ESTIMATING UNCERTAINTIES FOR ALPHA-EJECTION CORRECTIONS AND eU VALUES FOR THE ZIRCON (U-TH)/HE METHOD

Abstract:

The (U-Th)/He thermochronologic technique is widely used to constrain the timing of geologic events such as exhumation, erosion, and cooling of plutons. Zircon (U-Th)/He (ZHe) thermochronology can record thermal histories over a wide range of temperatures depending upon the amount of radiation damage in the grain. Calculating an accurate (U-Th)/He date requires accounting for the fraction of He atoms lost due to alpha-particle ejection. Effective uranium concentrations (eU) are also critical for ZHe data interpretation because eU can be a proxy for radiation damage, which influences the zircon temperature sensitivity. Alpha-ejection corrections and eU values are typically estimated using grain measurements made under an optical microscope and assuming idealized grain geometries. This study builds upon the work of Zeigler et al., (this meeting) on the apatite system and assesses the uncertainty associated with alpha-ejection corrections and eU values for zircon.

We performed a size analysis of >700 zircon grains analyzed in our lab from 2017-2018 to determine an appropriate distribution of representative grain sizes. We then evaluated a diverse set of samples to capture the range of typically analyzed sizes, morphologies, and radiation damage levels. The 7 selected samples used in our work encompass this spectrum and include four igneous samples with ages ranging from Archean to Oligocene, two detrital samples, and one metamorphic sample. We next will use a similar approach to that of Zeigler et al. (this meeting) in which we will first develop a Grain Evaluation Matrix, then acquire nano-CT data for ~400 grains to accurately determine zircon volumes and geometries, and followed by calculation of alpha-ejection corrections directly from their actual shapes using Monte Carlo techniques. We can then compare these “true” measurements to those determined with more traditional methods to better understand the accuracy and precision of zircon alpha-ejection corrections and eU estimates.