OXYGEN ISOTOPE AND TRACE ELEMENT COMPOSITIONS OF ZIRCON FROM ARCHEAN RHYOLITE, PANORAMA FORMATION, NORTH POLE DOME, PILBARA CRATON, WESTERN AUSTRALIA

We carried out in-situ analyses for oxygen isotope ratios and trace element concentrations by ion microprobe (IMS-1280) of Archean zircons from rhyolite of Panorama Formation in the North Pole Dome, Pilbara Craton, Western Australia to document intracrystalline patterns of variability, correlation of compositions, and to provide metrics to assess alteration. These zircons were previously analyzed by LA-ICP-MS yielding ages from 3.3–3.7 Ga within a single sample. Analyzed spot sizes are ~10µm in diameter for oxygen isotopes and ~25µm for trace elements. Trace element compositions were measured in the same domains as δ¹⁸O and U-Pb.

Values of δ¹⁸O show a tight range from 5.3 to 6.4‰ VSMOW and average 5.78 ± 0.45‰ (2SD, n = 32) in previously dated zircons. These values are similar to magmatic zircons throughout the Archean and there is no correlation of δ¹⁸O to U-Pb age. Values of [Ca, Fe, REE, Th and U] suggest variable amounts of radiation damage and alteration. There is no systematic variation between trace element concentrations and U-Pb age or concordance. A majority of spots have [Ca] ≥ 150 ppm, with values up to 1584 ppm, but SEM/CL show no evidence of mineral inclusions in the analysis pits. Concentrations of Ca considered as an index of alteration in zircon, correlate with P, Ti, Fe, Y and REE, and with cumulative dose by alpha-decay of Th and U (2.9E+15 to 1.8E+16 events/mg). Lithium concentration correlates with [Ca] and ranges from 8.5–25 ppm for high-[Ca] and 0.4–16 ppm for low-[Ca] (<150 ppm) spots . A majority of high-[Ca] zircons show enrichment in middle–heavy REE (M–HREE) and a peak at around Dy with concave-down trends in chondrite-normalized diagrams. Calculated excess [Dy] correlates with [Ca]. Low-[Ca], less altered zircons show normal REE patterns without M–HREE-enrichment. These correlations suggest that trace element compositions have been variably affected in many zircon domains by alteration, likely facilitated by radiation damage. The oxygen isotope ratios are more robust than trace element compositions and preserve the narrow range of magmatic δ¹⁸O values. We can assess degree of alteration in zircon and estimate more pristine trace element compositions including REE with monitoring Ca concentration by high-sensitivity in-situ analysis.