ESTIMATING UNCERTAINTIES ON ALPHA-EJECTION CORRECTIONS AND EFFECTIVE URANIUM VALUES RELEVANT FOR THE APATITE (U-TH)/HE METHOD

Apatite (U-Th)/He (AHe) dating is a widely-applied thermochronological technique used to decipher low-temperature thermal histories. Accurate dates require that the results are corrected for alpha-ejection because ⁴He atoms travel up to 20 microns during alpha-decay and a correction is required to account for He lost by this effect. Effective uranium concentrations (eU) are important for accurate AHe data interpretation because radiation damage scales with eU, which affects He retentivity. Both parameters are calculated on the basis of crystal size and idealized morphology, but their uncertainties are poorly characterized and typically not included in the reported uncertainties on AHe data. Our goal in this study is to develop a ‘rule of thumb’ for uncertainties associated with grain geometries and therefore error associated with alpha-ejection corrections and eU values by comparing manually measured grain size and actual grain size using micro-computed tomography (microCT).

We first analyzed the size distribution of 1100 apatite grains run in the CU TRaIL over the past 5 years to determine representative size categories for this study. We additionally characterized apatite morphology and surface relief using a quality matrix to derive a single parameter dubbed a “QUALM”. Morphology from prismatic to rounded is graded from A to D and surface relief from ‘least’ to ‘most’ is graded from 1 to 4 which allows for a single parameter (e.g. B3) to describe both the morphology and surface relief of a single crystal. Apatites were picked from 5 samples with a range of ages and lithologies and binned into their respective sizes and QUALM category. MicroCT analysis will be completed on a sample of approximately 400 grains representative of those usually analyzed for AHe dates.