Geochemistry and Petrology of the Bellecombe Lava Sequence, Enclos Fouqué Caldera, Piton de la Fournaise Volcano (Réunion, France)

Round 1

Reviewer 1 Report

The paper represents a detailed petrographic and petrochemical description of a series of recent 12 lavas in one of the caldera walls of the Piton de la Fournaise volcano. Petrography is done very well, rare nowadays, I have no issue with the paper but it reads like a “USGS-style” report about a certain famous locality.  Am not sure it means to provide any breakthrough advancement of science, but I cannot find anything wrong with this paper. 

Perhaps some MELTS modeling of crystallization conditions can be added, if not done before by others

 

If so it can be accepted with minimal modifications.

 

 Detailed comments

93 Are these related to rift or plateau lava magmatism? 

115 does it include interacting with seawater, high in 87Sr?

123 20% of rock volume or phenocryst portion?

139 Are the feldspars particularly large and homogenous and An rich? Also called allivalitic?

Fig 2 why Cr abundance drops in one of the samples ? Or other samples are enriched? Is this due to crusting by a metal ball grinder? 

344 you answered my comment on line 139

Fig. 7 Nice! Modern-day petrological imaging

Author Response

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Reviewer 2 Report

The manuscript presents study of the petrography and mineral composition of basalts at one of the sections of the Enclo-Fouque caldera of the Piton de la Fournaise volcano on Reunion Island in the Indian Ocean. The petrology of various fragments of this shield volcano has previously been well studied and described in many publications. The purpose of this article was to characterize the evolution of the magmatic system that fed the eruptive activity prior to the collapse of the Enclo Fouquet caldera. Section 1 is informative and with links to numerous published materials of current models about the evolution of volcanic activity and the petrology of the Piton de la Fournaise volcano.

Section 2 details the Materials and Methods. This section should be supplemented with a description of the method for obtaining 2D (area%) petrographic characteristics of lavas. The data in Table 2 (Results of 2D petrographic analysis of phenocrysts and vesicles) was obtained from thin sections (how many ?) or from SEM EDS (what area of BSE images was studied ?), what software was used ? In the absence of sufficiently large statistics of such measurements, it makes no sense to give values with an accuracy of 0.0n% of the area%.

I have a lot of comments on the presentation of data in section Results.

(1) Table 1 (Results of major and trace element analysis) should report LOI values and Total of Major elements (wt%) for all analyses.

(2) I recommend to show on the TAS diagram (SiO₂ vs. Na₂O+K₂O) the points of the studied basalts and the fields of the main varieties of rocks Enclos Fouqué caldera, Piton de la Fournaise.

(3) The spider diagram (Figure 4a) shows a non-typical sequence of elements. I propose to use the most commonly used series of elements for volcanic rocks: Cs, Rb, Ba, Th, U, K, Nb, Ta, La, Ce, Pb, Pr, Sr, P, Nd, Zr, Hf, Sm, Eu, Ti , Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu. The remaining trace elements, incl. Ni and Cr, along which basalts from 6-8 stratigraphic position are significantly distinguished, should be placed before this sequence.

(4) I also recommend showing REE patterns normalized to chondrite. On this diagrams, provide data for the main geochemical reservoirs (OIB, EMORB, and NMORB).

(5) Provide a link to the calculation of the alkalinity index AI = [K₂O + Na₂O – 0.37 × (SiO₂ – 39)]) and the corresponding diargamm for rocks.

(6) How is the porphyry index PI calculated?

(7) Mineral chemistry are poorly shown. Before Figure 6, I propose to use the common classification diagramms in mineral end-members (mol.%): Ab-An-Or for feldspars, Wol-En-Fs for clinopyroxenes, and Fa-Fo-Tf for olivines.

(8) In all compositions of minerals in Supplementary Material S1, sums of Major elements (wt%), crystal chemical formulas and end-members values in mol.% should be reported.

(9) Many clinopyroxene compositions (Table S1) are puzzling because of the very high Al₂O₃ admixture (10-27 wt%). Some analyzes of olivine contain 6-9 wt% Al₂O₃ with a large admixture of TiO₂ up to 1-3 wt%. Are these unusual mineral varieties or analytical artifacts of SEM EDS method? Please clarify.

(10) Why is the composition of oxides of Fe and Ti (titanomagnetite ?) not presented.

(11) I recommend to show on the BSE images megacrysts or phenocrysts of plagioclase, olivine and clinopyroxene with both direct and reverse zoning.

Only after significant editing of Section 3 will it be possible to discuss the findings. In general, the Discussion Section not contain new data or models, but only repeats (confirms) the conclusions previously obtained by other researchers about the petrology, processes, and conditions for the formation of the rocks of the Piton de la Fournaise volcano.

Author Response

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Reviewer 3 Report

Title is ok and the abstract too. Please make some minor modifications for the abstract as indicated in the attached annotated pdf.

Lines 28-29, Keywords: Your keywords would include "tholeiitic affinity" too. Also, you can delete this “Rempart de Bellecombe” and use "dynamic activity" instead.

Line 80: In the entire manuscript, as well as in the supplementary file, please use dot instead of commas when it comes to digits, e.g. here you need to report the info as (0.53-0.29 Ma) instead of (0,53-0,29 Ma). Please follow the same way you do in line 260.

