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

Cooperdock, Emily H. G.; Ketcham, Richard A.; Stockli, Daniel F.

(U–Th) inline-formula∕ He thermochronometry relies on the accurate and precise quantification of individual grain volume and surface area, which are used to calculate mass, alpha ejection (inline-formulaFT) correction, equivalent sphere radius (ESR), and ultimately isotope concentrations and age. The vast majority of studies use 2-D or 3-D 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, and inline-formulaFT correction derived from 2-D microscope and 3-D 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) inline-formula∕ He ages to assess the influence of 2-D versus 3-D inline-formulaFT correction on sample age precision and effective uranium (eU). The data illustrate that the 2-D approach systematically overestimates grain volumes and surface areas by 20 %–25 %, impacting the estimates for mass, eU, and ESR – important parameters with implications for interpreting age scatter and inverse modeling. inline-formulaFT factors calculated from 2-D and 3-D measurements differ by inline-formula∼2 %. This variation, however, has effectively no impact on reducing intra-sample age reproducibility, even on small aliquot samples (e.g., four grains). We also present a grain-mounting procedure for X-ray CT scanning that can allow hundreds of grains to be scanned in a single session and new software capabilities for 3-D inline-formulaFT and inline-formulaFT-based ESR calculations that are robust for relatively low-resolution CT data, which together enable efficient and cost-effective CT-based characterization.



Cooperdock, Emily H. G. / Ketcham, Richard A. / Stockli, Daniel F.: Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th) ∕ He age data and reproducibility. 2019. Copernicus Publications.


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Rechteinhaber: Emily H. G. Cooperdock et al.

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