the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Epidote dissolution–precipitation during viscous granular flow: a micro-chemical and isotope study
- 1Department of Geological Sciences, University of Bern, Bern, 3012, Switzerland
- 2Dipartimento di Scienze dell’Ambiente e della Terra, University of Milano-Bicocca, Milan, 20126, Italy
- 1Department of Geological Sciences, University of Bern, Bern, 3012, Switzerland
- 2Dipartimento di Scienze dell’Ambiente e della Terra, University of Milano-Bicocca, Milan, 20126, Italy
Abstract. Deformation of polymineralic aggregates can be accommodated by viscous granular flow, a process mediated by the interplay among intracrystalline plasticity and dissolution–precipitation, each active in specific minerals at given P–T conditions. Common rock-forming minerals like quartz, feldspars and sheet silicates have been intensively studied in terms of deformation processes. Instead, the deformation behavior of epidote and its role during viscous granular flow is not well investigated, although this mineral is ubiquitous in granitic rocks deforming at greenschist-facies conditions. In this contribution, we provide microstructural and geochemical evidence for the occurrence of dissolution–precipitation of epidote during deformation of an epidote-quartz vein. The main part of the vein is deformed producing a fold, which is visible due to relicts of primary-growth layering inside the vein. The deformation mechanisms active during deformation include dynamic recrystallization of quartz by subgrain rotation recrystallization, producing grain-size reduction of the primary vein quartz. This occurs contemporaneously with dissolution and (re)precipitation of epidote, and grain-boundary sliding, leading to a combined process described as viscous granular flow. The combination of intracrystalline plasticity, grain boundary sliding and dissolution locally and repeatedly produce creep cavities. These represent not only loci for nucleation of new epidote grains at the expenses of dissolved one, but they also allow fluid-mediated transport of elements. The same trace element patterns between old epidote relicts and newly formed grains, with much narrower variability, indicate a process of chemical homogenization. The nature of the fluid mediating deformation is investigated using Pb–Sr isotope data of epidote, which suggest that deformation is assisted by internally recycled fluids with the addition of a syn-kinematic external fluid component.
Veronica Peverelli et al.
Status: open (until 15 Jul 2022)
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RC1: 'Comment on egusphere-2022-311', Anonymous Referee #1, 23 Jun 2022
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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
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CC1: 'Comment on egusphere-2022-311', Matthias Konrad-Schmolke, 23 Jun 2022
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2 Geological setting
The geological setting as well as Fig. 1 are rather poor. It might not be necessary for the general understanding of the results and interpretations in this manuscript, but It would be benficial to the general quality of the work if the geological background would be a bit more detailed. The map is not very helpful once the reader would like to see the Aar massif's position in the Alpine orogen.
Lines 72ff.: "... and mainly records Alpine deformation ...". What else deformation does it record?
Lines 75ff.: What kind of shear zones are they? How wide? What does "a large number" mean? What orientations?
In general, this paragraph needs reworking!
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CC2: 'Comment on egusphere-2022-311', Matthias Konrad-Schmolke, 23 Jun 2022
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3 Field relations and sample description
Line 93: What kind of "rock laboratory" is the Grimsel Test Site?
Line 94: What is the "characteristic pattern of shear zones"?
Lines 95ff: Oblique to which direction? Fig. 2 is way too small to see either the geology or details of the shear zone/epidote vein.
Why are the other samples mentioned? None of them is considered in this work, or am I wrong?
Veronica Peverelli et al.
Veronica Peverelli et al.
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