Chromite Reference Materials and Matrix Effects in SIMS Oxygen Isotope Measurements

Secondary ion mass spectrometry (SIMS) measurements of δ 18 O 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 (FeOFe 2 O 3 ) and spinel (MgAl 2 O 4 ) 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 δ 18 O must be combined with electron probe microanalyses of the individual analysed spots.