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

Manabu Nishizawa, Shigenori Maruyama, Tetsuro Urabe, Naoto Takahata, Yuji Sano

    Research output: Contribution to journalArticle

    14 Citations (Scopus)

    Abstract

    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 34SH2S 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.

    Original languageEnglish
    Pages (from-to)1397-1404
    Number of pages8
    JournalRapid Communications in Mass Spectrometry
    Volume24
    Issue number10
    DOIs
    Publication statusPublished - May 30 2010

    ASJC Scopus subject areas

    • Analytical Chemistry
    • Spectroscopy
    • Organic Chemistry

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