Chemical Abrasion: The Mechanics of Zircon Dissolution

McKanna, Alyssa J.; Koran, Isabel; Schoene, Blair; Ketcham, Richard A.

doi:https://doi.org/10.5194/gchron-2022-19

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https://doi.org/10.5194/gchron-2022-19

Preprints

26 Jul 2022

| 26 Jul 2022

Status: a revised version of this preprint was accepted for the journal GChron and is expected to appear here in due course.

Chemical Abrasion: The Mechanics of Zircon Dissolution

Alyssa J. McKanna, Isabel Koran, Blair Schoene, and Richard A. Ketcham

Abstract. Chemical abrasion is a technique that combines laboratory annealing and partial dissolution in hydrofluoric acid (HF) to selectively remove radiation-damaged portions of zircon crystals prior to U-Pb isotopic analysis, and it is applied ubiquitously to zircon prior to U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of zircon dissolution in HF and the impact of different leaching conditions on the zircon structure, however, are poorly resolved. We present a microstructural investigation that integrates microscale X-ray computed tomography (µCT), scanning electron microscopy, and Raman spectroscopy to evaluate zircon dissolution in HF. We show that µCT is an effective tool for imaging metamictization and complex dissolution networks in three dimensions. We find that most grains do not dissolve predominantly from rim-to-core. Acid frequently reaches crystal interiors via fracture networks spatially associated with radiation damage zoning and inclusions to dissolve higher U zones, material in the vicinity of fractures, and some inclusions. Other acid paths to crystal cores include the dissolution of surface-reaching inclusions and the percolation of acid across zones with high defect densities. In highly crystalline samples dissolution is crystallographically-controlled with dissolution proceeding almost exclusively along the c-axis. Increasing the leaching temperature from 180 °C to 210 °C results in deeper etching textures, wider acid paths, more complex internal dissolution networks, and greater volume losses. We discuss the implications of our findings for zircon ID-TIMS U-Pb geochronology and paired trace element analyses, radiation damage annealing models, and for using µCT for imaging radiation damage zoning for (U-Th)/He thermochronology.

Received: 15 Jul 2022 – Discussion started: 26 Jul 2022

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Alyssa J. McKanna et al.

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RC1:
'Comment on gchron-2022-19', Fernando Corfu, 19 Aug 2022

The paper report the effects of annealing and HF partial dissolution experiments on four different zircon samples of different age and radiation damage. With the help of various imaging and measuring techniques the changes due to annealing and partial dissolution are monitored and form the basis for a discussion of the factors that control the geometry of the partial dissolution processes and implications for the use of this mineral in geochronology. The paper provides a systematic insight into the behaviour of zircon during the widely applied chemical abrasion procedure, and will be a welcome addition to the literature on the subject.

Overall the paper is well prepared and structured. I find, however, that the present version could be improved by a good weeding of unnecessary words and sentences. Some expressions that the authors like to use frequently should be reconsidered. One example is ‘compositional zone’. I have made some suggestions in the file and discuss specific points below.

Although the results are very relevant for zircon U-Pb geochronology, the studied zircons have not themselves been dated, and so the specific data do not have a direct connection with the U-Pb behavior of the samples. Consequently, the discussion of implications for U-Pb dating presented in the paper is very generalized and in part trivial. I suggest to concentrate on the main substance of the study and avoid meandering off in inefficient discussions.

Some specific points:

Line 64; ‘… poorly understood, and several outstanding questions remain. Do most zircon crystals predominantly dissolve from rim to core?’

Everyone who has done chemical abrasion will have noted very rapidly that zoned and metamict zircons do not dissolve that way. That can hardly be described as a poorly understood fundamental question.

Fig.2: The explanations are unclear. Are the grains shown in each of the four panels representing each of the sample? If yes, why does the second panel contain zircon from two different samples (blue and yellow circles). If not, what is the distinction? Overall I find this figure quite useless, not even as a decoration.

Fig.3: Not sure about using the term ‘metamict’ once zircon has been annealed. Can be confusing.

209: ‘typical magmatic growth patterns’   It is true that many magmatic zircons have oscillatory zoning, but the same pattern can in part be seen in metamorphic zircon. Better to use descriptive terms.

287: ‘ This could imply that increasing the duration of the leaching step results in a more crystalline zircon residue due to the progressive dissolution of higher damage domains.’ Why the ‘could’? It seems to be the most logical explanation.

And then the following sentence; ‘We note, however, that only a small number of AS3 crystals survived 12 h of chemical abrasion, and only a small number of Raman analyses were made. We recommend further study to better evaluate this possibility.’ Sounds rather trivial.

316: ‘As evidenced by our SE images and discussed further below, μCT does not capture radiation damage zoning that does not result in a strong density contrast such as variations in radiation damage below the ~1×1018 α/g threshold.’ Suggest rephrasing to avoid the double negatives.

Fig. 9, caption: ‘interior compositional zone’. Compositional zone? As opposed to what?

367: ‘ All observed dumbbells are oriented parallel to the grain’s c-axis’. That is a surprising statement. Looking at the figures I would have assumed that they are all normal to the elongation of the crystal (= c-axis). Please elaborate to avoid confusion

Fig 13 caption: ‘The yellow arrow highlights the grain’s shell-like appearance because of significant dissolution in the grain’s interior.’ I see a highly resorbed grain, not much left of the shell.

‘images of dog-chewed zircon residues ‘ maybe a bit too colloquial?

419: ‘In a visual game of connect-the-dots …’     ??

422: ‘We see dumbbell-like fracture patterns again in sample Zr36 (Fig. 13b-III) where crosscutting fractures connect different oscillatory zones removed by dissolution to one another and to the grain surface’. Rather convolute sentence, hard to understand. Please rephrase.

483: ‘… The long axes of deep, octahedral etch pits on (100) align with the crystal’s c-axis…’

I wonder why they are called octahedral. Those in Fig 16a look prismatic to me.

Fig. 19, caption: ‘ Projection on (100) looking down the a-axis. The c-axis is vertical to the page, and the a2-axis is horizontal’ Confusing: should it not be: ‘Projection on (100)

looking down the c-axis’ ? The same for (b): if the pane is 001 then the view must be parallel to a? Or not?

640: ‘… suggest that a crystal’s bulk radiation damage also plays an important role.’ That seems a rather trivial discovery. What else could one expect?

641: ‘Crystal morphology plays a lesser role in that crystals with very high aspect ratios dissolve more slowly than more equant grains’   ??? That is not apparent in Fig. 18e, and would seem to contradict the higher solubility along the c-axis than along a.

Section ‘4.2.1 Zircon U-Pb ages’ lines 648 – 726: I find the discussion on this topic very generalized and superfluous. The experiments in this paper demonstrate the great variability between zircon of different geneses, compositions and ages. Yet many of the reflections made here focus on some idealized magmatic zircon in young systems. Because of such a simplification the discussion is trivialized and almost meaningless. Clearly, the lessons from the present experiments must be considered separately for each set of zircon used for geochronology. I recommend removing this section, it just detracts from the paper.

778: ‘…chemically abraded residues are more crystalline than their annealed counter parts…’: Isn’t that the logical relationships since CA removed the less crystalline domains? And: ‘… radiation damage is annealed hydrothermally during HF leaching…’. Speculative?

867: ‘Increasing the leaching temperature from 180 °C to 210 °C or increasing the leaching duration leads to the development of more extensive dissolution networks in higher damage grains ‘ This is a rather trivial conclusion. Something CA-users did not observe before?

870: ‘More crystalline zircon samples lack fracturing related to radiation damage zoning.’ Another trivial statement. The conclusions should focus on the important aspects of the research, not on trivialities.

