Phlogopite ⁴⁰Ar/³⁹Ar Geochronology for Guodian Skarn Fe Deposit in Qihe–Yucheng District, Luxi Block, North China Craton: A Link between Craton Destruction and Fe Mineralization

Round 1

Reviewer 1 Report (Previous Reviewer 1)

Comments and Suggestions for Authors

I have thoroughly reviewed the manuscript titled "40Ar/39Ar Geochronology of Phlogopite for Guodian Skarn Fe Deposit in Qihe-Yucheng District, Luxi Block, North China Craton" and I must commend on the improvements made since the initial version. The quality of the manuscript is commendable, and it significantly enhances our understanding of this skarn system.

However, I have some constructive feedback that I believe will further strengthen the manuscript. In particular, I urge you to delve deeper into the discussion regarding the relationship between the ages of phlogopite and magnetite. While it is plausible that they are almost contemporaneous, the evidence presented in Figures 6 and 7, as well as the paragenetic table, suggests that magnetite postdates phlogopite. The distinction between the age of these two minerals needs to be clarified, as the current evidence does not conclusively support it. It is imperative to provide a more robust argument to establish the chronological sequence accurately.

Furthermore, I encourage you to expand on the connections between skarns and the tectonic collapse. Merely discussing ages does not necessarily imply a direct correlation with the same geological event. There could be multiple collapse events and magmatic stages within a few million years. A more detailed examination is warranted to elucidate and substantiate the expected impact of this contribution to the geological evolution of the region in China.

Detailed corrections and suggestions are referred in the attached PDF

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

Thank you very much for your time involved in reviewing the manuscript, and offering such detailed and helpful comments to further improve the manuscript. We appreciate your clear and detailed feedback and hope that the explanation has fully addressed all of your concerns. Detailed responses are referred in the attached PDF。

Author Response File: Author Response.pdf

Reviewer 2 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

This is my review of the revised manuscript entitled, "40Ar/39Ar geochronology of phlogopite for Guodian scars Fe deposit in Qihe-Yucheng district, Luxi Block, North China Craton: A link between destruction and Fe mineralization.

If line numbers are mentioned, they are in relation to either the "Track Changes" version of the manuscript or my original review, which will be noted when quoted.

The manuscript has been significantly re-written and the flow/grammar has been greatly improved. I also think the general conclusions are supported by the reported data. However, I unfortunately have to again recommend rejection and/or major revisions. I'm confused by the authors' responses to my original review. The "Author Responses" file accompanying this submission chooses to only address portions of my original review. It is also evident that the authors chose to only address a small portion of the detailed line-by-line comments that I provided. If the authors do not agree with any of the comments, or do not wish to not make certain changes, that is fine. However, it is standard practice to, at a minimum, address ALL the comments in the response to the reviewer AND justify when changes aren't made. A considerable amount of time gets spent on detailed reviews, and that effort should be reciprocated in the responses. Again, I think the general conclusions are probably supported (though additional detail is needed still), which adds to the confusion of why the original comments from my first review were not addressed in full.

That said, I will briefly highlight my remaining issues with the manuscript. The focus will be on the Author Response document I received, but I encourage a full re-reading of my original review. I'll also highlight where my original comments were clearly ignored and/or not addressed.

The "Original Major Comment" or "Original Minor Concern" headings below relate to the Author Response document I was provided. I want also note that the text associated with the numbered "Major comments" and "Minor Concerns" in the Author Response document were shortened from my original review and are lacking important context.

Original Major Comment 1 & 2: The authors have added the information needed to satisfy my original comments relating to the text describing experimental procedures, constants, and standard reference information. Though the data reporting and presentation is still inadequate (as described below).

Original Major Comment 3: The authors now state that the plateau age error is at the 2-sigma level. Please specify if this also relates to the isochron ages. Additionally, this still does not answer my original question about what is incorporated into this uncertainty. Is this just the internal uncertainty? Does it include the error on the J-value? Does it include the decay constant uncertainty? As mentioned in the original comment, this information is necessary before ages, especially from different isotopic systems (e.g. U-Pb in zircon), are compared against one another.

And while it's understood that the ArArCALC software calculates the plateau ages. This does not adequately describe the criteria that was used when deciding what heating steps were included in the plateau calculations for each sample. The user can normally force plateaus to be calculated on certain steps, or they can select a statistical method that allows the software to choose on its own. This criteria needs to be mentioned, as I suspect it will play a role in explaining the large MSWD values discussed later.

