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Geochronology Advances in geochronological science
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Discussion papers
https://doi.org/10.5194/gchron-2019-3
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-2019-3
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 27 May 2019

Submitted as: research article | 27 May 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Geochronology (GChron).

Resolving the effects of 2D versus 3D grain measurements on (U-Th)/ He age data and reproducibility

Emily H. G. Cooperdock1,2, Richard A. Ketcham1, and Daniel F. Stockli1 Emily H. G. Cooperdock et al.
  • 1Department of Geological Sciences, University of Texas at Austin, Austin, 78712, USA
  • 2Woods Hole Oceanographic Institution, Woods Hole, 02543, USA

Abstract. (U-Th)/He thermochronometry relies on accurate and precise quantification of individual grain volume and surface area, which are used to calculate mass, alpha ejection (FT) correction, isotope concentrations, equivalent sphere radius (ESR), and ultimately age. The vast majority of studies use 2D or 3D microscope dimension measurements and an idealized grain shape to calculate these parameters, and a long-standing question is how much uncertainty these assumptions contribute to observed intra-sample age dispersion and accuracy. Here we compare the results for volume, surface area, grain mass, ESR, effective uranium (eU) and FT correction derived from 2D microscope and 3D x-ray computed tomography (CT) length and width data for > 100 apatite grains. We analyzed apatite grains from two samples that exhibited a variety of crystal habits, some with inclusions. We also present 83 new apatite (U-Th)/He ages to assess the influence of 2D versus 3D FT correction on sample age precision. The data illustrate that the 2D approach systematically overestimates grain volumes and surface areas by 20–25 %, impacting the estimates for mass, eU, and ESR – all important parameters used for interpreting age scatter and inverse modeling. FT factors calculated from 2D and 3D measurements differ by ~ 2 %. This variation, however, has effectively no impact on reducing intra-sample age reproducibility. We also present a grain mounting procedure for x-ray CT scanning that can allow 100's of grains to be scanned in a single session, and new software capabilities for 3D FT and FT-based ESR calculations that are robust for relatively low-resolution CT data, that together enable efficient and cost-effective CT-based characterization.

Emily H. G. Cooperdock et al.
Interactive discussion
Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Emily H. G. Cooperdock et al.
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Resolving the effects of 2D versus 3D grain measurements on (U-Th)/He age data and reproducibility R. A. Ketcham, E. H. G. Cooperdock, and D. F. Stockli https://doi.org/10.17612/P7D115

Emily H. G. Cooperdock et al.
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Short summary
(U-Th)/He chronometry relies on accurate grain-specific size and shape measurements. We use > 100 apatite grains to compare assumed 2D versus true 3D grain shapes measured by microscope and x-ray computed tomography, respectively. We find that volume and surface area both differ by ~ 25 % between the two techniques and directly affect mass and concentration measurements. However, we found a very small effect on the FT correction (2 %), and no discernible impact on mean sample age or dispersion.
(U-Th)/He chronometry relies on accurate grain-specific size and shape measurements. We use  100...
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