Improved Accuracy and Precision in LA-ICP-MS U-Th/Pb Dating of Zircon through the Reduction of Crystallinity Related Bias

Abstract:

The accuracy of zircon U-Th/Pb ages generated by LA-ICP-MS is limited by systematic bias resulting from differences in crystallinity of the primary reference and that of the unknowns being analyzed. In general, the use of a highly crystalline primary reference will tend to bias analyses of materials of lesser crystallinity toward older ages. When dating igneous rocks, bias can be minimized by matching the crystallinity of the primary reference to that of the unknowns. However, the crystallinity of the unknowns is often not well constrained prior to ablation, as it is a function of U and Th concentration, crystallization age, and thermal history. Likewise, selecting an appropriate primary reference is impossible when dating detrital rocks where zircons with differing ages, protoliths, and thermal histories are analyzed in the same session.

We investigate the causes of systematic bias using Raman spectroscopy and measurements of the ablated pit geometry. The crystallinity of five zircon reference materials with ages between 28.2 Ma and 2674 Ma was estimated using Raman spectroscopy. Zircon references varied from being highly crystalline to highly metamict, with individual reference materials plotting as distinct clusters in peak wavelength versus Full-Width Half-Maximum (FWHM) space. A strong positive correlation (R²=0.69) was found between the FWHM for the band at ~1000 cm^-1 in the Raman spectrum of the zircon and its ablation rate, suggesting the degree of crystallinity is a primary control on ablation rate in zircons. A moderate positive correlation (R²=0.37) was found between ablation rate and the difference between the age determined by LA-ICP-MS and the accepted ID-TIMS age (ΔAge).

We use the measured, intra-sessional relationship between ablation rate and ΔAge of secondary references to reduce systematic bias. Rapid, high-precision measurement of ablated pit geometries using an optical profilometer and custom MatLab algorithm facilitates the implementation of the technique. The technique results in no decrease in the precision of individual measurements and an overall improvement of intra-sessional reproducibility of reference material ages, and thus a reduction in external error.