Chromite Reference Materials and Matrix Effects in SIMS Oxygen Isotope Measurements

Secondary ion mass spectrometry (SIMS) measurements of delta O-18 in nine chromite (Mg, Fe)(Al, Cr, Fe)(2)O-4 samples (Cr# = 0.58-0.79, Mg# = 0.39-0.72) from chromitites and harzburgite have confirmed their intra- and inter-grain isotopic homogeneity, making them suitable reference materials for in situ analysis. The SIMS measurements of the oxygen isotopic composition of these chromites exhibit a range of instrumental mass fractionation (IMF) as a function of the end-member compositions of the chromite, particularly the magnetite (FeOFe2O3) and spinel (MgAl2O4) components. Functional relationships have been developed to correct for IMF over the compositional range X-Mag = 0.01 to 0.05, and X-Spl = 0.14 to 0.22. We conclude that, to be accurate, SIMS measurements of delta O-18 must be combined with electron probe microanalyses of the individual analysed spots.