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

The MILES (Micro In situ Laser Extraction System) laser microprobe per mits high spatial resolution (< 10(-3) mm3, or <0.2 mumol S) in situ s ampling of geological material for sulfur isotope analysis. Sulfides a re 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 isoto pe ratio mass spectrometer, sulfur isotope analyses of powders of pyri te, galena, and sphalerite yield delta34S(CDT) values with a high prec ision, ranging from 0.03 to 0.09 parts per thousand. The sulfur isotop e 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 is otopic fractionations between multiple sulfur species apparently cause d by laser isotope separation and/or reaction with oxygen during analy sis. The precision and accuracy of delta34S(CDT) values from in situ a nalyses are good (less-than-or-equal-to 0.2 parts per thousand), but i sotopically homogeneous working standards or intercomparison materials are not available thus far. Sulfur isotope ratios derived by conventi onal-SO2 and laser-SF6 are well correlated (r2 = 0.99999), but a slope different from unity (m = 1.035) arises, probably due to inadequate c orrections to SO2 data for oxygen isobaric interferences. Sulfur isoto pe isopleths in a large, cubic metamorphic pyrite porphyroblast, deter mined from 79 in situ analyses, are discordant to crystallographic zon ation. Concordance between crystallographic and isotopic zonation need s be tested using high precision and spatial resolution analyses such as those described here. Sampling crystallographic zones in minerals c an result in erroneous conclusions if isotopic and crystallographic zo ning are not coincident.