An expanded workflow for detrital rutile provenance studies: An application from the Neotethys Orogen in Anatolia

Mueller, Megan A.; Licht, Alexis; Möller, Andreas; Condit, Cailey B.; Fosdick, Julie C.; Ocakoğlu, Faruk; Campbell, Clay

doi:https://doi.org/10.5194/egusphere-2023-1293

Preprints

https://doi.org/10.5194/egusphere-2023-1293

Preprints

28 Jun 2023

| 28 Jun 2023

An expanded workflow for detrital rutile provenance studies: An application from the Neotethys Orogen in Anatolia

Megan A. Mueller, Alexis Licht, Andreas Möller, Cailey B. Condit, Julie C. Fosdick, Faruk Ocakoğlu, and Clay Campbell

Abstract. Sedimentary provenance is a powerful tool for reconstructing convergent margin evolution. Yet single mineral approaches, like detrital zircon, have struggled to track sediment input from mafic and metamorphic sources. Sediment input from these lithologies is especially critical for reconstructing orogenic settings dominated by terrane accretion, ophiolite obduction, and forearc inversion. Rutile can form in metamorphic and igneous rocks and hydrothermal veins, and its U-Pb age and geochemistry often records cooling from the most recent medium to high grade metamorphic event. Thus, detrital rutile complements detrital zircon datasets by offering a path forward in sedimentary provenance reconstructions when metamorphic terranes are potential source regions. However, U-Pb geochronology in rutile can be difficult due to low uranium concentrations and high discordance. Here, we present detrital rutile U-Pb geochronology and trace element geochemistry results from the Late Cretaceous to Eocene Central Sakarya and Sarıcakaya Basins in Anatolia to reconstruct provenance during Neotethys orogenesis. The resulting detrital rutile U-Pb analyses are highly discordant due to the incorporation of non-radiogenic initial Pb. We present a new workflow that accounts for low-U rutile and is based on common Pb corrections and discordance filters. The resulting age spectra are similar for grains up to 40 % concordant (60 % discordant) and across the common Pb correction methods, thus providing a path forward to confidently interpret provenance from discordant rutile grains. Together, the detrital rutile trace element geochemistry and Zr-in-rutile thermometry indicate sediment was sourced from mixed metamafic and metapelitic units with low-grade metamorphic temperatures. Low-U concentration rutile are numerous and more discordant and were predominantly sourced from Late Triassic-Early Jurassic greenschist and blueschist facies rocks with both mafic and pelitic lithologies. This corresponds to sediment derived from the Karakaya Complex, a Paleozoic subduction-accretion complex or oceanic plateau that was accreted and metamorphosed in the Triassic-Jurassic, exhumed to the surface in the Jurassic, and then deformed during Neotethys suturing in the Late Cretaceous to Paleogene. Late Triassic-Early Jurassic ages are nearly absent from the detrital zircon record, emphasizing that a multi-mineral approach, especially inclusive of low-U rutile, provides a more holistic provenance reconstruction. These detrital rutile results serve as an additional layer of data often unexplored across convergent margins globally, and thus provide an exciting path forward in characterizing diverse provenance of orogenic settings.

Received: 20 Jun 2023 – Discussion started: 28 Jun 2023

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Journal article(s) based on this preprint

17 Jun 2024

Navigating the complexity of detrital rutile provenance: methodological insights from the Neotethys Orogen in Anatolia

Megan A. Mueller, Alexis Licht, Andreas Möller, Cailey B. Condit, Julie C. Fosdick, Faruk Ocakoğlu, and Clay Campbell

Geochronology, 6, 265–290, https://doi.org/10.5194/gchron-6-265-2024,https://doi.org/10.5194/gchron-6-265-2024, 2024

Short summary

Megan A. Mueller, Alexis Licht, Andreas Möller, Cailey B. Condit, Julie C. Fosdick, Faruk Ocakoğlu, and Clay Campbell

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1293', David M. Chew, 18 Sep 2023

see attached

Citation: https://doi.org/10.5194/egusphere-2023-1293-RC1
- RC3:
  'Reply on RC1', David M. Chew, 26 Sep 2023
  
