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  • Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th) ∕ He age data and reproducibility
Title: Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th) ∕ He age data and reproducibility
Abstract. (U–Th) ∕ He thermochronometry relies on the accurate andprecise quantification of individual grain volume and surface area, whichare used to calculate mass, alpha ejection (FT) correction, equivalentsphere radius (ESR), and ultimately isotope concentrations and age. The vastmajority of studies use 2-D or 3-D microscope dimension measurements and anidealized grain shape to calculate these parameters, and a long-standingquestion is how much uncertainty these assumptions contribute to observedintra-sample age dispersion and accuracy. Here we compare the results forvolume, surface area, grain mass, ESR, and FT correction derived from2-D microscope and 3-D X-ray computed tomography (CT) length and width datafor > 100 apatite grains. We analyzed apatite grains from twosamples that exhibited a variety of crystal habits, some with inclusions. Wealso present 83 new apatite (U–Th) ∕ He ages to assess the influence of 2-D versus 3-D FT correction on sample age precision and effective uranium(eU). The data illustrate that the 2-D approach systematically overestimatesgrain volumes and surface areas by 20 %–25 %, impacting the estimates formass, eU, and ESR – important parameters with implications for interpretingage scatter and inverse modeling. FT factors calculated from 2-D and 3-Dmeasurements differ by ∼2 %. This variation, however, haseffectively no impact on reducing intra-sample age reproducibility, even onsmall aliquot samples (e.g., four grains). We also present a grain-mountingprocedure for X-ray CT scanning that can allow hundreds of grains to be scannedin a single session and new software capabilities for 3-D FT andFT-based ESR calculations that are robust for relatively low-resolutionCT data, which together enable efficient and cost-effective CT-basedcharacterization.  more » « less
Award ID(s):
1762458
NSF-PAR ID:
10164875
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Geochronology
Volume:
1
Issue:
1
ISSN:
2628-3719
Page Range / eLocation ID:
17 to 41
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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