IMPROVING LA-ICPMS DATING TECHNIQUES: EXPERIMENTS ON ZIRCON FROM A 3.51 GA DIORITIC GNEISS, EAST PILBARA TERRANE, WESTERN AUSTRALIA

The “gold standard” U-Pb zircon dating method is ID-TIMS because highly precise and accurate ages obtain when Pb and U are purified, concentrated and calibrated using isotope dilution. Mattinson (2005) suggested that more concordant results ensue following annealing and chemical abrasion. “Spot” or in-situ dating methods are less accurate and precise because of “matrix effects”, a catchall for the subtle differences in composition among standards and unknowns. For LA-ICP-MS dating, Marillo-Sialer et al. (2014) and Crowley et al. (2014) have shown that a variety of factors result in age inaccuracies, but that the dominant effects stem from the ablation processes. We have studied the effect of annealing and/or chemical abrasion as compared to pristine ~3.5 Ga zircons in order to understand the impact of treatments on ancient zircons and the standards used to calibrate the isotope ratios (ages). We suggest that published reports of reverse discordance among Archean zircon commonly are caused by calibration artefacts and not common Pb. We propose improvements in the LA-ICP-MS dating workflow partially based on experiments conducted on large euhedral zircons from a dioritic gneiss of the Muccan Granitic Complex, East Pilbara Terrane. The weighted mean ²⁰⁷Pb/²⁰⁶Pb age (with annealed Temora2 as the reference standard) yielded 3.51 ± 0.03 Ga (n=37), making it the oldest documented coherent intrusion in the region. Zircon trace element concentrations help constrain petrogenesis. Ti-in-zircon thermometry yielded 720 ± 30 ^oC; Ca content shows positive correlation with domains of high alpha-dose and associated radiation-induced microstructural damage, and is useful for detecting discordant (Ca>200 ppm) grains.

Crowley, Q.G., Heron, K., Riggs, N., et al., 2014, Chemical abrasion applied to LA-ICP-MS U-Pb zircon geochronology, Minerals v.4, p. 503-518.

Marillo-Sialer, E., Woodhead, J., Hergt, J., et al., 2014, The zircon “matrix effect”: evidence for an ablation rate control on the accuracy of U-Pb age determinations by LA-ICP-MS, Journal of Analytical Atomic Spectrometry, v. 29, p. 981-989.

Mattinson, J.M., 2005, Zircon U-Pb chemical abrasion (“CA-TIMS”) method: combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages, Chemical Geology, v. 220, p. 47-66.