Preprints
https://doi.org/10.5194/gchron-2020-38
https://doi.org/10.5194/gchron-2020-38

  11 Dec 2020

11 Dec 2020

Review status: this preprint is currently under review for the journal GChron.

On the treatment of discordant detrital zircon U–Pb data

Pieter Vermeesch Pieter Vermeesch
  • Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT

Abstract. Detrital zircon U–Pb geochronology is a staple of sedimentary provenance analysis and crustal evolution studies. Constructing detrital age spectra is straightforward for concordant 206Pb / 238U- and 207Pb / 206Pb-compositions. But unfortunately, many detrital U–Pb datasets contain a significant proportion of discordant analyses. This paper investigates two decisions that must be made when analysing such discordant U–Pb data.

First, the analyst must choose whether to use the 206Pb / 238U- or the 207Pb / 206Pb-date. The 206Pb / 238U-method is more precise for young samples, whereas the 207Pb / 206Pb-method is better suited for old samples. However there is no agreement which cutoff should be used to switch between the two. This subjective decision can be avoided by using single grain concordia ages. These represent a kind of weighted mean between the 206Pb / 238U- and 207Pb / 206Pb-methods, which offers better precision than either of the latter two methods.

A second subjective decision is how to define the discordance cutoff between good and bad data. Discordance is usually defined as (1) the relative age difference between the 206Pb / 238U and 207Pb / 206Pb dates. However, this paper shows that several other definitions are possible as well, including (2) the absolute age difference; (3) the common-Pb fraction according to the Stacey-Kramers mantle evolution model; (4) the p-value of concordance; (5) the perpendicular logratio (or Aitchison) distance to the concordia line; and (6) the logratio distance to the maximum likelihood composition on the concordia line.

Applying these six discordance filters to a 10,000-grain dataset of detrital zircon U–Pb compositions reveals that: (i) the relative age discordance filter tends to suppress the young age components in U–Pb age spectra, whilst inflating the older age components; (ii) the Stacey-Kramers discordance filter is more likely to reject old grains and less likely to reject young ones; (iii) the p-value based discordance filter has the undesirable effect of biasing the results towards the least precise measurements; (iv) the logratio-based discordance filters are most strict for Proterozoic grains, and more lenient for Phanerozoic and Archaean 20 age components; (v) of all the methods, the logratio distance to the concordia composition produces the best results, in the sense that it yields nearly identical 206Pb / 238U and 207Pb / 206Pb age spectra without introducing an age bias. All the methods presented in this paper have been implemented in the IsoplotR toolbox for geochronology.

Pieter Vermeesch

 
Status: open (until 29 Jan 2021)
Status: open (until 29 Jan 2021)
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Pieter Vermeesch

Data sets

data.csv Pieter Vermeesch https://doi.org/10.5281/zenodo.4316558

Model code and software

IsoplotR Pieter Vermeesch https://doi.org/10.5281/zenodo.4316554

Pieter Vermeesch

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Short summary
This paper shows that the current practice of filtering discordant U-Pb data based on the relative difference between the 206Pb / 238U and 207Pb / 206Pb ages is just one of several possible approaches to the problem, and demonstrably not the best one. An alternative approach is to define discordance in terms of isotopic composition, as a logratio distance between the measurement and the concordia line. Application to real data indicates that this reduces the positive bias of filtered age spectra.