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| 16 May 2025
Technical note: Investigation into the mechanism of chemical abrasion using SHRIMP, Raman spectroscopy and atom probe tomography
Abstract. Chemical abrasion, a two-step process of annealing and partial dissolution, is routinely applied to zircon grains prior to U-Pb geochronology, to remove portions of the grains affected by Pb loss. The exact effect of this technique on the zircon structure and the distribution of radiogenic isotopes, however, remains elusive. Herein, grains of reference zircon OG1 were either fully or partially subjected to the chemical abrasion process and subsequently analysed by sensitive high mass resolution ion microprobe (SHRIMP) to determine the U-Pb systematics. Sputter craters from untreated, annealed, and fully chemically abraded aliquots were then mapped using Raman spectroscopy to determine the magnitude and distribution of radiation damage. For the untreated and annealed zircon, sputter craters from both concordant and discordant analyses were mapped. All chemically abraded zircon SHRIMP analyses were concordant. Raman mapping showed that the bottoms of discordant SHRIMP sputter craters generally had areas of higher and more heterogeneous Raman band width than concordant sputter pits for the same treatment. These results are consistent with previous scanning electron microscopy and micro computer tomography results. Based on the Raman maps, sputter craters with varying degrees of lattice damage and Pb loss were targeted for nanoscale analysis using atom probe tomography (APT) to assess the distribution of radiogenic isotopes at the lattice scale. APT reconstructions reveal a homogeneous distribution of all major components and radiogenic isotopes for all samples. These results indicate that APT is not able to detect elemental mobility or void formation arising from the lead loss, annealing or chemical abrasion in these samples. Thus the APT data do not provide additional constraints beyond the SHRIMP and Raman data on the mechanisms of Pb loss. Importantly, as the APT technique cannot distinguish between concordant and 5–10 % discordant zircon, it should not be used for this purpose.
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RC1: 'Comment on egusphere-2025-1810', Donald Davis, 29 May 2025
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The is an interesting study of the effect of the anneal-leach (chemical abrasion) procedure on the structure and U-Pb concordance of zircon using micro-Raman spectroscopy, SIMS and APT. The main revelation of the Raman data is the association of zones of high radiation damage with spots that gave discordant data for both annealed (but unleached) and unannealed samples, which is what might be expected. The APT data show no evidence of Pb mobility or zoning at the sub-micron scale although as the authors point out there could be sampling bias in that highly damaged material may have failed to produce suitable APT samples. Given the universal adoption if the chemical abrasion method in U-Pb geochronology for mitigating Pb loss in ancient zircon and the generally poorly understood mechanisms of Pb loss, I think that the work is worth publishing after a few improvements to the figures and perhaps a more balanced discussion of Pb loss as suggested below. The manuscript does not strike me as too long for a technical note but if it needs to be shortened there could be less discussion of the APT results given that these show a generally uniform distribution of U and Pb atoms.
Even after several generations of U-Pb geochronology there still seems to be uncertainty in the community about the mechanism(s) of Pb loss in zircon, with some associating Pb loss directly with metamictization. The following is my personal assessment of the problem, based partly on Das and Davis (2010, doi:10.1016/j.gca.2010.06.029) which I hope the authors and other readers will find useful. After a lifetime of experience, I have never found a case of low-temperature volume diffusion of Pb in zircon that was not associated with chemical alteration. I have analyzed many high-U zircon grains with metamict structure that gave concordant results from internally homogenous domains. Zircon alteration was documented by Tom Krogh (1975, Carnegie Institution of Washington Yearbook, 74, 619-623) who showed that it required a highly damaged crystal structure and an external source of fluid, which alters zircon from the surface inwards or along cracks generated in zoned zircon by differential expansion of the higher U zones.