  Sorry initially attached as a word file which may nit have worked - hopefully this pdf does
  
  Citation: https://doi.org/10.5194/egusphere-2023-1293-RC3
  - AC1: 'Reply on RC3', Megan Mueller, 15 Nov 2023
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1293/egusphere-2023-1293-AC1-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2023-1293-AC1
RC2:
'Comment on egusphere-2023-1293', Laura Bracciali, 25 Sep 2023

The manuscripts aims at presenting a workflow for detrital rutile provenance studies, based on U-Pb LA dating and trace elemental analysis.
Rutile geochemical data and LA U-Pb data are presented for reference materials (with R-10 rutile used as primary) and detrital samples from Anatolia as the case study to validate the workflow. Testing different common Pb correction approaches is one of the aims of the study.
Starting from the introduction, the authors seem unaware that rutile is an established provenance proxy. If U-Pb dating of detrital rutile was not a widespread technique until perhaps 10-15 year ago (particularly by laser ablation, primarily due to the lack of available reference materials at the time), it is nowadays established and routinely applied in combination with bulk and other single grain methods.
I have identified several issues regarding the U-Pb rutile data, summarised as follows (see detailed comments in the annotated manuscript):
- the data handling of the reference materials (unreported scatter in the measured and corrected isotopic ratios of the reference materials, inconsistency between uncertainties before and after the correction, lack of representation of the data as Concordia diagrams)
- the sample dataset which should serve as the case study to validate the workflow (rejection of >50% of the initial analysis due to “anomalous spiky patterns”, followed by further rejections due to large 7/6 uncertainty, leaving with only 30% of the initial data, with no possibility to carry out provenance interpretation at the sample level due to too small n per sample; last but not least vast majority of these remaining data being largely discordant despite the different common Pb correction approached used
- attempted provenance analysis based on such a largely discordant dataset, with age modes identified in the KDEs distributions of up to 60% discordant data and unsupported claim that the KDEs distributions of variably discordant data are similar hence meaningful, and can be used to constrain provenance
All the above cannot be taken as an example for best practice and certainly not recommended as a part of a workflow to be followed by others. Therefore I am afraid I cannot recommend the publication of the manuscript in its current form.
The large amount of work and effort behind the manuscript is appreciated (analytical work, data handling, care in drafting a range of informative figures), however the LA U-Pb sample dataset is affected by problems likely starting with the analytical set-up hence cannot serve as a case study, nor can the discordant data support any provenance interpretation.
I would suggest to either: i) reconsider the LA U-Pb analytical protocol and data handling/uncertainty propagation before repeating LA U-Pb dating on the same samples to integrate the existing geochemical data (which I have not reviewed in detail); ii) select a different set of samples to validate the workflow.
In any case the manuscript requires substantial restructuring and rewriting, hence a new submission is recommended.
Kind regards,
Laura Bracciali

Citation: https://doi.org/10.5194/egusphere-2023-1293-RC2
- AC2: 'Reply on RC2', Megan Mueller, 15 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1293/egusphere-2023-1293-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1293-AC2
RC4:
'Comment on egusphere-2023-1293', Inês Pereira, 30 Sep 2023

