Relative retention of trace element and oxygen isotope ratios in zircon from Archean rhyolite, Panorama Formation, North Pole Dome, Pilbara Craton, Western Australia

In-situ analyses for oxygen isotope ratio and trace element concentration by ion microprobe of 3.3–3.7 Ga zircons from rhyolite of Panorama Formation in the North Pole Dome, Pilbara Craton document intracrystalline patterns of variability, show compositional correlation, and provide metrics to assess alteration.Values of δ18O in the North Pole Dome (NPD) zircons average 5.78 ‰ with a tight range from 5.3 to 6.4 ‰ VSMOW. These values are similar to magmatic zircons throughout the Archean worldwide and there is no correlation of δ18O, UPb age, or trace elements. SEM/CL examination shows no evidence of mineral inclusions in the SIMS analysis pits. In-situ analysis of Ca concentration is useful to assess the degree of alteration in zircon and retention of trace element compositions. Most zircon analyses (74 %) have elevated [Ca] ≥ 120 ppm, with values up to 1614 ppm suggesting alteration, probably facilitated by radiation damage. Many high-Ca zircons show enrichment in middle–heavy REE (M–HREE), a peak near Dy, and concave-down trends in chondrite-normalized diagrams. The calculated Dy anomalies (Dy/Dy*) correlate with [Ca]. High Ca also correlates with P, Ti, Fe, Y and REE, and with cumulative (unannealed) α-doses from radiactive decay of Th and U (3.1 to 20 × 1015 events/mg). If not annealed, this α-dose could have caused radiation damage that exceeded the first percolation point. Radiation damage can also facilitate alteration of δ18O in zircon, but examples from other terranes show that alteration is heterogeneous and correlates to greater variability of δ18O within and among zircons. The tight range of oxygen isotope ratios in the NPD rhyolite zircons and their similarity to other Archean zircons, world-wide, suggests that the analyzed NPD zircons have retained original, magmatic oxygen isotope ratios.

► In-situ δ18O and trace element analysis of rhyolite zircons (3.3–3.7 Ga) by SIMS.
► [Ca] is the most obvious alteration index, and increases with radiation damage.
► δ18O shows a typical Archean igneous value (5.8 ± 0.5‰) unaffected by alteration.
► The oxygen isotopes are more robust and retentive than trace element compositions.