Apatite grains


  1. Apatite grains>
    . Resolving the effects of 2D versus 3D grain measurements on (U-Th)/He age data and reproducibility. Geochronology. .

    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.