High precision and spatial resolution sulfur isotope analysis using MILES laser microprobe

Abstract

The MILES ( Micro In situ Laser Extraction System) laser microprobe permits high spatial resolution (< 10 −3mm 3, or < 0.2 μmol S) in situ sampling of geological material for sulfur isotope analysis. Sulfides are combusted in F 2 by absorption of CO 2 laser radiation and converted to sulfur hexafluoride (SF 6). The product SF 6 is purified by cryogenic distillation. In combination with a high-sensitivity dualinlet isotope ratio mass spectrometer, sulfur isotope analyses of powders of pyrite, galena, and sphalerite yield δ 34S cdt values with a high precision, ranging from 0.03 to 0.09%. The sulfur isotope ratios measured are accurate and exhibit no matrix-dependent sulfur isotope effects over the range of 62%. A minimum F 2 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 δ 34S cdt values from in situ analyses are good ( $ ̌ 0.2‰), but isotopically homogeneous working standards or intercomparison materials are not available thus far. Sulfur isotope ratios derived by conventional-SO 2 and laser-SF 6 are well correlated ( r 2 = 0.99999), but a slope different from unity ( m = 1.035) arises, probably due to inadequate corrections to SO 2 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.