A COMPARISON OF SULFUR ISOTOPE RATIO MEASUREMENT USING 2 ION MICROPROBE TECHNIQUES AND APPLICATION TO ANALYSIS OF TROILITE IN ORDINARY CHONDRITES

Continual improvements in the precision of isotope ratio measurements made by ion microprobe, and the attempt to resolve smaller isotopic di fferences using this technique means that it is increasingly important to demonstrate the accuracy of calibration of this method, particular ly when it is impossible to mount a standard with the unknown being an alyzed. An assessment of the accuracy and precision using ion micropro be methods for the measurement of isotope ratios is made analyzing a p yrite standard mounted in several thin sections and troilite in ordina ry 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 mea surements 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 can not be mounted with the unknown. For analysis of a standard pyrite mou nted in different thin sections, the measured instrumental mass bias v aried by up to 7 parts per thousand. Analysis of the same samples usin g extreme energy filtering showed no variation in instrumental mass bi as within 0.5 parts per thousand. Similar results were observed in ana lysis of troilite. Whereas the extreme energy filtering results are wi thin error of the expected 0 parts per thousand, values obtained using high mass resolution differ by up to ?+/-3 parts per thousand. Our re sults indicate that it is possible to analyze delta(34)S values in sul fides using an ion microprobe with precision and accuracy of +/-0.25 p arts per thousand(1 sigma) in 12 min, with a spatial resolution under 20 mu m. Copyright (C) 1997 Elsevier Science Ltd.