⁴⁰Ar/³⁹Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province

Round 1

Reviewer 1 Report

Seamounts not contains polymetallic nodules. Only Co-rich crusts! Polymetallic nodules are distribution on abisal basins. 

The largest number of geological studies has been carried out for the Pacific Ocean, including deep-sea drilling of both the abyssal parts and seamounts of the Pacific . The authors' assumption about the poor knowledge of the Pacific Ocean is incorrect! 

Where is the description of the Ar/Ar dating method?

Why seamounts name an samples numbers is identic?

Why you study 5 samples and you presents coordinates for 3 samples? You must present the sampling depth!

You need present the seamounts bathymetry

line 168-172 - This is the figure caption 

You must review analytical results! Only 3 numbers. For example, table 2, Al2O3 content must be right as 14.3% (not 14.253%)

 

Author Response

Reviewer 1:

1. Seamounts not contains polymetallic nodules. Only Co-rich crusts! Polymetallic nodules are distribution on abisal basins.

Reply: Polymetallic nodules do not develop in the seamount area, which has been modified in this paper.

2. The largest number of geological studies has been carried out for the Pacific Ocean, including deep-sea drilling of both the abyssal parts and seamounts of the Pacific . The authors' assumption about the poor knowledge of the Pacific Ocean is incorrect!

Reply: As for the hypothesis of research progress in the Pacific Ocean, I revised it so that due to the large scope of the Pacific Ocean, the wide distribution of seamounts and the complex causes, the causes of many seamounts are not understood.

3. Where is the description of the Ar/Ar dating method?

Reply: The dating samples were experimentally tested at Oregon State University and VU University in Amsterdam. I have added the AR / Ar dating method to the article.

4. Why seamounts name an samples numbers is identic?

Reply: There are 6 samples in this paper, of which MPID201 and MPID202 are duplicate samples collected at the same sampling point.

5. Why you study 5 samples and you presents coordinates for 3 samples?

Reply: The sampling locations of the remaining three samples have been added .

6. You need present the seamounts bathymetry.

Reply: We don't have the bathymetry of seamounts.

7. You must review analytical results! Only 3 numbers. For example, table 2, Al2O3 content must be right as 14.3% (not 14.253%).

Reply: The data in this paper has been modified.

Reviewer 2 Report

Dear Authors,

I read with great interest your manuscript presenting new geochemical data and datings of basaltic samples from seamounts in the Western Pacific Province. I think that the Manuscript is well written and organized, though maybe too concise. However, I believe that it can be improved in some parts before being published. In my opinion, the weak point of this manuscript is the discussion section. I believe that you briefly describe the petrogenesis of the studied rocks without providing significant discussion to support that these rocks formed in association with a mantle plume. In addition, the discussion about the ages and their relationships with the neighboring seamount chains is worth to be done.

In conclusion, I suggest major revision.

Below I listed the major concern of the Manuscript, as well as some minor point. In addition, you will find comments in the annotated pdf. I hope you will find these suggestions helpful for improving the science and shape of your manuscript.

 

Major point

 

• Information about methods and instruments used for Ar/Ar dating is completely missing. You must add this information.

In general, all the methodological aspects should be improved by adding more information, such as for example the detection limits, etc. You can also provide this information as supplementary material.

• I have doubts about the interpretation of the sample MP5D15A as a calc-alkaline basalts. In figure 4 it plots in the alkaline/transitional field (see Nb/Y in the diagram of Winchester and Floyd 1977 and Pearce 1996). Accordingly, in the incompatible element spider diagram in figure 5, it does not show negative anomalies in Ta, Nb, and Ti, which suggest and alkaline/transitional affinity rather than a calc-alkaline affinity. In addition, calc-alkaline affinity for basaltic rocks doesn’t fit with your interpretation of the within-plate magmatism in association to a mantle plume activity and you do not discuss this inconsistency in the discussion.
• The discussion section is the very weak point of your manuscript and I suggest to significantly improve it. I may agree with the hypothesis of these basalts generated from a mantle plume, but I for sure noticed that it is not enough constrained and discussed. In fact, some authors also proposed that some seamounts can be formed around the main plume head or hot spot (e.g. Long et al., 2020 and reference therein). In addition, your ages are significantly older with respect to the close and active Hawaii volcanoes. You should discuss this age discrepancy.

