HIGH-PRECISION AND SPATIAL-RESOLUTION SULFUR ISOTOPE ANALYSIS USING MILES LASER MICROPROBE

The MILES (Micro In situ Laser Extraction System) laser microprobe permits high spatial resolution (< 10(-3) mm3, or <0.2 mumol S) in situ sampling of geological material for sulfur isotope analysis. Sulfides are combusted in F2 by absorption of CO2 laser radiation and converted to sulfur hexafluoride (SF6). The product SF6 is purified by cryogenic distillation. In combination with a high-sensitivity dual-inlet isotope ratio mass spectrometer, sulfur isotope analyses of powders of pyrite, galena, and sphalerite yield delta34S(CDT) values with a high precision, ranging from 0.03 to 0.09 parts per thousand. The sulfur isotope ratios measured are accurate and exhibit no matrix-dependent sulfur isotope effects over the range of 62 parts per thousand. A minimum F2 pressure of 20 kPa (for MILES) is required to mediate against small isotopic fractionations between multiple sulfur species apparently caused by laser isotope separation and/or reaction with oxygen during analysis. The precision and accuracy of delta34S(CDT) values from in situ analyses are good (less-than-or-equal-to 0.2 parts per thousand), but isotopically homogeneous working standards or intercomparison materials are not available thus far. Sulfur isotope ratios derived by conventional-SO2 and laser-SF6 are well correlated (r2 = 0.99999), but a slope different from unity (m = 1.035) arises, probably due to inadequate corrections to SO2 data for oxygen isobaric interferences. Sulfur isotope isopleths in a large, cubic metamorphic pyrite porphyroblast, determined from 79 in situ analyses, are discordant to crystallographic zonation. Concordance between crystallographic and isotopic zonation needs be tested using high precision and spatial resolution analyses such as those described here. Sampling crystallographic zones in minerals can result in erroneous conclusions if isotopic and crystallographic zoning are not coincident.