Monazite sample preparation and LA-ICPMS analyses

Monazite grains from samples 8628–5606, 8628–5638, 8628–5628 and 8628–6001were analysed in situ in polished blocks mounted in 2-inch round mounts. Monazite grains were identified using a FEI Quanta 600 SEM controlled by an automated software package (Mineral Liberation Analyser), and high resolution, high contrast BSE images (Supplementary Fig. ³) were obtained for individual monazite grains using a Hitachi SU-70 field emission (FE)-SEM at the Central Science Laboratory, University of Tasmania. Further details on sample preparation and in situ monazite identification can be found in Halpin et al.⁶⁶. U–Pb monazite analyses were performed on an Agilent 7500cs quadrupole ICPMS with a 193 nm Coherent Ar–F gas laser and the Resonetics S155 ablation cell at the University of Tasmania. LA-ICPMS setup and conditions, and monazite data reduction and reproducibility, are described in detail in Halpin et al.⁶⁶ and summarised below. Tera-Wasserburg diagrams and weighted mean age calculations (Fig. 4) were made using Isoplot v4.11⁶⁴. Error ellipses on Tera-Wasserburg plots are calculated at the two-sigma level and weighted mean and intercept ages are reported at 95% confidence limits. Full tabulation of monazite isotopic data is presented in Supplementary Table ³.

Each analysis was pre-ablated with 5 laser pulses to remove the surface contamination then the blank gas was analysed for 30 s followed by 30 s of monazite ablation at 5 Hz and ∼2 J/cm² using a spot size of 9 μm; keeping U and Th in the pulse counting mode of detection on the electron multiplier. Elements measured included ³¹P, ⁵⁶Fe, ⁸⁹Y, ²⁰²Hg, ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th and ²³⁸U with each element being measured sequentially every 0.16 s with longer counting time on the Pb isotopes compared to the other elements. The down hole fractionation, instrument drift and mass bias correction factors for Pb/U and Pb/Th ratios on monazites were calculated using analyses on the in-house primary standard (14971 Monazite) and secondary standard monazite grains (RGL4B and Banaeira) analysed at the beginning of the session and every 15–20 unknowns, using the same spot size and conditions as used on the samples to provide an independent control to assess accuracy and precision. The correction factor for the ²⁰⁷Pb/²⁰⁶Pb ratio was calculated using 8 analyses of the international glass standard NIST610 analysed throughout analytical session and corrected using the values recommended by Baker et al.⁶⁷. All data reduction calculations and error propagations were done within Microsoft Excel® via macros designed at the University of Tasmania and summarised in Halpin et al.⁶⁶. ²⁰⁷Pb corrected ²⁰⁶Pb/²³⁸U weighted mean age for the secondary monazite standard Banaeira is 507 ± 4 Ma (n = 5, MSWD = 0.50), within error of the reference ages of 507.7 ± 1.3 Ma⁶⁸. ²⁰⁷Pb corrected ²⁰⁶Pb/²³⁸U weighted mean age for the secondary monazite standard RGL4b is 1560 ± 13 Ma (n = 5, MSWD = 0.30), within error of the reference age of 1566 ± 3 Ma⁶⁹.