the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jun 2026
Advancing meteorite impact chronology with in situ mica Rb-Sr dating
Abstract. Meteorite impacts are highly energetic processes that drives deformation on Earth under extreme pressure and temperature conditions, far exceeding those of typical crustal processes. Consequently, it may promote environmental perturbations, blossoming/extinction of life and even produce suitable conditions for the formation of mineral resources. Thus, constraining the timing of impacts is pivotal to shedding light on its role in Earth’s biogeodynamics, yet less than ~20 % of the impacts worldwide are precisely dated. Here, we present novel in situ mica Rb–Sr isotopes from the Australian Acraman and Gosses Bluff impact sites collected via LA-ICP-MS/MS, including single- and multi-collector instruments to expand the chronological toolbox for dating meteorite impacts. Whilst monazite (980 ± 28 Ma) and apatite (1448 ± 79 Ma) yield older ages compared to the expected Acraman impact age of 588 ± 35 Ma, in situ Rb–Sr from muscovite-bearing domains from the Acraman ejecta layer (580 ± 8 Ma; multi-collector age) and associated fine-grained zircon (598 ± 16 Ma) are consistent with the expected impact age. Similarly, apatite (132 ± 14 Ma) and mica-bearing domains (137 ± 9 Ma; multi-collector age) are comparable to the proposed impact age of 133 ± 3 Ma, whilst zircon yields mostly discordant data. The Rb–Sr results comparison between single- and multi-collector ICP-MS/MS has shown that the latter yielded significantly more precise Sr measurements, likely due to measurement of St isotopes with high resistor Faraday cups (1013 Ω) resulting in improved signal-to-noise ratio, consequently yielding more precise isotopic ratios and isochron ages. Our findings show that in situ Rb–Sr dating of micas formed during impact metamorphism offers the means for determining the timing of meteorite impact events. This approach effectively addresses the textural complexities commonly present in impact-related rocks, which are often overlooked by bulk isotope-dilution techniques, and delivers accuracy sufficient to establish the age of impacts. The effectiveness of in situ Rb–Sr dating of impact-related micas may be attributed to their higher resistance to hydrothermal alteration compared to Ar isotopes, which often yield complex degassing spectra and younger apparent ages such as for the Acraman impact. This study shows that in situ Rb–Sr isotopes of newly grown micas in impact-related rocks refines the chronology of impacts, with potential to increase the number of dated impacts globally with a low-cost, speedy technique with minimum sample preparation. Such task is crucial for understanding the role of meteorite impacts, including their potential influence on environmental crises, mass extinctions, and the emergence of life.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-3441', Thomas Belgrano, 10 Aug 2026
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AC1: 'Reply on RC1', Bruno Vieira Ribeiro, 04 Sep 2026
Response to Tom Belgrano (R1)
The paper presents the application of a suite of modern, robustly applied geochronological methods to two Australian impact crater case studies. The writing/presentation, especially the abstract, need some editing and a thorough proof read is needed throughout. Some more details are needed for completeness in the methods section, but these are minor revisions easily implemented, as the methods as employed seem rigorous. My only significant content request is to consider how accurate Rb/Sr corrections applied to the volcanic glass pellets are – whether matrix-matched glass or compositionally similar mica is more appropriate, and whether your data allow you to assess this. I would also like to see a more detailed explanation of what was propagated into the uncertainties, and to bring this into line with conventional external uncertainty reporting (if this is not the case already).
We would like to thank Tom Belgrano for his considerations on our manuscript. In the revised version, we include additional method information (including discussion on the error propagation approach) and address specific comments on matrix-matching. We hope this contribution will provide the means to expand the chronology of impact structures with in situ Rb–Sr dating.
Section/Line by Line Comments.
Abstract: The abstract is long at >400 words. Can this be refined into a more user-friendly length (i.e. <250–300 words)? It is also peppered with grammatical/style errors that need a go-over before resubmission.
The revised abstract has been shortened (down to 350 words) and revised thoroughly to eliminate grammatical errors.
L8. According to the journal rules, LA-ICP-MS/MS should be spelled out in the abstract and again at first use in the text.
We have defined the mentioned abbreviation in the revised abstract.
L10. The complete geochronometers need to be stated: mineral host and isotopic system.