For added content, my original review included this comment in the line-by-line comments: "Lines 226-228: What criteria was used to calculate the plateau age? There are a number of published and well-used criteria to follow (e.g. Taylor, 1997; Dalrymple and Lanphere, 1969, 1974; Fleck et al., 1977). The choice of criteria is up to the authors, but it needs to be explicitly mentioned."

Original Major Comment 4:

The addition of moles of 40Ar to Table 1 is an improvement, but it is still not adequate. For example, there are no uncertainties for any of the ratios, so a reader would not be able to recreate the isochron plots from this data table.

As far as I'm aware, there are no space limitations for data tables in this journal. So there is no reason the table included in Reviewer Responses shouldn't be included the main manuscript. If there are space limitations, it should be provided as a supplementary file.

Also, as clearly stated in my original review... "The 2020 paper by Schaen et al. (2020) (Title: Interpreting and reporting ⁴⁰Ar/³⁹Ar geochronologic data) presented community agreed best practices for conducting and reporting ⁴⁰Ar/³⁹Ar data. Table 4 of that publication lists the minimum and recommended data that needs to report for ⁴⁰Ar/³⁹Ar data. I advise the authors to follow the guidance in this paper as it not only provides added confidence in the data produced, but also added capabilities for the data to be used by other researchers."

 

Original Major Comment 6:

Yes, the internal variability and the precision of the individual measurement steps add the increased MSWD value. And the authors are also correct that the isochron and inverse isochron values agree well with the plateau age. However, while that means the data are reliable, it does not necessarily mean the age and its reported uncertainty are reliable.

By definition, the high MSWD means you are underestimating the reported age. Again, I don't know what criteria was used to calculate the plateau ages, nor do the authors specify what steps were used to calculate the isochron ages. Were these the same steps as the ones used to calculate the plateau age?

Original Minor Concern 2:

Even if the grain is fresh, there is no discussion or explanation of the young apparent ages at the beginning of the step-heating experiment. If it's not contamination, and authors don't think it's re-heating, then what is it? It can't just be ignored. I mentioned in the original comment about checking the isochron regressions in more detail (i.e. what steps were used?) , but this was not addressed.

Detailed Comments:

Instead of providing new detailed comments, I've highlighted important instances where comments from my original review were either ignored or not addressed either in the manuscript or in the authors' responses to my review.

The below comment from my original review was not addressed:

"Line 216: It is mentioned here that the plateau age is given as 2s uncertainty, but is this just the internal uncertainty? Does it include the uncertainty in the calculated J value? Does it include the uncertainty in the chosen decay constants? Since the authors are comparing this age to U-Pb zircon ages, the fully propagated uncertainty needs to be used."

The below comment from my original review was not addressed:

"Lines 226-228: What criteria was used to calculate the plateau age? There are a number of published and well-used criteria to follow (e.g. Taylor, 1997; Dalrymple and Lanphere, 1969, 1974; Fleck et al., 1977). The choice of criteria is up to the authors, but it needs to be explicitly mentioned."

 

The below comments from my original review regarding Table 1 were not addressed:

"Table 1: The steps included in the plateau calculation and isochron regressions should be marked in a separate column or by bolding appropriate lines of data.

Table 1: See comments above about minimum reporting requirements."

The below comments from my original review regarding Figure 8 were not addressed:

"Figure 8a: Include the steps along with the temperature label for what’s included in the plateau age (e.g. steps 7-14). The level of uncertainty should be listed either on the figure or in the caption.

Figure 8b: As above, the level of uncertainty should be listed either on the figure or in the caption.

Figure 8c: The X and Y axis should be cropped appropriately so that the points on the isochron can be seen by the reader. The minimums should be ~ X=0.020 and ~Y=0.0010. As above, the level of uncertainty should be listed either on the figure or in the caption."

The below comment from my original review was not addressed:

"Line 278-281: This sentence repeats itself twice in stating the sensitivity of phlogopite to late-geological processes. Please re-word. Also specify or give examples of what these late-geological processes might be that can produce a diffusion loss spectrum."

The below comment from my original review was not addressed:

"Line 285-286: There’s no definitive evidence that this phlogopite never experienced a thermal disturbance above its closure temperature. In fact, there IS evidence of diffusive Ar-loss as indicated by the young apparent ages at the low temperature heating steps. There are only a few potential causes of this, with one of the simplest being a brief thermal perturbation over the grain’s closure temperature."

The below comment from my original review was not addressed:

"Line 298: What does centralized mean in terms of the decrepitation temperature? Is this a statistical term? If so, please explain it."

