Evaluating baddeleyite oxygen isotope analysis by secondary ion mass spectrometry (SIMS)

Two baddeleyite megacrysts were evaluated as potential reference materials (RMs) for SIMS oxygen isotope analysis, and utilized to understand and calibrate instrumental mass fractionation (IMF). A baddeleyite crystal (S0045) from the Phalaborwa carbonatite, South Africa has a mean delta O-18(VSMOW) = +4.6 +/- 0.3 parts per thousand (range 0.75 parts per thousand) measured using laser fluorination gas source mass spectrometry (LF-GMS) and one (S0069) from the Mogok metamorphic belt, Myanmar has delta O-18(VSMOW) = +22.2 +/- 0.4% (range 0.89%). SIMS standardization utilizing these inherently heterogeneous RMs is possible by analyzing a number of crystal fragments and utilizing one of them lying at the median of the range. Metamictization, lattice orientation, and chemical composition do not appear to be significant (< 0.5 parts per thousand) variables in matrix matching of RMs and unknowns. Propagation of errors while utilizing the imperfect RMs results in 10 mu m diameter spot uncertainties of about +/- 0.3 parts per thousand (2 sigma). SIMS oxygen isotope analysis of co-crystalline zircon and baddeleyite from the 2.2 Ga Duck Lake sill (DLS) in the Northwest Territories, Canada, yield predominant delta O-18(VSMOW) modes of +6.0 parts per thousand and +3.2 parts per thousand, respectively. This difference is consistent with preserving high-temperature isotopic equilibrium between zircon and baddeleyite. DLS baddeleyite delta O-18 data as a whole are negatively skewed (to 0.0 parts per thousand), and interpreted to reflect low temperature, open-system behaviour. Zircon delta O-18 are less affected, but also show hints of the same influences of secondary alteration and oxygen isotope exchange.