In order to better constraint your hypothesis, I suggest adding more graphs (e.g., Zr/Nb vs Zr/Y from LeRoux et al 1983; maybe one tectonic discrimination diagram) to discuss the petrogenesis of these rocks in terms of partial melting degree and depth, and possible mantle source composition. There is plenty of petrogenetic modeling in literature to model the composition and melting condition of the mantle source for generating alkaline basalts such as those studied in your paper (see for example Pearce, 2008). In addition, given the proximity of your studied area to other seamounts, I suggest to add the compositional field of them in your geochemical diagram. 

 

Minor points

• Throughout the text, there is confusion between the terms “seamounts” and “basalts” (see annotated pdf), and this can confuse the reader. For example, in the second line of the abstract, the geochemical characteristic should be referred to your samples (in this case basalts/basaltic samples) rather than the seamounts. Seamounts can also be composed by volcaniclastic sediments (e.g., Gutierrez et al 2006; Quartau et al., 2018), and their geochemistry is less significative for understanding the petrogenesis within the upper mantle and hot spot setting. Please, check this not-perfect terminology in all the text, figures, and captions.
• the petrographic description (Lines 78-82) should be extended and improved using codified texture. I suggest using for example aphyric vs porphyritic texture, specifying the average dimension of the phenocrysts, and using the appropriate name for anisotropic textures (I think that directional texture is not codified).
• I suggest you to use caution in drawing petrogenetic conclusions (lines 124-128) based on contents and variation of relatively highly mobile elements. In fact, elements such as Sr, Ba, and K can be strongly remobilized during sea-floor hydrothermal alteration.
• Some of the samples mentioned in the text (line 62) are not shown in figure 1.
• Figure 1: I think the caption of this figure must be revised. This is not a geological map; I suggest to use “Bathymetric map of the study area in the Western Pacific showing the location of the studied seamounts. The radiometric ages of three basalts are also shown”.
• Figure 3: add the name of the dated samples in each graph.
• it is not clear what is the meaning of “latest ages” for the seamount formation.
• You should spend some words to discuss the significance of ca 35Ma age gap in such close seamounts. This discussion must be improved in my opinion.
• The graphical quality of the geochemical graphs is very poor. Please improve them

 

Reference cited in this review

Barbero, E., Zaccarini, F., Delavari, M., Dolati, A., Saccani, E., Marroni, M., & Pandolfi, L. (2021). New evidence for Late Cretaceous plume-related seamounts in the Middle East sector of the Neo-Tethys: Constraints from geochemistry, petrology, and mineral chemistry of the magmatic rocks from the western Durkan Complex (Makran Accretionary Prism, SE Iran. Lithos, 396–397, 106228. https://doi.org/10.1016/j.lithos.2021.106228)

Pearce, J. A. (2008). Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust. Lithos, 100(1–4), 14–48. https://doi.org/10.1016/j.lithos.2007.06.016

Gutierrez, M., Casillas, R., Fernandez, C., Balogh, K., Ahijado, A., Castillo, C., Colmenero, J.R., Garcia-Navarro, E., 2006. The submarine volcanic succession of the basal complex of Fuerteventura, Canary Islands: a model of submarine growth and emergence of tectonic volcanic islands. Geol. Soc. Am. Bull. 118, 785–804. https://doi.org/10.1130/B25821.1.

Quartau, R., Ramalho, R.S., Madeira, J., Santos, R., Rodrigues, A., Roque, C., Carrara, G., Brum da Silveira, A., 2018. Gravitational, erosional and depositional processes on volcanic ocean islands: insights from the submarine morphology of Madeira Archipelago. Earth Planet. Sci. Lett. 482, 288–299. https://doi.org/10.1016/j.epsl.2017.11.003.

Pearce, J. (1996). A User ’ s Guide to Basalt Discrimination Diagrams. Geological Association of Canada Special Publication, 12, 79–113.