I was asked to review the manuscript entitled “An expanded workflow for detrital rutile provenance studies: An application from the Neotethys Orogen in Anatolia” by Mueller et al., where the authors have tackled the issues related with detrital rutile provenance, mainly those arising from U-Pb dating, to provide a new workflow to be more widely applied by others. They further explore their data in view of the Neotethys orogen in Anatolia.
Objectively, I think the manuscript is interesting, mostly well-written and logic, overall, with good figures, but does not bring elements of novelty concerning rutile U-Pb geochronology. The authors have a biased starting point: “most people apply a U threshold before U-Pb rutile dating”. It is true that some authors have done it, but digging in the literature, it is not that frequent anymore. Some may do it, to avoid having to deal with very complicated datasets, where very little Pb is radiogenic, but most analyse all grains up to about 150 grains per sample, give or take. Then, that may discard some data.
I liked how detailed the authors present their common-Pb corrections, which is useful to the community. But again, this is not expanding a workflow, it is mostly literature review applied to a dataset and explored a bit further.
My main concern related to U-Pb data acquisition/treatment came from the rather large number of discarded rutile grains, due to inclusions, lamellae, etc. In the end, they discarded many rutile grains. How did it affect or biased the final dataset? In these circumstances, how robust is it to average all samples and plot them together, especially in a dynamic sedimentary environment? What about source variability through time weighted on each of your sample populations (n)? Figure A3 is clear in illustrating the issue here.
The geochemistry is used, but PCA is not really adding anything new or rather interesting. I do not see the purpose of keeping it in its current form. I am afraid that by stating strongly very early on that you would be "expanding the workflow" of rutile in provenance studies, but then not combining it well with TE systematics is misleading.
My suggestion, if you will, is that you make a full twist and turn of your manuscript. Place the focus on using detrital rutile in reconstructing convergent margin evolution, using Anatolia and your sample set. I recommend you acquire more data (more grains) to overcome the reduced number of dates you have from each individual sample. Focus first on your case study, then move on into the rutile dating challenges affecting provenance, and how your approach can enhance the level of information we obtain from U-Pb rutile dating and from convergent settings. This requires major rewriting, but you can reuse most of your very nice figures. Unfortunately, I do not recommend publication in its current form, but you should feel encouraged to a re-submission.
I added a few more comments below, summarising parts of my assessment, and more detailed comments can be found in the annotated .pdf file.
Section 2.1.
Provenance: Don’t place all the focus on Nb-Cr and Zr thermometry. You can be more concise in your sentences and ideas, and then you could highlight how other more recent tools can be effectively used in provenance studies (see my comments in the pdf). I also find that this subsection’s title is not suitable. You should re-think better how 2.1 related to the other subsection under section 2 and propose an improved structure and matching headings. You have provenance in 2, and then a “synopsis”, followed by “challenges” that actually just relate to U-Pb. So, in my view you are emphasising the application of detrital rutile geochronology as a provenance tool. This is fine, but the subheadings need to clarify this.
Sections 2.3.1 and 2.3.2
You should provide at least a short review on the ²⁰⁴Pb-based correction of common Pb. You can discuss why it is sometimes very difficult to apply it, but omitting it is not satisfactory in a paper meant to cover common Pb corrections.
I suggest you include a figure with two panels, in one showing a common Pb-bearing analysis and the corresponding corrected age using the ²⁰⁸Pb correction and in the second the 207. These can the theoretical if you will, but you can add arrows and annotations illustrating the assumptions.
Section 6.2. Careful with rushing into conclusions. While it may be true that in your dataset your U<4ppm is vastly metamafic, this does not necessarily imply lower T. For a robust assessment of lower Ts, you need to rely on your metapelitic detrital grains. Rephrase.
Section 6.3
If you are not exploring your data further, I would remove it, as it is not bringing anything really new, and it does not add to any relevant information to your main objective. Since they are detrital grains, I would have difficulty to saying much more.
If you decide to follow my suggestion of major rewriting, you could do a bit more work on detrital rutile trace element geochemistry, try to interpret their chemistries a bit more, and then it would be fine to use the PCA as a starting point of such a subsection.
I hope you find my assessment fair, even if against your best expectations, and consider my comments as a means to improve your current work.
Best wishes,
Inês

Citation: https://doi.org/10.5194/egusphere-2023-1293-RC4
- AC3: 'Reply on RC4', Megan Mueller, 15 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1293/egusphere-2023-1293-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1293-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1293', David M. Chew, 18 Sep 2023

see attached

Citation: https://doi.org/10.5194/egusphere-2023-1293-RC1
- RC3:
  'Reply on RC1', David M. Chew, 26 Sep 2023
  
  Sorry initially attached as a word file which may nit have worked - hopefully this pdf does
  
  Citation: https://doi.org/10.5194/egusphere-2023-1293-RC3
  - AC1: 'Reply on RC3', Megan Mueller, 15 Nov 2023
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1293/egusphere-2023-1293-AC1-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2023-1293-AC1
RC2:
'Comment on egusphere-2023-1293', Laura Bracciali, 25 Sep 2023

