Evaluating Downhole Fractionation Corrections in LA-ICP-MS U-Pb Zircon Geochronology

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Among the most significant challenges in maximizing the precision and accuracy of U-Pb zircon geochronology by LA-ICP-MS is minimizing the impact of downhole fractionation, the time-dependent evolution of Pb/U ratios caused mainly by complex differences in the volatility and chemical properties of elements as they are excavated from the ablation site. To produce meaningful dates for unknown materials, downhole fractionation is typically quantified in a reference zircon and a time-based correction factor subsequently employed to yield constant Pb/U in both standard and unknown zircons. This assumes that both the reference and unknown zircon exhibit similar downhole behaviour. As a test of this assumption, downhole fractionation trends were characterized and quantified in three common zircon reference materials (Plešovice, 337 Ma; Temora-2, 417 Ma; 91500; 1065 Ma) and in three low-U (< 300 ppm) zircon samples with coherent U-Pb systematics from mafic intrusions (Laramie, 1436 Ma; Bushveld, 2057 Ma; Stillwater, 2710 Ma). Using an exponential downhole correction model based on each of the untreated zircon reference materials, the corrections were applied to each of the “unknowns” and the resulting time-dependent Pb/U ratios and final ages were compared. The effectiveness of pre-treatment protocols was also evaluated by comparing downhole fractionation trends for untreated grains, annealed grains, and grains that were annealed and leached (i.e., chemical abrasion). Each of the three zircon reference materials exhibited distinct downhole fractionation and their calculated correction factors had variable influence in correcting each of the unknowns. Application of the fit parameters either over-corrected or under-corrected the Pb/U ratios of the unknowns due to differences in slope for the different downhole rates at a given time. In most cases, annealing zircon prior to LA-ICP-MS analysis lessened the magnitude of U-Pb mass fractionation during laser ablation, whereas chemical abrasion did not significantly change ablation behaviour beyond simply annealing the grains. Based on the relative effectiveness of the downhole correction that was applied, the resulting U-Pb dates of zircon from the Precambrian mafic intrusions can vary significantly when compared to the CA-ID-TIMS ages established for these samples. The results of this study indicate that a robust downhole correction method for LA-ICP-MS U-Pb geochronology of zircon, particularly for application to magmatic zircon and igneous crystallization ages, involves characterization of downhole fractionation in different reference materials and then applying a correction to the unknown zircon based on a reference zircon that behaves similarly during ablation.