Aug. 13, 2022   F. Corfu

Citation: https://doi.org/10.5194/gchron-2022-19-RC1
- AC1: 'Reply on RC1', Alyssa McKanna, 13 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://gchron.copernicus.org/preprints/gchron-2022-19/gchron-2022-19-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/gchron-2022-19-AC1
CC1:
'Comment on gchron-2022-19 (McKanna et al.2022 https://doi.org/10.5194/gchron-2022-19 )', Charles Magee, 29 Aug 2022

McKanna et al. describe a morphological study of the changes wrought on zircon by the chemical abrasion process. They use a combination of SEM imaging, microscale X-ray computed tomography, and Raman spectroscopy to characterize individual zircon crystals as they go through this process.
The study is important because even though chemical abrasion (CA) has been used for 17 years to improve the apparent closed system behaviour of zircons during U-Pb isotopic analyses, nobody really understands how it works. As the technique is necessary for modern high precision zircon isotope dilution thermal ionization mass spectrometry (ID-TIMS) based geochronology, a better understanding of the physical processes involved should improve geochronologists’ ability to interpret the results of CA ID-TIMS.
The study is well designed, well executed, and clearly explained. The use of techniques which don’t involve destructive sample preparation or analysis on the intermediate steps is smart, as it allows the same individual grains to be followed through the process where the only thing damaging the grains is chemical abrasion.
There are three particular areas where the manuscript could be improved.
Section 2.1
Firstly, the introduction says that the second described sample- SAM-47- is from the Corunna Downs granitoid complex. The Australian Stratigraphic Unit database describes this term as informal. See:
https://asud.ga.gov.au/search-stratigraphic-units/results/34394
It has been replaced by the Corunna Downs Granitic Complex:
https://asud.ga.gov.au/search-stratigraphic-units/results/72996
Note, however, that this term also is obsolete. As shown in the links above, Pilbara Granitic Complexes are not stratigraphic units, but are geological provinces, so neither of these descriptors is particularly informative. More importantly, the SAM-47 description is the only description that does not have any references associated with it, making it difficult to understand the geologic background of this sample, or any associated information that would allow readers to interpret the results.
Furthermore, the latitude and longitude (89°59’55.97”, 100°08’2.38”) given are in the Arctic Ocean near the north pole, and are not in the Pilbara craton.
In summary, it would be helpful if the authors could more accurately locate the sample site, and relate it to a local, named stratigraphic unit, and provide appropriate reference(s) to previous work that provides geological context.
Section 3.2.1
The use of a synthetic zircon (which is not described in the samples section of the methods) may not be the most appropriate measure of a full annealing natural zircon. Lattice strain can be caused by factors other than radiation damage, such as the incorporation of variably incompatible trace elements into the lattice structure. If the synthetic zircon is pure ZrSiO4, instead of being grown with levels of P, Y, REE, and other trace elements typical of zircons from basic to felsic host rocks, then the ability of chemical abrasion to repair lattice strain may be underestimated due to the lack compositionally related lattice strain in the chemically pure synthetic crystal.
Similarly, we can’t tell from the Raman data whether the narrower peak widths of KR18-04 and BOM2A are due to damage or composition, although the narrower peaks for the younger zircons, and excellent choice of one mafic and one felsic zircon from both the ‘old’ and ‘young’ groups does suggest irradiation is important.
On a related note, when estimating the accumulated lattice strain, a U/Th/He age or a fission track age may be more appropriate than a crystallization age, depending on the ability of moderate-to-high temperature zircon to self-anneal radiation damage over geological time. The lack of location data for SAM-47 (see above) makes estimating this difficult, but to use a well-studied East Pilbara Archean example, the Owen’s Gully Diorite has a crystallization age of 3467 Ma (Stern et al. 2009), but a helium age of only about ~750 Ma (Magee et al. 2017).
Figure 2
Finally, it might be worth specifically pointing out that 2b is a colour photomicrograph, as the annealing out of radiation damaged colour centres is an important but sometimes overlooked part of CA. This illustration is so dramatic that readers might not appreciate that the second image is a colour image in which all the colour has been annealed out of the zircons, leaving them almost colourless.
Overall, this is an excellent study which provides copious data and interpretation of the chemical abrasion process, and I look forward to seeing the final version.
Charles Magee
References:
Magee, C. W. Jr., Danišík, M., and Mernagh, T.: Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology. Geoscientific Instrumentation, Methods, and Data Systems. 6, 2, 523-536, doi:10.5194/gi-6-523-2017, 2017.
Stern, R. A., Bodorkos, S., Kamo, S. L., Hickman, A. H., and Corfu, F.: Measurement of SIMS instrumental mass fractionation of Pb isotopes during zircon dating, Geostandards and Geoanalytical Research, 33, 145-168, doi:10.1111/j.1751-908X.2009.00023.x, 2009.

Citation: https://doi.org/10.5194/gchron-2022-19-CC1
- AC4: 'Reply on CC1', Alyssa McKanna, 13 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://gchron.copernicus.org/preprints/gchron-2022-19/gchron-2022-19-AC4-supplement.pdf
  
  Citation: https://doi.org/10.5194/gchron-2022-19-AC4
CC2:
'Comment on gchron-2022-19', Magdalena Huyskens, 09 Nov 2022

The manuscript “Chemical Abrasion: The Mechanics of Zircon Dissolution” by McKanna and others is presenting microstructural investigations of zircon behaviour during chemical abrasion treatment using scanning electron microscopy, Raman spectroscopy, and X-ray computer tomography, in order to understand the mechanics behind the CA technique and aims at giving recommendations to remove Pb loss from zircon samples.
They have documented dissolution structures associated with compositional zones, fractures, inclusions and crystallographic orientation. These observations support that CA is not just removing the outer parts of a zircon crystal for intermediately damaged zircons, potentially biasing the age, but most inclusions and radiation damaged zones are reached by the treatment. For zircons with a low radiation damage, the CA treatment mostly dissolved the zircons from the outside, especially along the c axis.
Overall, the study is very detailed and well written, adding to the understanding of the chemical abrasion technique. While many of the observations are known by practitioners of the CA technique, they have never been properly documented and quantified, which can lead to conclusions like the crystallographic orientation dictating the dissolution direction. However, there are some major points that I would like to see addressed.
There are quite a few qualitative statements that can and should be backed up with statistical analyses. For example, claiming changes in the slopes between v3(SiO4) and Eg peak after annealing and leaching (lines 254-256) and reporting the average changes and range for the peak width in a raman spectrum for the individual samples and temperature steps.
For the estimates of volume loss, the method needs to be described in more detail. Right now, I am not sure if it is including interior dissolution features or not.
One of the findings is that some compositional zones are preferentially dissolved. Do you have any compositional data for these? It would been great to know what the difference in composition is and if the solubility is solely based on radiation damage, or some composition otherwise is more soluble.
The one recommendation that is put forward for the CA technique is to increase the temperature: “In most samples regardless of initial damage content, we find that chemical abrasion at 210 °C is more effective at mining out soluble zones from crystal interiors. Based on our mechanistic blueprint, we predict that hotter leaching temperatures are thus more likely to better mitigate Pb-loss in geochronological datasets.” I do not find this supported in the data. Yes, the solubility increases with increasing temperature. Increasing it a little bit more or increasing the time will completely dissolve the entire zircon grain. Since there is no U-Pb data associated with this study, the mitigation of Pb loss is speculative. In addition, there are many zircons that will completely dissolve with such a treatment, in which case no U-Pb date could be collected.
Minor comments:
Misspelling of reference “Bowring and Schmidtz, 2003” (multiple times in the introduction)
Figure 2: Is there any way to track which conditions were used for which grains? It would be helpful to get an idea overall how the different samples are behaving under the different conditions. In addition, it would be nice to have images of the zircons before annealing. There are often colour changes associated with this step.
Figure 5: All panels should have the sample name in them, at roughly the same position. It is confusing that c) is within a). Use the same font type. If one panel has the alpha dose, the other one should have this too.
L 254-260: “We note that relationship between the v3(SiO4) and Eg peak widths is steeper after annealing in each of the four samples, since the two Raman peaks have different temperature sensitivities (Hartel et al., 2021). This observation suggests that laboratory annealing is not simply the inverse of radiation damage accumulation. As such, we caution against using the Váczi and Nasdala (2017) calibration to derive alpha dose estimates from v3(SiO4) peak widths for annealed or chemically abraded samples and omit alpha dose axes from Figures 5b and 6b.” This is inconsistent. The alpha dose is noted for the annealed samples, but not the partially leached ones.
Figure 7: The choice of color for the 180 °C for 12 h for AS3 & SAM-47 is odd, since it fits the color scheme of samples KR18-04 & BOM2A.
Lines 274-281: “Notably, SAM-47 and BOM2A residues each have at least one data point with a narrower v3(SiO4) and Eg peak width than their solely annealed counterparts suggesting that some residues have a higher degree of crystallinity. Further, we find that the residue datapoints for these two samples largely plot below (at lower v3 for a given Eg) the annealed datapoints indicating a change in the relationship between the v3(SiO4) and Eg peaks. Taken together, these observations could suggest that additional structural recovery occurs in some zircon samples during HF leaching even after dry annealing at significantly higher temperatures.”
Is there any reason that this observation can’t just be explained by the removal of more damaged zones that were not annealed during the high temperature annealing step? Structural recovery during HF leaching seems impossible to me and would need some further explanation.
Line 405: “… many most …” remove one of those words
Line 777- 780: “There is also an apparent change in the relationship between the widths of the v3(SiO4) and Eg peak after partial dissolution in HF acid in some samples, and a small number of Raman analyses for chemically abraded residues are more crystalline than their annealed counter parts” Same as for Lines 274-281. Does this not just mean that the parts that were not annealed due to larger radiation damage are still present before leaching?
The section 4.2.2 Inclusions and zircon trace element analyses is not discussing the impact of dissolving compositional zones within zircons that are observed in this study.
The section “4.4 Imaging radiation damage zoning: Implications for (U-Th)/He thermochronology” seems disconnected from the main point of the paper and is a little distracting.

