ANALYSIS OF SULFIDES IN WHOLE ROCK MATRICES BY ELEMENTAL ANALYZER-CONTINUOUS FLOW ISOTOPE RATIO MASS SPECTROMETRY

Abstract

Potential matrix effects in the sulfur isotopic analysis of whole rocks by elemental analyzer-continuous-flow isotope ratio mass spectrometry (EA-CFIRMS) have been investigated by measuring mixtures of a well-calibrated laboratory sulfide reference mineral and common rock-forming minerals. Mixtures of sulfide and quartz, orthoclase, plagioclase, kaolinite, olivine, magnetite, ilmenite, garnet, halite, fluorite, calcite, and graphite show consistent S yields and δ34S values generally within +/-0.2%% of the measured pure sulfide value. However, erratic results are obtained for mixtures of reference sulfide and pyroxene, amphibole, or biotite. These minerals may suppress sample peaks, cause extreme peak tailing, and prohibit isotope ratio measurements. Pure sulfide minerals introduced after the mixtures also showed suppressed sample peaks, suggesting SO2 adsorption on mineral and residual ash surfaces. Removal of the residual ash resulted in normal measurements for reference sulfide minerals. Whole rocks with S concentrations in excess of ~100 ppm can be successfully analyzed using EA-CFIRMS provided that measurements are carefully monitored for evidence of SO2 retention during and after combustion.