the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Feb 2026
Past the first date: Resolving successive lead-loss episodes in zircon
Abstract. Zircon uranium–lead (U–Pb) geochronology is a cornerstone of Earth science for resolving the timing of deep-time processes, yet the U–Pb system in zircon does not always behave as a closed chronometer. Selective loss of radiogenic Pb can shift isotopic ages and generate discordant analyses that record later alteration, fluid-rock interaction, or heating. Many zircon datasets preserve evidence for more than one Pb-loss episode, yet many Pb-loss modelling approaches return a single "best" loss time. Here, we extend the concordant-discordant comparison (CDC) framework to recover multi-episode Pb-loss histories by using concordant analyses as a reference age distribution and scoring candidate Pb-loss times by how well reconstructed discordant ages reproduce that reference.
The updated workflow can partition discordant analyses into internally coherent sub-arrays, retain reproducible local optima across Monte Carlo realisations rather than collapsing each realisation to a single optimum, and summarise statistically supported candidates as an ensemble catalogue with empirical 95 % uncertainty intervals and support values that quantify run-to-run stability.
Synthetic benchmarks spanning single-stage and two-stage discordance geometries across three scatter tiers show that CDC achieves lower overall median absolute error and higher event-wise coverage than a discordia-likelihood discordance-dating (DD) approach. CDC performs best for single-stage benchmarks and for mixtures in which episodes remain well separated, whereas DD variants are more accurate and attain higher coverage in higher-scatter two-stage cases where likelihood surfaces are broad and competing modes occur. By reporting reproducible local optima rather than a single optimum, the CDC ensemble catalogue enables explicit recovery of multi-episode Pb-loss histories from discordant zircon U–Pb populations. Future work will focus on strongly overlapping multi-episode scenarios that remain difficult to deconvolve when candidate Pb-loss ages are tested one at a time.
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Status: open (until 01 Apr 2026)
- RC1: 'Comment on egusphere-2026-280', Donald Davis, 19 Feb 2026 reply
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RC2: 'Comment on egusphere-2026-280', Anonymous Referee #2, 26 Feb 2026
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The presence of discordance and Pb loss is an unavoidable aspect of many U–Pb datasets. In igneous and metamorphic contexts, Pb loss generates discordia lines in U–Pb isotope space, whose upper and lower intercepts constrain both the protolith age and the timing of Pb loss.
In detrital settings, where the goal of U–Pb geochronology is to obtain age spectra, the traditional strategy for dealing with discordance has been to reject discordant analyses based on a chosen criterion and to retain only concordant data. This approach can be frustrating when most analysed grains are discordant. Moreover, excluding discordant analyses may bias the chronological record, potentially obscuring source regions that predominantly yield discordant zircons.
It is therefore unsurprising that geochronologists have sought alternative approaches. Reimink et al. (2016) proposed a numerical algorithm that fits lines through clusters of discordant U–Pb data and identifies the lower and upper intercepts of the lines passing through the largest number of analyses.
The authors of the present manuscript have published several papers on this topic (Mathieson et al., 2025a, 2025b), modifying the approach of Reimink et al. (2016) by first considering the distribution of the concordant subpopulation and using its modes to constrain the upper intercepts before searching for lower intercepts. They refer to this as the "Concordant Discordant Comparison" (CDC) approach.
Pb loss is linked to radiation damage, as clearly explained by Don Davis in his review of the manuscript. This process can produce fanning arrays of discordant zircons (Case 3 of 7 in the manuscript). Reimink et al. (2025) dated a quartzite by fitting the lower intercept of such a fanning array of discordant zircons that were reset by a metamorphic event approximately 25 Ma ago.
Andersen et al. (2019, 2022) demonstrated that tectonically driven metamorphism is not required to induce Pb loss. Even surface weathering can cause a metamict zircon to lose its Pb, leading to the well-known phenomenon of apparent "modern Pb loss". Andersen et al. (2019, 2022)'s examples likewise resemble fanning arrays of discordant data with a single lower intercept and multiple upper intercepts.