We have added the complete information in the revised abstract.
L13/14. Which impact is the second sentence referring to?
The second sentence refers to the Gosses Bluff impact, which is now mentioned in the revised abstract.
L17. Misspelling of the symbol Sr? Enhanced precision is unlikely to be solely due to the amplification, as Faradays are generally similarly or even less sensitive than ion-counting QQQ detectors. The major precision jump derives from simultaneous, correlated detection of isotopes and higher ion yields.
Typo corrected. We agree that simultaneous isotopic measurements with higher ion yields would support greater precision through better counting statistics, which will be acknowledged in the revised version.
L30. It’s difficult to foresee how Rb-Sr could inform research into the emergence of life. How many unmetamorphosed impact related rocks are there from the Eoarchean? The paper is justified well-enough without this claim, or perhaps ‘extinction and diversification of life’ is a better fit?
Fair point, we were point towards the potential link between impacting and developing suitable locations for complex prebiotic to perhaps ultimately biological geochemistry. In any case this is peripheral to our thesis here and we have removed this sentence.
Section 2.
Readers of this section would benefit from splitting into sub-sections labelled by each impact.
We have sub-divided this section into sub-sections as suggested.
L76. *The age of the Acraman structure
Fixed.
Section 3.
L130. More details are needed on the uncertainty propagation method. If only one uncertainty is reported, this should be the fully propagated external uncertainty including errors on the calibration materials, decay constants, inter-session variance, etc. following e.g. Rosel & Zack, 2022, Gilbert and Zack 2024.
We have expanded on the error propagation in the revised version. To clarify, the uncertainty propagation for U-Pb data was carried out using Iolite’s MSWD approach as described in Paton et al. (2010), whereas the propagation for Rb–Sr follows the approach from Rosel and Zack (2022).
L148 + L169. State the reference values and how they were measured. The ages are missing units on L148.
Fixed.
L174. *Thermo Scientific. The acronym ICP-MS has already been introduced? Take the opportunity to introduce MC-ICP-MS/MS.
We have fixed this sentence.
L216-217. Need to state the results and reference values for the key secondaries here, as in the other method sections. Again, specify the uncertainty treatment (should follow the recommendations of Horstwood et al. 2016).
We have added the required information in the revised version.
Treatment of common Pb, even if uncorrected for, should be explained explicitly given the mix of minerals dated.
No common-Pb correction has been applied to any U–Pb data, as mentioned in the revised version.
Some Neoma labs experience ZrO-YO related interferences when analysing with NIST and biotite. This seems to have been overcome here, judging by the secondary ages, but its worthing mentioning why you are confident they are not an issue, as if present they would affect the standards but not the unknowns.
That is correct. We have observed a systematic influence of Zr-O oxide interference on the SrF peaks, which has been significantly reduced by the continuous flushing of the mass flow controller with ultrahigh-purity N2. This has been added to the text accordingly.
Some discussion of is needed as to matrix correction and validation of the glass pellet ages. Surely NIST would be a better calibration material than a mica? Can the authors show some downhole patterns (e.g. in the supplement at least) to support this one way or the other?
I believe this might be a confusion, as we have not dated glassy, amorphous materials. Instead, we targeted neocrystallized fine-grained micas from melt-impact rocks. In this sense, the adopted approach using mica reference materials to calibrate the matrix effects is appropriate, returning expected ages for the Gosses Bluff impact.
Section 4
Fig. 4./QQQ data: Why not combined the two labs data into a third isochron? If uncertainties are propagated conservatively using the higher for each parameter from each session, this should be defensible and looks like it would yield better-constrained ages. The one outlying btu included analysis from Gosses bluff (Fig. 4c) could be justifiably rejected and would likely improve the age correspondence between the two samples? It looks like a postulated secondary isochron could even be fit to all those outlying analysis, probably recording a younger devitrification/alteration event.
We prefer to maintain the results from distinct labs separately to assess their reproducibility instead of trying to fit a single regression through a large dataset.
The mineral ‘aliettite’ is uncommon enough it needs introduction at first use.
Aliettite has been simply described in the manuscript at its first appearance (section 4.1).
Section 5.
L342. We have experienced the same, it’s worth setting out why explicitly – superior low abundance sensitivity of the QQQ compared to multi-collection but higher LOD by Neoma (or whatever the analysts attribute this to).