 

The below comment from my original review was not addressed:

 

"Line 299: The closure temperature of biotite/phlogopite atually has a much larger range (mainly lower than the range reported in this manuscript). This is partly due to composition (more Fe-rich biotite has lower closure temperatures than phlogopite) and grain size, as well as cooling rate. The cooling equation of Dodson (1973) shows that under a fast cooling regime, the closure temperature of a mineral increases. Under a slow cooling regime, the closure temparture of a mineral decreases. Is there compositional data on the phlogopite that can help constrain this closure temperature better."

The below comment from my original review was not addressed:

Line 309: These two deposits are closely related in formation process, but not timing correct? There’s a ~7 Myr gap between the two. Just clarify this point.

 

 

 

Comments on the Quality of English Language

The quality and grammar of English has been greatly improved in this version of the manuscript.

Author Response

Dear Reviewer,

Thank you very much for your time involved in reviewing the manuscript, and offering such detailed and helpful comments to further improve the manuscript. We appreciate your clear and detailed feedback and hope that the explanation has fully addressed all of your concerns. Detailed responses are referred in the attached PDF。

Author Response File: Author Response.pdf

Reviewer 3 Report (Previous Reviewer 2)

Comments and Suggestions for Authors

Despite the fact that the authors did a lot of work to improve the manuscript, one of my comments remained ignored - respective Ca/K spectra didnt presented.

Author Response

Dear Reviewer,

Thank you very much for your time involved in reviewing the manuscript, and offering such detailed and helpful comments to further improve the manuscript. We appreciate your clear and detailed feedback and hope that the explanation has fully addressed all of your concerns. Detailed responses are referred in the attached PDF。

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report (Previous Reviewer 1)

Comments and Suggestions for Authors

All my suggestions were accurately addressed.

This work contributes significantly to understanding the metallogenic evolution of skarn deposits in China. It provides valuable insights into the link between these deposits and the crustal processes involved. By shedding light on these connections, the research enhances our comprehension of the geological mechanisms according to the new data of age got it in the phlogopite crystals.

Author Response

Dear Reviewer,   Thank you very much for your time involved in reviewing the manuscript, and offering such detailed and helpful comments to further improve the manuscript. We appreciate your clear and detailed feedback and hope that the explanation has fully addressed all of your concerns. Detailed responses are referred in the attached PDF。

Author Response File: Author Response.pdf

Reviewer 2 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

The authors have adequately answered my major concerns with the manuscript and updated the data tables to meet the minimum reporting requirements. I thank them for their time in updating the manuscript and recommend acceptance after the very minor edits listed here. I encourage them to upload the data tables as excel files to the supplementary material or to a publicly accessible repository. I also encourage additional English Language editing during the editorial stage before publishing.

Two minor edits in the Ar/Ar section:

Line 250: Change "14 stages of heating processes" to "14 heating steps".

Line 252: Throughout the manuscript, use "young" and "old" instead of "small" and "big" when referring to the apparent ages individual heating steps.

 

Comments on the Quality of English Language

As mentioned above and in previous reviews, there have been major improvements in the quality of the English Language. I still recommend some additional edits during the editorial process.

Author Response

Dear Reviewer,   Thank you very much for your time involved in reviewing the manuscript, and offering such detailed and helpful comments to further improve the manuscript. We appreciate your clear and detailed feedback and hope that the explanation has fully addressed all of your concerns. Detailed responses are referred in the attached PDF。

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

the manuscript still does not align with the feedback I provided in my previous review.

 

As I previously suggested, the manuscript requires significant changes in both its content and focus in order to be considered for publication. My initial recommendation to reject the manuscript was based on the belief that, in its current state, it lacks the necessary elements to be publishable. To clarify, the authors should consider one of the following options:

1. Complete Rewrite: The manuscript would benefit from a thorough overhaul and restructuring. The current content and approach are not suitable for publication in its current form. The authors should consider reworking the entire article to make it more coherent, focusing on a specific regional context or presenting it as a detailed case study.

2. Regional Focus: Alternatively, the authors could choose to narrow their focus to a specific regional context, delving deeper into the geological significance within that area. This would require significant modifications to the manuscript's content and methodology.

3. Case Study Approach: Another option would be to transform the manuscript into a comprehensive case study, which would require substantial revisions, including the addition of new data, analysis, and interpretations.

The revisions made by the authors do not adequately address these fundamental issues. It is essential to emphasize that what the authors have changed in the manuscript does not align with the specific needs I outlined in my previous review. The manuscript, as it stands, does not meet the standards required for publication.

Given the extensive nature of the revisions required, I would recommend that the authors take the time to thoroughly rework the manuscript to meet the necessary criteria for publication. At this time, I cannot endorse its publication.