The manuscripts aims at presenting a workflow for detrital rutile provenance studies, based on U-Pb LA dating and trace elemental analysis.
Rutile geochemical data and LA U-Pb data are presented for reference materials (with R-10 rutile used as primary) and detrital samples from Anatolia as the case study to validate the workflow. Testing different common Pb correction approaches is one of the aims of the study.
Starting from the introduction, the authors seem unaware that rutile is an established provenance proxy. If U-Pb dating of detrital rutile was not a widespread technique until perhaps 10-15 year ago (particularly by laser ablation, primarily due to the lack of available reference materials at the time), it is nowadays established and routinely applied in combination with bulk and other single grain methods.
I have identified several issues regarding the U-Pb rutile data, summarised as follows (see detailed comments in the annotated manuscript):
- the data handling of the reference materials (unreported scatter in the measured and corrected isotopic ratios of the reference materials, inconsistency between uncertainties before and after the correction, lack of representation of the data as Concordia diagrams)
- the sample dataset which should serve as the case study to validate the workflow (rejection of >50% of the initial analysis due to “anomalous spiky patterns”, followed by further rejections due to large 7/6 uncertainty, leaving with only 30% of the initial data, with no possibility to carry out provenance interpretation at the sample level due to too small n per sample; last but not least vast majority of these remaining data being largely discordant despite the different common Pb correction approached used
- attempted provenance analysis based on such a largely discordant dataset, with age modes identified in the KDEs distributions of up to 60% discordant data and unsupported claim that the KDEs distributions of variably discordant data are similar hence meaningful, and can be used to constrain provenance
All the above cannot be taken as an example for best practice and certainly not recommended as a part of a workflow to be followed by others. Therefore I am afraid I cannot recommend the publication of the manuscript in its current form.
The large amount of work and effort behind the manuscript is appreciated (analytical work, data handling, care in drafting a range of informative figures), however the LA U-Pb sample dataset is affected by problems likely starting with the analytical set-up hence cannot serve as a case study, nor can the discordant data support any provenance interpretation.
I would suggest to either: i) reconsider the LA U-Pb analytical protocol and data handling/uncertainty propagation before repeating LA U-Pb dating on the same samples to integrate the existing geochemical data (which I have not reviewed in detail); ii) select a different set of samples to validate the workflow.
In any case the manuscript requires substantial restructuring and rewriting, hence a new submission is recommended.
Kind regards,
Laura Bracciali

Citation: https://doi.org/10.5194/egusphere-2023-1293-RC2
- AC2: 'Reply on RC2', Megan Mueller, 15 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1293/egusphere-2023-1293-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1293-AC2
RC4:
'Comment on egusphere-2023-1293', Inês Pereira, 30 Sep 2023

