Oxygen isotopic heterogeneity in the Temora-2 reference zircon

For the past decade and a half, Geoscience Australia has distributed zircon from a portion of the Middledale Gabbroic Diorite under the label Temora-2. This reference zircon was originally developed as a reference material for use in ion microprobe U-Th-Pb geochronological analyses. As ion probe capability has increased to allow the measurements of other isotopic systems at geologically useful precision and accuracy, the Temora-2 zircon has remained a convenient reference material to use for those systems. However, the suitability of this material for non-geochronological applications must be continuously reassessed. This study demonstrates that some (but not all) aliquots of the Temora-2 zircon, distributed by Geoscience Australia to analytical laboratories worldwide, have δ18O values up to 1 lower than the reference laser fluorination δ18O value quoted in Black et al. (2004). Although the long and complex collection history of this material makes it difficult to pinpoint the cause of this discrepancy, we suspect it relates to material from two or more boulders from the Temora-2 site, with different δ18O values, being sampled and mixed together in the field prior to storage at Geoscience Australia. Therefore, oxygen isotope measurements on SIMS mounts where Temora is the only reference zircon may be biased towards heavier values by up to 1, unless there are additional constrains on the actual δ18O value of the specific aliquot of Temora-2 zircons placed on that particular ion probe mount. We recommend that future oxygen isotope work should use a reference zircon other than Temora-2, until Geoscience Australia can replace the current stock of heterogeneous Temora-2 material with zircon which has a uniform δ18O value.