U-Pb-Hf zircon data from Wilkes/Queen Mary lands

Brief description

This dataset file is unpublished. Details of analytical methods: U-Pb from Daczko et al. (2018) and Lu-Hf from Halpin et al. (2020).

U-Pb analysis was via SHRIMP following procedure outlined in Daczko et al. (2018). Zircon grains were hand-picked and mounted into a 25-mm diameter epoxy resin disc along with grains of reference zircons BR266 (559 Ma, 909 ppm U; Stern and Amelin, 2003) and OGC-1 (3465 Ma; Stern et al., 2009) and a fragment of NBS610 glass (used to center the 204Pb peak). The mount was polished to expose the zircon grains and reference materials, then carbon-coated for cathodoluminescence imaging on a TESCAN Mira 3 scanning electron microscope in the John de Laeter Centre, Curtin University. The carbon coat was removed and the mount gold-coated prior to U-Pb isotope analysis on the SHRIMP II sensitive high resolution ion microprobe at the John de Laeter Centre, Curtin University.

Analytical procedures for the Curtin SHRIMP II facility were described by Kennedy and De Laeter (1994) and De Laeter and Kennedy (1998) and are similar to those described by Compston et al. (1984) and Williams (1998). A mass-filtered primary beam of O2– ions at 10 keV with 25–30 μm diameter was used to sputter secondary ions from the target material. The primary beam current measured at the mount surface was ~2.0 nA, and the beam was rastered over each analysis site for 3–4 minutes to remove surface contamination before secondary ions were collected in 6 scans through the following masses: 196 (90Zr216O+, 2 seconds), 204 (204Pb+, 10 seconds), 205.5 (background, 10 seconds), 206 (206Pb+, 20 seconds), 207 (207Pb+, 30 seconds), 238 (238U+, 3 seconds), 248 (232Th16O+, 2 seconds) and 254 (238U16O+, 3 seconds). Values of 206Pb/238U in zircons from 8628–5807 and 8628–6006 were calibrated using analyses of reference zircon BR266, assuming a power law relationship between 206Pb+/238U+ and 238U16O+/238U+ and a fixed exponent of 2 (Claoué-Long et al., 1995). External spot-to-spot uncertainty (1σ) in 238U/206Pb values in BR266 over the analytical session was 1.03%. Values of 207Pb/206Pb were monitored using the OGC-1 reference zircon which yielded an error-weighted mean 207Pb/206Pb date (95% confidence) of 3466.3 ± 4.8 Ma for the analytical session, within uncertainty of the reference value (3465.4 Ma). Data were processed and displayed using the Excel add-ins SQUID 2.50.09.08.06 (Ludwig, 2009) and Isoplot 3.76.12.02.24 (Ludwig, 2012). All analyses were corrected for common Pb based on measured 204Pb (Compston et al., 1984) and common Pb isotope ratios appropriate for the approximate age of zircon crystallization according to the Stacey and Kramers (1975) model of Pb isotope evolution.

Lu-H analysis was via LAM-ICPMS following procedure in Halpin et al. (2020). Hf isotope analyses were performed in situ on the same grains analysed for U-Pb using a Photon Machines Excimer 193 nm Ar-F laser ablation micro-probe attached to a Nu Plasma multi- collector (MC)-ICPMS system at Macquarie University GeoAnalytical (MQGA)(see Griffin et al., 2004 for a detailed methodology). A gas blank was analysed for 30 s followed by up to 120 s of ablation at a beam diameter of 40–50 μm, 5 Hz and ~7.5 J/cm2. Zircon CL images were used to ensure that Hf isotope analyses overlapped the same domain analysed for U-Pb. The Mud Tank and Temora-2 zircon standards were used as a reference standard for Hf analysis; our weighted average 176Hf/177Hf values for these standards are 0.282525 ± 07 (n = 11, MSWD = 1.09) and 0.282649 ± 20 (n = 2), respectively, within error of the published values of 0.282523 ± 43 (Mud Tank; Griffin et al., 2006) and 0.282680 ± 24 (Temora-2; Woodhead et al., 2004). Uncertainties quoted are the internal measured uncertainty and do not include any propagation of error from the reference standard.

The initial 176Hf/177Hf value (Hfi) in zircon is calculated using the measured 176Lu/177Hf, 176Hf/177Hf and apparent 207Pb/206Pb age and the 176Lu decay constant of Scherer et al. (2001) of 1.865 x 10-11. Model age calculations (TDM) are based on a depleted-mantle source with Hfi = 0.279718 and 176Lu/177Hf = 0.0384. This provides a value of 176Hf/177Hf (0.28325) similar to that of average mid-ocean ridge basalt over 4.56 Ga. The calculated TDM ages use the measured 176Lu/177Hf of the zircon and give a minimum age for the source material of the magma from which the zircon crystallised. Two-stage model ages (TDM2) are calculated assuming that the parental magma was derived from the average continental crust (176Lu/177Hf = 0.015), which in turn was originally derived from the depleted mantle.