I was asked to review the manuscript entitled “An expanded workflow for detrital rutile provenance studies: An application from the Neotethys Orogen in Anatolia” by Mueller et al., where the authors have tackled the issues related with detrital rutile provenance, mainly those arising from U-Pb dating, to provide a new workflow to be more widely applied by others. They further explore their data in view of the Neotethys orogen in Anatolia.
Objectively, I think the manuscript is interesting, mostly well-written and logic, overall, with good figures, but does not bring elements of novelty concerning rutile U-Pb geochronology. The authors have a biased starting point: “most people apply a U threshold before U-Pb rutile dating”. It is true that some authors have done it, but digging in the literature, it is not that frequent anymore. Some may do it, to avoid having to deal with very complicated datasets, where very little Pb is radiogenic, but most analyse all grains up to about 150 grains per sample, give or take. Then, that may discard some data.
I liked how detailed the authors present their common-Pb corrections, which is useful to the community. But again, this is not expanding a workflow, it is mostly literature review applied to a dataset and explored a bit further.
My main concern related to U-Pb data acquisition/treatment came from the rather large number of discarded rutile grains, due to inclusions, lamellae, etc. In the end, they discarded many rutile grains. How did it affect or biased the final dataset? In these circumstances, how robust is it to average all samples and plot them together, especially in a dynamic sedimentary environment? What about source variability through time weighted on each of your sample populations (n)? Figure A3 is clear in illustrating the issue here.
The geochemistry is used, but PCA is not really adding anything new or rather interesting. I do not see the purpose of keeping it in its current form. I am afraid that by stating strongly very early on that you would be "expanding the workflow" of rutile in provenance studies, but then not combining it well with TE systematics is misleading.
My suggestion, if you will, is that you make a full twist and turn of your manuscript. Place the focus on using detrital rutile in reconstructing convergent margin evolution, using Anatolia and your sample set. I recommend you acquire more data (more grains) to overcome the reduced number of dates you have from each individual sample. Focus first on your case study, then move on into the rutile dating challenges affecting provenance, and how your approach can enhance the level of information we obtain from U-Pb rutile dating and from convergent settings. This requires major rewriting, but you can reuse most of your very nice figures. Unfortunately, I do not recommend publication in its current form, but you should feel encouraged to a re-submission.
I added a few more comments below, summarising parts of my assessment, and more detailed comments can be found in the annotated .pdf file.
Section 2.1.
Provenance: Don’t place all the focus on Nb-Cr and Zr thermometry. You can be more concise in your sentences and ideas, and then you could highlight how other more recent tools can be effectively used in provenance studies (see my comments in the pdf). I also find that this subsection’s title is not suitable. You should re-think better how 2.1 related to the other subsection under section 2 and propose an improved structure and matching headings. You have provenance in 2, and then a “synopsis”, followed by “challenges” that actually just relate to U-Pb. So, in my view you are emphasising the application of detrital rutile geochronology as a provenance tool. This is fine, but the subheadings need to clarify this.
Sections 2.3.1 and 2.3.2
You should provide at least a short review on the ²⁰⁴Pb-based correction of common Pb. You can discuss why it is sometimes very difficult to apply it, but omitting it is not satisfactory in a paper meant to cover common Pb corrections.
I suggest you include a figure with two panels, in one showing a common Pb-bearing analysis and the corresponding corrected age using the ²⁰⁸Pb correction and in the second the 207. These can the theoretical if you will, but you can add arrows and annotations illustrating the assumptions.
Section 6.2. Careful with rushing into conclusions. While it may be true that in your dataset your U<4ppm is vastly metamafic, this does not necessarily imply lower T. For a robust assessment of lower Ts, you need to rely on your metapelitic detrital grains. Rephrase.
Section 6.3
If you are not exploring your data further, I would remove it, as it is not bringing anything really new, and it does not add to any relevant information to your main objective. Since they are detrital grains, I would have difficulty to saying much more.
If you decide to follow my suggestion of major rewriting, you could do a bit more work on detrital rutile trace element geochemistry, try to interpret their chemistries a bit more, and then it would be fine to use the PCA as a starting point of such a subsection.
I hope you find my assessment fair, even if against your best expectations, and consider my comments as a means to improve your current work.
Best wishes,
Inês

Citation: https://doi.org/10.5194/egusphere-2023-1293-RC4
- AC3: 'Reply on RC4', Megan Mueller, 15 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1293/egusphere-2023-1293-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1293-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (28 Nov 2023) by Pieter Vermeesch

Dear Dr. Mueller,

All three reviewers agree that your manuscript on "An expanded workflow for detrital rutile provenance studies: An application from the Neotethys Orogen in Anatolia" contains good elements but is not suitable for publication in Geochronology in its present form. One of the reviewers suggests major revisions, whereas the other two reviewers recommend rejection.

All three reviewers question the novelty of your study. The workflow described in your manuscript is commonly used in the detrital rutile U-Pb geochronology community. This is a problem because GChron is a journal that emphasises methodological aspects of geochronology. Pure applications should be published elsewhere.

The reviewers have all noticed some technical issues with the data, and with the way that you have applied the common-Pb correction. However, they disagree on the seriousness of these issues. You have discarded 70% of your analyses. Reviewer 2 thinks that this is too much and therefore finds your dataset unsuitable for geological interpretation. I can see their point and even agree with it. However, I also appreciate the honesty and transparency of your paper. I don't want to set a precedent whereby GChron would only publish 'perfect' datasets, as this would incentivise geochronologists to cherry pick data. In fact, the frank and open discussion of your imperfect dataset is the main appeal of your manuscript to me.