Fair point! This is now acknowledged in the revised version.
L404. What is meant by the lower mass resolution issues referred to here? I struggle to see how this would have an effect. Lower sensitivity and sequentially detected isotopic ratios are more likely culprits.
Thanks for pointing it out. We meant to refer to lower sensitivity instead of mass resolution, which is now fixed in the revised manuscript.
Sect. 5.3 is good, really useful stuff.
Thank you!
Sect 5.4. Worth pointing out the lack of mineral reference materials for some of the envisioned work.
Indeed, this is worth mentioning. We have added this warning in the revised version.
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AC1: 'Reply on RC1', Bruno Vieira Ribeiro, 04 Sep 2026
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RC2: 'Comment on egusphere-2026-3441', Alicia Cruz-Uribe, 26 Aug 2026
Ribiero et al. present Rb-Sr dates determined by single and multicollector tandem mass spectrometry of impact rocks from the Acraman and Gosses bluff impact sites. I agree with the comments made by Thomas Belgrano, though in some cases I think there are a few more fundamental aspects of this work that need to be addressed in a major revision in order for it to be suitable for publication.
First and foremost, it is worth reiterating the lack of proofreading of the manuscript. Careful attention should be paid to the entire text, including subject headers (example: 3.1 automate mineralogy mapping). This is not grammatically sound in its present state; please address carefully in review. Thanks!
This also does not just apply to the basics of the grammar, but to some of the things that are said (or not). For example, lines 13-15 in the abstract are lacking the words “from the Gosses Bluff impact site”, or at least I think that’s what that sentence is referring to. Another example from line 251: “We collected 46 Rb–Sr data at the Adelaide University”. This is fairly nonsensical, and these mistakes are prevalent throughout the manuscript. Line 255: “At 1’Curtin University, we collected 53 Rb–Sr data”. Eek.
In situ should be italicized throughout.
Some notes about the methodologies presented: There is insufficient detail regarding the experimental setups of any of these labs. At a minimum, the ICP-MS parameters for every machine need to be included as supplemental tables. It is extremely difficult to assess the validity of the data, most particularly the multicollector data, without knowing the analytical setup. Additionally, in the text, the multicollector workflow needs to be detailed, including how the instrument was tuned. I feel this is a basic requirement for an untested system from this lab. Additionally, the analysis of only a single secondary mineral for Rb-Sr (LaPosta) is not sufficient, particularly given that the isochron from one session is 87 +- 4 Ma. This is not terribly convincing. Also, why were there no white mica reference materials run on the Neoma? The Acraman impact sample is dominated by white mica; reference to biotite is thus moot.
As the authors point out, matrix correction of Rb/Sr ratios is critical to account for the differences in fractionation between matrices (ala Giuliani et al. 2024). However, I do not believe there is sufficient evidence given to justify the use of the age correction on these materials in the way that I have to assume they have been done (as this is not well detailed in the text). It seems as though both samples are highly heterogeneous within each laser spot. The need for matrix correction arises from multiple processes, the sum total of which constitute the overall matrix effects in a sample. The problem with ablating domains that contain both muscovite and/or glass and/or feldspar and/or quartz is that each of these phases has 1) a distinct ablation threshold and behavior, and 2) distinct Rb-Sr fractionation behavior, leading to multiple differences in the ablation response and Rb-Sr fractionation compared to the two types of reference materials used (glass for baseline subtraction, biotite for age correction). The problems that can arise from this approach are massive differences in matrix, none of which can be characterized, and which are not directly comparable to any of the reference materials. So, how can we trust an age from a mixed sample that has been matrix corrected to a mica (and possibly the wrong mica, for instance, by using biotite to correct muscovite). The fundamental question is, do the materials analyzed behave like glass, or like mica? Or potentially, like neither?
Now, my understanding of the magnitude of fractionation on an ASI laser system is that it is less so than on an ESI or Teledyne machine due to primarily the difference in the objective lens, which effectively dictates how the focus of the laser changes downhole. However, there is no mention of the scale of the correction factors applied to any of the data. Again, please include this information in the supplementary material. Regardless of the magnitude, we know that there is still a correction that has to be done, and the authors need to demonstrate that the corrections they have done are valid for the materials in question. This is not insignificant, as the entire study hinges upon this.