Citation: https://doi.org/10.5194/gchron-2022-19-CC2
- AC5: 'Reply on CC2', Alyssa McKanna, 13 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://gchron.copernicus.org/preprints/gchron-2022-19/gchron-2022-19-AC5-supplement.pdf
  
  Citation: https://doi.org/10.5194/gchron-2022-19-AC5
EC1:
'Comment on gchron-2022-19', Daniel Condon, 05 Dec 2022
In addition to the two community comments and 1 referee comments I am presenting my review of the manuscript prior to an editorial decision.
McKanna et al., present a detailed study of the impact of annealing and leaching (i.e., ‘chemical abrasion) on zircon, using a workflow that exploits backscattered electron imagining, raman spectrometry and micro X-ray computed tomography (mCT), applied to a four samples. These four samples cover a range of ages (from ~3 Ga to 66 Ma) and thus radiation damage histories. These samples also have differing internal zonation patterns and mineral inclusions. These samples are subjected to thermal annealing (900C for 48 hours) and leaching in HF (180 and 210C for 12 hours), with characterisation prior to and following this treatment.
The paper is aimed at those using zircon for U-Pb (and to a lesser extent, U-Th/He) geochronology as this ‘annealing and leaching’ process is employed to ‘effectively eliminate’ Pb-loss in the residual zircon that is then often used for high-precision U-Pb dating. As discussed in the third paragraph, this pre-treatment is universally applied but the mechanistic understanding is limited, and this impacts how datasets are interpreted. How confident can we be in single ‘youngest’ U-Pb dates from a zircon, that may or may not be the youngest zircon. Has Pb-loss been 100.00% eliminated, or do we still need to keep that as an option when interpreting dataset, assigning age uncertainties? How does the leaching work and how does this impart a bias in the age of the material left for analyses after annealing and leaching?
This paper is a step towards having a more mechanistic understanding of the chemical abrasion process and how this varies, related to accumulated radiation damage, presence/lack of inclusions, crystal morphology and internal zonation etc.
The paper mixes a set of quantitative data that is linked to observations and qualitative data derived from it. As a result, the description of the data is rather wordy and at times difficult to follow – wonder if some form of tabulation wouldn’t help? Also, lots of the discussion/generalisation seems reasonable, its worth making it clear that this if for four samples and a wider range of samples need to be characterised, and perhaps a wider range of parameters (i.e., the annealing in addition to the leaching that his manuscript focusses on).
Some general comments:
Fractures – the use of this term implies that planar features are a result of stress that is applied to the material. There is a good argument to be made for this process and it seems likely to be a common occurrence in some samples. My comment is that calling all planar features fractures implies a certain causative process (differential stress). Are all planar features fractures? Maybe they are.

Rim to core dissolution. A lot of different mechanisms and processes are discussed and the authors do a good job of introducing these and tracking them through the discussions. Whilst the images/analyses support that rim to core dissolution is not typical the authors do state (line 680) that there is a progressive rim to core dissolution

Much of the focus in the discussion, and in the community, is around the leaching temperature as being the thing that is most significant. Perhaps it is but what above the duration of the annealing, or the rate of cooling? This paper focusses on samples that have bene annealed for 48 hours but it should be acknowledged in the manuscript that practitioners quote a range of annealing durations, typically 48 or 60 hours. I assume this is the time between turning the furnace on and off, and often cooling can take several hours although the rate of cooling can be increased by opening the furnace.

The zircon crystals studied have not been analysed for high-precision U-Pb – but data does exist for the samples (AS3 – Schoene et al., GCA, 2005, coherent U-Pb; SAM-47 – no U-Pb data published? KR18-04 – MacLennan et al., Sci Adv, 2020 – overdispersion, Pb loss? BOM2A, Basu et al., 2020, single population). Have these analyses been conducted at experimental conditions analogous to those deployed in this study? I appreciate that the precision/resolution may not be at the level to preclude Pb-loss but presenting the data might help frame the discussion around the implications for zircon U-Pb systematics and age interpretations

The paper seems focussed around leaching mechanisms/processes applied to whole crystals – however the process will often be applied to fragments and/or grains that has been polished for CL, on both cases exposing the interior of the grains. Would be useful to mention for the non-practitioners that not all zircons will come as complete crystals.

It is a long paper and much of the qualitative observational data based upon examination of many observations from the four samples, which a subset of representative images presented. One issue is around readability – could some of the generalised observational data/interpretations be tabulated to make it more accessible? Personally, I found it challenging, going back and forth to try and compare what is said for the different samples and leaching temperatures. I felt the use of tables may be helpful for compiling this qualitative information and making it more readily accessible.

Conclusion section – is it possible to draw out the observations and how they record a progression of processes?

Some minor comments (tied to line number in the manuscript).
Bowring and Schmitz, not Bowring and Schmidtz

also mention the rare occurrences of reverse discordance seen in some samples?

thermal annealing instead of laboratory annealing

remove more soluble

bias, yes, but more realistically this should be considered an additional source of uncertainty in the assigned age

also prompted the community to question/explore a range of interpretative frameworks for such datasets

and “How does the duration/temperature of the annealing impact the crystal structure and precondition it for reaction to leaching?”

Line 96 – could the sample information be tabulated?
Line 146 – what portion (percentage of grains) didn’t survive the leaching and was their anything distinctive about those grains? Did any grains break apart?
Figure 2 A is reflected light, what is the light source for B? I assume AS3 is top left etc., for Panel B but there is space to add label, or state this in the figure caption. The images are low resolution – will higher resolution version be submitted as a supplement? Also, the top right panel indicated the images contain residues that have been leached for varying times (4 and 12 hours) – how does the reader distinguish these different grains?
Line 251 – how does this look for 60 hours? Is there any published data for this?
Line 262 – were replicate raman determinations made on any of the crystals to assess variation within a crystal?
Figure 8 – what is the 2D nature of the 3D rendering in figure 8?
376 – remove interestingly.
678 – ID-TIMs analyses represent an integrated analyses of the residue post-leaching – this could be core-rim, or more core, or more rim – seems that is will be sample and duration dependent?
687 – “hot leaching” (210C)? Or should this be hotter leaching? Or longer leaching?
702 – yes, but in lower radiation damaged samples it may also impact an age-bias towards the core/older material
723 – and could this be a mechanism that results in the rare cases of reverse discordance we see in CA ID-ITMS data for some old zircons? Could we be seeing this in samples but not at a resolvable level, where the zonation is favourable?
741 – yes but sometimes the inclusion rich zircon may be the ones we want to date…
781 – is hydrothermal annealing a thing? Can you provide a reference? Looking at the literature I couldn’t find anything in materials science – do you mean hydrothermal treatment? This sounds odd – how robust are the few data this discussion is based up?
784 - Widmann.
865 – Okay – then what impact does the annealing temperature/duration/cool down rate have on the formation of micro fractures? All the discussion is around varying the leaching parameters but should we also be considering the annealing step?
839 – four samples covering a range of ages and radiation damage accumulation.
888 - … removal of excess closed system material AND potential age bias in lower radiation damaged materials?
Citation: https://doi.org/10.5194/gchron-2022-19-EC1
- AC3: 'Reply on EC1', Alyssa McKanna, 13 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://gchron.copernicus.org/preprints/gchron-2022-19/gchron-2022-19-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/gchron-2022-19-AC3
RC2:
'Comment on gchron-2022-19', Daniel Condon, 10 Jan 2023

