12 Apr 2023
 | 12 Apr 2023
Status: a revised version of this preprint is currently under review for the journal GChron.

Modeling apparent Pb loss in zircon U-Pb geochronology

Glenn R. Sharman and Matthew A. Malkowski

Abstract. Although the loss of radiogenic Pb from zircon is known to be a major factor that can cause inaccuracy in the U-Pb geochronological system, the distribution of Pb loss in natural samples has not been well characterized. Treatment of zircon by chemical abrasion (CA) has become standard practice in isotope dilution-thermal ionization mass spectrometry (ID-TIMS), but CA is much less commonly employed prior to in-situ analysis via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) or secondary ionization mass spectrometry (SIMS). Differentiating the effects of low levels of Pb loss in Phanerozoic zircon with relatively low precision in-situ U-Pb dates, where the degree of Pb loss is insufficient to cause discernible discordance, is challenging. We show that U-Pb dates that have been perturbed by Pb loss may be modeled by convolving a Gaussian distribution, that represents the unperturbed U-Pb date distribution, with a distribution that characterizes Pb loss. We apply this mathematical framework to model the distribution of apparent Pb loss in 10 igneous samples that have both non-CA LA-ICP-MS or SIMS U-Pb dates and an estimate of the crystallization age, either through CA U-Pb or 40Ar/39Ar geochronology. All but one sample showed negative age offsets that were unlikely to have been drawn from an unperturbed U-Pb date distribution. Of the eight continuous distribution types we considered, modeling apparent Pb loss using the Weibull distribution produced, on average, the closest match with the non-CA U-Pb date distributions. We show two contrasting patterns in apparent Pb loss: samples where most zircon U-Pb dates undergo a bulk shift and samples where most zircon U-Pb dates exhibited low age offset but fewer grains had more significant offset. Our modeling framework allows comparison of relative degrees of apparent Pb loss between samples of different age, with the first and second Wasserstein distances providing useful estimates of the total magnitude of apparent Pb loss. Given that the large majority of in-situ U-Pb dates are acquired without the CA treatment, this study highlights a pressing need for improved characterization of apparent Pb loss distributions in natural samples to aid in interpreting non-CA in-situ U-Pb data and to guide future data collection strategies.

Glenn R. Sharman and Matthew A. Malkowski

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Review of gchron-2023-6', Anonymous Referee #1, 08 May 2023
  • RC2: 'Review of gchron-2023-6', Anonymous Referee #2, 11 May 2023

Glenn R. Sharman and Matthew A. Malkowski

Data sets

Supplemental materials for "Modeling apparent Pb loss in zircon U-Pb geochronology" Glenn R. Sharman and Matthew A. Malkowski

Model code and software

Pb_loss_modeling (GitHub) Glenn R. Sharman

Video supplement

Supplemental Video 1 Glenn R. Sharman and Matthew A. Malkowski

Glenn R. Sharman and Matthew A. Malkowski


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Short summary
The mineral zircon is widely used to determine the age of rocks based on the radioactive decay of U to Pb, but the measured U-Pb date can be too young if the zircon loses Pb. We present a new method for estimating the distribution of apparent Pb loss by mathematical convolution. Applying this approach to 10 samples illustrates contrasting patterns of apparent Pb loss. This study highlights the importance of quantifying Pb loss to better understand its potential effects on zircon U-Pb dates.