Developing “Rule-of-Thumb” Uncertainties on α-ejection Corrections and eU for the Apatite (U-Th)/He Method Using Nano-CT

Abstract:

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 α-ejection because ⁴He atoms travel ~20 µm during α-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 eU and the α-ejection correction parameter (Ft) are calculated on the basis of crystal size and assuming an idealized morphology. However, the uncertainty stemming from the calculations’ assumptions depends on how much the real crystal geometry deviates from that assumed, and this uncertainty is typically not included in the propagated uncertainties on AHe data. Our goal for this study was to develop a ‘rule of thumb’ for Ft and eU uncertainties associated with the full range of commonly analyzed apatite geometries by comparing manually measured grain size and actual grain size using nano-computed tomography (nano-CT).

Apatite geometry and roughness were characterized using a Grain Evaluation Matrix (GEM). Geometry from prismatic (hexagonal) to rounded is graded from A to C and surface roughness from ‘least’ to ‘most’ is graded from 1 to 2 which allows for a single parameter (eg. B2) to describe both of these parameters. High resolution nano-CT scans of 270 grains representative of those usually analyzed for AHe dates were completed and processed using Dragonfly and Blob3D. Initial analysis suggests that grain size and geometry (rather than surface roughness) exert the strongest controls on the uncertainty associated with Ft corrections and eU. Additional analysis is underway to develop rules of thumb for assigning uncertainties to Ft and eU as a function of apatite size and GEM value.