Comments on the manuscript “Chemical abrasion: The mechanics of zircon dissolution” by Alyssa J. McKanna and co-authors, submitted for publication in Geochronology (#gchron–2022–19)

Lutz Nsdala

Generalities:

This is an interesting contribution whose results deserve to be brought to the attention of the international community. During the last 15 years, several teams have undertaken extensive dry-annealing experiments in order to understand why applying Mattinson´s CA–TIMS technique is advantageous in many cases; that is, why dry annealing prior to HF etching improves the selective removal of more radiation-damaged zircon during HF etching? The present study focuses more on the second step: What exactly happens during the HF attack? Observations such as preferred dissolution and preferred volume shrinking along a certain crystallographic direction, and the obvious contribution of fluid-driven recovery processes, are novel and valuable. The results are of interest and for the most part sound, and I am happy to suggest acceptance for publication after minor (mostly technical) revision.

Main comments:

The main shortcoming of the present research is perhaps that there is no chemical information on the samples studied. The present paper presents results of rather sophisticated studies including micro-CT and precise dimension estimates, but the most basic information remains hidden. There are quite a few journals that would refuse to publish a study on samples whose chemical compositions are unknown; for good reasons, as I think.

In particular, there are no U and Th concentrations available that, along with U–Pb ages, could have been used to calculate realistic time-integrated alpha doses. Instead, authors fiddle with equation #2 of Váczi and Nasdala (2017) in attempting to estimate self-irradiation doses from Raman FWHMs. Which is based on a serious fallacy of thinking: The equation of Váczi and Nasdala (2017) – that by the way has never been proposed as a “calibration”, as present authors claim in line 223 – refers solely to Sri Lankan gem zircon with its particular damage-annealing history, whereas other zircon populations may have appreciably different FWHM-dose relationships.

In contrast, comparing alpha doses that were calculated from U, Th and age, with the observed FWHM values, would have opened up valuable opportunities for characterising the initial samples and their annealing histories. This chance was missed. Also, because of the unavailability of chemical compositions, it cannot be evaluated whether HF etching has removed completely some interiors of the grains but has left the rest fully unchanged, or did un-dissolved remnants experience some chemical leaching?

Minor comments:

- Present authors rely on the results of Palenik et al. (2003) and Váczi and Nasdala (2017) who both have claimed that the FWHM of the n₃(SiO₄) Raman band of Sri Lankan zircon has a maximum (“saturation”) value of about 35 cm^–1. On the other hand, sample AS3 yielded FWHMs of about 48 cm^–1 (Fig. 5a, Table S1). This apparent contradiction should be discussed, and actually such extremely broadened Raman spectra should be shown (at least in a supplementary figure).

It has been suspected that Sri Lankan zircon of elevated degrees of radiation damage have experienced preferred annealing, which might explain the “saturation” as a particular feature of Sri Lankan zircon. In conclusion, it is not surprising that other zircon, not affected by the particular “Sri Lankan” annealing history, may indeed show further band broadening. This has been rarely observed thus far but is valuable; so it should been shown and discussed.

- Line 183, it is not really of relevance for the reader to learn about the equipment of the Princeton Raman system; instead, it should merely be stated which particular laser was actually used in the present study. Presumably red, as the green laser tends to induce Er³⁺-related photoluminescence obscuring the Raman spectrum? The laser power at the sample surface (so not laser output but power measured behind the objective) needs to be reported.

- Line 185: I wished Horiba would stop implementing the main silicon band in their auto-calibration procedure. However, if authors believe the friendly Horiba service person who did so was right, they should provide a published reference for the 520.7 cm^–1 value they used for calibration. Other authors have used 520.5 or 521.0 cm^–1, and none of these values has ever been appropriately supported. Note that the oriented Si wafer is provided for alignment purposes but pretty unsuitable for wavenumber calibration: Visible light penetrates only a few µm into Si, which is why increasing the laser power, or changing the laser colour, changes absorption-induced heating and causes a shift of the Raman band. Why not using a truly known signal instead? The Rayleigh line has a Raman shift of 0 cm^–1; a perfect calibration means.

- Line 189–190, quoting ~2 µm spatial resolution upon using a “” is too general and imprecise. With the confocal hole set to 400 µm, the spatial resolution will be much poorer than 2 µm lateral, and with a 100 µm confocal hole, the LabRAM Evolution should – provided the beam alignment is well done – reach its maximum performance close to the Rayleigh or Sparrow criterions, well below 1 µm (see detailed study by Kim et al., 2020, Current Applied Physics 20:71–77). The above resolution estimate does of course only apply if the Olympus 100× / n.a. 0.90 objective was used (this essential information is missing as well).

- Page 8, caption of Fig. 3a: The wording “” is too imprecise. State explicitly which images are CL, BSE, SE.

- Lines 218–219, the expression “” is a contradiction in itself. To vibrate asymmetrically, it takes at least two bonds, whereas the stretching vibration of a single Si–O bond does not have any symmetry. Should be reworded to “”.

- Fig. 18, in subfigures a–d the main x-axis label is missing.

- Lines 768–769, ”; and again lines 891–892, “”. These statements are presented as if they were a result of the present study, which is decidedly not the case. In contrast, it has been emphasised may times already during the last 20 years that annealing does not inverse the damage accumulation process (mismatch of Raman FWHMs and Raman shifts, mismatch of unit-cell parameters a and c, etc.). Something like “” would be more appropriate and more honest.

- Same in lines 250–252, “”. This is not a new finding but merely another well-known fact, here merely reconfirmed again. People who wish to achieve near-complete annealing of zircon do this at 1400°C for four days or so.

By the way, Raman analyses do not suggest anything; only the results of Raman analyses suggest something (wording in line 891 should be improved).

-Line 800, a typo? Instead of “” there should be “”.

- Line 983, the second author is Dr Chutimun (first name) Chanmuang N. (family name), so her name should be abbreviated “Chanmuang N., C.”; same in line 1012. Similarly, in line 1050 the second author should be quoted as “Van den haute, P.”

- Table S1, the “empirical correction” of Raman shift values for band downshifts that are claimed to be caused by laser-induced heating remains fully vague (unclear). Something like this has never been observed and reported before; so at least some comprehensible documentation is needed. How about simply decreasing the laser-power density, to reduce laser-induced heating to a negligible extent??

- Table S1, in view of the fact that many authors present and discuss uncorrected FWHMs, applying FWHM correction in the present article is meritorious. However, a correction assuming a FWHM of the IPF (instrumental profile function) of 1.5 cm^–1 (I assume this is the meaning of the unitless “” in footnote c?) most likely has led to an overcorrection. The FWHM of the IPF of the LabRAM Evolution with its 800 mm focal length, in case the 1800 grooves per millimetre diffraction grating is used, is about 0.8 cm^–1 in the red and about 1 cm^–1 in the green range of the electromagnetic spectrum. Authors should test this – and hence the experimental band broadening of their system – by measuring the width of the Rayleigh line, Kr lamp or similar emission lines, or even better narrow Raman bands of known widths (unstressed diamond, Ba nitride).

