Biotite Reference Materials for Secondary Ion Mass Spectrometry 18O/16O Measurements

Five new biotite reference materials were calibrated at the SwissSIMS laboratory (University of Lausanne) for oxygen isotope determination by secondary ion mass spectrometry (SIMS) and are available to the scientific community. The oxygen isotope composition of the biotites, UNIL_B1 to B5, was determined by laser-heating fluorination to be 11.4 +/- 0.11 parts per thousand, 8.6 +/- 0.15 parts per thousand, 6.1 +/- 0.04 parts per thousand, 7.1 +/- 0.05 parts per thousand and 7.6 +/- 0.04 parts per thousand, respectively. SIMS analyses on spots smaller than 20m gave a measurement repeatability of 0.3 parts per thousand (2 standard deviation, 2s). The matrix effect due to solid solution in natural biotite could be expressed as a linear function of X-Mg and X-F for biotite. No effect was found for different crystallographic orientations. SIMS analysis allows the oxygen isotope composition of biotite to be measured with a measurement uncertainty of 0.3-0.4 parts per thousand (2s) for biotites with similar major element compositions. A measurement uncertainty of 0.5 parts per thousand (2s) is realistic when F poor biotites (lower than 0.2% m/m oxides) within the compositional range of X-Mg of 0.3-0.9 were compared from different sessions. The linear correlation with F content offers a reasonable working curve for F-rich biotites, but additional reference materials are needed to confirm the model.