A COMPARISON OF SULFUR ISOTOPE RATIO MEASUREMENT USING TWO ION MICROPROBE TECHNIQUES AND APPLICATION TO ANALYSIS OF TROILITE IN ORDINARY CHONDRITES

Abstract

Continual improvements in the precision of isotope ratio measurements made by ion microprobe, and the attempt to resolve smaller isotopic differences using this technique means that it is increasingly important to demonstrate the accuracy of calibration of this method, particularly when it is impossible to mount a standard with the unknown being analyzed. An assessment of the accuracy and precision using ion microprobe methods for the measurement of isotope ratios is made analyzing a pyrite standard mounted in several thin sections and troilite in ordinary chondrite meteorites. We compare two methods, the traditional high mass resolution approach and the more recent extreme energy filtering technique. The results indicate that both techniques offer precise measurements for analyses made within a single thin-section. However, the accuracy of the high mass resolution results are significantly worse than those obtained using extreme energy filtering when a standard cannot be mounted with the unknown. For analysis of a standard pyrite mounted in different thin sections, the measured instrumental mass bias varied by up to 7%%. Analysis of the same samples using extreme energy filtering showed no variation in instrumental mass bias within 0.5%%. Similar results were observed in analysis of troilite. Whereas the extreme energy filtering results are within error of the expected 0%%, values obtained using high mass resolution differ by up to +/-3%%. Our results indicate that it is possible to analyze δ34S values in sulfides using an ion microprobe with precision and accuracy of +/-0.25%% (1σ) in 12 min, with a spatial resolution under 20 μm.