[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-P11] Microscale sulfur isotope imaging analysis with NanoSIMS: A new methodology for arbitrary area selection

*Kohei Sasaki¹, Akizumi Ishida², Kenichirio Sugitani³, Naoto Takahata¹ (1.Atmosphere and Ocean research Institute, University of Tokyo, 2.Department of Earth Science, Tohoku University, 3.Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University)

Keywords:Archean, Pyrite, Sulfur isotope, NanoSIMS

We present a method for in situ sulfur isotope analysis in arbitrary small areas (< 1 µm²) within pyrite using ion imaging mode of nanoscale secondary ion mass spectrometry (NanoSIMS). We evaluated the precision and accuracy of δ34S values obtained by this method using hydrothermally formed pyrite with homogeneous sulfur isotope ratios. The in−situ ion imaging method developed here allowed for the arbitrary selection of any ROI less than 1 µm2 within a raster areas and was accurate and precise enough to detect δ34S variations in small pyrite (<10 µm) common in ancient sedimentary rocks. This technique was applied to measure the δ34S values of sedimentary pyrite in early Archean cherts. The results were consistent with those obtained through conventional spot analysis. The δ34S values of small pyrite showed a fractionation of over 20‰ from those of Archean seawater sulfate. This could be explained by the production of H2S by microbial sulfate reduction or the microbial sulfur disproportionation of elemental sulfur. Although more data are needed to strengthen explanations for the origin of analyzed pyrite, our results indicate that the imaging mode of NanoSIMS is a powerful tool for high spatial resolution isotope analysis, which could provide insights into sulfur metabolizing activity on the early Earth.