Abstract. (U–Th) ∕ He thermochronometry relies on the accurate and
precise quantification of individual grain volume and surface area, which
are used to calculate mass, alpha ejection (FT) 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 FT 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) ∕ 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 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. FT factors calculated from 2-D and 3-D
measurements differ by ∼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 FT and
FT-based ESR calculations that are robust for relatively low-resolution
CT data, which together enable efficient and cost-effective CT-based
characterization.
Low-Temperature Synthesis of Monolithic Titanium Carbide/Carbon Composite Aerogel
