Accurate Measurements of H2O, F and Cl Contents in Biotite Using Secondary Ion Mass Spectrometry

摘要

Five biotite reference materials (RMs) currently used for oxygen isotope measurements at the SwissSIMS laboratory (University of Lausanne) were investigated for their H2O, F and Cl contents using secondary ion mass spectrometry in negative mode. The homogeneity of these RMs, as well as that of the international reference material NBS30, was tested by exploiting the high sensitivity of the SIMS technique. No crystal orientation dependency was discovered at the level of the homogeneity of the RMs. Normalisation schemes with Si-28(-), Si-30(-) O-16(-) and O-18(-) were explored. A matrix effect is documented here for H2O measurements, which depended on SiO2 and X-Mg compositions of the different biotite RMs. A successful multivariate fit was obtained that reproduces the H2O content of the six biotite RMs, this within error of 0.15% m/m and with a reduced chi(2) (or MSWD) of 0.672. Fluorine and Cl contents were calibrated using a linear fit for F content between 0% and 4% m/m and Cl content below 0.3% m/m. Accuracy for H2O measurements is expected to be 3-4% (1SE). Accuracy for F content was about 3% (1s) and typically about 15% (1s) for Cl content.