In their new manuscript, Mathieson and Kirkland propose a generalisation of the CDC method to accommodate multiple episodes of Pb loss. These are summarised in Figures 1 and 2 as seven "Cases". Case 1 corresponds to a simple discordia line, solvable by York regression. Case 3 represents the fanning arrays previously discussed by Reimink et al. (2025) and Andersen et al. (2019, 2022).
Cases 2, 4, 5, 6, and 7 are new and involve various combinations of upper and lower intercepts. I have several concerns regarding these scenarios and the algorithm used to resolve them:
1. Although the five new scenarios are straightforward to simulate numerically, they are problematic from a geological perspective. It is difficult to envisage a Pb-loss event that completely resets one zircon population but leaves another unaffected. The authors suggest that such discrete arrays could correspond to zircon cores and rims. However, I have not encountered such clear separation between two populations in real datasets.
2. The manuscript presents only one real-world example (inset in Figure 8), which is not particularly convincing. There is no demonstration that the two arrays are statistically robust. Furthermore, roughly one third of the discordant analyses fall on neither array. If this is the strongest available example, then fitting multiple arrays is a straw man problem.
3. In the title of Mathieson et al. (2025b), the authors celebrate their algorithm's ability to turn "trash into treasure". This is acceptable only if there is a reliable way to distinguish trash from treasure. I am concerned that such a mechanism is lacking in the new algorithm. Broad application of this method risks introducing spurious interpretations into the literature. I do not question the quality of the Gawler Craton data shown in Figure 8; I assume the isotopic ratios are accurately plotted on the concordia diagram. However, in this case, the discordant analyses appear to contain little or no chronometric information. It would be risky to claim otherwise.
4. The description of the algorithm indicates that the fitting procedure requires the user to define a modelling window for the upper intercepts to guide estimation of the lower intercepts. Zircons are classified as "concordant" or "discordant" based on an arbitrary concordance threshold. Discordant grains are then deemed "valid" or "invalid" depending on whether a line connecting them to a proposed lower intercept falls inside or outside the modelling window. These binary classifications impose an artificial dichotomy on what is inherently continuous data. In reality, concordance and validity exist along a spectrum. The authors have developed a highly sophisticated algorithm, but sophistication alone does not guarantee correctness.
Unfortunately, I do not think that this contribution is ready for publication.
References:
Andersen T, Elburg MA, Magwaza BN. Sources of bias in detrital zircon geochronology: Discordance, concealed lead loss and common lead correction. Earth-Science Reviews. 197:102899, 2019.
Andersen T, Elburg MA. Open-system behaviour of detrital zircon during weathering: an example from the Palaeoproterozoic Pretoria Group, South Africa. Geological Magazine.159(4):561-76, 2022.
Mathieson, L. M., Kirkland, C., Bodorkos, S., and Daggitt, M.: From discordance to discovery: extracting fluid–rock interac-
tion timescales through zircon U–Pb analyses in the Arunta region, Central Australia, Journal of the Geological Society, 182,
https://doi.org/10.1144/jgs2024-111, 2025a.Mathieson, L. M., Kirkland, C., and Daggitt, M.: Turning trash into treasure: Extracting meaning from discordant data via a dedicated875
application, Geochemistry, Geophysics, Geosystems, 26, https://doi.org/10.1029/2024GC012066, 2025bReimink, J. R., Davies, J. H., Waldron, J. W., and Rojas, X.: Dealing with discordance: a novel approach for analysing U–Pb detrital zircon
datasets, Journal of the Geological Society, 173, 577–585, https://doi.org/10.1144/jgs2015-114, 2016.Reimink, J. R., Beckman, R., Schoonover, E., Lloyd, M., Garber, J., Davies, J. H. F. L., Cerminaro, A., Perrot, M. G., and Smye, A.:
Discordance dating: A new approach for dating alteration events, Geochronology, 7, 369–385, https://doi.org/10.5194/gchron-7-369-2025,
2025.Citation: https://doi.org/10.5194/egusphere-2026-280-RC2
Data sets
LeadLoss reproducibility data for “Past the first date: Resolving successive Pb-loss episodes in zircon” (Zenodo release paper-2025-peak-picking-v1.2.1) Lucy M. Mathieson and Christopher L. Kirkland https://doi.org/10.5281/zenodo.18217446
Model code and software
LeadLoss: Pb-loss peak picking tools and manuscript reproduction workflow (paper-2025-peak-picking-v1.2.1) Lucy M. Mathieson and Christopher L. Kirkland https://doi.org/10.5281/zenodo.18217446
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This manuscript presents a statistical approach, which the authors call concordant–discordant comparison (CDC), for processing discordant U-Pb data to derive estimates for discrete ages of partial resetting. Several efforts to derive useful geologic information from discordant data sets have previously been published by the authors and others. This presents an approach for treating data sets that involve multiple secondary ages. It seems like an impressive application of statistical theory that could lead to useful information if the hypotheses on which it is based were correct. In my opinion, however, at least one assumption is generally not correct in the case of zircon, as explained below.
