Micro-scale (1.5μm) sulphur isotope analysis of contemporary and early archean pyrite

We present a method for in situ sulphur (S) isotopic analysis of significantly small areas (1.5μm 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 δ³⁴S value was 1.5% 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% at the level of 1 sigma of standard deviation (named 1SD, n1/425). For each separate sample, the average δ ³⁴S value was comparable with that measured by a conventional method, and the accuracy was better than 2.3%. 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 mm) 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 ³⁴S-depleted core and ³⁴S-enriched rim, suggesting isotopic evolution of the source H₂S from 15 to 5%. Production of H₂S by microbial sulphate reduction (MSR) in a closed system provides a possible explanation for both the ³⁴S-depleted initial H₂S and the progressive increase in the δ ³⁴S_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.