Unfortunately I was unable to find Raman shifts and corrected FWHMs of synthetic zircon as measured by present authors. However, data points in Figs. 5 and 6 imply that synthetic zircon would have a FWHM of the n₃(SiO₄) band of about 1 cm^–1, which is well below the real value of about 1.7–1.8 cm^–1. This seems to support my above suspicion of FWHM overcorrection. Let me assume authors have fitted an FWHM of 2.1 cm^–1. This value, corrected for a (too large) 1.5 cm^–1 IPF, yields an overcorrected FWHM of 1.0 cm^–1. In contrast, a fitted FWHM of 2.1 cm^–1, corrected for a 0.8 cm^–1 IPF, yields a quite realistic FWHM of 1.8 cm^–1. Anyway, data for synthetic zircon should be added to Table S1.

- Table S1: Reporting FWHM values with two decimal digits fakes an unrealistic accuracy. As the uncertainty is 10% for narrow FWHMs, reporting 4.4 instead of 4.38 cm^–1 will do the job.

Citation: https://doi.org/10.5194/gchron-2022-19-RC2
- AC2: 'Reply on RC2', Alyssa McKanna, 13 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://gchron.copernicus.org/preprints/gchron-2022-19/gchron-2022-19-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/gchron-2022-19-AC2

Status: closed

RC1:
'Comment on gchron-2022-19', Fernando Corfu, 19 Aug 2022

The paper report the effects of annealing and HF partial dissolution experiments on four different zircon samples of different age and radiation damage. With the help of various imaging and measuring techniques the changes due to annealing and partial dissolution are monitored and form the basis for a discussion of the factors that control the geometry of the partial dissolution processes and implications for the use of this mineral in geochronology. The paper provides a systematic insight into the behaviour of zircon during the widely applied chemical abrasion procedure, and will be a welcome addition to the literature on the subject.

Overall the paper is well prepared and structured. I find, however, that the present version could be improved by a good weeding of unnecessary words and sentences. Some expressions that the authors like to use frequently should be reconsidered. One example is ‘compositional zone’. I have made some suggestions in the file and discuss specific points below.

Although the results are very relevant for zircon U-Pb geochronology, the studied zircons have not themselves been dated, and so the specific data do not have a direct connection with the U-Pb behavior of the samples. Consequently, the discussion of implications for U-Pb dating presented in the paper is very generalized and in part trivial. I suggest to concentrate on the main substance of the study and avoid meandering off in inefficient discussions.

Some specific points:

Line 64; ‘… poorly understood, and several outstanding questions remain. Do most zircon crystals predominantly dissolve from rim to core?’

Everyone who has done chemical abrasion will have noted very rapidly that zoned and metamict zircons do not dissolve that way. That can hardly be described as a poorly understood fundamental question.

Fig.2: The explanations are unclear. Are the grains shown in each of the four panels representing each of the sample? If yes, why does the second panel contain zircon from two different samples (blue and yellow circles). If not, what is the distinction? Overall I find this figure quite useless, not even as a decoration.

Fig.3: Not sure about using the term ‘metamict’ once zircon has been annealed. Can be confusing.

209: ‘typical magmatic growth patterns’   It is true that many magmatic zircons have oscillatory zoning, but the same pattern can in part be seen in metamorphic zircon. Better to use descriptive terms.

287: ‘ This could imply that increasing the duration of the leaching step results in a more crystalline zircon residue due to the progressive dissolution of higher damage domains.’ Why the ‘could’? It seems to be the most logical explanation.

And then the following sentence; ‘We note, however, that only a small number of AS3 crystals survived 12 h of chemical abrasion, and only a small number of Raman analyses were made. We recommend further study to better evaluate this possibility.’ Sounds rather trivial.

316: ‘As evidenced by our SE images and discussed further below, μCT does not capture radiation damage zoning that does not result in a strong density contrast such as variations in radiation damage below the ~1×1018 α/g threshold.’ Suggest rephrasing to avoid the double negatives.

Fig. 9, caption: ‘interior compositional zone’. Compositional zone? As opposed to what?

367: ‘ All observed dumbbells are oriented parallel to the grain’s c-axis’. That is a surprising statement. Looking at the figures I would have assumed that they are all normal to the elongation of the crystal (= c-axis). Please elaborate to avoid confusion

Fig 13 caption: ‘The yellow arrow highlights the grain’s shell-like appearance because of significant dissolution in the grain’s interior.’ I see a highly resorbed grain, not much left of the shell.

‘images of dog-chewed zircon residues ‘ maybe a bit too colloquial?

419: ‘In a visual game of connect-the-dots …’     ??

422: ‘We see dumbbell-like fracture patterns again in sample Zr36 (Fig. 13b-III) where crosscutting fractures connect different oscillatory zones removed by dissolution to one another and to the grain surface’. Rather convolute sentence, hard to understand. Please rephrase.

483: ‘… The long axes of deep, octahedral etch pits on (100) align with the crystal’s c-axis…’

I wonder why they are called octahedral. Those in Fig 16a look prismatic to me.

Fig. 19, caption: ‘ Projection on (100) looking down the a-axis. The c-axis is vertical to the page, and the a2-axis is horizontal’ Confusing: should it not be: ‘Projection on (100)

looking down the c-axis’ ? The same for (b): if the pane is 001 then the view must be parallel to a? Or not?

640: ‘… suggest that a crystal’s bulk radiation damage also plays an important role.’ That seems a rather trivial discovery. What else could one expect?

641: ‘Crystal morphology plays a lesser role in that crystals with very high aspect ratios dissolve more slowly than more equant grains’   ??? That is not apparent in Fig. 18e, and would seem to contradict the higher solubility along the c-axis than along a.

Section ‘4.2.1 Zircon U-Pb ages’ lines 648 – 726: I find the discussion on this topic very generalized and superfluous. The experiments in this paper demonstrate the great variability between zircon of different geneses, compositions and ages. Yet many of the reflections made here focus on some idealized magmatic zircon in young systems. Because of such a simplification the discussion is trivialized and almost meaningless. Clearly, the lessons from the present experiments must be considered separately for each set of zircon used for geochronology. I recommend removing this section, it just detracts from the paper.

778: ‘…chemically abraded residues are more crystalline than their annealed counter parts…’: Isn’t that the logical relationships since CA removed the less crystalline domains? And: ‘… radiation damage is annealed hydrothermally during HF leaching…’. Speculative?

867: ‘Increasing the leaching temperature from 180 °C to 210 °C or increasing the leaching duration leads to the development of more extensive dissolution networks in higher damage grains ‘ This is a rather trivial conclusion. Something CA-users did not observe before?

870: ‘More crystalline zircon samples lack fracturing related to radiation damage zoning.’ Another trivial statement. The conclusions should focus on the important aspects of the research, not on trivialities.

Aug. 13, 2022   F. Corfu

Citation: https://doi.org/10.5194/gchron-2022-19-RC1
- AC1: 'Reply on RC1', Alyssa McKanna, 13 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://gchron.copernicus.org/preprints/gchron-2022-19/gchron-2022-19-AC1-supplement.pdf
  
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CC1:
'Comment on gchron-2022-19 (McKanna et al.2022 https://doi.org/10.5194/gchron-2022-19 )', Charles Magee, 29 Aug 2022