Line 169: Please insert longitudes and latitudes for the location map shown in Fig. 1a.

Line 308, Fig. 4: This spider diagram is very crowded and normally you can use the least immobile elements only and not all trace elements. Please reconstruct, if possible.

Line 345: The anorthite content (An_73-20) of the plagioclase as microlite is very wide, do you have any explanation for this?.

In the supplementary file, please give a footnote to include abbreviations of spots in minerals in which you made a microanalysis, e.g. gm, ph, …. etc. Other alternative is to include them among the file caption.

Comments for author File: Comments.pdf

Acceptable English and just needs fine polishing.

Author Response

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Round 2

Reviewer 2 Report

 Unfortunately, the authors did not take into account several of my significant comments on the Results section. Therefore, I have doubts about the quality of the analytical data.

(1) Why in Table 1 do the Totals of almost all analyzes when accounting for LOI do not exceed 95-96 wt%? This is due to analytical problems? In this regard, please provide statistics on the standards measured for major and trace element analysis in your laboratories (provide data in the Supplementary Table).

(2) On spider diagram (Figure 4a) provide all trace elements incl. Ni and Cr from Table 1 and patterns of main geochemical reservoirs (OIB, EMORB, NMORB).

(3) In all compositions of minerals in Supplementary Material S1, Totals of Major elements (wt%), crystal chemical formulas and end-members values in mol.% should be reported.

The authors comment on my proposal: “This would imply to dramatically increase the size of the table, making it difficult to read. We believe that the original dataset in oxides can be easily used by interested scientists that can calculate quickly the chemical formulas and end member using the most common spreadsheet for petrologic applications.”

I recommend presenting this Table in EXEL format (the file size is not limited in the supplementary materials).

 (4) Before Figure 6 or instead of this Figure, I propose to use the common classification diagramms mineral end-members (mol.%): for feldspars, clinopyroxenes, olivines, and Fe-Ti oxides. This can be done on one Figure in the form of a collage of classification diagrams.  

(5) I repeat once again that the chemistry of minerals is presented poorly and with numerous errors in phase diagnostics. This is unacceptable in a specialized mineralogical journal.

The authors comment on my proposal: “The high Al2O3 in olivine and cpx are due to the presence of not very usual mineral varieties, we rule out the hypothesis of analytical artifacts by SEM since they are found only in specific lavas and with very similar compositions.”

This is wrong. Some analyzes are erroneously diagnosed (plagioclases among pyroxenes) or are mixtures of phases (for example, olivines with anomalous Al and Ca). There are a lot of such analyzes (red lines in the data file).

Comments for author File: Comments.docx

Author Response

In the following, we provided a point-by-point reply (in red) to all the reviewer comments (in black).

 

Unfortunately, the authors did not take into account several of my significant comments on the Results section. Therefore, I have doubts about the quality of the analytical data.

We apologize for submitting an old supplementary table including some wrong analysis and we are very thankful to the reviewer for pointing it out.

• Why in Table 1 do the Totals of almost all analyzes when accounting for LOI do not exceed 95-96 wt%? This is due to analytical problems? In this regard, please provide statistics on the standards measured for major and trace element analysis in your laboratories (provide data in the Supplementary Table).

The data of measurement on the standard are now reported in Supplementary materials.

• On spider diagram (Figure 4a) provide all trace elements incl. Ni and Cr from Table 1 and patterns of main geochemical reservoirs (OIB, EMORB, NMORB).

Figure 4a now includes Ni and Cr. We also added the patterns of the main geochemical reservoirs normalized to the mantle. Although Ni and Cr are not usually included in incompatible elements spider diagrams normalized to the mantle, we are reporting them for comparison as suggested by the reviewer. However, as far as we know there is no univocal Ni and Cr typical content reported for NMORB and EMORB, so normalization to primitive samples for these geochemical reservoirs was not reported.

(3) In all compositions of minerals in Supplementary Material S1, Totals of Major elements (wt%), crystal chemical formulas and end-members values in mol.% should be reported.

We thank the reviewer for noticing the mistake in the supplementary table. We fixed it, submitting a new one in Excel format including chemical formulas and end members.

 (4) Before Figure 6 or instead of this Figure, I propose to use the common classification diagramms mineral end-members (mol.%): for feldspars, clinopyroxenes, olivines, and Fe-Ti oxides. This can be done on one Figure in the form of a collage of classification diagrams.  

We made new ternary diagrams of plagioclase, pyroxene, and olivine compositions as requested by the reviewer. Since we do not discuss the composition of Fe-Ti oxides, we did not include their diagram. All classification diagrams are now reported in the revised manuscript as 3 new figures (Fig. 6-7-8) in place of the old Figure 6.

(5) I repeat once again that the chemistry of minerals is presented poorly and with numerous errors in phase diagnostics. This is unacceptable in a specialized mineralogical journal.

We fixed table S1, deleting the mistakenly reported analysis.

We hope that the revised version of the manuscript meets the reviewer's requirements and apologize for the error in submitting an incorrect version of the supplementary table.

Author Response File: Author Response.docx