Round 1

Reviewer 1 Report

Review of “Emplacement and segment geometry of large, high-viscosity magmatic sheets” 

I’ve made a number of comments on the draft pdf (uploaded) but here emphasize my main concerns. The paper is very interesting and important, but I don’t think the results and major conclusions are well communicated.

I. What is the big picture?

The first thing that comes to this reader’s mind is what is the relationship between the dykes, especially Sosa, and Cerro Bayo? You may not have a good answer, but they are collocated in time, space, and chemical composition. You suggest that magma in Sosa actually flowed toward Cerro Bayo, the opposite of what one would expect. Can you discuss the possibilities? Can you offer a 3-D perspective of how these intrusions and the caldera structure fit together?

II. Chemical zonation of the dykes

This interests me a lot. I published a paper about it in Nature. Many high-viscosity dykes are chemically zoned with a center more silicic than the margins. You say that’s the case for the Great Dyke but not for Sosa but your chemistry figure shows it’s true for Sosa also. This makes sense because if everything were flowing at once the margin would be lower viscosity and essentially lubricate flow of the viscous center. Alternatively, maybe the dykes started out like what remains in the margins but continued flow in the center introduced more silicic magma. What do you think?

Carrigan, C.R. and J.C. Eichelberger, Zoning of magmas by viscosity in volcanic conduits, Nature, 343, 248-251, 1990.

III. Water in magma

It seems highly unlikely that such old and partially crystallized rock would preserve magmatic water contents in glass, which hydrates rapidly. The only thing that could show otherwise would be H and O isotope data, but the chances of the glass containing magmatic rather than meteoric water is so remote I wouldn’t bother. Better to just estimate a depth of emplacement and then assume that magmatic vapor is mostly water and at lithostatic pressure. And along that line, you need to say how the P and T estimates were made, not just cite them. And please write “[name, ref #] reported that...” not “[ref #] reported that...”

Taylor, B.E., J.C. Eichelberger, and H.R. Westrich, Hydrogen isotope evidence of rhyolitic magma degassing during shallow intrusion and eruption, Nature, 306, 541-545. 1983

Eichelberger, J.C., C.R. Carrigan, H.R. Westrich, and R.H. Price, Non­explosive silicic volcanism, Nature, 323, 598-602, 1986

IV. Rock magnetism data

This is a specialized field and many people interested in dykes will not be familiar with the parameters (including myself). They need to be defined and what the figures show must be clearly conveyed. My first question is should we even worry about the difference between an anisotropy of 1.02 and 1.04? This is the bulk of the new data reported in the paper but its importance does not jump out at the reader.

You’ll also note that I suggested a change in title to make it – to my mind – more reflective of the content of the paper. Your composite drone imagery of the dykes is truly amazing (though I think they could be tweaked to make the dykes stand out better)! I hope these comments are helpful.

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for the constructive comments. For details on the comments, please see the attached file: "ResponseToReviewer1.pdf"

Best regards.

Author Response File: Author Response.pdf

Reviewer 2 Report

Title:                      Emplacement and segment geometry of large, high-viscosity magmatic sheets

Authors:               Tobias Schmiedel, Steffi Burchardt, Tobias Mattson, Frank Guldstrand, Olivier Galland, Octavio Palma, and Henrik Skogby

Journal:               Minerals

Methods: photogrammetry, microstructural analysis, igneous petrology, Fourier-Transform-Infrared-Spectrometry, Anisotropy of Magnetic Susceptibility (AMS)

General assessment

This paper is a fine contribution to the ongoing revision on the mechanisms of magma emplacement in shallow magma plumbing systems. The overall manuscript is sound and the interpretations and conclusions are supported by observations and data. Provided that the authors rule out or include the Sosa Dyke as being a composite dyke and derive the implications in terms of interpretation (magma emplacement etc) and conclusions, I am recommending this manuscript for publication. Depending on what the author find about the Sosa Dyke the revision work may range from minor to major. 