McKanna et al. describe a morphological study of the changes wrought on zircon by the chemical abrasion process. They use a combination of SEM imaging, microscale X-ray computed tomography, and Raman spectroscopy to characterize individual zircon crystals as they go through this process.
The study is important because even though chemical abrasion (CA) has been used for 17 years to improve the apparent closed system behaviour of zircons during U-Pb isotopic analyses, nobody really understands how it works. As the technique is necessary for modern high precision zircon isotope dilution thermal ionization mass spectrometry (ID-TIMS) based geochronology, a better understanding of the physical processes involved should improve geochronologists’ ability to interpret the results of CA ID-TIMS.
The study is well designed, well executed, and clearly explained. The use of techniques which don’t involve destructive sample preparation or analysis on the intermediate steps is smart, as it allows the same individual grains to be followed through the process where the only thing damaging the grains is chemical abrasion.
There are three particular areas where the manuscript could be improved.
Section 2.1
Firstly, the introduction says that the second described sample- SAM-47- is from the Corunna Downs granitoid complex. The Australian Stratigraphic Unit database describes this term as informal. See:
https://asud.ga.gov.au/search-stratigraphic-units/results/34394
It has been replaced by the Corunna Downs Granitic Complex:
https://asud.ga.gov.au/search-stratigraphic-units/results/72996
Note, however, that this term also is obsolete. As shown in the links above, Pilbara Granitic Complexes are not stratigraphic units, but are geological provinces, so neither of these descriptors is particularly informative. More importantly, the SAM-47 description is the only description that does not have any references associated with it, making it difficult to understand the geologic background of this sample, or any associated information that would allow readers to interpret the results.
Furthermore, the latitude and longitude (89°59’55.97”, 100°08’2.38”) given are in the Arctic Ocean near the north pole, and are not in the Pilbara craton.
In summary, it would be helpful if the authors could more accurately locate the sample site, and relate it to a local, named stratigraphic unit, and provide appropriate reference(s) to previous work that provides geological context.
Section 3.2.1
The use of a synthetic zircon (which is not described in the samples section of the methods) may not be the most appropriate measure of a full annealing natural zircon. Lattice strain can be caused by factors other than radiation damage, such as the incorporation of variably incompatible trace elements into the lattice structure. If the synthetic zircon is pure ZrSiO4, instead of being grown with levels of P, Y, REE, and other trace elements typical of zircons from basic to felsic host rocks, then the ability of chemical abrasion to repair lattice strain may be underestimated due to the lack compositionally related lattice strain in the chemically pure synthetic crystal.
Similarly, we can’t tell from the Raman data whether the narrower peak widths of KR18-04 and BOM2A are due to damage or composition, although the narrower peaks for the younger zircons, and excellent choice of one mafic and one felsic zircon from both the ‘old’ and ‘young’ groups does suggest irradiation is important.
On a related note, when estimating the accumulated lattice strain, a U/Th/He age or a fission track age may be more appropriate than a crystallization age, depending on the ability of moderate-to-high temperature zircon to self-anneal radiation damage over geological time. The lack of location data for SAM-47 (see above) makes estimating this difficult, but to use a well-studied East Pilbara Archean example, the Owen’s Gully Diorite has a crystallization age of 3467 Ma (Stern et al. 2009), but a helium age of only about ~750 Ma (Magee et al. 2017).
Figure 2
Finally, it might be worth specifically pointing out that 2b is a colour photomicrograph, as the annealing out of radiation damaged colour centres is an important but sometimes overlooked part of CA. This illustration is so dramatic that readers might not appreciate that the second image is a colour image in which all the colour has been annealed out of the zircons, leaving them almost colourless.
Overall, this is an excellent study which provides copious data and interpretation of the chemical abrasion process, and I look forward to seeing the final version.
Charles Magee
References:
Magee, C. W. Jr., Danišík, M., and Mernagh, T.: Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology. Geoscientific Instrumentation, Methods, and Data Systems. 6, 2, 523-536, doi:10.5194/gi-6-523-2017, 2017.
Stern, R. A., Bodorkos, S., Kamo, S. L., Hickman, A. H., and Corfu, F.: Measurement of SIMS instrumental mass fractionation of Pb isotopes during zircon dating, Geostandards and Geoanalytical Research, 33, 145-168, doi:10.1111/j.1751-908X.2009.00023.x, 2009.

Citation: https://doi.org/10.5194/gchron-2022-19-CC1
- AC4: 'Reply on CC1', Alyssa McKanna, 13 Jan 2023
  
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CC2:
'Comment on gchron-2022-19', Magdalena Huyskens, 09 Nov 2022

The manuscript “Chemical Abrasion: The Mechanics of Zircon Dissolution” by McKanna and others is presenting microstructural investigations of zircon behaviour during chemical abrasion treatment using scanning electron microscopy, Raman spectroscopy, and X-ray computer tomography, in order to understand the mechanics behind the CA technique and aims at giving recommendations to remove Pb loss from zircon samples.
They have documented dissolution structures associated with compositional zones, fractures, inclusions and crystallographic orientation. These observations support that CA is not just removing the outer parts of a zircon crystal for intermediately damaged zircons, potentially biasing the age, but most inclusions and radiation damaged zones are reached by the treatment. For zircons with a low radiation damage, the CA treatment mostly dissolved the zircons from the outside, especially along the c axis.
Overall, the study is very detailed and well written, adding to the understanding of the chemical abrasion technique. While many of the observations are known by practitioners of the CA technique, they have never been properly documented and quantified, which can lead to conclusions like the crystallographic orientation dictating the dissolution direction. However, there are some major points that I would like to see addressed.
There are quite a few qualitative statements that can and should be backed up with statistical analyses. For example, claiming changes in the slopes between v3(SiO4) and Eg peak after annealing and leaching (lines 254-256) and reporting the average changes and range for the peak width in a raman spectrum for the individual samples and temperature steps.
For the estimates of volume loss, the method needs to be described in more detail. Right now, I am not sure if it is including interior dissolution features or not.
One of the findings is that some compositional zones are preferentially dissolved. Do you have any compositional data for these? It would been great to know what the difference in composition is and if the solubility is solely based on radiation damage, or some composition otherwise is more soluble.
The one recommendation that is put forward for the CA technique is to increase the temperature: “In most samples regardless of initial damage content, we find that chemical abrasion at 210 °C is more effective at mining out soluble zones from crystal interiors. Based on our mechanistic blueprint, we predict that hotter leaching temperatures are thus more likely to better mitigate Pb-loss in geochronological datasets.” I do not find this supported in the data. Yes, the solubility increases with increasing temperature. Increasing it a little bit more or increasing the time will completely dissolve the entire zircon grain. Since there is no U-Pb data associated with this study, the mitigation of Pb loss is speculative. In addition, there are many zircons that will completely dissolve with such a treatment, in which case no U-Pb date could be collected.
Minor comments:
Misspelling of reference “Bowring and Schmidtz, 2003” (multiple times in the introduction)
Figure 2: Is there any way to track which conditions were used for which grains? It would be helpful to get an idea overall how the different samples are behaving under the different conditions. In addition, it would be nice to have images of the zircons before annealing. There are often colour changes associated with this step.
Figure 5: All panels should have the sample name in them, at roughly the same position. It is confusing that c) is within a). Use the same font type. If one panel has the alpha dose, the other one should have this too.
L 254-260: “We note that relationship between the v3(SiO4) and Eg peak widths is steeper after annealing in each of the four samples, since the two Raman peaks have different temperature sensitivities (Hartel et al., 2021). This observation suggests that laboratory annealing is not simply the inverse of radiation damage accumulation. As such, we caution against using the Váczi and Nasdala (2017) calibration to derive alpha dose estimates from v3(SiO4) peak widths for annealed or chemically abraded samples and omit alpha dose axes from Figures 5b and 6b.” This is inconsistent. The alpha dose is noted for the annealed samples, but not the partially leached ones.
Figure 7: The choice of color for the 180 °C for 12 h for AS3 & SAM-47 is odd, since it fits the color scheme of samples KR18-04 & BOM2A.
Lines 274-281: “Notably, SAM-47 and BOM2A residues each have at least one data point with a narrower v3(SiO4) and Eg peak width than their solely annealed counterparts suggesting that some residues have a higher degree of crystallinity. Further, we find that the residue datapoints for these two samples largely plot below (at lower v3 for a given Eg) the annealed datapoints indicating a change in the relationship between the v3(SiO4) and Eg peaks. Taken together, these observations could suggest that additional structural recovery occurs in some zircon samples during HF leaching even after dry annealing at significantly higher temperatures.”
Is there any reason that this observation can’t just be explained by the removal of more damaged zones that were not annealed during the high temperature annealing step? Structural recovery during HF leaching seems impossible to me and would need some further explanation.
Line 405: “… many most …” remove one of those words
Line 777- 780: “There is also an apparent change in the relationship between the widths of the v3(SiO4) and Eg peak after partial dissolution in HF acid in some samples, and a small number of Raman analyses for chemically abraded residues are more crystalline than their annealed counter parts” Same as for Lines 274-281. Does this not just mean that the parts that were not annealed due to larger radiation damage are still present before leaching?
The section 4.2.2 Inclusions and zircon trace element analyses is not discussing the impact of dissolving compositional zones within zircons that are observed in this study.
The section “4.4 Imaging radiation damage zoning: Implications for (U-Th)/He thermochronology” seems disconnected from the main point of the paper and is a little distracting.

