Textural Identification of Polycrystalline Magmatic, Tectonically-Deformed, and Shock-Related Zircon Aggregates

Round 1

Reviewer 1 Report

Dear author, I have read the text and I have many doubts as to the legitimacy of the arguments.
I will start with the fact that there is a lack of a set of tests, a clear one indicating what type of rock it was, where the zircons were taken from, how many were etc.
But the matter of constructing arguments seems more important to me. Examples of several zircons are cited and on this basis the possibility of distinguishing their origin is determined. There are no statistics, no data to examine many features, compare them ...
In addition, I see quotes from literature with similar results in other publications. I would like to ask for their separate character.
The arguments you postulated should be based on a larger sample population, they relate to an attempt to characterize zircons.
There is probably an error on line 133, should it be Figure 1C? but 8C?

I am sorry to write such an opinion but I think, that the issues you raise
however, it requires a much broader number of samples tested in different rocks not only in the Alps, unless you indicate in the title that the case concerns the characteristics of telko zircons from the Alps but you have already published such texts in the past, which you quote.

Author Response

Dear Reviewer, 

Thank you for your valuable comments. Please, see the document attached, where I replied to your questions point-by-point.

I hope that the explanations are reasonable, and the information added to the paper has improved the text to a satisfactory level.

Thank you

Best regards

Elizaveta

Author Response File: Author Response.pdf

Reviewer 2 Report

Review on the manuscript “On the differences between “granular” zircon types: magmatic, tectonic, and shock-produced” by E. Kovaleva

The paper is very well written, and it contains valuable information and criteria to distinguish between different genetic types of zircons. This work is especially valuable as the zircon is the most powerful tool in the U-Pb age dating of rocks. The scientific content, the argument approach, the structure of the paper, the separation of results from interpretation, the graphic presentation, and the English language are all very good. I strongly recommend publishing this paper in Minerals, as a valuable paper to the journal. However, minor corrections or clarifications should be made before accepting the manuscript for publication. Please see below some recommendations and comments.

Recommended modifications and general comments

1. Using of term “conflate” is confusing. This term means “growing into one” which implies for the granular zircon that several grains can grow to make one complex, polygranular zircon. The synonyms of conflate are amalgamate, consolidate, converge, blend, etc. However, from the text, the meaning of the term “conflated” is rather “confused”. A granular igneous zircon can be conflated (or confused) with a metamorphic or shock-related zircon. The word conflate should be removed and replaced if “confused” is the intent of the meaning.
2. I suggest replacing “Shock-produced” with “shock-related” in the title, as shock-produced strictly suggests the zircon which is formed (crystallized) from an impact-melt. “Shock-related” term is more general and allows discussions on shock-produced and shock-affected zircons (pre-impact zircons which were affected by an impact).
3. D-grain: A definition/description (in textural terms, not genetic terms) is needed where the term is used for the first time in the text.
4. Specify in more detail (e.g. on “specific orientation”) the criteria to recognize a FRIGN zircon.
5. The conclusions and recommendations, even very important and valuable, are vague and difficult to apply. They are rather warnings than a useful tool in applying the results. I suggest the author synthesize the observed textural criteria (grain shape, boundary types, subgrains, low-angle vs high-angle features/inclusions, degree of misorientation, etc) as indicators for a certain type of grains (D, Z, FRIGN, igneous, regional metamorphic/metasomatic, shear-zone related, impact-related (textural criteria to recognize a pre-impact zircon which presents deformation features created by an impact), or impact-produced zircon. The coagulation of such structured information in a table format would be an extraordinary tool to be used by any researcher studying zircons or wanting to better understand the use of U-Pb age data. A researcher can use such criteria in understanding composite/conflated grains (detrital relict zircon which was affected by metamorphism, or metasomatism, or a zircon which preserve igneous features, and show regional metamorphic imprint followed by later localized deformation, which can be related to a shear zone (or tectonic as it is intended herein) or/and impact related features. Such a table can also contain specific mechanisms attributed to each microtextural observation/genetic type of zircon.
6. I suggest that “composite” zircons should also be introduced. Composite or conflated grains (here conflated used as several grains agglomerated into one) should be very common, as the paper also shows (e.g. anhedral core, followed by igneous euhedral zoning, which is turned into subgrains with various misorientation, followed by overgrowth around the aggregate, or individual grains, etc. A composite zircon would be one polygranular zircon which shows 2 or more different genetic overprints. However, not any polygranular zircon is composite, as sometime recrystallization or shock-induced process can make all the individual grains in the aggregate to be similar and any older features are erased.
7. A shorter discussion on the U-Pb diffusion during the above-mentioned processes and conditions where the U-Pb clock can be reset would be useful. Any possible link between textural observation and temperature?

Comments and recommendations on terminology

I would say that some terms are confusing at best. Some of them (granular zircon) is unfortunately still used in the literature, while others are pushed into use with this paper. Maybe it is the time to challenge this and come back to the meaning and rules of using textural vs genetic terms.