Floyd, P. A., & Winchester, J. A. (1978). Identification and discrimination of altered and metamorphosed volcanic rocks using immobile elements. Chemical Geology, 21(3–4), 291–306. https://doi.org/10.1016/0009-2541(78)90050-5

LeRoex, A.P., Dick, H.J.B., Erlank, A.J., Reid, A.M., Frey, F.A., Hart, S.R., 1983. Geochemistry, Mineralogy and Petrogenesis of Lavas Erupted along the Southwest Indian Ridge between the Bouvet Triple Junction and 11 degrees East. J. Petrol. 24, 267–318.

Long, X., Geldmacher, J., Hoernle, K., Hauff, F., Wartho, J., & Garbe‐Schönberg, C. ‐Diete. (2020). Origin of isolated seamounts in the Canary Basin (East Atlantic): The role of plume material in the origin of seamounts not associated with hotspot tracks. Terra Nova, 32(5), 390–398. https://doi.org/10.1111/ter.12468

Comments for author File: Comments.pdf

Author Response

Reviewer 2:

1. Information about methods and instruments used for Ar/Ar dating is completely missing. You must add this information.

Reply:The dating samples were experimentally tested at Oregon State University and VU University in Amsterdam. I have added the AR / Ar dating method to the article.

2. Query about the alkalinity / Calc alkalinity of  MP5D15A sample.

Reply:The lithology of MP5D15A sample has been modified to subalkaline basalt.

3. Improvement of discussion part.

Reply: We refer to your opinion, add the discussion on rock source in the discussion of magmatic genesis, refer to the article of Pearce J A et al., 2007, and add the Nb / Yb versus Th / Yb diagram.

4. There is confusion between the terms “seamounts” and “basalts” (see annotated pdf), and this can confuse the reader.

Reply: We have adopted a more accurate vocabulary to distinguish seamounts from samples, and carefully distinguish them.

5. the petrographic description (Lines 78-82) should be extended and improved using codified texture. I suggest using for example aphyric vs porphyritic texture, specifying the average dimension of the phenocrysts, and using the appropriate name for anisotropic textures (I think that directional texture is not codified).

Reply: The text has been modified to "some samples show a fluidal texture aligned with feldsapar laths".

6. Some of the samples mentioned in the text (line 62) are not shown in figure 1.

Reply: The remaining three sample sampling points have been marked on the picture

7. Figure 1: I think the caption of this figure must be revised. This is not a geological map; I suggest to use “Bathymetric map of the study area in the Western Pacific showing the location of the studied seamounts. The radiometric ages of three basalts are also shown”.

Reply: The name of Figure 1 has been changed to “Bathymetric map of the study area in the Western Pacific (a) and sampling stations (b)”. The numbers marked in the figure are the age of seamount basalt.

8. Figure 3: add the name of the dated samples in each graph.

Reply: The name of the dated sample has been added to each figure.

9. it is not clear what is the meaning of “latest ages” for the seamount formation.

Reply: The “latest” has been deleted

10. You should spend some words to discuss the significance of ca 35Ma age gap in such close seamounts. This discussion must be improved in my opinion.

Reply:  On the issue of age difference, we refer to the documents of kopper et al., 2003 and long et al., 2020 and added a discussion. It is considered that this age difference may be formed by small plumes in the Western Pacific.

11. The graphical quality of the geochemical graphs is very poor. Please improve them.

Reply: The original picture has been replaced with a higher definition geochemical picture.

 

Reviewer 3 Report

Dear authors,

I have read your article and I think it deserve the publication after some minor revisions. The only thing that leaves me doubtful is the small number of samples on which the analysis were made, I really think that it would have been a more valid work. Anyway, the work is good, and the English needs only some minor revision. You will find, in the attached file, some useful suggestions.

Comments for author File: Comments.pdf

Author Response

Reviewer 3:

1. Grammatical errors.

Reply: The syntax errors you listed have been corrected.

2. you have only one sample image?

Reply: These photos are representative of the samples in this paper.

3. It is recommended to add N-MORBand E-MORB modes in figures 5 and 6. 

Reply: E-MORB and N-MORB modes have been added for figures 5 and 6.

