Identification and elimination of a matrix-induced systematic error in LA-ICP-MS ²⁰⁶Pb/ ²³⁸U dating of zircon

Link references in abstract. Black et al. (2004) identified a systematic difference between LA-ICP-MS and TIMS measurements of ²⁰⁶Pb/ ²³⁸U in zircons, which they correlated with the incompatible trace element content of the zircon. We show that the offset between the LA-ICP-MS and TIMS measured ²⁰⁶Pb/ ²³⁸U correlates more strongly with the total radiogenic Pb than with any incompatible trace element. This suggests that the cause of the ²⁰⁶Pb/ ²³⁸U offset is related to differences in the radiation damage (alpha dose) between the reference and unknowns. We test this hypothesis in two ways. First, we show that there is a strong correlation between the difference in the LA-ICP-MS and TIMS measured ²⁰⁶Pb/ ²³⁸U and the difference in the alpha dose received by unknown and reference zircons. The LA-ICP-MS ages for the zircons we have dated can be as much as 5.1% younger than their TIMS age to 2.1% older, depending on whether the unknown or reference received the higher alpha dose. Second, we show that by annealing both reference and unknown zircons at 850°C for 48h in air we can eliminate the alpha-dose-induced differences in measured ²⁰⁶Pb/ ²³⁸U. This was achieved by analyzing six reference zircons a minimum of 16 times in two round robin experiments: the first consisting of unannealed zircons and the second of annealed grains. The maximum offset between the LA-ICP-MS and TIMS measured ²⁰⁶Pb/ ²³⁸U for the unannealed zircons was 2.3%, which reduced to 0.5% for the annealed grains, as predicted by within-session precision based on counting statistics. Annealing unknown zircons and references to the same state prior to analysis holds the promise of reducing the 3% external error for the measurement of ²⁰⁶Pb/ ²³⁸U of zircon by LA-ICP-MS, indicated by Klötzli et al. (2009), to better than 1%, but more analyses of annealed zircons by other laboratories are required to evaluate the true potential of the annealing method.