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

Abstract

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 28Si−, 30Si− 16O− and 18O− were explored. A matrix effect is documented here for H2O measurements, which depended on SiO2 and XMg 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 χ2 (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.

H2O Content Measurement in Phengite by Secondary Ion Mass Spectrometry: A New Set of Reference Materials

“…Hence, we explored the possible effect of crystal orientation of the measured 16 O 1 H − / 16 O − ratio. No effect was found based on preliminary tests on randomly oriented white micas of similar FeO contents measured in two different thin sections, in agreement with the observations of Siron et al (). The resulting calculated H 2 O fraction in the hydroxyl site was homogeneous within uncertainty in each sample; therefore, we concluded that the crystal orientation does not have a strong effect on the resulting 16 O 1 H − / 16 O − ratio.…”

“…A matrix effect has also been reported for major and minor element measurements (Steele et al , Bulle and Layne , Siron et al ). Measured ion intensities compared with the reference values, referred to as relative sensitivity factors (RSFs), exhibit trends that can vary between different mineral groups (Ottolini et al ), or can be correlated to changes in major element composition for one particular mineral group (Ottolini and Hawthorne , Siron et al ). Siron et al () showed a correlation between the measured 16 O 1 H − / 28 Si − RSF in biotite RMs and their X Mg compositions.…”

“…Normalisation of the 16 O 1 H − signal to a measurable ion is required in order to account for the possible changes in secondary ion yield due to sputtering and extraction during the progress of the analysis. For H 2 O content measurement, 28 Si − and 16 O − are the most commonly used for normalisation (Mosenfelder et al , , Hu et al , Siron et al ). Both normalisations were tested in this study.…”

“…Measured ion intensities compared with the reference values, referred to as relative sensitivity factors (RSFs), exhibit trends that can vary between different mineral groups (Ottolini et al ), or can be correlated to changes in major element composition for one particular mineral group (Ottolini and Hawthorne , Siron et al ). Siron et al () showed a correlation between the measured 16 O 1 H − / 28 Si − RSF in biotite RMs and their X Mg compositions. Koga et al () reported differences between calibrations for H 2 O content determination of Fe‐rich and Fe‐poor amphiboles and micas.…”

“…Analyses consisted of 240 s of pre-sputtering, followed by 1 min of automatic centring of transfer deflectors and twenty-four cycles with counting times of 8 s. Each session started with four to six analyses of UNIL_B2 in order to check the measurement repeatability and was followed by several sequences of two or three white mica RM analyses and one UNIL_B2 analysis. As shown by Siron et al (2018b), the quality of the vacuum inside the analysis chamber influences the measured 16 O 1 H -/ 28 Siratio in biotite. Hence, a chemically homogeneous orthopyroxene (OPX) was used as an 'internal' reference material for background intensity correction in each session.…”