Micro-Scale (1.5 Mu M) Sulphur Isotope Analysis Of Contemporary And Early Archean Pyrite

We present a method for in situ sulphur (S) isotopic analysis of significantly small areas (1.5 mu m in diameter) in pyrite using secondary ion mass spectrometry (NanoSIMS) to interpret microbial sulphur metabolism in the early earth. We evaluated the precision and accuracy of S isotopic ratios obtained by this method using hydrothermal pyrite samples with homogeneous S isotopic ratios. The internal precision of the delta S-34 value was 1.5 parts per thousand at the level of 1 sigma of standard error (named 1SE) for a single spot, while the external reproducibility was estimated to be 1.6 parts per thousand at the level of 1 sigma of standard deviation (named 1SD, n = 25). For each separate sample, the average delta S-34 value was comparable with that measured by a conventional method, and the accuracy was better than 2.3 parts per thousand. Consequently, the in situ method is sufficiently accurate and precise to detect the S isotopic variations of small sample of the pyrite (less than 20 mu m) that occurs ubiquitously in ancient sedimentary rocks. This method was applied to measure the S isotopic distribution of pyrite within black chert fragments in early Archean sandstone. The pyrite had isotopic zoning with a S-34-depleted core and S-34-enriched rim, suggesting isotopic evolution of the source H2S from -15 to -5 parts per thousand. Production of H2S by microbial sulphate reduction (MSR) in a closed system provides a possible explanation for both the S-34-depleted initial H2S and the progressive increase in the delta S-34(H2S) value. Although more extensive data are necessary to strengthen the explanation for the origin of the MSR, the results show that the S isotopic distribution within pyrite crystals may be a key tracer for MSR activity in the early earth. Copyright (C) 2010 John Wiley & Sons, Ltd.