My main criticism revolves around the Sosa Dyke. And although I rank the overall manuscript Scientific Soundness as high, you need to consider the following point. Field observation provided and obtained AMS data lead me to suggest that this dyke is composite, i.e. made of two successive injections. Ruling out this option has large implications for the later interpretations. I encourage the author to investigate this option. I provide later (comment to section 5.2), what I found as compelling hints supporting the two magma injections. If correct the ratio t/w and the radius of the tip (rtip) to the dyke length (w) will change in the final figure 9. Where would the two subsequent injections plot in Fig. 9? Unfolding this, only the first injection of the magma would be emplaced in the true host environment, while the second injection would be cased-in the two walls of the previously split magma injection. Can this explain the contrasting AMS signal at the margin (first injection) and at the centre of the section III (second injection) for the Sosa Dyke?

Additionally, I found that the Figure 1 needs some improvements to provide a clearer understanding of the geological setting.

Comment on the interpretation (section 5.2.)

The field picture of the Sosa Dyke – Transverse 2 shows two massive parts on either side of the dyke. The middle is marked by a depression (negative erosion). Both walls on either side have the same thickness. This type of observations are typical markers of multiple injections where the later magma pulse uses the same plane as the previous dyke injection and split it in exactly half (the weakest point of the plane where both cooling fronts from either side meet).

Your AMS samples in this case clearly show a nice symmetry supporting two subsequent injections. I wonder if your geochemistry is also supporting that.

Looking at Transverse 1, although less clear, I discern a difference in the fracture distribution (denser fractures near the walls), and the colour of the rock is lighter in the centre. The AMS here is not clearly supporting two injections, but could it be a bias from the sampling. Is the dip of the dyke section changing here? And what is the geochemistry telling about the centre vs the edge, can it support two injections?

Comment on High-viscosity sheet intrusion (section 5.3)

Here I would make a table to summarize the values of sheet intrusions in the world and show the one you document here. Please define parameter t and w in the text, I presume thickness is “t” and length is “w”.

Please indicate the value you collect for the Sosa Dyke and Great Dyke in the table.

What would be the value if the Sosa Dyke is made of two injections? Using the picture of Fig. 8 I measure that the thickness of the first injection for the Segment III would be ~16 m and the second ~10 m. Where is this plotting on figure 9 and does it change the classification of the dyke? Doing the same for the Segment I, where does it plot? Do the three Segments fold on the same line then?

Comment on section 5.4.

If you confirm that there are indeed two subsequent magma injections at the Sosa Dyke, the second injection’s host is the previous magma injection. This could change your interpretation onward. You would have to make sure that your argument still holds and if you can still question the common practice of using dyke segment geometries as proxy for single intrusion emplacement mechanism.   

Figure 1 needs some improvement.

At the moment, the snapshot of the volcano is too close from the caldera. This picture does not allow to appreciate the dykes documented here with respect to the volcano.

I suggest to take a wider view picture of the volcano and next to it to draw a structural outline of the volcano. The caldera wall should be represented with a structural indicator showing the direction of cliff dip. Having a wider view of the caldera will allow to appreciate the significance of the dykes with respect to the volcano centre. At the moment we cannot really say from the picture that they are radial dykes (radial would mean that one end points towards the centre of the volcano). Currently, we do not know where is the centre of the volcano with respect to the image.

There are many more dykes that one can spot from the GoogleEarth image, I think it would be good to add them on the structural map to see which direction they are pointing to.

Figure 2 The orientation of both Transverses on the mapped dyke segments is not consistent with orientation of the North on the AMS Figures 7 and 8. Please correct that.

See further comments on the text in the annotated pdf of the manuscript provided.

See attached commented manuscript for further comments.

Best wishes to fix quickly this manuscript for publication.

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for the constructive comments. Please find the detailed reply to the comments in the attached document "ResponseToReviewer2.pdf"

Best regards.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

General assessment

After re-submission, I found that the present article is ready for publication as it is and in the present form!

I thank the author for their comprehensive response to my comments. I am very satisfied with each response.

This article show-case one of the first examples of quantified application of photogrammetry and digitalized outcrop. This alone greatly contribute to the high-impact of this paper.

Author Response

We thank the reviewer for the positive reply to our revisions!