Characterization and use of isotopically homogeneous standards for in situ laser microprobe analysis of³⁴S/³²S ratios

Conventional and laser microprobe measurements of S isotope ratios were made on a suite of eight natural sulfide samples to develop a well-characterized, isotopically homogeneous set of standards for in situ laser microprobe analysis. All the samples are from massive sulfide deposits in either high-grade metamorphic terranes or igneous systems. The data indicate that at the conventional sampling scale (> 1 mm), the samples are isotopically homogeneous within the analytical uncertainty of the conventional technique (::1:0.20/00). At the laser microprobe sampling scale (70-100 /-Lm), the samples are slightly heterogeneous, reflecting either actual isotope variation, imprecision associated with the laser microprobe technique, or both. A set of calibration curves was developed to standardize accurately laser microprobe data generated on unknown samples. This represents a fundamental advance in in situ laser microprobe technology and is a rigorous analytical method for accurately determining 034SCDT values from extremely small sample domains within sulfide minerals.