Developing a Correction to Remove Systematic Bias in U-Pb LA-ICP-MS Zircon Ages Related to Zircon "Matrix Effects"

Abstract:

For more than a decade the occurrence of systematic discrepancies between the U-Pb ages of zircons determined by LA-ICP-MS and ID-TIMS has been acknowledged. Trace element concentrations, crystallographic orientation and damage to the crystal lattice by radioactive decay have all been cited as possible causes for the discrepancy termed the “matrix effect”. Recent studies have concluded that differences in Laser Induced Elemental Fractionation (LIEF) between zircon reference materials results from variations in the ablation rate between the primary reference, which is used to build a model for LIEF during data reduction, and the unknowns. These variations are likely related to physical differences in the crystal lattice caused by alpha particle ejection. We tested this hypothesis by measuring the ablation rate for ~200 individual ablation pits in a variety of reference materials using an optical profilometer. Our data demonstrate a clear relationship between delta age (the difference between the age calculated by LA-ICP-MS and the accepted ID-TIMS age, expressed as a percentage) and ablation rate. The relationship between calculated alpha dosage for each ablation and delta age is less clear. This suggests that the zircon’s thermal history may play an important role in controlling ablation rate through annealing of crystal lattice defects. However, alpha dosage is readily quantifiable during routine zircon U-Pb analyses and therefore its relationship to delta age may provide a useful first order correction to remove systematic biases from U-Pb ages. Raman spectroscopy could provide a more robust measure of radiation damage in the zircon lattice and could help to refine our understanding of the processes involved.