Some comments on the single versus multicollector data: what are the internal precision numbers based on? Given the precision of the Sr isotope data, it appears that the precision on the Neoma is at least an order of magnitude too low for that instrument. However, it is difficult to assess the tuning protocol without the details given, so I think this can only be assessed after the manuscript has been revised. The point raised by Thomas about YO and ZrO interferences is valid, though again, impossible to assess with what is given in the methods. Please expand.
One small comment about the use of the word triple quadrupole: this was a terrible misnomer at the introduction of the 8800. None of these machines is a triple quadrupole. It really is more correct to just say quadrupole-based tandem mass spectrometer, or just ICP-MS/MS. Ok, soapbox rant over.
My final comment (since I realize that all of these comments are criticisms): I think this is really cool, and has potential. I really like the impact structure story, and the idea of using Rb-Sr to look at these systems. It will be a nice contribution once the details are cleaned up a bit.
Citation: https://doi.org/10.5194/egusphere-2026-3441-RC2 -
AC2: 'Reply on RC2', Bruno Vieira Ribeiro, 04 Sep 2026
Response to Alicia Cruz-Uribe (R2)
- Ribeiro et al. present Rb-Sr dates determined by single and multicollector tandem mass spectrometry of impact rocks from the Acraman and Gosses bluff impact sites. I agree with the comments made by Thomas Belgrano, though in some cases I think there are a few more fundamental aspects of this work that need to be addressed in a major revision in order for it to be suitable for publication.
We revise the manuscript to address the comments from the reviewer and correct some grammatical errors, improving the presentation.
- In situ should be italicized throughout.
The majority of publishers and journals accept the expression “in situ” as Roman, thus becoming unnecessary to italicized it. Given this is an established expression in the literature, we maintain this as is.
- There is insufficient detail regarding the experimental setups of any of these labs. At a minimum, the ICP-MS parameters for every machine need to be included as supplemental tables. It is extremely difficult to assess the validity of the data, most particularly the multicollector data, without knowing the analytical setup. Additionally, in the text, the multicollector workflow needs to be detailed, including how the instrument was tuned. I feel this is a basic requirement for an untested system from this lab.
We agree that including the ICP-MS parameters is helpful to demonstrate the tuning and analytical conditions of each instrument. Thus, we include them in the revised supplementary material. We however consider that a much more important and accessible method to access the “validity of the data” is through the secondary reference materials analysed, not the tuning settings of gas, lens nor collectors.
Both Curtin and Adelaide labs have well-established and published analysis routines, which has been appropriately referenced in the manuscript. Moreover, the analytical conditions applied to the multicollector ICP-MS analysis are similar to existing literature (e.g., Bevan et al., 2021; Dauphas et al., 2022; Cruz‐Uribe et al., 2023). To make the instrument settings even clearer, for the interested readership, we have included additional specific analytical conditions in the revised manuscript.
We disagree with the reviewer’s choice of “untested” with reference to the analytical approach used in this work. The GeoHistory Facility has produced published high-quality Rb–Sr data from micas and feldspar using the Neoma ICP-MS/MS system (e.g., Kirkland et al., 2025 GEOLOGY; Olierook et al., 2026 GGR; Rodrigues et al., 2026 JSG; and many other in preparation and under review).
- Additionally, the analysis of only a single secondary mineral for Rb-Sr (LaPosta) is not sufficient, particularly given that the isochron from one session is 87 ± 4 Ma. This is not terribly convincing. Also, why were there no white mica reference materials run on the Neoma? The Acraman impact sample is dominated by white mica; reference to biotite is thus moot.
