Articles | Volume 5, issue 1
© Author(s) 2023. This work is distributed underthe Creative Commons Attribution 4.0 License.
Examination of the accuracy of SHRIMP U–Pb geochronology based on samples dated by both SHRIMP and CA-TIMS
- Final revised paper (published on 11 Jan 2023)
- Supplement to the final revised paper
- Preprint (discussion started on 04 Aug 2022)
- Supplement to the preprint
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor |
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RC1: 'Comment on gchron-2022-20', Anonymous Referee #1, 23 Aug 2022
- AC1: 'Author responce to RC 1 and 2', Charles Magee, 17 Oct 2022
RC2: 'Comment on gchron-2022-20', Yuri Amelin, 08 Sep 2022
- AC1: 'Author responce to RC 1 and 2', Charles Magee, 17 Oct 2022
Peer review completion
AR: Author's response | RR: Referee report | ED: Editor decision
ED: Publish subject to revisions (further review by editor and referees) (25 Oct 2022) by Sandra Kamo
AR by Charles Magee on behalf of the Authors (05 Dec 2022)  Author's response Author's tracked changes Manuscript
ED: Publish subject to technical corrections (13 Dec 2022) by Sandra Kamo
ED: Publish subject to technical corrections (13 Dec 2022) by Klaus Mezger(Editor)
This study reviews published and to lesser extent new U-Pb zircon geochronology data generated by two methods: secondary ionization mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS) without and with pre-treatment of zircon by chemical abrasion (CA), respectively. The SIMS analyses were carried out over approximately the past 15 years using the SHRIMP II instrument at Geoscience Australia, and they targeted felsic plutonic and volcanic rocks with ages mostly falling into the Permo-Triassic to Cambrian age range. The exceptions are one Cretaceous “calcilutite” and Archean secondary reference zircon OG1, which was analyzed in most of the sessions to monitor Pb-isotopic fractionation. The goal of the comparison is to re-assess the reproducibility of SHRIMP U-Pb geochronology considering updated and improved methodology implemented over the past two decades. This is done in reference to CA-TIMS ages obtained for the same samples with an uncertainty that is considered negligible compared to that of SHRIMP.
This sample-based comparison is for an age range and geological provenance of zircon that is typical for the Geoscience Australia lab: the samples fall into an age range, where uncertainties for Pb-Pb ages are typically larger than those of U-Pb dating. Therefore, uncertainties are dominated by how well the U-Pb calibration curve can be defined and reproduced. The materials are mostly igneous rocks with seemingly “simple” crystallization histories (e.g., compared to metamorphic zircon). In comparing both methods, this includes zircon where the penalty of TIMS with its indiscriminate averaging over multiple growth domains in individual zircon is minor, and seemingly negligible compared to analytical uncertainties. The CA pre-treatment selectively removes zircon, but this is commonly regarded as non-detrimental in obtaining the original crystallization age of zircon.
The approach of comparing zircon ages generated by different methods from such materials, however, necessarily has limitations due to a lack of knowledge and temporal resolution regarding the age homogeneity of the samples. Although some aspect of zircon longevity in magma systems are probably indeed negligible, there are other concerns for zircon age heterogeneity even at the limit of TIMS resolution, that are less clear-cut to dismiss. Only with sufficient sampling, which is however rarely achieved in CA-TIMS studies, might this become adequately constrained. Another critical aspect is that CA-untreated and CA-treated zircon is compared; this – at least for zircon with younger SHRIMP relative to CA-TIMS ages – leaves some doubts whether indeed the same materials are compared in case SHRIMP ages would overlap on zircon areas affected by Pb-loss that would have otherwise been removed by CA. What the manuscript is also missing is a presentation and discussion of U abundances in the analyzed zircons, as this would control potential metamictization, and there are also documented matrix effects for high-U zircon in SIMS analysis.