The problem of zircon U-Pb discordance from what should be a robust mineral was a puzzle to early geochronologists. We now know that the chemical and mechanical stability of zircon (ZrSiO4) derives from its structure and not its chemical composition, as in the case of baddeleyite (ZrO2). The zircon structure is subject to degradation due to alpha recoil events following decay of U and its alpha-emitting daughter radionuclides. A high alpha dosage in an old or very high-U zircon gradually destroys the crystal structure leaving a disordered or metamict state. This significantly expands the volume of the crystal which, in the case of a non-uniform U distribution, introduces internal stress that cracks the crystal. This by itself does not affect the U-Pb budget. I have obtained concordant data from many uniformly highly damaged zircon crystals after abrading off the surfaces. However, any water that encounters metamict domains results in alteration and loss of most or all radiogenic Pb accumulated up to that time. Fluid can enter the crystal along cracks produced by metamictization but alteration is commonly incomplete, which suggests that the source of fluid is often the small amount of magmatic water left over from magma crystallization. Most analyses are done on an assemblage of altered and unaltered domains within the same grain, and data often scatter about a discordia line between the crystallization age and a younger age of alteration. The age of Pb loss may be controlled by the time at which a given crystal domain achieves sufficient radiation damage to allow alteration, which should be variable since zircon is commonly zoned in U. In such a case one should see an array of discordant data fanning out from an older age on concordia representing crystallization and bounded by a lower intercept age of zero, which would be a horizontal line on the inverse concordia plots used by the authors. In cases where older inherited zircon has been reheated in a younger magma or subjected to a prolonged high-temperature metamorphic event, the data may form a discrete array where lower and upper intercept ages represent real geologic events but variable low-temperature Pb loss will be superimposed if later alteration has not been completely removed.
The authors first hypothesis is the assumption that alteration-related discordance in zircon represents disturbance caused by an interesting (regional) geologic event, which in some cases may be true. The second hypothesis is that such data reflect a series of Pb loss lines having a single upper concordia intercept age and a discrete number of lower intercept ages, with the data being otherwise scattered only by measurement errors. They seem to successfully test the method by generating a random synthetic data set according to the above constraints and extracting the ages of disturbance. It seems to me that a major problem with natural samples comes from the fact that in most cases one does not sample discrete Pb-loss domains, even with in-situ methods. The scale of most sampling probably includes multiple domains that may have lost Pb at different times. This should effectively destroy information on discrete lower intercept ages if it were present to begin with. The authors could challenge this view if they could apply their method to a natural data set where results (ages of disturbance) are clear and correlate with already known events. They have attempted this in a few previous publications using a previous version of the CDC approach, but it is not clear at least to me that resetting events were clearly resolved and if the present procedure would work any better.
I admire the work involved in developing this approach and am sorry to be so critical of its potential usefulness for zircon data, but any statistical analysis is only as good as the assumptions on which it is based. I hope that the authors can show my opinions to be wrong.
One additional minor point is to not use the word ‘deconvolve’ (lines 18 and 76). Convolution is a specific mathematical procedure that is not relevant in this application.
Don Davis