Recrystallisation and Trace-Element Mobility in Zircons: Implications for U-Pb Dating
Round 1
Reviewer 1 Report
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Reviewer 2 Report
SUMMARY:
In the manuscript entitled “Recrystallisation and Trace-element Mobility in Zircons: Implications for U-Pb Dating” Huijsmans and co-authors present the results of a detailed microstructural, crystallographic and chemical study of zircons with complex zoning. The authors review the current state of the literature regarding the interpretation and the potential processes involved with such complex zircons and compare it to their results obtained from a multi-technique approach (Cathodoluminescence, EBSD, FSE and electron microprobe). The zircons studied come from 3 different localities, including 1 locality (W34) previously investigated by Nasdala et al. (1998) for similar purposes. The authors propose a novel hybrid model to explain the generation of such complex internal microstructures and zonation; model based on the combination of several previously proposed mechanisms. They finally emphasise the importance of such microstructural study for the interpretation of U-Pb-Th dating.
The manuscript is of extremely high quality, both in terms of form and content. The scientific approach is very sound and thorough. In light of the results presented and the existing literature discussed, the interpretations are adequate and relevant. As mentioned by the authors, this topic is of uttermost importance for the interpretation of U-Pb dating in complex zircons and this manuscript will help the community to deal with such samples. I really appreciated the detailed observations presented and the model proposed. I did find the graphical model in figure 6 well-made and very useful to understand such a multi-stage model.
Next step, if not for this manuscript, would be doing careful U-Pb dating of the different domains on these specific grains, in order to have completely matching datasets. I would even suggest doing in situ Hf in order to look at the 176Hf/177Hf ratios in each domain to further check if it is indeed the same source.
Specific comments:
Figure 1: This figure is too dark. I would suggest cropping out the background around the zircons and to enhance the images by balancing contrast and brightness levels. It would also be better if this figure was slightly bigger in the published version.
Figure 3 caption:
Line 228: “C) enhanced migration…” should be “B) Enhanced...”
Figure 4: Similar issue than Fig. 1., the contrast for the CL image for grain RC12_g10 (D) should be enhanced. The focus of the paper being complex microstructures and the use of detailed mapping techniques, make sure that in the final version of the manuscript this kind of figures is allocated as much space as possible. The figure in this current display is really too small. Maybe bringing the block of pictures (D, E, F) from the right panel below A, B, C would help dealing with the lateral restrictions.
Lines 254-259: Despite a lesser spatial resolution, it would have been interesting to get some insights from LA-ICP-MS.
Table 1.: I would put measurement units in brackets next to the element header. E.g. Hf (wt%) U (ppm) etc
Figure 5.: It is very faint, but you can somehow distinguish lower U counts coinciding with brighter CL domains. This seems also supported by spot analysis. Again, a larger figure would be better.
Lines 324-325: “absence of these recrystallisation microstructures in W65 can be explained by only minor incorporation of trace elements in the crystal lattice” Do you have chemical evidence for this? E.g. spot analyses, maps?
Line 437: “granitic”
Author Response
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Reviewer 3 Report
The article is concerned with the relationship between microstructures, compositional variations and trace-element mobility in zircons from the Jack Hills Metasedimentary Belt in Western Australia, and the granitic injection complex of Harris in north-west Scotland. The article is clearly written, the results are very well presented.
I have small question regarding the incorporation of hafnium into the zircon crystal structure. On lines 386-387 the authors wrote: "The same pipe diffusion mechanisms allows for Hf to enter the zircon structure due to the increased partition coefficient of Hf at lower temperatures [29]." The work [29] (Wang et al., 2010) concerns the incorporation of hafinium into the structure of crystallizing zircon. But I don't understand - how hafnium can penetrate into the crystal structure of zircon that has already crystallized? As a result of the dislocations movement in the opposite direction?I think the article needs some clarification on this.
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Reviewer 4 Report
A paper that presents some lovely microtextural bservations and addresses some interesting problems in interpreting zircon, a key mineral in geochronology and igneous petrology. In particular the band contrast observations were very interesting to me as someone more familiar with compositional measurement techniques. However, I think a few points need to be addressed before publication.
The authors argue that high trace elements in primary igneous zircon cause so much strain that the zircon recrystallises. However, many zircons have high to very high trace element concentrations yet don't undergo this spontaneous breakdown. Why not? Diffusion of most trace elements in zircon has been found to be very slow, yet stage 2 (lines 380-387) don't state the conditions under which it might occur faster.
Furthermore, if the recrystalisation is due to the trace element concentrations, then why does the recrystallisation cross cut the zoning rather than follow the zones (which presumably have the same high concentrations throughout).
It's potentially misleading to say that the partition coefficient fr Hf increases with decreasing temperature. It does, but this is mostly due to the decrease in solubility of Zr and Hf in silicate melts with decreasing T. And Zr and Hf are essential structural components of zircon so they behave differently to other elements such as REE. The important question is whether the exchange coefficient (DHf/DZr) increases with decreasing temperature, and it would be nice to see a better reference than that one by Bill Griffin cited in support of this idea.
The introduction would be improved by describing the W34 zircon U-Pb data in a little more detail. What are the upper and lower intercepts of the discordia (lines 79-80)?
What is the difference between bimodal zoning and oscillatory zoning (lines 150-155)?
A few queries about the methods. 1) line 110 says "an additional carbon coat": additional to what? I think you mean a "new carbon coat" as there was presumably no C coat on there after the polishing. 2) The EPMA method is poorly described and, quite frankly, illogical. Why would you use zircon at all as a standard for Al, Ca and Fe which at most occur at a few ppm levels? Why would you use Temora zircon when every single grain of it has a different composition, and not even homogeneous within a single grain? Which elements were calibrated using which zircon reference material (z6266 vs Temora)? (I note that the literature references cited don't report Al, Ca, Y or Fe for either zircon. So what values were used for these?)
Table 1 - too many significant figures for some elements especially Y.
Figure 3 - scale bar for panel A would be better placed in the top right and not right next to Fig. 3B.
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