Ion imaging of ancient zircon

Analytical methods SIMS U–Pb Sample AC13 was collected by Stephen Moorbath from the University of Oxford in July 1995, from 500 metres NNW of the Acasta camp. Zircon crystals from sample AC13 were analysed for U–Th–Pb isotopes using the SHRIMP II ion probe at Curtin University following standard operating procedures (Wingate and Kirkland, 2014). The zircon surface was sputtered with a primary, mass-filtered (O2) − beam with ~ 2 nA current, focused to a ~ 15 μm spot. The mass resolution, M/ΔM, was better than 5000. Twenty-two analyses of the 91500 zircon reference material (Wiedenbeck et al., 1995) were obtained during the session, all of which indicate an external spot-to-spot (reproducibility) uncertainty of 1.33% (1σ) and a U/Pb calibration uncertainty of 0.45% (1σ). These calibration uncertainties are included in the calculated uncertainties on U/Pb* ratios and dates listed in Table S1. The OG1 zircon reference material was analysed as an unknown and yielded a weighted mean Pb/Pb age of 3458 ± 7 Ma (MSWD = 0.51, n = 5), within accepted values (Stern et al., 2009). No fractionation correction on Pb/Pb was deemed necessary. Common-Pb corrections were applied to all analyses using contemporaneous common Pb determined according to the model of Stacey and Kramers (1975) based on Pb counts. The Excel-based program Squid 2 (Ludwig, 2001) was used for data processing and data were plotted using Isoplot (Ludwig, 2003).