Gravity Data Reveal New Evidence of an Axial Magma Chamber Beneath Segment 27 in the Southwest Indian Ridge

Round 1

Reviewer 1 Report

GENERAL COMMENTS

In this study it is used a 3D gravity inversion methodology to confirm the existence of an axial magma chamber (AMC) beneath the southwest Indian Ridge (SWIR). However, this AMC already was detected from wide-angle/tomographic seismic experiments. My main criticism to this study is that authors almost completely omit these previous results in order to provide initial constraints (i.e., from velocity structure) to starting inversion models, finally validate their inversion results and to discuss the numerical coherence with other geophysical models in the region which is basic in geophysical modeling.

Because the existence of the AMC was already confirmed by wide angle/tomographic seismic experiments, authors need to improve the manuscript better justifying they aims of the study and answering several questions:

What are the new findings of this study related to the understanding of hydrothermal systems and the implications for the mineral exploration?

Are they providing a better 3D detection and geometrical characterization of the AMC?  

What are the advantages of the gravity inversion relating the previous seismic studies?

What is the numerical coherence between gravity and seismic models?

Compare and discuss inversion results with other studies in analogous spreading ridges

In the Section 3.1, the methodological explanation is supported in papers 42, 43 and 44. They are manuscripts written by Chinese scientist and in my understanding they mostly unknow in detail by the broad scientific community because they are published in Chinese journals (Not high rated journals). These manuscripts are in Chinese and only the abstract is in English. Therefore, although some explanations result familiar to me, I cannot review properly this section. Authors should provide more details of the methodology and/or describe other studies where this methodology were successfully validated.  

The dataset and cruise/s details are poorly described. Authors mention several China DAYANG cruises but any description about are omitted. Because the gravity inversion uncertainties it would be useful to provide information for complimentary data from the surveys (seismic, magnetic, heat flux…..). These complimentary data would be necessary to validate the gravity inversion results.

The paper would be of interest for the scientific community if these issues are properly addressed.

COMMENTS ON SPECIFIC LINES

Lines 21-22 “The results indicate that a thickened crust and low-density crustal materials exist beneath segment 27”. Is there any explanation for the crustal thickening in the axial zone ? This is not a general feature in an axial zone of an active spreading system. Is this being totally or partially coherent with seismic tomography results? Please discuss and clarify.

Lines 24-25. “The density results identify several faults, which provide potential channels for magma migration”. Discuss the fault geometry inferred from inversion related to spreading system activity?

Lines 46-48 “As the largest and most completely developed ultraslow-spreading ridge, the Southwest Indian Ridge (SWIR) has complex structural characteristics, special crustal accretion patterns and frequent hydrothermal activities.” Discuss this with gravity and seismic inversion results identifying faults patterns and hydrothermal anomalies.

Lines 150-158. This section needs to be completed with details about the residual and regional anomalies using spectral decomposition. Authors show a spectral curve as supplementary material (A1) but they do not say anything about.

Lines 188-189. Authors say they did not use a seismic tomography model or other data as our initial density model in the process of gravity inversion. Instead, they use the smoothest model. From the point of view of the geophysical inversion methodology this is counterintuitive and needs to be detailed explained considering the usual non-unique solution provided by potential field data.

Lines 150-158. This section needs to be completed with details about the residual and regional anomalies using spectral decomposition. Authors show a spectral curve as supplementary material (A1) but they do not describe the interpretation.

Lines 188-189. Authors say they did not use a seismic tomography model or other data as our initial density model in the process of gravity inversion. Instead, they use the smoothest model. When there is not seismic data available it should be assumed, but it is not the case. From the point of view of the methodology this is not proper and needs to be detailly explained considering the usual non-unique solution provided by potential field data.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The authors present the inversion of gravity data over segment 27 of the southwest Indian ridge, demonstrating a possible axial magma chamber. The paper is well written, with only some minor concerns/questions.