We disagree that reference to biotite is moot. The relevant issue for Rb–Sr matrix correction is the fractionation behaviour of the matrix during ablation and analysis, not whether the reference material and unknown belong to exactly the same mica species. Published experimental work demonstrates that the principal mica varieties, including phlogopite, biotite and muscovite, exhibit closely comparable downhole Rb–Sr fractionation behaviour, as reinforced by direct comparisons among these matrices and glass (Giuliani et al., 2024; Glorie et al., 2023; Olierook et al., 2026). Accordingly, biotite, phlogopite or muscovite reference materials can be used to characterise and correct the relevant mica matrix effect. This is not an assumption specific to the present study, but an approach that has been widely and successfully applied in published Rb–Sr geochronology (Hogmalm et al., 2017; Li et al., 2020; Tillberg et al., 2020; Rösel and Zack, 2022; Ribeiro et al., 2022, 2023a, 2023b, 2026; Gyomlai et al., 2022; Kirkland et al., 2023; Larson et al., 2023; Almeida et al., 2023; Giuliani et al., 2024; Kellett et al., 2024; Nicomedes et al., 2025; Tollefson et al., 2025; Caxito et al., 2025; Perret et al., 2026; Jakobsson et al., 2026; Olierook et al., 2026; among many others).
Most mica reference materials can be analysed successfully by quadrupole ICP-MS/MS instruments using ion-counting detectors. In contrast, precise Rb–Sr analysis using the Neoma ICP-MS/MS is more strongly limited by signal-to-noise on the Sr isotopes. Many commonly used mica reference materials, particularly biotite and phlogopite materials, contain very low Sr concentrations (e.g., Rösel and Zack, 2021; Glorie et al., 2023; Giuliani et al., 2024; Olierook et al., 2026), making them challenging to analyse precisely on the Neoma. Long-term analytical testing at Curtin has demonstrated that Mount Dromedary biotite provides sufficiently high Sr signals and reproducible isotopic measurements to serve as a robust mica reference material on this instrument. Its suitability for matrix correction therefore derives from its demonstrated analytical behaviour, not from an assumption that the Acraman mica is biotite. This approach and its analytical performance are discussed in detail by Olierook et al. (2026).
We also disagree that the analytical validity of the dataset rests on the La Posta secondary reference material alone. The 87 ± 4 Ma result from one analytical session is admittedly relatively imprecise, but La Posta is an independent secondary monitor rather than the basis of the mica matrix correction. The validity of the analytical approach is instead supported by the demonstrated behaviour and long-term reproducibility of the mica reference material, the established equivalence of fractionation behaviour among mica varieties, and, importantly, the reproducibility and geological coherence of the ages obtained from the unknown samples. We have revised the manuscript to make the respective roles of the primary reference material and secondary monitor, and the basis for the matrix correction, substantially clearer.
- As the authors point out, matrix correction of Rb/Sr ratios is critical to account for the differences in fractionation between matrices (ala Giuliani et al. 2024). However, I do not believe there is sufficient evidence given to justify the use of the age correction on these materials in the way that I have to assume they have been done (as this is not well detailed in the text).
Matrix correction of Rb/Sr ratios, as applied in this study, follows established procedures that have been widely described and validated in the literature (e.g., Giuliani et al., 2024; Glorie et al., 2023, and references therein). The relevant methodology and supporting references were included in the original manuscript (e.g., lines 168–170 and 197–198). We agree, however, that the implementation of the correction in the present study was not described in sufficient detail to make the procedure fully transparent. We have therefore revised the Methods section to provide a clearer description of how the matrix correction was applied to these materials, including the basis of the age correction and additional reference to the original methodological studies.
- It seems as though both samples are highly heterogeneous within each laser spot. The need for matrix correction arises from multiple processes, the sum total of which constitute the overall matrix effects in a sample. The problem with ablating domains that contain both muscovite and/or glass and/or feldspar and/or quartz is that each of these phases has 1) a distinct ablation threshold and behavior, and 2) distinct Rb-Sr fractionation behavior, leading to multiple differences in the ablation response and Rb-Sr fractionation compared to the two types of reference materials used (glass for baseline subtraction, biotite for age correction). The problems that can arise from this approach are massive differences in matrix, none of which can be characterized, and which are not directly comparable to any of the reference materials. So, how can we trust an age from a mixed sample that has been matrix corrected to a mica (and possibly the wrong mica, for instance, by using biotite to correct muscovite). The fundamental question is, do the materials analyzed behave like glass, or like mica? Or potentially, like neither?
We agree that uncontrolled ablation of mixed mineral domains could generate complex matrix effects. However, this does not describe the analyses used to define the reported ages. Petrographic characterisation was used to determine what analyses could be grouped and yield meaningful geochronology. The Acraman ejecta is dominated by muscovite in the analysed domains, whereas the Gosses Bluff age is defined by pure aliettite domains; mixed aliettite–feldspar analyses were identified and excluded from the age interpretation.
