Every zircon deserves a date: selection bias in detrital geochronology

Maximilian Dröllner,Christopher L Kirkland,Milo Barham,Bryant Ware

doi:10.1017/s0016756821000145

Maximilian Dröllner, Christopher L Kirkland + Show 2 more

Open Access

https://doi.org/10.1017/s0016756821000145

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Journal: Geological magazine	Publication Date: Apr 12, 2021
Citations: 28	License type: CC BY 4.0

Affiliation: Curtin University

Abstract

AbstractDetrital zircon geochronology can help address stratigraphic- to lithospheric-scale geological questions. The approach is reliant on statistically robust, representative age distributions that fingerprint source areas. However, there is a range of biases that may influence any detrital age signature. Despite being a fundamental and controllable source of bias, handpicking of zircon grains has received surprisingly little attention. Here, we show statistically significant differences in age distributions between bulk-mounted and handpicked fractions from an unconsolidated heavy mineral sand deposit. Although there is no significant size difference between bulk-mounted and handpicked grains, there are significant differences in their aspect ratio, circularity and colour, which indicate inadvertent preferential visual selection of euhedral and coloured zircon grains. Grain colour comparisons between dated and bulk zircon fractions help quantify bias. Bulk-mounting is the preferred method to avoid human-induced selection bias in detrital zircon geochronology.

Highlights

Detrital zircon U–Pb geochronology is a powerful tool in deciphering Earth’s sedimentary archive, able to answer a myriad of research questions including: sediment transfer (e.g. Luo et al 2014); maximum depositional ages (e.g. Nelson, 2001); tectonomagmatic processes (e.g. Wotzlaw et al 2011); palaeogeographic correlations (e.g. Samson et al 2005); or crustal evolution (e.g. Amelin et al 1999)
According to Fedo et al (2003), we can distinguish between two strategies in detrital zircon geochronology: (i) qualitative analysis that strives for representation of every age mode within the detrital record, regardless of their relative abundance (e.g. Gehrels & Ross, 1998); and (ii) quantitative analysis that endeavours to obtain representative age distributions (e.g. Li et al 2019), or a combination of both strategies (e.g. McWilliams et al 2010)
No experimental design to test selection bias existed a priori, that is, age data were primarily acquired during conventional zircon U–Pb geochronology sessions for the purpose of sedimentary provenance analysis

Summary

Introduction

Detrital zircon U–Pb geochronology is a powerful tool in deciphering Earth’s sedimentary archive, able to answer a myriad of research questions including: sediment transfer (e.g. Luo et al 2014); maximum depositional ages (e.g. Nelson, 2001); tectonomagmatic processes (e.g. Wotzlaw et al 2011); palaeogeographic correlations (e.g. Samson et al 2005); or crustal evolution (e.g. Amelin et al 1999). The underlying assumption for a geologically meaningful interpretation of inter-sample comparison of detrital zircon age distributions is that the analysed samples are a true reflection of the sediment sampled and that this can be used as a proxy for the relative proportion of crystalline rocks in the source region. This foundational assumption may be undermined by a number of biases that can be simplified to those associated with (i) geological processes, and (ii) methodological approaches (Chew et al 2020; Fig. 1).

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