Considering these problems and omissions, three critical aspects stand out, where I find that the interpretation overstepped what can be extracted from the data:
Moreover, a clear identification of a bimodal distribution requires differences between the modes of 3–5 times the standard error (Keller et al. 2018). This is not the case in a distribution where the two modes only differ by 0.7% if the SHRIMP uncertainty was 1%; the standard error of each subpopulation would then be between 0.2 and 0.3% (because of division by square-root of n). Applying the conservative criterion of 4-times the standard error, the difference between the two modes should be at least 0.8 to 1.2%, which is larger than the postulated difference. I therefore do not believe that it is statistically justified to discriminate between plutonic and volcanic samples regarding the apparent deviations between ages determined by different methods; if the authors think that the bimodality in the data is robust, it is their onus to provide an adequate statistical analysis to demonstrate the validity of their assessment.
The manuscript refers to another study in preparation, where a direct comparison of CA-treated zircon analyzed with both methods, and involving reference zircons will be made. I actually expect more insight on the analytical comparability and the assessment of realistic uncertainties from this future study. Due to the significant uncertainties regarding the homogeneity of the natural samples studied here, especially those of pyroclastic origin, there is little new insight, and in fact, the main conclusion stated in this study is that an uncertainty <0.7 % for SHRIMP data is over-optimistic. Hence, in essence, the 20 year old estimate for SHRIMP reproducibility at ~1% appears to be still valid.
Line 23: This apparent bimodality needs to be statistically verified.
Line 25: “better single-grain age-resolution of TIMS” = this is a bit awkward to read, as the integration of multiple age domains is the main drawback of TIMS. I also doubt if CA-TIMS can resolve genuine pre-eruptive zircon crystallization in the same magma system (= antecrysts) in the age range presented. Even if it did, what would this mean for dating a geological event such as deposition of a tephra (see Keller et al., 2018)?
Line 298: is not included
Line 310: This section is repetitive and tedious to read; this can be condensed to summarizing the main points in a table. In fact, I think section 4.1 which is presently in the discussion, should be presented as the main result.
Line 402: Pb-loss after 3-5 million years seems highly speculative, and not supported by any experimental data on zircon interaction with fluids. As U abundances are not discussed, there is no way to gauge timescales for metamictization, but this is something that the authors should look into and add to the presentation.
Line 405: Yes, I totally agree that this is not statistically robust.
Line 408: I disagree that the shape of the distribution has been assessed in a statistically robust way.
Line 416. Not sure if “p-hacking” is an adequate term; in any case, it has a negative connotation.
Line 456: Why would this be more likely? There seem to be some underlying assumptions here that should be explicitly stated.
Lien 457: This sentence is awkward: What are natural ages? What are chemically abraded ages?
Line 465: I am not convinced that this is a valid interpretation.
Line 469: “SIMS geochronology is not the best method in geologic settings where grains may have real differences in crystallization age that are smaller than the precision of a single spot, but larger than the precision of the final age of the pooled spot values.” I don’t agree with this statement, as a bulk method will create artificially small uncertainties for an age that may not have any geological significance (see discussion in Keller et al., 2018, and elsewhere).
Line 478: “improvements in SHRIMP manufacturing and installation may have reduced the fundamental uncertainty associated with the calibration equation” “May have” reads awkward; the data and interpretation in this paper at least do not support this.
Fig. 2: The PDF is based on assigned uncertainties that may or may not be adequate (see comment to Fig. 4)
Fig. 3: I would omit this plot; the fit has a probability of only 0.003, the slope generated is probably an artifact of the data selection, and the results for OG1 show that this relation is invalid (including younger reference zircon would probably also confirm this). “Cherry picking” and “p-hacking”: why even go there?
Fig. 4: MSWD and probability of fit suggest that there is overdispersion/underestimation of uncertainties for the SHRIMP results.
Fig. 5: Statistical testing of the difference/equivalence of both distributions would be required to demonstrate that this distinction is significant (e.g., using a Kolmogorov—Smirnov comparison).
Burgess, S. D., Coble, M. A., Vazquez, J. A., Coombs, M. L., & Wallace, K. L. (2019). On the eruption age and provenance of the Old Crow tephra. Quaternary Science Reviews, 207, 64-79.
Keller, C. B., Schoene, B., & Samperton, K. M. (2018). A stochastic sampling approach to zircon eruption age interpretation. Geochemical Perspectives Letters (Online), 8(LLNL-JRNL-738859).