[MTT51-10] Recent progress of in situ sulfur 2- and 4-isotope measurements at Kochi Institute, JAMSTEC
Keywords:Sulfur isotope, SIMS, Pyrite, Seafloor hydrothermal deposits, Bacterial sulfate reduction
The sulfur 4-isotope test analyses of the pyrite standard, UWPy-1, were performed using a 20 kV accelerated Cs+ primary beam of ~5.4 nA and ~15 µm in diameter. Four sulfur isotope ion signals were accelerated at 10 kV and detected with 4 FC detectors, simultaneously. The mass resolving power (M/DM) was set at ~2200 for 32S- and 36S-, and ~5000 for 33S- and 34S-, respectively. The typical count rate of the 32S- signal was 5.8×109 cps. Reproducibility of spot-to-spot analyses was ±0.35‰, ±0.05‰, and ±0.91‰ (2 SD) for δ34S, Δ33S, and Δ36S, respectively. These are enough to investigate the sulfur mass independent fractionation signature of Archean sulfide samples.
The sulfur 2-isotope analyses with a small beam were performed using a 20kV Cs+ primary beam of ~0.1 nA and ~2 µm in diameter. Two sulfur isotope ion signals (32S- and 34S-) were accelerated at 10 kV and detected with 2 FC detectors, simultaneously. The mass resolving power was set at ~2200 for 32S- and ~5000 for 34S-, respectively. The typical count rate of the 32S- signal was ~7×107 cps, and the typical spot-to-spot reproducibility (2 SD) of the δ34S value was 0.38‰. The seafloor hydrothermal deposit samples frequently contain fine-grained framboidal, colloform, and euhedral pyrites, and we recognized that framboidal pyrites have distinctly lower δ34S values (down to -38.9‰) than those of other type pyrites, indicating that framboidal pyrites are formed by the bacterial sulfate reduction and other pyrites are grown by separate processes. The present results indicate that the small beam sulfur isotope analysis can be strong tool for better understanding of the origin of the seafloor hydrothermal deposits. The whole dataset and their interpretations of seafloor hydrothermal deposit samples will be reported in the other presentation of this conference .
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