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

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