After considering your manuscript, the reviews, and your detailed response to these reviews, I am willing to reconsider your manuscript for publication in GChron after major revisions. If you decide to resubmit, then I would request that you choose a new title so that it is clear that you are not describing a new method, but are presenting a detailed account of a challenging but representative detrital rutile U-Pb dataset. It is important to remain focussed on the methodological aspects of the study. The geological implications of the method are of secondary importance.

I also have some additional comments on the manuscript. I apologise for not giving these to you sooner, but I have been travelling a lot over the past few months. Please clarify if and how you have addressed these points in your cover letter, should you decide to go ahead with a resubmission:

1. You use the percentage of common Pb (using the Stacey-Kramers model) as a discordance filter. As mentioned on line 195 of your manuscript, this filter skews the age distribution towards younger ages. Have you tried the logratio definition of discordance? You may find that this has less of an effect on the shape of the age distributions.

2. Please remove the use of similarity, likeness and cross-correlation from your study. As a rule of thumb, it is best to avoid dissimilarity measures that were invented by geologists, not statisticians.

3. Please make a cleaner separation between results and interpretation. For example, on lines 338-339 (in Section 5: U-Pb Results):

"As the U-threshold filter is both instrumentation sensitivity dependent and biases provenance results, we suggest that a U-threshold protocol is not appropriate for provenance studies."

and on lines 356-358 (in Section 6: Trace Element Results):

"Therefore, the exclusion of low-U rutile in provenance studies likely misses sediment sourced from lower grade metamorphic units and metamafic sources."

These are conclusions, not results. Please move them to a later point in the manuscript.

4. I agree with the reviewers that PCA is potentially powerful, but hasn't been used to its full potential in your study. However, there is another issue that needs to be fixed first. According to line 372, you used OriginPro to do these calculations. I haven't used Origin before, but I understand that it is a general purpose statistics package. Therefore, I suspect that it ignores the fact that trace element concentrations are compositional data. The important difference between 'regular' PCA and 'compositional' PCA is explained by Aitchison (1983, doi:10.1093/biomet/70.1.57). There are lots of computer programs that implement compositional PCA (using logratio statistcs), including CoDaPack (in Excel) and the compositions, robCompositions and provenance packages in R. The interpretation of compositional biplots is succinctly described in Raimon Tolosana-Delgado's "CoDa in a nutshell" document:

http://www.sediment.uni-goettingen.de/staff/tolosana/extra/CoDaNutshell.pdf

5. Your response to reviewer 2 attributes the poor performance of the Kragero secondary standard to a pulse-analog conversion issue. Does the summary table of Figure A2 represent all the data or only the 'good' data?

6. I have never used DetritalPy, but the cumulative distributions of Figure 5 look wrong. Cumulative distributions should not be smooth but should consist of discrete steps.

7. Lines 261-262 of your paper claims that "Detrital rutile U-Pb raw data are given in the data repository". However, when I go to the data repository, I get a spreadsheet with isotopic ratios and concentrations. This is not what I understand as raw data. It would be great if you could share your raw mass spectrometer data. Your response to Reviewer 2 contains some interesting plots showing the raw mass spectrometer data to illustrate what you mean with 'spiky' data. I would be interested to see some of this detail incorporated in the paper.

Thank you for submitting your work to GChron.

Pieter Vermeesch

Hide

AR by Megan Mueller on behalf of the Authors (20 Feb 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (28 Feb 2024) by Pieter Vermeesch

RR by Anonymous Referee #4 (08 Apr 2024)

Suggestions for revision or reasons for rejection

This paper from Mueller et al. details complexities around the dating of detrital rutile in rocks from Anatolia. It provides some information that are commonly not provided in geochronological datasets, to provoke discussion about how we can best treat these kinds of data and materials (i.e. detrital Pbc–bearing minerals that contain provenance-diagnostic trace elements and that can be dated by the U–Pb method). As this paper has already been reviewed, I have few comments about the abstract, introduction and “synopsis” (section 2), which have been altered quite heavily from the initial submitted manuscript and which appear to me to be a useful and concise discussion of relevant topics. The Geological context (section 3) is also perfectly followable in its current format, and the methodology (section 4) can also be easily parsed and appears robust to me. And, overall, the paper provides several interesting diagrams and discussion on topics related to dating of Pbc bearing phases.

