Mineralization and Skarn Formation Associated with Alkaline Magma Chambers Emplaced in a Limestone Basement: A Review

Round 1

Reviewer 1 Report

Dear Editor, I have now completed my revision of the paper : “Mineralization and skarn formation associated with alkaline magma chambers emplaced in a limestone basement: a review “ by Marco Knuever, Daniela Mele and Roberto Sulpizio.

This review paper primarily centers on the topic of mineralization and skarn formation within three distinct volcanic systems: Somma-Vesuvius and Colli Albani in Italy, and Merapi in Indonesia. These systems share common characteristics, namely the presence of alkaline magma chambers situated within limestone basements. The compositional spectrum of the examined systems spans from tephrite to phonolite for Somma-Vesuvius and Colli Albani, and from high-K basalt to basaltic andesite for Merapi. Furthermore, the studied eruptions exhibit varying levels of explosivity, encompassing Plinian, subplinian, and strombolian activities for Somma-Vesuvius, hydrovolcanic activity for Colli Albani, and ash flow and lava dome dynamics for Merapi.

Notably, the authors observe no apparent correlations between magma composition and the mineral assemblages found in skarns. Instead, these assemblages are predominantly influenced by the nature of the host rock and its capacity for assimilating calcium (as Ca-rich limestone) or both calcium and magnesium (as dolomite). Within alkaline magma chambers, the assimilation of Ca-rich limestones leads to the formation of wollastonite, plagioclase, and clinopyroxene-rich mineral compositions. Conversely, the assimilation of dolomite results in clinopyroxene, olivine, and spinel-rich assemblages. Additional accessory mineral phases in the magma result from the accumulation of incompatible elements within magmatic brines during the cooling process. The specific minerals that crystallize are determined by the dominant volatile phase, with fluorite and apatite indicating a fluorine-rich phase, cotunnite, stibnite, and scheelite suggesting a chlorine-rich fluid, and pyrrhotite, anhydrite, and ellestadite forming in the context of sulfur-rich magmatic brine.

An important aspect regards the CO2 release related to skarn formation by decarbonation of assimilated carbonate wall-rock; authors evidenced that, as suggested in the literature, this additional volatile phase in the magma's gas budget could lead to increased mass eruption rates and prolonged eruptions.

I found the manuscript well written, interesting, and certainly valuable for enhancing our understanding of carbonate assimilation by magmas and skarn formation. I have only few minor suggestions:

1)      While the authors indeed consider eruptions of varying degrees of chemical evolution, explosivity, and eruptive mechanisms, encompassing lava dome to hydro-magmatic explosions to Plinian eruptions, the discussion suggests a lack of discernible disparities in the processes of assimilation and skarn formation across these distinct eruptive types. Consequently, the manner by which this process, alongside the ensuing release of CO2, potentially influences the extent of eruption explosivity remains somewhat ambiguous. To offer further clarity, a more comprehensive discussion of this aspect would be advantageous.

2)      Given the review nature of the paper, meticulous attention should be devoted to ensuring the inclusion of all requisite references. I point out these two missing ones on the Vesuvian magmatic system and the carbonate assimilation processes in Vesuvian eruptions:

- Buono, G.; Pappalardo, L.; Harris, C.; Edwards, B. R.; Petrosino, P. (2020). Magmatic stoping during the caldera-forming Pomici di Base eruption (Somma-Vesuvius, Italy) as a fuel of eruption explosivity. Lithos, 370-371, 105628, https://doi.org/10.1016/j.lithos.2020.105628.

- Pappalardo and Mastrolorenzo (2010). Short residence times for alkaline Vesuvius magmas in a multi-depth supply system: Evidence from geochemical and textural studies, Earth and Planetary Science Letters, 296, 1–2, https://doi.org/10.1016/j.epsl.2010.05.010.

3) All figures incorporated within the manuscript are sourced from other publications. It would be prudent to clarify whether appropriate permissions were sought from the respective journals for their inclusion.

I hope my suggestions can help.

Author Response

Dear Reviewer,

thank you for the comments that improved the quality of the manuscript. All line numbers correspond to the new manuscript with marked changes. Here are the responses to your comments:

1) Thanks for the comment. We have re-written the last part of the discussion to clarify how assimilation and skarn formation can lead to higher eruption explosivities in the three volcanic systems. As of now we had only discussed the potential effects in Vesuvian plinian eruptions, and we added some sentences about hydro-magmatic explosions (Colli Albani) and lava dome extrusion (Merapi). Lines 533-553 

2) Thanks for the suggestions: we have added both references (buono et al. 2020 in line 253, and pappalardo & mastrolorenzo 2010 in line 75.

3) For 5 of 6 figures we currently have the permissions (see the attached Permissions PDF-file). At the moment we lack the permission for Figure 02 (from: Pascal, M.-L.; Fonteilles, M.; Boudouma, O.; Principe, C. Qandilite from Vesuvius Skarn Ejecta: Conditions of Formation and Miscibility Gap in the Ternary Spinal - Qandilite - Magnesioferrite. The Canadian Mineralogist 2011, 49, 459–485,
doi:10.3749/canmin.49.2.459.). A permission has been requested, but to date we have received no answer.

 

We hope that with the corrections, the manuscript now fulfils the requirements for publication.

On behalf of the authors,

Sincerely,

Marco Knüver

Reviewer 2 Report

The work is a compilation of studies on different objects under a single topic - Mineralization and skarn formation associated with alkaline magma chambers emplaced in a limestone basement. All drawings are taken from previously published works. The tables contain information according to a single template. The article is replete with references to contemporary literature. Despite the lack of original data, the article will be useful to the readers of Minerals.

Incomprehensible places that need to be corrected are marked in the text of the manuscript:

1) Table 1 lacks the mineral composition of non-skarn igneous rocks. You can put it in the same column (magma composition / magma chamber ). This will help you see the whole situation. This is a wish that the authors can ignore.

2) Very unfortunate gray scale of figure 1. In the original article from which this figure is taken, it is also in gray tones. The authors could do a very useful job for all readers - translate this drawing into color. This is a wish that the authors can ignore.

3) Line 238. “Ca-richer melt” or “Ca-Mg richer melt”.  

4) It is necessary to correct the symbols in Figure 3. The sign of the skarns. Explain Ca-Ma melts are Ca-Mg-CO2-Si melts or Ca-Mg-richer Si-melts ?

5) The text in lines 256 and 263,264 contradicts each other. We must agree.

6) The conclusion in lines 265-267 does not follow from the information given. It is not obvious that the chloride phase predominates.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

thank you for the comments that improved the quality of the manuscript. All line numbers correspond to the new manuscript with marked changes. Here are the responses to your comments:

1) Due to lack of time (only 5 days were granted to incorporate the comments) we chose to ignore this wish.

2) Indeed the gray scale of figure 1 is quite unfortunate and we therefore agreed to re-draw and colorize the figure.

3) Line 239: Corrected to "Ca-Mg-richer melts".

4) Figure 3: Skarn sign corrected (added the black strokes in the legend). Ca-Mg-richer melts are Ca-Mg-richer Si-melts, CO2 due to its low solubility is depicted as exsolved fluid phase. Added a sentence in the figure description.

5) Lines 259-260 and 267-268: Corrected the mistake and added a reference to clarify.

6) Lines 270-273: Corrected the concluding sentence in light of the above made correction. Corrected points 5+6 also in the discussion part lines 500-501

 

We hope that with the corrections, the manuscript now fulfils the requirements for publication.

On behalf of the authors,

Sincerely,

Marco Knüver