have the label of inner aureole on the figure itself – that was helpful on the previous two figures. The same would be helpful for Figure 8 (melt film pseudomorphs at contact)

Lines 350-355. I think it’s also worth mentioning here that the chemical potential gradient that is the driver for diffusion might induce back-diffusion of Sr into biotite if the Sr is more compatible in biotite than in garnet (or result in basically no chemical potential gradient). I don’t know the relative compatibilities of Sr in grt and bt off the top of my head, but in measuring both, my instinct tells me that the Sr is perfectly happy in bt, and so there might not be a driver for diffusion out of the biotite. Then it doesn’t matter what the diffusion rates are, if there is nothing actually driving diffusion. Now, biotite in the matrix would be a completely different story, because now we have potential fluids within grain boundaries, minerals like plag, etc, that certainly have higher distribution coefficients for Sr than biotite would.

Lines 390-405. This is very interesting. Cruz-Uribe et al. (2023) address this concept as well, and I think a citation of Davies et al. (2018) would not go amiss here.