Determining high precision, in situ, oxygen isotope ratios with a SHRIMP II: Analyses of MPI-DING silicate-glass reference materials and zircon from contrasting granites

The development of new techniques and instrumentation on the ANU SHRIMP II ion microprobe has made it possible to measure the oxygen isotope ratios of insulating and conducting phases (e.g. silicates, carbonates, phosphates and oxides) on a 25 mu m scale with better than 0.4 parts per thousand precision and accuracy at 95% confidence. Instrumentation changes include the installation of a multiple collector, charge neutralization using an oblique-incidence low-energy electron gun, and the addition of Helmholtz coils to counter mass dispersion by the Earth's magnetic field. A redesign of sample mounts and mount holders has effectively eliminated differences in variable isotope fractionation across the mount surface during analysis. Techniques have been developed to minimize the effect of electron-induced secondary ionization of oxygen. During a 6-minute analysis involving 100-140 s of data collection, delta O-18 values can be measured on one 25 mu m spot with an internal precision of better than 0.2 parts per thousand (2 standard errors). Analyses of MPI-DING silicate-glass reference material demonstrate that the external reproducibility of single spots can be better than 0.4 parts per thousand at 95% confidence, and that for matrix-matched samples and reference material, accuracy is commensurate with precision. MPI-DING glasses are acceptable ion microprobe reference materials for oxygen isotope measurements of glasses, although KL2-G is possibly heterogeneous. Zircon reference materials TEMORA 2 and FC1 appear to be acceptable as preliminary oxygen isotope reference materials. SHRIMP II analyses of FC1 indicate that it has a delta O-18 value of 5.4 parts per thousand (VSMOW). Analyses of zircon oxygen isotopic compositions from a gabbro, a tonalite and a granodiorite from southeastern Australia are presented. Zircon from the gabbro has a delta O-18 value of 5.6 parts per thousand, the tonalite has an I-type affinity and slightly heterogeneous delta O-18 values around 6.6 parts per thousand, and the granodiorite has an S-type affinity and a range of igneous, melt precipitated zircon delta O-18 values between 8.2 and 10.2 parts per thousand. These results suggest that the gabbro is mantle-derived and slightly contaminated with crustal material, and that the I-type granodiorite has evolved in a similar manner from a mantle-derived source. The delta O-18 values of the zircon from the S-type granodiorite are not only higher than from the I-type, but also more heterogeneous, consistent with partial melting of a poorly-mixed, metasedimentary source. (C) 2008 Elsevier B.V. All rights reserved.