4. The ordinate title of Figure 4 needs to be modified.

Reply: Figure 4 has been modified

5. The size of Figure 3 is too small.

Reply: The size of Figure 3 has been changed to a larger one

6. The annotation of figure 8 should be inserted into its title.

Reply: The annotation of figure 8 has been inserted into its title.

7. A border should be added to figure 6.

Reply:  A border has been added for Figure 6.

 

Round 2

Reviewer 1 Report

Thank you for your revises.

I have one question. You present two time of volcanism in seamounts which located very close. In conclusion you wright "... formed from magma that originated in the deep mantle and resulted from the presence of a mantle plume in the Western Pacific". The mantle plume is stay, the plate is moving. Why age is different? Is mantle plume movement too? The geochemistry of sample MP5D11 is different (table 2, low K2O, high SiO2, high TiO2 etc ) from MP5D15A. You not discuss this point in text.  What reason of defferent ages?

Author Response

Reviewer1

• What reason of defferent ages?The mantle plume is stay, the plate is moving. Why age is different?

Reply:  On the issue of age difference, we refer to the documents of kopper et al., 2003 and long et al., 2020 and added a discussion. It is considered that this age difference may be formed by small plumes in the Western Pacific.As for whether the mantle plume is stable, we believe that the mantle plume is stable, but the rising plume at its head is not fixed. These plumes at the upper part of the mantle plume is easy to be attracted by the expansion ridge and offset(Wei-dong S, et al.2021)

2、 The geochemistry of sample MP5D11 is different (table 2, low K2O, high SiO2, high TiO2 etc ) from MP5D15A.

Reply: Samples mp5d11 and mp5d15a are not taken at the same sampling point, and some elements in the rock may migrate due to the influence of seawater or other factors.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Dear Authors,

I read the revised version of your manuscript that deals with new geochemical data and datings of basaltic samples from seamount in the Western Pacific Province.

I think that the new version is improved in large parts of the manuscript. Nevertheless, I think that it still suffers of minor problems. The most important of them are listed below. In particular, I believe that you do not present enough data to constrain the depth of partial melting. In addition, you will find some further suggestions in the attached annotated pdf file of your submission.

In conclusion, I suggest minor revision.

I hope you will find these suggestions helpful for improving your manuscript.

 

Major point

• Lines 180 and 207: what do you intend for deep mantle? I think this term is too generic and is a major issue, deep mantle could be 300 km or 60km. A petrogenetic estimation would help you to evaluate, for example, if partial melting occurred in the spinel or garnet facies (or both) and estimate better the depth of partial melting. Otherwise, you have no convincing data to hypothesize the depth of partial melting. Your sentences in the cited lines of the text are not fully supported and may be modified using more caution and more hypothetic language and explain what is for you “deep mantle”.

 

Minor points

• I appreciate that you add a discussion about the different ages in the section 5.1. I suggest adding a short sentence also in the conclusion section to summarize your idea about the small plumelets in the western Pacific.
• Lines 54 and 152: it is not clear what is the meaning of “latest”
• Please in figures 5 and 6 use the same symbol 8in term of colour and symbol type) for each sample to allow an easy evaluation of spider and REE diagrams
• Line 130: I think that usually, the abbreviation of large-ion lithophile elements is LILE

Comments for author File: Comments.pdf

Author Response

Reviewer 2

• Sufficient data are not provided to limit the depth of partial melting.

Reply:Referring to Wang Xi Kui, 1993, we have used K2O which has a good linear relationship with magma depth to calculate the depth of magma source of the samples.

• It is suggested to add a short sentence to the conclusion to summarize your views on small feathers in the Western Pacific.

Reply:A summary of the age difference has been added in the conclusion section.

• Please in figures 5 and 6 use the same symbol 8in term of colour and symbol type) for each sample to allow an easy evaluation of spider and REE diagrams.

Reply:Modifications have been made in figures 5 and 6.

• Line 130: I think that usually, the abbreviation of large-ion lithophile elements is LILE

Reply:It has been modified in the corresponding part of the article.

 

Author Response File: Author Response.docx