We however disagree that use of biotite to correct muscovite represents an inappropriate matrix correction strategy. Published work demonstrates that biotite, phlogopite and muscovite exhibit closely comparable Rb–Sr downhole fractionation behaviour, including direct comparison with glass standards (e.g., Olierook et al., 2026). This supports the interchangeable use of mica reference materials for matrix correction. A more important matrix issue is the actual nature of the mica is it sheets, a powered, orientated grains among others.
Finally, the resulting Rb–Sr ages agree with independently established impact ages and with U–Pb zircon and apatite geochronology. This provides an important external validation that any residual matrix effects have not materially biased the reported ages.
- Now, my understanding of the magnitude of fractionation on an ASI laser system is that it is less so than on an ESI or Teledyne machine due to primarily the difference in the objective lens, which effectively dictates how the focus of the laser changes downhole. However, there is no mention of the scale of the correction factors applied to any of the data. Again, please include this information in the supplementary material. Regardless of the magnitude, we know that there is still a correction that has to be done, and the authors need to demonstrate that the corrections they have done are valid for the materials in question. This is not insignificant, as the entire study hinges upon this.
We have included the correction factor to account for the matrix effects in the revised manuscript. In our view, the results from secondary reference materials are sufficient to demonstrate that the matrix correction was appropriately carried out. These results, along with reference ages for each reference material, have also been included in the revised manuscript, following Thomas Belgrano’s suggestions. Such inclusion only reinforces the robustness of the employed analytical approach and data.
- Some comments on the single versus multicollector data: what are the internal precision numbers based on? Given the precision of the Sr isotope data, it appears that the precision on the Neoma is at least an order of magnitude too low for that instrument. However, it is difficult to assess the tuning protocol without the details given, so I think this can only be assessed after the manuscript has been revised. The point raised by Thomas about YO and ZrO interferences is valid, though again, impossible to assess with what is given in the methods. Please expand.
For comparison reasons, the internal precision on the isotopic ratios is a purely associated to the internal uncertainty disregarding external uncertainties. This provides the means to establish a direct comparison between single-collector and multi-collector performance. Expanding on the YO-ZrO comment from Thomas Belgrano, we have included additional technical information on the method description such as the continuously flushing the reaction cell with high-purity N2 (99.999%) to reduce oxide levels minimizing the interference effects of YO-ZrO.
- One small comment about the use of the word triple quadrupole: this was a terrible misnomer at the introduction of the 8800. None of these machines is a triple quadrupole. It really is more correct to just say quadrupole-based tandem mass spectrometer, or just ICP-MS/MS.
We acknowledge that “triple quadrupole” may not be strictly accurate terminology. However, the term is the most widely used term for this form of analysis in the literature and thus is well understood within the analytical community to refer to this instrument configuration. We have therefore retained the terminology used in the manuscript for consistency with common usage.
- My final comment (since I realize that all of these comments are criticisms): I think this is really cool, and has potential. I really like the impact structure story, and the idea of using Rb-Sr to look at these systems. It will be a nice contribution once the details are cleaned up a bit.
Thank you. We appreciate the constructive comments and have taken them seriously in revising and clarifying the manuscript. We hope our responses address the concerns raised and demonstrate why we remain confident that the reported results are robust and accurate within their stated uncertainties.
Citation: https://doi.org/10.5194/egusphere-2026-3441-AC2
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AC2: 'Reply on RC2', Bruno Vieira Ribeiro, 04 Sep 2026
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Review of Ribeiro et al. “Advancing meteorite impact chronology with in situ mica Rb-Sr dating” for Geochronology, August 2026
The paper presents the application of a suite of modern, robustly applied geochronological methods to two Australian impact crater case studies. The writing/presentation, especially the abstract, need some editing and a thorough proof read is needed throughout. Some more details are needed for completeness in the methods section, but these are minor revisions easily implemented, as the methods as employed seem rigorous. My only significant content request is to consider how accurate Rb/Sr corrections applied to the volcanic glass pellets are – whether matrix-matched glass or compositionally similar mica is more appropriate, and whether your data allow you to assess this. I would also like to see a more detailed explanation of what was propagated into the uncertainties, and to bring this into line with convetional external uncertainty reporting (if this is not the case already).