For example:

1. Granular zircon. The term granular is a textural term in use for more than 100 years, and it strictly applies to rocks, not to grains in the rock. In this paper, the term “granular” suggests that one grain of zircon is made of several other grains. The term should be polygranular, not granular. The term “polygranular” applies to grains of one mineral species agglomerated together (conflated, yes!) in one polygranular aggregate. I find this term “granular zircon” very confusing, as there can be grains of zircon made of one single “grain”, and being into a rock showing granular texture. If at all possible, as a recommendation, not a requirement, I suggest change the terminology of granular zircon to polygranular zircon. If the author wants to keep “granular zircon” because it was used in the literature before, then please at least give a definition containing microtextural definition in the introduction (not a genetic definition).
2. Tectonic zircon. This term is very confusing and ill-used, and I strongly recommend against using it. We use the "tectonic" term for a totally different scale of observation. Just imagine we will start to define tectonic garnet, tectonic quart, tectonic kyanite, tectonic mica, etc. because we can observe subgrains, and polygranular aspects, or kink bands. This isthe wrong way to proceed with terminology. Microtextural terms should be used, and not genetic terms as this will introduce huge confusion starting with describing samples. A similar misleading way was proposed in the 80’ for rocks, with the term tectonites, wanting to make a distinction between regional metamorphic rocks showing deformation and rocks related to shear zones. On the way, it became clear that most of the foliated metamorphic rocks were… tectonites. The term became redundant and was practically abandoned. A "tectonic" zircon can show cataclastic features, the existence of subgrains, subgrains boundary migration, recovery features to recrystallization, meaning that it shows that the mechanisms are the same as for any other minerals deformed during metamorphism. Such minerals can have a different pre-metamorphic origin (igneous, sedimentary). In other words, tectonic zircon here is equivalent to a metamorphic zircon (e.g. a magmatic zircon which suffered deformation during x metamorphic conditions). The overgrowth on euhedral facets can be an indicator of the igneous stage, while subgrain development, polygranular aspect, recovery, combined with overgrowth of the polygranular zircon can be an indicator for metamorphism and re-equilibration of some traces (Y, REE) during zircon-fluid interaction. One should be able to separate between defining and describing features that one can observe, versus genetic interpretation in describing a mineral/sample.

There can be zircons affected by regional metamorphism, without being in a shear-zone, and still presenting subgrains and having subgrains with various misorientation. The zircons deformed in a shear zone are still metamorphic. Then what is different between the two? “Tectonic” can also suggest a brittle deformation, as a fault. Zircon along such faults can be brittle-deformed or can totally escape this deformation due to localization of deformation. Then what is a tectonic zircon? However, cataclastic microtexture and polygranular aspect due to deformation, a higher degree of sub-grains misorientation can characterize zircons in different regimes of deformation, different strain rates, etc. How do we make the difference between detrital zircons (with a complex history) and regional metamorphism-related zircons, or local metamorphism (shear-zone or mylonite-related) zircon? The differences should be based on microtextural criteria.

Suggestion: cataclastic polygranular zircon (this is what one can observe) instead of tectonic zircon (this is what one might interpret, trying to choose between regional metamorphic, relict detrital, shear-zone related, etc). “Tectonic zircon” term is wrong and confusing and the use of such terms should not be encouraged.

A shock-related zircon should also have the same microtextural features as a metamorphic zircon, but here comes your paper to show the differences in the observations related to microtextures (shape/size/misorientation/low or high angle boundaries, subgrains) and zonation type, overgrowth type, etc. The set of criteria to distinguish between magmatic, metamorphic, shock-related, etc zircons can be better synthesized.

1. Shock-produced zircon. This term suggests that zircon is actually produced (crystallized) from an impact-resulted melt. What about other zircons which pre-existed the impact and were affected by impact without melting (deformation, recrystallization, all related to the great problem of U-Pb mobility and clock-re-setting)? How can one make a difference between shock-produced zircon, shock-related recrystallized zircon, and shock-related deformed zircon? Individual information or examples can be extracted from the text (points 1-7 from lines 388-413) but a general understanding or distinguishable criteria are not offered (see Comments 5 and 7 from above).

Please also note the comments on the pdf manuscript (as sticky notes).

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

Thank you for your useful contribution. 

Please, see my reply attached. I have also double-checked my English, as recommended, using Grammarly.

I hope that the paper is much improved now.

Thank you

Best wishes

Elizaveta

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

However, I still don't think enough statistics are available. The title also does not reflect the population of zircons tested. Please change it.

Author Response

Dear Editor and Reviewers,

Two comments were received from reviewer 1 in the last round of review (below). I have addressed them point-by-point with as much detail as possible.

Thank you

Best wishes

Elizaveta

1. However, I still don't think enough statistics are available.

While I am not opposed to adding statistical calculations in general, I am still a bit puzzled about what statistical processing to apply, as this is a textural study, and no quantitative data is presented (except the EBSD data that already have embedded statistics in it, as described in Methods). As a standard practice, textural studies on accessory minerals do not present statistics of any kind, and I was trying to find something relevant in the literature, but could not. For example, Atlas of Zircon Textures (Corfu et al. 2003) is a classic work that I always cite as an example, but no statistics are used there.

It is even more difficult because some examples I use in my work, are only known from 1-2 samples total, such as Z-grains described by Piazolo et al. (2012), or the polycrystalline shocked grains with large granules, only known from 3 or 4 studies with a handful of studied grains. 

The presented study has a diagnostic value only and is aimed to describe the variety of internal features that characterize polycrystalline zircon. It shows how such features can be used to extract information concerning the isotopic ages, geological history, and petrogenetic context of zircon. In that context, statistical evaluations are not going to be of great significance.

2. The title also does not reflect the population of zircons tested. Please change it.

The title is corrected accordingly.