Alteration can only occur after the degree of damage becomes high enough and its age should be approximated by the lower intercept of the Pb loss line. The alteration is usually associated with an increase in elements not normally found at high abundance in zircon such as Ca. I routinely measure 88Sr as a proxy for alteration when doing LA-ICPMS analyses, although normal zircon has small amounts of Sr and a peak could also correspond to an inclusion. Alteration of metamict domains is usually incomplete, suggesting that the source of the fluid is limited, perhaps from residual magmatic water. Alteration is readily recognizable, usually as an amoeboid front penetrating into the metamict phase, which is dark under both BSE and CL. Since, as shown by Nasdala et al (2006, doi: 10.2138/am.2006.2241), highly damaged zircon is bright under BSE but dark under CL, alteration is much easier to recognize under BSE. This is problematic for some communities of geochronologists who exclusively use CL. Provided one can recognize and avoid alteration, analyzing only unaltered zircon no matter how damaged should give a concordant result. This was the idea behind the air abrasion method where exposed surfaces of crack-free zircon grains are mechanically removed hopefully leaving unaltered material. Since many ancient zircon populations are extensively cracked, air abrasion usually results on a highly selected sample that may not be representative of the whole population. Mattinson (2005) solved this problem by first annealing a picked fraction to partially remove the radiation damage and then leaching in HF, using the same procedure as for dissolution but for a shorter period of time to differentially dissolve alteration. This has the advantage that alteration along internal cracks can be removed, not just on the grain surface. One problem is that the annealing process does not completely restore the crystalline structure so the previously metamict but unaltered phase will also dissolve although at a slower rate than altered zircon. Because the Raman emission from a single undamaged crystal is polarized but is unpolarized from a damaged but annealed crystal, I assume that damaged domains are reconstituted after annealing as a polycrystalline assemblage that remains relatively soluble. This can result in a great deal of mischief when applied to zoned Archean zircon. Most zircon from felsic rocks shows micron-scale oscillatory zonation presumably because of the high magma viscosity and differential diffusion rates of U vs Zr. High-U zones will become metamict but crucially, significant daughter Pb will be displaced by alpha recoil into adjacent low U, relatively undamaged zones. Applying chemical abrasion to such a sample results on a comb structure where the high-U zones have been dissolved. The result of analysis is a reversely discordant datum because of excess Pb recoiled from the high-U zones. Because the 238U decay chin emits 8 alpha particles while the 235U decay chain emits 7, the average recoil distance of a 206Pb atom will be larger than for 207Pb, which means that the measured 207Pb/206Pb age of the reversely discordant point can be slightly younger than the true age. Cases like this make me very careful about applying the chemical abrasion to highly damaged or very ancient zircon. Chemical abrasion was originally developed to remove discordance from Phanerozoic zircon where it succeeded brilliantly. Over most of this age range one relies completely on the 206Pb/238U system and the degree of radiation damage is usually relatively limited. The leach procedure is more effective than air abrasion but should not result in extensive dissolution of the sample grain.
Although I realize that SHRIMP is used here as an analytical tool, CA treated zircon is meant to be analyzed using ID-TIMS. As noted above, discordant and concordant domains should typically coexist in the same grain and the in situ nature of SHRIMP, especially its 1 micron penetration depth, should allow alteration-free domains to be targeted based in imaging without pre-treatment of the sample.
The sampling depth of Raman appears similar to that for SHRIMP (Fig 1). Was there any reason not to perform micro-Raman spectroscopy on the spots before SHRIMP analysis? Presumably the temperature of the pit during sputtering is low enough that there is no annealing.
Line 35: ‘vanishingly’, non-scientific term.
Line 139-140: Regarding the common Pb correction in SHRIMP, there should be no significant primary common Pb in zircon, especially not for near-concordant early Archean aged grains. As I understand it, the small 204Pb signal measured by SHRIMP is derived from Pb in the Au coating, which should have a fixed isotopic composition.
Line 238-239: ‘This reconfirms that radiation damage lowers the Pb retention performance of zircon.’ This statement seems to imply that radiation damage leads to discordance, for which I see no direct evidence. It is a pre-condition necessary for low temperature Pb loss following alteration. It might have been worthwhile to search for any evidence of alteration using high resolution BSE images of the SHRIMP target areas associated with discordant data.
Line 276: ‘We know from the U-Pb data that the discordant spots in this study have lost Pb.’ This is a tautology.
Line 277: ‘…these data’.
Line 286-287: ‘Our results are hence consistent with radiation-damage-enhanced volume diffusion of Pb out of the zircon.’ Same comment as for Line 238-239 above. If radiation damage did lead to diffusive loss of Pb then it is hard for me to see how annealing damaged zones followed by leaching would consistently remove discordant domains while partially preserving those with damage. Alteration is very soluble in HF and should be selectively removed.
Fig. 1 This figure is confusing and could be composed with more care. It is hard to associate subfigures with their labels. What I presume is the scale bars look like a sub-figure.
Fig. 2 Again, more care should be put into this figure. The colours for same of the groupings make the ellipses virtually invisible. The figure contains a great deal of information that might be easier to absorb using a legend rather than having to read the caption to interpret which points are associated with which process.
Fig 3: Along with the textural maps in Fig 3, it would be useful to show high resolution CL and BSE images of both the spots and their whole grains before SHRIMP analysis to see the context of the spots.
Don Davis
Citation: https://doi.org/10.5194/egusphere-2025-1810-RC1
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