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 δ 18 O VSMOW  = +4.6 ± 0.3‰ (range 0.75‰) measured using laser fluorination gas source mass spectrometry (LF-GMS) and one (S0069) from the Mogok metamorphic belt, Myanmar has δ 18 O 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‰) variables in matrix matching of RMs and unknowns. Propagation of errors while utilizing the imperfect RMs results in 10 μm diameter spot uncertainties of about ±0.3‰ (2σ). 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 δ 18 O VSMOW modes of +6.0‰ and +3.2‰, respectively. This difference is consistent with preserving high-temperature isotopic equilibrium between zircon and baddeleyite. DLS baddeleyite δ 18 O data as a whole are negatively skewed (to 0.0‰), and interpreted to reflect low temperature, open-system behaviour. Zircon δ 18 O are less affected, but also show hints of the same influences of secondary alteration and oxygen isotope exchange. [ABSTRACT FROM AUTHOR]