With these minor revisions and touch ups throughout, this will be a useful contribution and no doubt will be made use of by further studies looking to reapply and develop impact-dating.
Dr. Thomas Belgrano, University College Dublin
Section/Line by Line Comments.
Abstract: The abstract is long at >400 words. Can this be refined into a more user friendly length (i.e. <250–300 words)? It is also peppered with grammatical/style errors that need a go-over before resubmission. Examples are listed below:
In the first sentence: ‘processes that drives’.
In the second sentence, reference to the previous sentence's subject as ‘it’, but there are several nouns in the previous sentence this could ambiguously refer to, and the one the authors mean is plural in any case, so ‘it’ is not appropriate.
L8. According to the journal rules, LA-ICP-MS/MS should be spelled out in the abstract and again at first use in the text.
L10. The complete geochronometers need to be stated: mineral host and isotopic system.
L13/14. Which impact is the second sentence referring to?
L17. Misspelling of the symbol Sr? Enhanced precision is unlikely to be solely due to the amplification, as Faradays are generally similarly or even less sensitive than ion-counting QQQ detectors. The major precision jump derives from simultaneous, correlated detection of isotopes and higher ion yields.
L30. It’s difficult to foresee how Rb-Sr could inform research into the emergence of life. How many unmetamorphosed impact related rocks are there from the Eoarchean? The paper is justified well-enough without this claim, or perhaps ‘extinction and diversification of life’ is a better fit?
Section 2.
Readers of this section would benefit from splitting into sub-sections labelled by each impact.
L76. *The age of the Acraman structure
L93. It would be good to introduce the relatively uncommon term suevite (e.g. on L89 at mention of impact melt breccia).
Section 3.
L130. More details are needed on the uncertainty propagation method. If only one uncertainty is reported, this should be the fully propagated external uncertainty including errors on the calibration materials, decay constants, inter-session variance, etc. following e.g. Rosel & Zack, 2022, Gilbert and Zack 2024.
L148 + L169. State the reference values and how they were measured. The ages are missing units on L148.
L174. *Thermo Scientific. The acronym ICP-MS has already been introduced? Take the opportunity to introduce MC-ICP-MS/MS.
L216-217. Need to state the results and reference values for the key secondaries here, as in the other method sections. Again, specify the uncertainty treatment (should follow the recommendations of Horstwood et al. 2016; https://doi.org/10.1111/j.1751-908X.2016.00379.x).
Treatment of common Pb, even if uncorrected for, should be explained explicitly given the mix of minerals dated.
Some Neoma labs experience ZrO-YO related interferences when analysing with NIST and biotite. This seems to have been overcome here, judging by the secondary ages, but its worthing mentioning why you are confident they are not an issue, as if present they would affect the standards but not the unknowns.
Some discussion of is needed as to matrix correction and validation of the glass pellet ages. Surely NIST would be a better calibration material than a mica? Can the authors show some downhole patterns (e.g. in the supplement at least) to support this one way or the other?
Section 4
Fig. 4./QQQ data: Why not combined the two labs data into a third isochron? If uncertainties are propagated conservatively using the higher for each parameter from each session, this should be defensible and looks like it would yield better-constrained ages. The one outlying btu included analysis from Gosses bluff (Fig. 4c) could be justifiably rejected and would likely improve the age correspondence between the two samples? It looks like a postulated secondary isochron could even be fit to all those outlying analysis, probably recording a younger devitrification/alteration event.
The mineral ‘aliettite’ is uncommon enough it needs introduction at first use.
Section 5.
L.331-335. See comment on additional sources of increased precision for the abstract.
L342. We have experienced the same, it’s worth setting out why explicitly – superior low abundance sensitivity of the QQQ compared to multi-collection but higher LOD by Neoma (or whatever the analysts attribute this to).
L404. What is meant by the lower mass resolution issues referred to here? I struggle to see how this would have an effect. Lower sensitivity and sequentially detected isotopic ratios are more likely culprits.
Sect. 5.3 is good, really useful stuff.
Sect 5.4. Worth pointing out the lack of mineral reference materials for some of the envisioned work.