Please convey my feedback to the authors and encourage them to carefully consider the suggestions outlined above. I would be willing to reassess the manuscript once these fundamental concerns have been addressed.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

The paper presents new data on the age of iron skarn from the Guodian deposit. No doubt that precise geochronological data are critical for understanding the sources of the ore and the regional tectonic correlations.

Presented data are high-quality data generated by the 40Ar/39Ar step heating method. However, I strongly recommend to plot Ca/K spectra which give useful hints towards the chemical–mineralogical composition of the degassed sample.

Table 3 – needs in correct references for the Zircon U-Pb age of diorite from the Guodian intrusion. Ref# 23 doesn’t contain any appropriate information. Therefore, its hard to compare the new 40Ar/39Ar data with previously reported for the host magmatic complexes.

Reviewer 3 Report

Comments and Suggestions for Authors

Review for “40Ar-39Ar geochronology of phlogopite and its geological significance for Guodian skarn Fe deposit in Qihe-Yucheng area, Shandong Province” by Feng et al.

 

Summary:

 The current study focuses primarily on elucidating the age of ore formation in the Guodian iron deposit in the context of other Mesozoic skarn Fe deposits in the North China Craton. The study goes through considerable tectonic and field geological descriptions of the region and associated skarn Fe deposits, which provide some relative age constraints. An absolute age of Fe ore formation comes from a new ⁴⁰Ar/³⁹Ar age calculated from an incremental step-heating experiment of a single phlogopite grain. This age is then incorporated into an interpretation of a weakening of the North China Craton and the formation of other seemingly coeval Mesozoic skarn Fe deposits.

 

The manuscript is well though-out, but there are some writing and data reportion/interpretation issues that need to be addressed before publication. I recommend major revisions before publication.

 

Major Concerns:

My major concerns center around the data reporting, calculation, and interpretation of ⁴⁰Ar/³⁹Ar plateau and isochron ages. While the step-heating experiment seems performed as needed, there is needed detail and context for the method constants, calculations, and interpretations… and as a result a re-examination of how the measured isotopic values fit in the context of previous chronology of skarn Fe deposits.

 

Context/References: As a first note, there are method constants and references that are necessary for readers to make informed interpretations of the data reported in this study. This goes for present day, but especially for future researchers that aim to use the results when the constants used in ⁴⁰Ar/³⁹Ar geochronology eventually change, as they have many times in the past two decades (e.g. ⁴⁰K decay constants). This also includes references for the standard used to calculate the J value… as the current manuscript lists a ZBH-25 standard and age but no reference to where this standard and/or age was developed. Additionally, the ratios used for trapped argon correction, the total 40K decay constants, and other irradiation information are missing. For irradiation information, the shape of the irradiation holder as well as whether the measured samples were isolated or combined during irradiation are incredibly important. This context can help the authors potentially differentiate between numerous scenarios (i.e. recoil during irradiation, re-heating, slow cooling) that may have produced the young apparent ages during early, low-temperature heating steps. Most of these issues are noted as detailed comments below within Section 5. “Test Results”.

 

Raw Data Reporting: In addition to needed context and references in Section 5, the data presented in Table 1 is not adequate. The 2020 paper by Schaen et al. (2020) (Title: Interpreting and reporting ⁴⁰Ar/³⁹Ar geochronologic data) presented community agreed best practices for conducting and reporting ⁴⁰Ar/³⁹Ar data. Table 4 of that publication lists the minimum and recommended data that needs to report for ⁴⁰Ar/³⁹Ar data. I advise the authors to follow the guidance in this paper as it not only provides added confidence in the data produced, but also added capabilities for the data to be used by other researchers. Many of these items are discussed in the detailed comments below, as well as in the continued discussion major and minor concerns.

 

One added priority item for raw data reporting is that individual isotopic abundances should be reported (in any units: V, cps, moles, etc.). This should be reported at a minimum, with calculated isotopic ratios being reported in addition, if warranted. Reporting raw isotopic abundances of individual Ar isotopes allows the reader to reproduce the calculated plateau and isochron diagrams with ease. The current data table only reports 3 ratios, with no uncertainties, and no mention of ³⁸Ar (which may play an important role depend on whether there is any Cl in the phlogopite – a likely possibility). Additionally, not a requirement, but it’s beneficial to also report the ratios used to plot the normal and inverse isochrons along with the respective correlation coefficient for each heating step.

 

Age Calculations & Reporting:

There needs to be clarification throughout the manuscript on what the reported level of uncertainty is for the measured ages, as well as the reported previously published ages. Additionally, if the ⁴⁰Ar/³⁹Ar ages are being compared to U-Pb zircon ages, then the uncertainty on the decay constant and the measured standard both need to be included into the final reported age.