Section 5.1 has my first comment. Here you state that 665 of your 1,277 analyses have been rejected (line 395). I am familiar with detrital rutile dating, and this seems to be uncommonly high. In general, however, I think that anyone would agree that methodologies that screen data must be applied lightly and applied only to improve the accuracy of the resulting dataset, and also that any screen that is too onerous can run counter to that purpose. As this is the most significant screen in your data processing workflow, it thus needs to be rigorously and extensively justified. Figure 3 attempts to do this, but it is a blunt tool for this task. Qualitative descriptions of unsuitable raw ICPMS signals are given, but reproducible thresholds to include or not to include a grain aren't provided. You identify three problems that are cause for you to discard grains. These are “spikiness”, inclusions, and low-signals. Firstly, which of these three reasons is most significant, and in what proportion do these three categories occur? Secondly, a ‘spiky’ raw ICPMS signal can be a sign of poor gas flow in the analytical set-up, cones that need to changed, or other build-up in the system’s cells and tubing. For completeness, you should consider and discuss this. Thirdly, and most seriously – screening grains on the basis of low raw counts on U and Pb is philosophically just as incorrect as screening grains on the basis of low U in ppm, similar to the authors whose methodology you disagree with (i.e. Okay et al., 2011). The only difference is that, instead of discarding grains on the processed signal, you now reject grains on the basis of the raw signal. On those grounds, I fundamentally disagree with this approach. Why not use the Chew et al. power-law filter for all grains, regardless of a screen for raw background corrected counts on 238, 206, 207 etc.? Such an approach would at least treat all grains in the same way.

My second comment relates to interpretations of the trace elements. Firstly, point 4) in the abstract could be due to an artefact in your detrital dataset. In particular, figure 10 does not convince me that you have sufficient data to determine whether metapelitic or metamafic rutile in general contains proportionally more U. It could be that the metamafic rutile have lower average U due to random chance due to low numbers of analysed grains. Additionally, your finding only holds within the confines of your dataset, which is not globally representative. Secondly, exploration of the trace element data is underdeveloped. PCA is an extremely useful tool for exploratory geochemical data analysis, but it inherently results in loss of information, as all geochemical variation is condensed into a 2-dimensional space – it is possible that scatterplots etc. may reveal useful information not shown on PCA diagram. In section 6.3, you make the interpretation that the trace element data derives from protolith (Cr, Nb etc.) and temperature (Zr, Hf) factors. However, the vectors on figure 12 demonstrate that PC1 is dominated by Tungsten (W). Why is this? What is the significance of that finding? And why is it not discussed? Additionally, why not colour the points by their metapelitic/metamafic categorisation, and/or Zr-in-rutile T? This would indicate whether PC2 really is discriminating on the basis of protolith, or PC1 on the basis of T. Thirdly, plots of PCA loadings are discussed in section 6.3, but not such plots are shown. Fourthly, and lastly, consideration is not given to the fact that some rutile derive from igneous rocks. It is possible that some rutile labelled as metamafic or metapelitic using the scheme of Triebold et al. (2012) may derive from a plutonic rocks (e.g. Huang et al., 2024 – igneous rutile dated from an Archean pluton; Pe-Piper et al., 2019 – igneous rutile dated from a syenite; Janousek and Gerdes, 2003 – igneous rutile dated from a granitoid pluton). Indeed, on line 76 it is stated that rutile is a common accessory mineral in metamorphic *and igneous* rocks. How might this affect interpretations on the basis of a metamorphic/igneous division, especially WRT high-T concordant "metapelitic" grains on figure 11. Might these be igneous?

My third and last comment relates to the discussion. From line 599 several examples are given of papers where a significant proportion of rutile ages were rejected from published studies (Caracciolo et al., 2021; Govin et al., 2021; Shannan et al., 2020). Firstly, it is not clear from this section whether these rates of rejection are typical of detrital rutile studies, or whether perhaps they are instead related to the specific source regions of these rutile. And secondly, it is noted that the study of Shannan et al. (2020) uses a discordance filter, which is a method that is discounted by the present study and thus not particularly useful for comparative purposes. Consequently, I don’t find this section convincing as an argument to support the interpretation from line 607: “Together these studies illustrate Together these studies illustrate that there is a formidable methodological hurdle in trying to scale up detrital rutile U-Pb to large-n provenance applications”.

