Evaluation of the effects of composition on instrumental mass fractionation during SIMS oxygen isotope analyses of glasses

Abstract

Significant instrumental mass fractionation (IMF) occurs during measurements of oxygen isotope ratios in magmatic glasses by SIMS. In order to characterise and correct for this fractionation, we measured oxygen isotope ratios in a range of international and internal glass standards ranging in composition from basalt (47wt.% SiO2) to rhyolite (72wt.% SiO2) and with known major element compositions. Oxygen isotope ratios were determined by laser fluorination at SUERC, East Kilbride, or taken from previously published values. A total of 1105 δ18O measurements were made over nine sessions on a Cameca IMS-1270 ion microprobe at the University of Edinburgh. SIMS measurements on glass standards had external precision better than ±0.36‰ (1σ), and the reference material analysed alongside the unknown samples, USGS synthetic glass GSA-1G, had an average external precision of ±0.14‰. The selected standards are thus sufficiently homogeneous in δ18O to be suitable calibration standards. In terms of δ18O, the SIMS measurements show that, within a single session, IMF may vary by up to 4.7‰ from one glass standard to another. IMF is strongly correlated with SiO2 and CaO. A least squares regression calculation was used to explore potential univariate and multivariate correction schemes. For each correction scheme, the correction coefficients determined for each session were then used to calculate the IMF and correct the measured isotopic ratio of each glass standard. A univariate correction scheme using only SiO2 to correct for IMF reproduced 75% of the glass standards to within ±0.2‰ of their true δ18O, and 95% to within ±0.4‰. Bivariate correction schemes using SiO2–CaO and FeO–CaO produced similar results, but did not significantly improve on the SiO2 correction. The correction schemes were applied to δ18O measurements made on melt inclusions and glasses from the Askja volcanic system, North Iceland. The uni- and bivariate correction schemes tested produced δ18O values within the published range for Icelandic basalts. We recommend a simple correction scheme based on the SiO2 content of appropriate standards, which should span a suitable compositional range from basalt to rhyolite.