 

Additionally, the criteria used to calculate the plateau age needs to be explicitly mentioned. There is considerable excess scatter in the apparent plateau age, as made clear by the elevated MSWD of ~36. If the current criteria for calculating the plateau age remains the same, then there needs to be some comment about the cause of the data dispersion and about how the uncertainty is significantly underestimated. This also goes for the isochron calculated ages. One way to approach this is to increase the uncertainty until the MSWD would be equal to 1 (e.g. Vermeesch, 2018).

 

Conversely, although there are young apparent ages at the beginning of the step-heating experiments… why not report the total integrated age of the phlogopite sample. This would give the authors a minimum age to also add into their interpretations.

 

Minor Concerns:

There are some issues with the writing style. Mostly repeated fragmented sentences. I’ve tried to add edits where I can, hopefully the comments are helpful. Overall, additional proof-reading for punctuation and spelling errors is needed.

 

Is there any evidence of alteration within the phlogopite (e.g. interlayered chlorite)? This type of non-K phase in-between the phlogopite layers can cause recoil during irradiation which may lead to apparent ages spectra that start young before reaching a plateau. Often that plateau is older than the true value, due to the recoil, which would be made more conclusive if the same plateau only steps resulted in isochron ⁴⁰Ar/³⁶Ar initial values that are well above atmosphere.

 

Was ³⁸Ar measured? This can have some important implications if there’s a large amount of Cl in the phlogopite sample.

 

Please reference the map figures when describing the tectonic scale geologic features. It also may be useful to have a large-scale map of the different ore deposits listed in Fig. 9, with the ages reported on the map for a better spatial reference for the reader.

 

 

Line by Line Comments:

 

Line 20: Change “an” to “a”.

Line 25: Change “closely intergrowth” to just “intergrowths”.

Line 28: What level of uncertainty are the two ages the same?

Line 29: Change “U-Pb of diorite” to “U-Pb ages from diorite”.

Line 47: Change “clusters” to “cluster”.

Line 51: Add “in” between “mineralization” and “the”.

Line 54: Delete “which is”.

Line 67: What mineral phase was used in the U-Pb geochronology studies?

Line 71: Delete “as the research object”.

Section 2: Please reference the map figures when describing the geologic setting.

Line 188: Delete “-“ between words “magnetite” and “containing”.

Line 194: Give sieve sizes in mm as well as mesh.

Line 196: It’s standard practice to denote the name of the reactor used during irradiation of samples as different reactors produce slightly different interference corrections.

Line 198: What is the citation/reference for the ZBH-25 standard?

Line 199: Is this 1 or 2 sigma uncertainty? Standard deviation or standard error?

Line 207: Replace “rare” with “noble”.

Lines 208-210: What value for atmospheric argon correction was used? Be sure to provide a citation/reference for this value.

Line 214: What is the citation/reference for the 40K decay constant used?

Line 216: It is mentioned here that the plateau age is given as 2s uncertainty, but is this just the internal uncertainty? Does it include the uncertainty in the calculated J value? Does it include the uncertainty in the chosen decay constants? Since the authors are comparing this age to U-Pb zircon ages, the fully propagated uncertainty needs to be used.

Line 220: Add “step-heating” before “experiment”.

Line 221: Replace “14 stages of heating processes” with “14 incremental heating steps”.

Lines 226-228: What criteria was used to calculate the plateau age? There are a number of published and well-used criteria to follow (e.g. Taylor, 1997; Dalrymple and Lanphere, 1969, 1974; Fleck et al., 1977). The choice of criteria is up to the authors, but it needs to be explicitly mentioned.

Lines 226-231: What heating steps are used in calculating the isochron ages? Also, related to the previous point, the MSWD values for these ages are extremely high. This means the uncertainties are being significantly underestimated, which is a concern for the validity of the age interpretation.

Table 1: The steps included in the plateau calculation and isochron regressions should be marked in a separate column or by bolding appropriate lines of data.

Table 1: See comments above about minimum reporting requirements.

Figure 8a: Include the steps along with the temperature label for what’s included in the plateau age (e.g. steps 7-14). The level of uncertainty should be listed either on the figure or in the caption.

Figure 8b: As above, the level of uncertainty should be listed either on the figure or in the caption.

Figure 8c: The X and Y axis should be cropped appropriately so that the points on the isochron can be seen by the reader. The minimums should be ~ X=0.020 and ~Y=0.0010. As above, the level of uncertainty should be listed either on the figure or in the caption.

 

Line 270: Delete “of the effective”.

Line 272-274: Isn’t the importance here that phlogopite is K-bearing, not H2O-bearing? It’s the potassium that makes it useful as a chronometer.

