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Micro‐scale (1.5 µm) sulphur isotope analysis of contemporary and early Archean pyrite
Author(s) -
Nishizawa Manabu,
Maruyama Shigenori,
Urabe Tetsuro,
Takahata Naoto,
Sano Yuji
Publication year - 2010
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.4517
Subject(s) - pyrite , archean , chemistry , sulfur , sedimentary rock , mineralogy , isotope analysis , isotope , analytical chemistry (journal) , hydrothermal circulation , stable isotope ratio , geology , geochemistry , environmental chemistry , paleontology , oceanography , physics , organic chemistry , quantum mechanics
We present a method for in situ sulphur (S) isotopic analysis of significantly small areas (1.5 µm 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 δ 34 S 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, n = 25). For each separate sample, the average δ 34 S 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 µm) 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 34 S‐depleted core and 34 S‐enriched rim, suggesting isotopic evolution of the source H 2 S from −15 to −5‰. Production of H 2 S by microbial sulphate reduction (MSR) in a closed system provides a possible explanation for both the 34 S‐depleted initial H 2 S and the progressive increase in the δ 34 S 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 © 2010 John Wiley & Sons, Ltd.