Citation: https://doi.org/10.5194/gchron-2022-19-CC2
- AC5: 'Reply on CC2', Alyssa McKanna, 13 Jan 2023
  
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  Citation: https://doi.org/10.5194/gchron-2022-19-AC5
EC1:
'Comment on gchron-2022-19', Daniel Condon, 05 Dec 2022
In addition to the two community comments and 1 referee comments I am presenting my review of the manuscript prior to an editorial decision.
McKanna et al., present a detailed study of the impact of annealing and leaching (i.e., ‘chemical abrasion) on zircon, using a workflow that exploits backscattered electron imagining, raman spectrometry and micro X-ray computed tomography (mCT), applied to a four samples. These four samples cover a range of ages (from ~3 Ga to 66 Ma) and thus radiation damage histories. These samples also have differing internal zonation patterns and mineral inclusions. These samples are subjected to thermal annealing (900C for 48 hours) and leaching in HF (180 and 210C for 12 hours), with characterisation prior to and following this treatment.
The paper is aimed at those using zircon for U-Pb (and to a lesser extent, U-Th/He) geochronology as this ‘annealing and leaching’ process is employed to ‘effectively eliminate’ Pb-loss in the residual zircon that is then often used for high-precision U-Pb dating. As discussed in the third paragraph, this pre-treatment is universally applied but the mechanistic understanding is limited, and this impacts how datasets are interpreted. How confident can we be in single ‘youngest’ U-Pb dates from a zircon, that may or may not be the youngest zircon. Has Pb-loss been 100.00% eliminated, or do we still need to keep that as an option when interpreting dataset, assigning age uncertainties? How does the leaching work and how does this impart a bias in the age of the material left for analyses after annealing and leaching?
This paper is a step towards having a more mechanistic understanding of the chemical abrasion process and how this varies, related to accumulated radiation damage, presence/lack of inclusions, crystal morphology and internal zonation etc.
The paper mixes a set of quantitative data that is linked to observations and qualitative data derived from it. As a result, the description of the data is rather wordy and at times difficult to follow – wonder if some form of tabulation wouldn’t help? Also, lots of the discussion/generalisation seems reasonable, its worth making it clear that this if for four samples and a wider range of samples need to be characterised, and perhaps a wider range of parameters (i.e., the annealing in addition to the leaching that his manuscript focusses on).
Some general comments:
Fractures – the use of this term implies that planar features are a result of stress that is applied to the material. There is a good argument to be made for this process and it seems likely to be a common occurrence in some samples. My comment is that calling all planar features fractures implies a certain causative process (differential stress). Are all planar features fractures? Maybe they are.

Rim to core dissolution. A lot of different mechanisms and processes are discussed and the authors do a good job of introducing these and tracking them through the discussions. Whilst the images/analyses support that rim to core dissolution is not typical the authors do state (line 680) that there is a progressive rim to core dissolution

Much of the focus in the discussion, and in the community, is around the leaching temperature as being the thing that is most significant. Perhaps it is but what above the duration of the annealing, or the rate of cooling? This paper focusses on samples that have bene annealed for 48 hours but it should be acknowledged in the manuscript that practitioners quote a range of annealing durations, typically 48 or 60 hours. I assume this is the time between turning the furnace on and off, and often cooling can take several hours although the rate of cooling can be increased by opening the furnace.

The zircon crystals studied have not been analysed for high-precision U-Pb – but data does exist for the samples (AS3 – Schoene et al., GCA, 2005, coherent U-Pb; SAM-47 – no U-Pb data published? KR18-04 – MacLennan et al., Sci Adv, 2020 – overdispersion, Pb loss? BOM2A, Basu et al., 2020, single population). Have these analyses been conducted at experimental conditions analogous to those deployed in this study? I appreciate that the precision/resolution may not be at the level to preclude Pb-loss but presenting the data might help frame the discussion around the implications for zircon U-Pb systematics and age interpretations

The paper seems focussed around leaching mechanisms/processes applied to whole crystals – however the process will often be applied to fragments and/or grains that has been polished for CL, on both cases exposing the interior of the grains. Would be useful to mention for the non-practitioners that not all zircons will come as complete crystals.

It is a long paper and much of the qualitative observational data based upon examination of many observations from the four samples, which a subset of representative images presented. One issue is around readability – could some of the generalised observational data/interpretations be tabulated to make it more accessible? Personally, I found it challenging, going back and forth to try and compare what is said for the different samples and leaching temperatures. I felt the use of tables may be helpful for compiling this qualitative information and making it more readily accessible.

Conclusion section – is it possible to draw out the observations and how they record a progression of processes?

Some minor comments (tied to line number in the manuscript).
Bowring and Schmitz, not Bowring and Schmidtz

also mention the rare occurrences of reverse discordance seen in some samples?

thermal annealing instead of laboratory annealing

remove more soluble

bias, yes, but more realistically this should be considered an additional source of uncertainty in the assigned age

also prompted the community to question/explore a range of interpretative frameworks for such datasets

and “How does the duration/temperature of the annealing impact the crystal structure and precondition it for reaction to leaching?”

Line 96 – could the sample information be tabulated?
Line 146 – what portion (percentage of grains) didn’t survive the leaching and was their anything distinctive about those grains? Did any grains break apart?
Figure 2 A is reflected light, what is the light source for B? I assume AS3 is top left etc., for Panel B but there is space to add label, or state this in the figure caption. The images are low resolution – will higher resolution version be submitted as a supplement? Also, the top right panel indicated the images contain residues that have been leached for varying times (4 and 12 hours) – how does the reader distinguish these different grains?
Line 251 – how does this look for 60 hours? Is there any published data for this?
Line 262 – were replicate raman determinations made on any of the crystals to assess variation within a crystal?
Figure 8 – what is the 2D nature of the 3D rendering in figure 8?
376 – remove interestingly.
678 – ID-TIMs analyses represent an integrated analyses of the residue post-leaching – this could be core-rim, or more core, or more rim – seems that is will be sample and duration dependent?
687 – “hot leaching” (210C)? Or should this be hotter leaching? Or longer leaching?
702 – yes, but in lower radiation damaged samples it may also impact an age-bias towards the core/older material
723 – and could this be a mechanism that results in the rare cases of reverse discordance we see in CA ID-ITMS data for some old zircons? Could we be seeing this in samples but not at a resolvable level, where the zonation is favourable?
741 – yes but sometimes the inclusion rich zircon may be the ones we want to date…
781 – is hydrothermal annealing a thing? Can you provide a reference? Looking at the literature I couldn’t find anything in materials science – do you mean hydrothermal treatment? This sounds odd – how robust are the few data this discussion is based up?
784 - Widmann.
865 – Okay – then what impact does the annealing temperature/duration/cool down rate have on the formation of micro fractures? All the discussion is around varying the leaching parameters but should we also be considering the annealing step?
839 – four samples covering a range of ages and radiation damage accumulation.
888 - … removal of excess closed system material AND potential age bias in lower radiation damaged materials?
Citation: https://doi.org/10.5194/gchron-2022-19-EC1
- AC3: 'Reply on EC1', Alyssa McKanna, 13 Jan 2023
  
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RC2:
'Comment on gchron-2022-19', Daniel Condon, 10 Jan 2023

Comments on the manuscript “Chemical abrasion: The mechanics of zircon dissolution” by Alyssa J. McKanna and co-authors, submitted for publication in Geochronology (#gchron–2022–19)

Lutz Nsdala

Generalities:

This is an interesting contribution whose results deserve to be brought to the attention of the international community. During the last 15 years, several teams have undertaken extensive dry-annealing experiments in order to understand why applying Mattinson´s CA–TIMS technique is advantageous in many cases; that is, why dry annealing prior to HF etching improves the selective removal of more radiation-damaged zircon during HF etching? The present study focuses more on the second step: What exactly happens during the HF attack? Observations such as preferred dissolution and preferred volume shrinking along a certain crystallographic direction, and the obvious contribution of fluid-driven recovery processes, are novel and valuable. The results are of interest and for the most part sound, and I am happy to suggest acceptance for publication after minor (mostly technical) revision.