My main comment is on the blockiness on the inversion misfit in figure A3. How sensitive is the model to this misfit and what caused this pattern? There does seem to be some correlation between misfit boundaries and your interpreted faults. 

line 118 - what line spacing was the survey done at?

line 122 - add a reference for ship-borne gravity processing.

line 128- 137 - you may want to show terrain correction strategy on a figure, since this is confusing. Also, is this correction responsible for the artifacts seen in figure A3?

line 156 - I think it would be good to see the power spectrum, with the matched filter fitted lines and cut-offs. Figure a1 is missing these extra details.

line 237-239 - This is basically a repeat of the preceding sentences.

Figure 6 - indicate directions (e.g. N and S) on the profiles. 

Figure 6 - is it possible to indicate the crust-mantle boundary on the models?

line 303 - can these boundaries be explained by something other that faults?

I recommend changing the colors of gravity and density maps so that low values are blue, and high values are red, which is what many readers will expect.

What causes the square artifacts in the data misfit in figure A3.

How can we be sure that the faults discussed (seen on B and D anomalies) are not part of the square misfit anomalies seen in figure A3? It seems to coincide with these artifacts.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Dear authors,

A valiant attempt at coming to grips with many aspects you do not seem to have encontered before.

see my comments in the attached

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

A brief summary 

·         The authors used gravity data to confirm the existence of an axial magma chamber under a hydrothermal field along the Southwest Indian Ridge. They use inversion of gravity data to study the crustal thickness and density structure in an area with potential for metallic mineral resources.

General concept comments

·         The extensive use of gravity data inversion is increasing due to improvements in data-processing power and availability of large datasets, and it is relevant and important to publish these new results.

·         There are 10 references in Chinese, all of them more than five years old. This reviewer cannot read Chinese and would suggest the authors avoid citing so many references in this language and replace them with other similar articles written in English, if possible, e.g. reference #2 is cited as reference for biogeochemistry influence in hydrothermal systems related to mid-ocean ridges, and I’m sure there are many other references on this topic. I’d also suggest the editors include some Chinese speaking reviewers to review the pertinent sections of the manuscript.

·         Some of the articles in Chinese are authored or co-authored by the co-authors of the present manuscript.

·         It is difficult for this reviewer to evaluate the first part of the Data and Methods sections (lines 116-152) as all the references are in Chinese, but the methods apparently are improvements from the standards applied worldwide and the results are coherent.

Specific comments 

Line 15: the authors should explain where segment 27 is located the first time they mention it.

Lines 55 and 191: What is “exploration contract of polymetallic sulfides”? I’d suggest the authors briefly explain what is this contract, if it is relevant, or replace with “area of interest” or other suitable expression.

Line 217: what characteristics? Variations? Lows? Highs?

Lines 188 to 195: I’d suggest the authors elaborate a little more this paragraph on why they did not use the seismic or other data as initial model for the inversion? Why did they choose that depth range?

Lines 317-330: I’d suggest that the authors change the sentences to the past tense, e.g. “magma has migrated” rather than “magma migrates”

Figures: I’d suggest the authors consistently explain in all legends what the stars, white lines, etc. represent.

Line 462-464 (Reference #34): there is a paper in English with the same title in the same journal, with different page numbers:  Tao C H, Li H M, Huang W, et al. Mineralogical and geochemical features of sulfide chimneys from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences. Chinese Sci Bull, 2011, 56: 2828−2838, doi: 10.1007/s11434-011-4619-4

Line 473-475 (Reference #38): there is a paper in English with the same title in the same journal, with different page numbers:  Zhang T, Lin J, Gao J Y. Interactions between hotspots and the Southwest Indian Ridge during the last 90 Ma: Implications on the formation of oceanic plateaus and intra-plate seamounts. Sci China Earth Sci, 2011, 54: 1177–1188, doi: 10.1007/s11430-011-4219-9

Line 502-504 (Reference #52): there is a paper in English with the same title in the same journal, with different page numbers:  Zhang T, Lin J, Gao J Y. Magmatism and tectonic processes in Area A hydrothermal vent on the Southwest Indian Ridge. Science China: Earth Sciences, 2013, 56: 2186–2197, doi: 10.1007/s11430-013-4630-5

NOTE: There are a few suggestions of (minor) changes in the attached file.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Authors have made a significant effort to address most of my comments and suggestions. They have introduced changes where needed improving the scientific content and quality of the manuscript. I think the manuscript is of interest for the scientific community and it can be accepted in the present form.