The study of Peverelli and co-authors entitled “Epidote dissolution–precipitation during viscous granular flow: a micro-chemical and isotope study” investigates an epidote and quartz vein from the Aar Massif with several analytical techniques. The authors conclude that during deformation quartz experienced dynamic recrystallization by subgrain rotation that decreased its grain size. Consequently, grain boundary sliding, creep cavitation, and viscous granular flow produced the investigated microstructure. During these processes, epidote experienced dissolution and (re)precipitation responsible for pinning quartz grain size at grain boundaries.

Although these sequence and association of deformation mechanisms is coherent with what has been proposed in literature in several rock types and geological settings, at the present the petrographic and microstructural description and data are not sufficient to support these interpretations. In my opinion, this can be fixed with major revisions, aimed at expanding this section of the manuscript. Furthermore, discussion should be expanded, adding more references of relevant articles from other research groups and discussing novel data in the light of those works.

Below a few major comments, several comments are present in the attached pdf.

• Streamline of the text: interpretations of deformation mechanisms appear too early in the text and are not supported by documentation. I suggest moving those in the discussion section, expanding this session in the light of relevant literature (at the moment largely missing)
• Section 3 need to be largely implemented, both for text and microphotos. At the moment the reader cannot assess what it is written and cannot place the different epidote vein generations, shear zone and host rock in context
• EBSD documentation need to be expanded both in data and text, following comments in the attached PDF (e.g. only one EBSD map is present in appendix C, without low and high angle boundaries displayed in the map and text explaining it).
• The authors conclude that subgrain rotation recrystallization was the main deformation mechanism of quartz in results section. They state that CPO formed due to this mechanism. What would have happened to this CPO when grain boundary sliding, creep cavitation, and viscous granular flow occurred? Would you expect an attenuation of this CPO, as several authors suggested? This discussion is completely absent in the manuscript. Please, add EBSD documentation, such as GROD or GOS or KAM grain size maps along your map in appendix C, pole figures also for , misorientation angle distribution. Afterward, please discuss more thoroughly all those deformation mechanisms in the light of relevant literature.
• Dissolution precipitation in epidote: in the conclusions the authors state: “we have demonstrated the occurrence of epidote dissolution–precipitation processes”. However, I fear that diss-prec is just a proposition and that there could be other explanations. This is because the authors did not present any microstructural characterization of Epidote A grains pointing to diss-prec processes acting, such as lobate edges, microporosity,…. Please, refer to Putnis 2009 Reviews in Mineralogy & Geochemistry for microstructures indicative of diss-prec processes. Regarding another possible interpretation, as you imply the presence of an external fluid why you cannot just have new nucleation of epidote in creep cavities (thus syn-deformation) related to the percolating fluid, without dissolving previous Epidote A?

Concluding, I think that with major revisions aimed at including additional data, expanding results, expanding discussion section in the light of relevant literature, the authors could substantiate their interpretations and conclusions.

Best wishes

Comments on the manuscript: «Epidote dissolution–precipitation during viscous granular flow: a micro-chemical and isotope study» by Peverelli et al.

The manuscript describes observations and geochemical data of a deformed vein in granitoid rocks. The data and observations are of good quality and are presented in a clear and concise way. The material is novel and original and of great importance for the inference of deformation processes in granitoids and other plagioclase-bearing rocks. I have not checked the geochemical details and isotopic parts, as these are not my field of expertise. However, the arguments presented make a lot of sense to me. From my point of view, the manuscript should be published after minor revisions. Some points in detail are listed below, some points are only suggestions:

Line 17: “Recrystallization” instead of “This”

Line 20: «ones» instead of «one»

Line 34: “… and strain rates” (not only temperature). “Interpretations” instead of “interpretation”.

Line 35: “modal amount” instead of “relative abundance”

Line 42: “in the presence of a fluid dissolved material may precipitate…” (the fluid does not have to circulate)

Line 60: I suggest to use “epidote group minerals” here, because clinozoisite, zoisite and epidote, etc. form a considerably larger group of minerals than just epidote, and they all seem to behave in the same way mechanically in terms of crystal plasticity, i.e., they are very strong. Perhaps you want to insert a sentence to make this point.

Line 70: “polymetamorphic” instead of “polycyclic”?

Line 100: “yielded” instead of “returned”

Line 101: insert paragraph break

Line 136: in the table, is there any reason, why “SGR + dislocation glide” should be different from “dislocation glide + SGR”? If not, please use the same word order.

Lines 249-253: fluid inclusions are not visible in the figures – you are referring to fluid inclusions inside qtz grains, but are epidote grains also included in qtz grains? Probably not (?), but the sentence indicates this. Please clarify.

Line 255: the microstructure appears completely recrystallized – is it possible to identify unrecrystallized qtz grains at all?

Fig. 12: the CL contrast is difficult to see – can perhaps both images (BSE and CL) be shown separately and next to each other?

Line 386: perhaps better: “location”? In addition, “dynamic granular fluid pump” is not really a scientific term but rather a sort of a “buzz word” to sound fancy. I would avoid such terms. The reference is appropriate, of course, but something like “cavitation as nucleation sites” would be a more neutral descriptive term.

Lines 394-401: there is circumstantial evidence for grain boundary processes involving grain boundary sliding and solution precipitation in plastically deformed qtz (dislocation creep) in Nègre et al. 2021 and Pongrac et al. 2022 based on experiments and in analogy to wet ice dislocation creep.

Lines 421-423: the compositions are not identical. Better: “the epidote B composition represents a narrower range of the compositional spectrum of epidote A” or something like this.

Line 431: “difference” instead of “heterogeneity”?

Line 453: again “granular fluid pump” could be replaced by descriptive terms like “combined grain boundary sliding, cavitation, and nucleation” or so.

Line 489: admittedly, it is a somewhat obscure reference, but Stunitz 1993 describes crystal plastic deformation (or better: the absence thereof) in clinozoisite.

If there are any questions, the authors may contact me.

Best regards

Holger Stunitz