Main comments:

The main shortcoming of the present research is perhaps that there is no chemical information on the samples studied. The present paper presents results of rather sophisticated studies including micro-CT and precise dimension estimates, but the most basic information remains hidden. There are quite a few journals that would refuse to publish a study on samples whose chemical compositions are unknown; for good reasons, as I think.

In particular, there are no U and Th concentrations available that, along with U–Pb ages, could have been used to calculate realistic time-integrated alpha doses. Instead, authors fiddle with equation #2 of Váczi and Nasdala (2017) in attempting to estimate self-irradiation doses from Raman FWHMs. Which is based on a serious fallacy of thinking: The equation of Váczi and Nasdala (2017) – that by the way has never been proposed as a “calibration”, as present authors claim in line 223 – refers solely to Sri Lankan gem zircon with its particular damage-annealing history, whereas other zircon populations may have appreciably different FWHM-dose relationships.

In contrast, comparing alpha doses that were calculated from U, Th and age, with the observed FWHM values, would have opened up valuable opportunities for characterising the initial samples and their annealing histories. This chance was missed. Also, because of the unavailability of chemical compositions, it cannot be evaluated whether HF etching has removed completely some interiors of the grains but has left the rest fully unchanged, or did un-dissolved remnants experience some chemical leaching?

Minor comments:

- Present authors rely on the results of Palenik et al. (2003) and Váczi and Nasdala (2017) who both have claimed that the FWHM of the n₃(SiO₄) Raman band of Sri Lankan zircon has a maximum (“saturation”) value of about 35 cm^–1. On the other hand, sample AS3 yielded FWHMs of about 48 cm^–1 (Fig. 5a, Table S1). This apparent contradiction should be discussed, and actually such extremely broadened Raman spectra should be shown (at least in a supplementary figure).

It has been suspected that Sri Lankan zircon of elevated degrees of radiation damage have experienced preferred annealing, which might explain the “saturation” as a particular feature of Sri Lankan zircon. In conclusion, it is not surprising that other zircon, not affected by the particular “Sri Lankan” annealing history, may indeed show further band broadening. This has been rarely observed thus far but is valuable; so it should been shown and discussed.

- Line 183, it is not really of relevance for the reader to learn about the equipment of the Princeton Raman system; instead, it should merely be stated which particular laser was actually used in the present study. Presumably red, as the green laser tends to induce Er³⁺-related photoluminescence obscuring the Raman spectrum? The laser power at the sample surface (so not laser output but power measured behind the objective) needs to be reported.

- Line 185: I wished Horiba would stop implementing the main silicon band in their auto-calibration procedure. However, if authors believe the friendly Horiba service person who did so was right, they should provide a published reference for the 520.7 cm^–1 value they used for calibration. Other authors have used 520.5 or 521.0 cm^–1, and none of these values has ever been appropriately supported. Note that the oriented Si wafer is provided for alignment purposes but pretty unsuitable for wavenumber calibration: Visible light penetrates only a few µm into Si, which is why increasing the laser power, or changing the laser colour, changes absorption-induced heating and causes a shift of the Raman band. Why not using a truly known signal instead? The Rayleigh line has a Raman shift of 0 cm^–1; a perfect calibration means.

- Line 189–190, quoting ~2 µm spatial resolution upon using a “” is too general and imprecise. With the confocal hole set to 400 µm, the spatial resolution will be much poorer than 2 µm lateral, and with a 100 µm confocal hole, the LabRAM Evolution should – provided the beam alignment is well done – reach its maximum performance close to the Rayleigh or Sparrow criterions, well below 1 µm (see detailed study by Kim et al., 2020, Current Applied Physics 20:71–77). The above resolution estimate does of course only apply if the Olympus 100× / n.a. 0.90 objective was used (this essential information is missing as well).

- Page 8, caption of Fig. 3a: The wording “” is too imprecise. State explicitly which images are CL, BSE, SE.

- Lines 218–219, the expression “” is a contradiction in itself. To vibrate asymmetrically, it takes at least two bonds, whereas the stretching vibration of a single Si–O bond does not have any symmetry. Should be reworded to “”.

- Fig. 18, in subfigures a–d the main x-axis label is missing.

- Lines 768–769, ”; and again lines 891–892, “”. These statements are presented as if they were a result of the present study, which is decidedly not the case. In contrast, it has been emphasised may times already during the last 20 years that annealing does not inverse the damage accumulation process (mismatch of Raman FWHMs and Raman shifts, mismatch of unit-cell parameters a and c, etc.). Something like “” would be more appropriate and more honest.

- Same in lines 250–252, “”. This is not a new finding but merely another well-known fact, here merely reconfirmed again. People who wish to achieve near-complete annealing of zircon do this at 1400°C for four days or so.

By the way, Raman analyses do not suggest anything; only the results of Raman analyses suggest something (wording in line 891 should be improved).

-Line 800, a typo? Instead of “” there should be “”.

- Line 983, the second author is Dr Chutimun (first name) Chanmuang N. (family name), so her name should be abbreviated “Chanmuang N., C.”; same in line 1012. Similarly, in line 1050 the second author should be quoted as “Van den haute, P.”

- Table S1, the “empirical correction” of Raman shift values for band downshifts that are claimed to be caused by laser-induced heating remains fully vague (unclear). Something like this has never been observed and reported before; so at least some comprehensible documentation is needed. How about simply decreasing the laser-power density, to reduce laser-induced heating to a negligible extent??

- Table S1, in view of the fact that many authors present and discuss uncorrected FWHMs, applying FWHM correction in the present article is meritorious. However, a correction assuming a FWHM of the IPF (instrumental profile function) of 1.5 cm^–1 (I assume this is the meaning of the unitless “” in footnote c?) most likely has led to an overcorrection. The FWHM of the IPF of the LabRAM Evolution with its 800 mm focal length, in case the 1800 grooves per millimetre diffraction grating is used, is about 0.8 cm^–1 in the red and about 1 cm^–1 in the green range of the electromagnetic spectrum. Authors should test this – and hence the experimental band broadening of their system – by measuring the width of the Rayleigh line, Kr lamp or similar emission lines, or even better narrow Raman bands of known widths (unstressed diamond, Ba nitride).

Unfortunately I was unable to find Raman shifts and corrected FWHMs of synthetic zircon as measured by present authors. However, data points in Figs. 5 and 6 imply that synthetic zircon would have a FWHM of the n₃(SiO₄) band of about 1 cm^–1, which is well below the real value of about 1.7–1.8 cm^–1. This seems to support my above suspicion of FWHM overcorrection. Let me assume authors have fitted an FWHM of 2.1 cm^–1. This value, corrected for a (too large) 1.5 cm^–1 IPF, yields an overcorrected FWHM of 1.0 cm^–1. In contrast, a fitted FWHM of 2.1 cm^–1, corrected for a 0.8 cm^–1 IPF, yields a quite realistic FWHM of 1.8 cm^–1. Anyway, data for synthetic zircon should be added to Table S1.

- Table S1: Reporting FWHM values with two decimal digits fakes an unrealistic accuracy. As the uncertainty is 10% for narrow FWHMs, reporting 4.4 instead of 4.38 cm^–1 will do the job.

Citation: https://doi.org/10.5194/gchron-2022-19-RC2
- AC2: 'Reply on RC2', Alyssa McKanna, 13 Jan 2023
  
  The comment was uploaded in the form of a supplement: https://gchron.copernicus.org/preprints/gchron-2022-19/gchron-2022-19-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/gchron-2022-19-AC2

Alyssa J. McKanna et al.

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https://doi.org/10.5194/gchron-2022-19-supplement

Alyssa J. McKanna et al.

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Short summary

Acid leaching is commonly used to remove damaged portions of zircon crystals prior to U-Pb dating. However, a basic understanding of the microstructural processes that occur during leaching is lacking. We present the first 3D view of zircon dissolution based on X-ray computed tomography data acquired before and after acid leaching. These data are paired with images of etched grain surfaces and Raman spectral data. We also reveal exciting opportunities for imaging radiation damage zoning in 3D.


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