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

Kouki Kitajima, Takayuki Ushikubo, Noriko T. Kita, Shigenori Maruyama, John W. Valley

    Research output: Contribution to journalArticle

    7 Citations (Scopus)

    Abstract

    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 δ 18O 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 δ 18O, 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]≥120ppm, with values up to 1614ppm 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 20×10 15events/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 δ 18O in zircon, but examples from other terranes show that alteration is heterogeneous and correlates to greater variability of δ 18O 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.

    Original languageEnglish
    Pages (from-to)102-115
    Number of pages14
    JournalChemical Geology
    Volume332-333
    DOIs
    Publication statusPublished - Nov 25 2012

    Keywords

    • Archean
    • Oxygen isotope ratio
    • Pilbara craton
    • SIMS
    • Trace elements
    • Zircon

    ASJC Scopus subject areas

    • Geology
    • Geochemistry and Petrology

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