Figure comments

Figures 5, 8 – a cumulative age distribution plot would be more useful than a cumulative KDE plot, which is simply a repetition of the data below it in a more awkward form.
Figure 11 – near-concordant grains are often very high T grains. Is it possible that these are primary igneous rutile?
Figure 12 – colour by protolith according to the scheme of Triebold et al., 2012.

ENDS

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ED: Publish subject to minor revisions (further review by editor) (09 Apr 2024) by Pieter Vermeesch

Your revised manuscript has been reviewed by an additional independent reviewer, who deems it suitable for publication pending further revision. The reviewer raises some important points, which you should address before submitting your final version. I will review the changes myself, so the paper won’t go through another formal round of review.

I won’t repeat all the reviewer’s comments here, but would like to highlight two points in particular. The reviewer raises the possibility that the abnormally high proportion of “unsuitable” measurements could be due to analytical issues such as “poor gas flow in the analytical set-up, cones that need to be changed, or other build-up in the system’s cells and tubing”. It is important that you address this concern, not only for the sake of this paper, but also for future studies from your lab. If you haven’t done so already, it may be useful to carry out some sensitivity tests to quantify the effect of new cones or different gas handling on the spikiness of your time-resolved ICP-MS signals.

The reviewer also picked up on a comment that I had made in my decision letter. You clarified that “The figures with cumulative distributions look smooth because they are cumulative KDE distributions.” Cumulative KDEs are NOT a good way to visualise data. Please replace them with ECDFs, which can be generated using any statistical software (including R or Matlab), as well as IsoplotR (as CADs).

In my decision letter, I had suggested that you try Aitchison’s logratio distance as a discordance criterion. In the cover letter of your revision, you responded that:

“many detrital rutile U-Pb datasets include analyses that are highly discordant with some analyses plotting close to common Pb compositions (i.e., Govin et al., 2018 Geology). The logratio distances are smallest for analyses closest to the concordia at both the upper and lower intercepts; a discordance filter based on this metric would exclude the ‘middle space’ analyses.”

You are right. The logratio distance is not useful for your dataset. It may be best to remove it altogether, especially since you appear to have used the original definition of the logratio distance, which has since been amended (https://doi.org/10.5194/gchron-3-247-2021-corrigendum). My apologies for the confusion.

Please try to address the reviewer’s comments without further adding to the length of your paper. Your manuscript currently has 15 figures, which is excessive. Consider removing some of the text and figures (e.g. the bits about the logratio distance), or move them to an appendix or online supplement.

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AR by Megan Mueller on behalf of the Authors (19 Apr 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (23 Apr 2024) by Pieter Vermeesch

ED: Publish as is (26 Apr 2024) by Klaus Mezger (Editor)

AR by Megan Mueller on behalf of the Authors (28 Apr 2024)

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17 Jun 2024

Navigating the complexity of detrital rutile provenance: methodological insights from the Neotethys Orogen in Anatolia

Megan A. Mueller, Alexis Licht, Andreas Möller, Cailey B. Condit, Julie C. Fosdick, Faruk Ocakoğlu, and Clay Campbell

Geochronology, 6, 265–290, https://doi.org/10.5194/gchron-6-265-2024,https://doi.org/10.5194/gchron-6-265-2024, 2024

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Megan A. Mueller, Alexis Licht, Andreas Möller, Cailey B. Condit, Julie C. Fosdick, Faruk Ocakoğlu, and Clay Campbell

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Sedimentary provenance utilizes the composition of sedimentary rocks to investigate the sources of sediment and the geologic processes controlling it. Common techniques struggle to track mafic igneous and metamorphic rock sources, yet rutile forms in these rock types. We use rutile from ancient sedimentary rocks in Türkiye to present a new workflow for integrating rutile U-Pb ages and chemical composition into an accurate sedimentary provenance reconstruction.


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An expanded workflow for detrital rutile provenance studies: An application from the Neotethys Orogen in Anatolia

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