Sample preparation, imaging and rejection criteria
The zircons in this study came from JH samples of Jack Hills metaconglomerate from the Discovery Outcrop on Eranondoo Hill1, from outcrops up to 900 m to the east, and from one cross-bedded metasandstone (01JH-36). Samples were reduced to sand size by electropulse disaggregation. Zircons were concentrated hydraulically on a gold table and by heavy liquids before separation of the low magnetic susceptibility zircon fraction by Frantz48,49. Zircons were hand-picked by binocular microscope, cast into 25.4 mm diameter epoxy mounts with analysis standards, and ground or polished at their approximate mid-sections.
The JH zircons of this study are divided into two suites. Sample mounts JH- and W- (01JH-13(a,b), 01JH-36, 01JH-42, 01JH-47, 01JH-54(a,b), 01JH-60(a,b) and W-74-3 and W-74-4) were extensively studied previously1,12,26,50,51 and in this study. Samples 01JH-12 and 03JH-141 were collected 1 m from 01JH-13, 01JH-54, W-74-3, and W-74-4. Zircons in mounts ERC-2 through ERC-10 were separated from 01JH-12 in 2022.
Before mounting, the zircons in mounts ERC-2 to ERC-10 were experimentally heated to 1,100 °C at 0.4 GPa of Ar atmosphere for 6 h in a rapid-quench internally heated gas apparatus to fuse melt inclusions for subsequent study. The zircons in sample mounts with no prefix ERC were not experimentally heated. Differences are observed in levels of retained radiation damage seen by Raman (effective dose) and cathodoluminescence, none of which influence the results reported here. No differences are observed between experimentally annealed zircons and not annealed zircons for age, oxygen isotope ratio or trace elements in the selected low-magnetic-susceptibility zircons with >95% concordant U–Pb ages52. This is consistent with many studies of chemically abraded zircons that were experimentally heated to 800–1,100 °C or above53 and further attests to the refractory and retentive nature of crystalline zircons.
Procedures for SIMS analysis of age (U–Pb), oxygen isotope ratio (δ18O and 16OH/16O) and trace element concentrations (REEs, Nb, Sc, Al, P, Ca, Ti, Fe, Y, Hf, Ta, Th and U) have been described elsewhere and are summarized below. A new aspect of these data is the analysis of Nb, Sc and Ta in zircon, which requires higher mass resolution than routinely used in studies with forward-geometry SIMS instruments30.
The polished mid-sections of zircons were imaged optically and by SEM (back-scattered electrons, secondary electrons and cathodoluminescence) before and after SIMS analysis. Pre-SIMS imaging allowed selection of analysis spots to avoid obvious alteration and inclusions, and targeting of sub-domains in zoned zircons. Post-SIMS imaging evaluated whether pits were irregular (that is, placed on inclusions, cracks and alteration zones) and in the selected domain50,54,55. Data from irregular pits were not considered further.
Zircon analyses were further filtered by composition. Acceptance criteria were conservatively set as follows: >95% concordant U–Pb ages; low concentrations of non-formula elements (<30 µg g−1 Al or Ca; <50 µg g−1 Fe); La < 0.25 µg g−1; Pr N < 10; (Sm/La) N > 10; Th/U > 0.1; Ce/Ce* > 3; and for δ18O, 16O/1H16O < 0.0004 (refs. 26,50,55,56,57,58,59). These zircons all passed the LREE-I test55, which is less stringent.
Geochronology
All SIMS analyses were made by CAMECA IMS 1280 in the WiscSIMS Lab, University of Wisconsin–Madison. Zircons in mounts ERC-8, ERC-9 and ERC-10 were surveyed for quick ages in automatic mode. Samples were cleaned and carbon-coated to minimize common lead contamination and a Hyperion-II RF-source produced a 5-nA 16O 2 − beam focused to around 15 μm on the sample surface. Simultaneous analysis of 204Pb+, 206Pb+, 207Pb+ and 92Zr 2 O+ took about 1.3 min per analysis (Supplementary Table 1). These quick model ages are based on 207Pb/206Pb.
Full U–Pb analyses were measured in this study for selected zircons in the ERC-2 to ERC-10 mounts. The data for JH and W mounts are compiled from earlier sources1,12,26. The ERC mounts were repolished to remove carbon and coated with Au for full U-Pb analysis. Gold coating (rather than C) yields higher and more stable count rates on Pb, but can include Pb from the Au, which must be removed by pre-sputtering before analysis and corrected for common Pb60. The primary beam of 16O 2 − (Hyperion-II RF-source) was accelerated at 10 keV for an impact energy of 23 keV and a spot size of about 20 μm. An oxygen leak was used at a chamber pressure of about 1–2 × 10−5 mbar to improve the Pb yield. Each counting cycle proceeded through the masses: 92Zr 2 16O+ [C], 92Zr 2 16O+ [L1], 200.5 (blank), simultaneous collection of Pb isotopes 204Pb+, 206Pb+ and 207Pb+, simultaneous collection of 238U+ and 238U16O+ and 238U16O 2 + at MRP ~ 8,000. A total of six cycles were measured and the last five were integrated for age calculations. For all sessions except January 2024, the measured values of 206Pb/238U were corrected based on the 206Pb/238U compared with 238U16O 2 /238U trend as measured on reference zircon 91500 (refs. 61,62), which was also used to determine and correct the instrumental mass fractionation of Pb isotopes. In January 2024, calibration of U–Pb isotope ratios used zircon reference material M127 (ref. 63), which was also used to determine and correct the instrumental mass fractionation of Pb isotopes. Temora-1, Temora-2 (ref. 64), 91500 and M127 were run as secondary reference materials in each session. Measured 204Pb was used to correct for common Pb using the two-stage Pb evolution model in ref. 60. Ages are based on 207Pb/206Pb in the >95% concordant analyses.
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