Evaluation of the effects of composition on instrumental mass fractionation during SIMS oxygen isotope analyses of glasses M.E. Hartley a, ⁎, T. Thordarson a , C. Taylor b , J.G. Fitton a , EIMF c a School of GeoSciences, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh, EH9 3JW, UK b Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride G75 0QF, UK c Edinburgh Ion Microprobe Facility, School of GeoSciences, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh, EH9 3JW, UK abstract article info Article history: Received 26 April 2012 Received in revised form 10 October 2012 Accepted 11 October 2012 Available online 23 October 2012 Editor: L. Reisberg Keywords: SIMS Oxygen isotopes Matrix effects Glass standards Melt inclusions Significant instrumental mass fractionation (IMF) occurs during measurements of oxygen isotope ratios in mag- matic 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 (47 wt.% SiO 2 ) to rhyolite (72 wt.% SiO 2 ) and with known major element compositions. Oxygen isotope ratios were deter- mined by laser fluorination at SUERC, East Kilbride, or taken from previously published values. A total of 1105 δ 18 O 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 δ 18 O to be suitable calibration standards. In terms of δ 18 O, 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 SiO 2 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 SiO 2 to correct for IMF reproduced 75% of the glass standards to within ±0.2‰ of their true δ 18 O, and 95% to within ±0.4‰. Bivariate correction schemes using SiO 2 –CaO and FeO–CaO produced similar results, but did not significantly improve on the SiO 2 correction. The correction schemes were applied to δ 18 O measurements made on melt inclusions and glasses from the Askja volcanic system, North Iceland. The uni- and bivariate correction schemes tested produced δ 18 O values within the published range for Icelandic basalts. We recom- mend a simple correction scheme based on the SiO 2 content of appropriate standards, which should span a suit- able compositional range from basalt to rhyolite. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction The development of in situ analytical techniques with micrometre spacing has significantly contributed to the understanding of the dynamics of geological systems. Secondary Ion Mass Spectrometry (SIMS) allows the analysis of oxygen isotopes in geological materials with spatial resolution down to 1 μm (e.g. Page et al., 2007) and preci- sion down to ± 0.3‰ (2σ; Valley and Kita, 2009). However, instrumen- tal mass fractionation (IMF) occurs during such measurements. Part of this fractionation is dependent on mineral type, composition and crystallographic orientation (Lyon et al., 1998; Huberty et al., 2010), and is commonly referred to as a ‘matrix effect’ (e.g. Eiler et al., 1997). An understanding of the matrix effect is required to improve the accuracy of SIMS measurements. This is particularly important for min- erals with multiple solid solutions (e.g. Vielzeuf et al., 2005). The complexities of cation site occupancies in mineral solid solutions and crystallographic orientation are removed when considering mag- matic glasses, since a true quenched glass with no microlite forma- tion upon cooling is an amorphous material. The compositional end- members of magmatic glasses may be considered to be silica-poor and magnesium-rich (e.g. komatiite, picrite) and silica-rich and magnesium- poor (rhyolite), but within this general trend a wide range of composi- tions is possible. Thus it is not advantageous to describe magmatic glasses in terms of a solid solution, but rather as a compositional continuum. Therefore, matrix effects are strictly compositional in nature, and crystal- lographic considerations such as cation site occupancy are not important. SIMS analysis of magmatic glasses is complex, since the potential ef- fect of compositional variability on mass fractionation of oxygen isotope ratios cannot be corrected using a single standard, unless that standard is compositionally identical, within error, to the sample of interest. However, corrections based on simple linear interpolations between Chemical Geology 334 (2012) 312–323 ⁎ Corresponding author at: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK. E-mail address: meh43@cam.ac.uk (M.E. Hartley). 0009-2541/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2012.10.027 Contents lists available at SciVerse ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo