Preprints
https://doi.org/10.5194/egusphere-2023-3046
https://doi.org/10.5194/egusphere-2023-3046
12 Jan 2024
 | 12 Jan 2024
Status: this preprint is open for discussion.

The Geometric Correction Method for zircon (U-Th)/He chronology: correcting systematic error and assigning uncertainties to alpha-ejection corrections and eU concentrations

Spencer D. Zeigler, Morgan Baker, James R. Metcalf, and Rebecca M. Flowers

Abstract. The conventional zircon (U-Th)/He (ZHe) method typically uses microscopy measurements of the dated grain together with the assumption that the zircon can be appropriately modeled as a geometrically perfect tetragonal or ellipsoidal prism in the calculation of volume (V), alpha ejection correction (FT), equivalent spherical radius (RFT), effective uranium concentration (eU), and corrected (U-Th)/He date. Here, we develop a set of corrections for systematic error and determine uncertainties to be used in the calculation of the above parameters for zircon, using the same methodology as Zeigler et al. (2023) for apatite. Our approach involved acquiring both “2D” microscopy measurements and high resolution “3D” nano-computed tomography (CT) data for a suite of 223 zircon grains from nine samples showcasing a wide range of morphology, size, age, and lithological source, calculating the V, FT, and RFT values for the 2D and 3D measurements, and comparing the 2D vs. 3D results. We find that the values derived from the 2D microscopy data overestimate the true 3D V, FT, and RFT values for zircon, with one exception (V of ellipsoidal grains). Correction factors for this misestimation determined by regressing the 3D vs. 2D data range from 0.81–1.04 for V, 0.97–1.0 for FT, and 0.92–0.98 for RFT, depending on zircon geometry. Uncertainties (1σ) derived from the scatter of data around the regression line are 13–21 % for V, 5 %–1 % for FT, and 8 % for RFT, again depending on zircon morphologies. Like for apatite, the main control on the magnitude of the corrections and uncertainties is grain geometry, with grain size being a secondary control on FT uncertainty. Propagating these uncertainties into a real dataset (N = 28 ZHe analyses) generates 1σ uncertainties of 12–21 % in eU and 3–7 % in the corrected ZHe date when both analytical and geometric uncertainties are included. Accounting for the geometric corrections and uncertainties is important for appropriately reporting, plotting, and interpreting ZHe data. For both zircon and apatite, the Geometric Correction Method is a practical and straightforward approach for calculating more accurate (U-Th)/He data, and for including geometric uncertainty in eU and date uncertainties.

Spencer D. Zeigler, Morgan Baker, James R. Metcalf, and Rebecca M. Flowers

Status: open (until 23 Feb 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-3046', Florian Hofmann, 06 Feb 2024 reply
  • RC2: 'Comment on egusphere-2023-3046', Anonymous Referee #2, 12 Feb 2024 reply
Spencer D. Zeigler, Morgan Baker, James R. Metcalf, and Rebecca M. Flowers
Spencer D. Zeigler, Morgan Baker, James R. Metcalf, and Rebecca M. Flowers

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Short summary
(U-Th)/He chronology relies on accurate measurements of zircon grain dimensions, but the systematic error and uncertainty associated with those measurements have been unquantified, until now. We build on the work of Zeigler et al. (2023) and present the zircon Geometric Correction Method, a simple solution to correcting the error and quantifying the geometric uncertainty in eU and dates. Including this geometric correction and uncertainty matters for data evaluation and interpretation.