Line 274: Change end of sentence to “precipitates simultaneously with magnetite”.

Line 275: Delete “In this study” and change “is” to “was”.

Line 278: Start a new paragraph with the sentence starting with “The Ar-Ar isotopic system of phlogopite”. Also change “of” to “in” in this sentence.

Line 278-281: This sentence repeats itself twice in stating the sensitivity of phlogopite to late-geological processes. Please re-word. Also specify or give examples of what these late-geological processes might be that can produce a diffusion loss spectrum.

Line 280: Delete “the”.

Line 280-281: Re-word. Maybe something like, “As menioned in section 5, the measure apparent age of phlogopite becomes relatively uniform at medium- to high-temperature steps (Fig. 8a). This uniform age pattern indicates…”

Line 184: “Ar isotopic ratio” not “composition”.

Line 285-286: There’s no definitive evidence that this phlogopite never experienced a thermal disturbance above its closure temperature. In fact, there IS evidence of diffusive Ar-loss as indicated by the young apparent ages at the low temperature heating steps. There are only a few potential causes of this, with one of the simplest being a brief thermal perturbation over the grain’s closure temperature.

Lines 286-289: See MSWD & uncertainty comments/concerns above.

Line 298: What does centralized mean in terms of the decrepitation temperature? Is this a statistical term? If so, please explain it.

Line 299: The closure temperature of biotite/phlogopite atually has a much larger range (mainly lower than the range reported in this manuscript). This is partly due to composition (more Fe-rich biotite has lower closure temperatures than phlogopite) and grain size, as well as cooling rate. The cooling equation of Dodson (1973) shows that under a fast cooling regime, the closure temperature of a mineral increases. Under a slow cooling regime, the closure temparture of a mineral decreases. Is there compositional data on the phlogopite that can help constrain this closure temperature better.

Line 300: 40Ar-39Ar is sometimes written 40Ar/39Ar. Either way is fine, just be consistent.

Line 299-302: Delete “based on the principle of isotopic dating”. Delete “all of” and change “indicate” to “indicates”. Also just note that the plateau age is the best estimate of when a grain passes through its closure temperature… it’s not always that though (e.g. 39Ar recoil during irradiation).

Line 303: As alluded to above, there may be a significant amount of time between crystallization and passing through the closure temperature depending on the composition/grain size of the phlogopite and the cooling rate.

Line 309: Insert space between “Ma” and “[22]”.

Line 309: These two deposits are closely related in formation process, but not timing correct? There’s a ~7 Myr gap between the two. Just clarify this point.

Line 312: Delete “which” and change “reveal” to revealing”.

 

 

 

 

Comments on the Quality of English Language

Review for “40Ar-39Ar geochronology of phlogopite and its geological significance for Guodian skarn Fe deposit in Qihe-Yucheng area, Shandong Province” by Feng et al.

 

Summary:

 The current study focuses primarily on elucidating the age of ore formation in the Guodian iron deposit in the context of other Mesozoic skarn Fe deposits in the North China Craton. The study goes through considerable tectonic and field geological descriptions of the region and associated skarn Fe deposits, which provide some relative age constraints. An absolute age of Fe ore formation comes from a new ⁴⁰Ar/³⁹Ar age calculated from an incremental step-heating experiment of a single phlogopite grain. This age is then incorporated into an interpretation of a weakening of the North China Craton and the formation of other seemingly coeval Mesozoic skarn Fe deposits.

 

The manuscript is well though-out, but there are some writing and data reportion/interpretation issues that need to be addressed before publication. I recommend major revisions before publication.

 

Major Concerns:

My major concerns center around the data reporting, calculation, and interpretation of ⁴⁰Ar/³⁹Ar plateau and isochron ages. While the step-heating experiment seems performed as needed, there is needed detail and context for the method constants, calculations, and interpretations… and as a result a re-examination of how the measured isotopic values fit in the context of previous chronology of skarn Fe deposits.

 

Context/References: As a first note, there are method constants and references that are necessary for readers to make informed interpretations of the data reported in this study. This goes for present day, but especially for future researchers that aim to use the results when the constants used in ⁴⁰Ar/³⁹Ar geochronology eventually change, as they have many times in the past two decades (e.g. ⁴⁰K decay constants). This also includes references for the standard used to calculate the J value… as the current manuscript lists a ZBH-25 standard and age but no reference to where this standard and/or age was developed. Additionally, the ratios used for trapped argon correction, the total 40K decay constants, and other irradiation information are missing. For irradiation information, the shape of the irradiation holder as well as whether the measured samples were isolated or combined during irradiation are incredibly important. This context can help the authors potentially differentiate between numerous scenarios (i.e. recoil during irradiation, re-heating, slow cooling) that may have produced the young apparent ages during early, low-temperature heating steps. Most of these issues are noted as detailed comments below within Section 5. “Test Results”.

