Secondary ion mass spectrometry for the determination of δ37Cl : Part II. Intercalibration of SIMS and IRMS for aluminosilicate glasses

Abstract

Secondary ion mass spectrometry (SIMS) offers unparalleled capability for the in situ determination of chlorine stable isotope ratios ( δ 37Cl) in small objects, such as igneous melt inclusions. However, SIMS analyses must be carefully calibrated for instrumental mass fractionation (IMF) of 37Cl/ 35Cl, which is highly dependent on the major-element composition of the sample. For the purpose of IMF calibration, eight synthetic aluminosilicate glasses, ranging from mafic to felsic composition, were fused from natural and synthetic rock powders with an internally heated pressure vessel (IHPV). This collection of standard materials was supplemented with one natural rhyolite glass. We used alkali fusion, followed by gaseous dual inlet isotope ratio mass spectrometry (AF-IRMS) as the reference technique for δ 37Cl determination of these glasses. Major-element and volatile contents of the glasses were measured by electron microprobe (EMPA) and infrared spectroscopy (FTIR). The absolute difference in IMF between basalt and rhyolite composition glasses can exceed 10‰. However, a strong linear correlation ( R 2=0.99) exists between IMF and expressions of the form (aSiO 2−bAl 2O 3−cCaO+dFeO), where oxides, based on EMPA analysis, are expressed in weight percent. Thus, calibrations based on a small set of glass standards allow SIMS determinations of δ 37Cl over a large range of silicate glass compositions, with an overall reproducibility of single spots better than ±1.0‰ (2 σ). Moreover, δ 37Cl data can be obtained even if the sample does not exactly match the composition of an individual reference glass.