 

Raw Data Reporting: In addition to needed context and references in Section 5, the data presented in Table 1 is not adequate. The 2020 paper by Schaen et al. (2020) (Title: Interpreting and reporting ⁴⁰Ar/³⁹Ar geochronologic data) presented community agreed best practices for conducting and reporting ⁴⁰Ar/³⁹Ar data. Table 4 of that publication lists the minimum and recommended data that needs to report for ⁴⁰Ar/³⁹Ar data. I advise the authors to follow the guidance in this paper as it not only provides added confidence in the data produced, but also added capabilities for the data to be used by other researchers. Many of these items are discussed in the detailed comments below, as well as in the continued discussion major and minor concerns.

 

One added priority item for raw data reporting is that individual isotopic abundances should be reported (in any units: V, cps, moles, etc.). This should be reported at a minimum, with calculated isotopic ratios being reported in addition, if warranted. Reporting raw isotopic abundances of individual Ar isotopes allows the reader to reproduce the calculated plateau and isochron diagrams with ease. The current data table only reports 3 ratios, with no uncertainties, and no mention of ³⁸Ar (which may play an important role depend on whether there is any Cl in the phlogopite – a likely possibility). Additionally, not a requirement, but it’s beneficial to also report the ratios used to plot the normal and inverse isochrons along with the respective correlation coefficient for each heating step.

 

Age Calculations & Reporting:

There needs to be clarification throughout the manuscript on what the reported level of uncertainty is for the measured ages, as well as the reported previously published ages. Additionally, if the ⁴⁰Ar/³⁹Ar ages are being compared to U-Pb zircon ages, then the uncertainty on the decay constant and the measured standard both need to be included into the final reported age.

 

Additionally, the criteria used to calculate the plateau age needs to be explicitly mentioned. There is considerable excess scatter in the apparent plateau age, as made clear by the elevated MSWD of ~36. If the current criteria for calculating the plateau age remains the same, then there needs to be some comment about the cause of the data dispersion and about how the uncertainty is significantly underestimated. This also goes for the isochron calculated ages. One way to approach this is to increase the uncertainty until the MSWD would be equal to 1 (e.g. Vermeesch, 2018).

 

Conversely, although there are young apparent ages at the beginning of the step-heating experiments… why not report the total integrated age of the phlogopite sample. This would give the authors a minimum age to also add into their interpretations.

 

Minor Concerns:

There are some issues with the writing style. Mostly repeated fragmented sentences. I’ve tried to add edits where I can, hopefully the comments are helpful. Overall, additional proof-reading for punctuation and spelling errors is needed.

 

Is there any evidence of alteration within the phlogopite (e.g. interlayered chlorite)? This type of non-K phase in-between the phlogopite layers can cause recoil during irradiation which may lead to apparent ages spectra that start young before reaching a plateau. Often that plateau is older than the true value, due to the recoil, which would be made more conclusive if the same plateau only steps resulted in isochron ⁴⁰Ar/³⁶Ar initial values that are well above atmosphere.

 

Was ³⁸Ar measured? This can have some important implications if there’s a large amount of Cl in the phlogopite sample.

 

Please reference the map figures when describing the tectonic scale geologic features. It also may be useful to have a large-scale map of the different ore deposits listed in Fig. 9, with the ages reported on the map for a better spatial reference for the reader.

 

 

Line by Line Comments:

 

Line 20: Change “an” to “a”.

Line 25: Change “closely intergrowth” to just “intergrowths”.

Line 28: What level of uncertainty are the two ages the same?

Line 29: Change “U-Pb of diorite” to “U-Pb ages from diorite”.

Line 47: Change “clusters” to “cluster”.

Line 51: Add “in” between “mineralization” and “the”.

Line 54: Delete “which is”.

Line 67: What mineral phase was used in the U-Pb geochronology studies?

Line 71: Delete “as the research object”.

Section 2: Please reference the map figures when describing the geologic setting.

Line 188: Delete “-“ between words “magnetite” and “containing”.

Line 194: Give sieve sizes in mm as well as mesh.

Line 196: It’s standard practice to denote the name of the reactor used during irradiation of samples as different reactors produce slightly different interference corrections.

Line 198: What is the citation/reference for the ZBH-25 standard?

Line 199: Is this 1 or 2 sigma uncertainty? Standard deviation or standard error?

Line 207: Replace “rare” with “noble”.

Lines 208-210: What value for atmospheric argon correction was used? Be sure to provide a citation/reference for this value.

Line 214: What is the citation/reference for the 40K decay constant used?

Line 216: It is mentioned here that the plateau age is given as 2s uncertainty, but is this just the internal uncertainty? Does it include the uncertainty in the calculated J value? Does it include the uncertainty in the chosen decay constants? Since the authors are comparing this age to U-Pb zircon ages, the fully propagated uncertainty needs to be used.

Line 220: Add “step-heating” before “experiment”.

Line 221: Replace “14 stages of heating processes” with “14 incremental heating steps”.

Lines 226-228: What criteria was used to calculate the plateau age? There are a number of published and well-used criteria to follow (e.g. Taylor, 1997; Dalrymple and Lanphere, 1969, 1974; Fleck et al., 1977). The choice of criteria is up to the authors, but it needs to be explicitly mentioned.

Lines 226-231: What heating steps are used in calculating the isochron ages? Also, related to the previous point, the MSWD values for these ages are extremely high. This means the uncertainties are being significantly underestimated, which is a concern for the validity of the age interpretation.

Table 1: The steps included in the plateau calculation and isochron regressions should be marked in a separate column or by bolding appropriate lines of data.

Table 1: See comments above about minimum reporting requirements.

Figure 8a: Include the steps along with the temperature label for what’s included in the plateau age (e.g. steps 7-14). The level of uncertainty should be listed either on the figure or in the caption.

Figure 8b: As above, the level of uncertainty should be listed either on the figure or in the caption.

Figure 8c: The X and Y axis should be cropped appropriately so that the points on the isochron can be seen by the reader. The minimums should be ~ X=0.020 and ~Y=0.0010. As above, the level of uncertainty should be listed either on the figure or in the caption.

 

Line 270: Delete “of the effective”.

Line 272-274: Isn’t the importance here that phlogopite is K-bearing, not H2O-bearing? It’s the potassium that makes it useful as a chronometer.

Line 274: Change end of sentence to “precipitates simultaneously with magnetite”.

Line 275: Delete “In this study” and change “is” to “was”.

Line 278: Start a new paragraph with the sentence starting with “The Ar-Ar isotopic system of phlogopite”. Also change “of” to “in” in this sentence.

Line 278-281: This sentence repeats itself twice in stating the sensitivity of phlogopite to late-geological processes. Please re-word. Also specify or give examples of what these late-geological processes might be that can produce a diffusion loss spectrum.

Line 280: Delete “the”.

Line 280-281: Re-word. Maybe something like, “As menioned in section 5, the measure apparent age of phlogopite becomes relatively uniform at medium- to high-temperature steps (Fig. 8a). This uniform age pattern indicates…”

Line 184: “Ar isotopic ratio” not “composition”.

Line 285-286: There’s no definitive evidence that this phlogopite never experienced a thermal disturbance above its closure temperature. In fact, there IS evidence of diffusive Ar-loss as indicated by the young apparent ages at the low temperature heating steps. There are only a few potential causes of this, with one of the simplest being a brief thermal perturbation over the grain’s closure temperature.

Lines 286-289: See MSWD & uncertainty comments/concerns above.

Line 298: What does centralized mean in terms of the decrepitation temperature? Is this a statistical term? If so, please explain it.

Line 299: The closure temperature of biotite/phlogopite atually has a much larger range (mainly lower than the range reported in this manuscript). This is partly due to composition (more Fe-rich biotite has lower closure temperatures than phlogopite) and grain size, as well as cooling rate. The cooling equation of Dodson (1973) shows that under a fast cooling regime, the closure temperature of a mineral increases. Under a slow cooling regime, the closure temparture of a mineral decreases. Is there compositional data on the phlogopite that can help constrain this closure temperature better.

Line 300: 40Ar-39Ar is sometimes written 40Ar/39Ar. Either way is fine, just be consistent.

Line 299-302: Delete “based on the principle of isotopic dating”. Delete “all of” and change “indicate” to “indicates”. Also just note that the plateau age is the best estimate of when a grain passes through its closure temperature… it’s not always that though (e.g. 39Ar recoil during irradiation).

Line 303: As alluded to above, there may be a significant amount of time between crystallization and passing through the closure temperature depending on the composition/grain size of the phlogopite and the cooling rate.

Line 309: Insert space between “Ma” and “[22]”.

Line 309: These two deposits are closely related in formation process, but not timing correct? There’s a ~7 Myr gap between the two. Just clarify this point.

Line 312: Delete “which” and change “reveal” to revealing”.