[MTT51-10] 高知コア研における硫黄２同位体比及び４同位体比局所分析の取り組み
キーワード：硫黄同位体、二次イオン質量分析計、黄鉄鉱、海底熱水鉱床、バクテリア硫酸還元
Secondary Ion Mass Spectrometry with a multiple collector system (MC-SIMS) has capability to perform accurate in situ stable isotope analyses with sub-permil precision and down to a few mm spatial resolution [1]. Sulfur isotope systematics of sediments and sedimentary/igneous rocks provides constraints on the biological activity, the origin of the sulfur reservoirs, the evolution of Earth’s atmosphere, and behaviors of volatile elements in the Earth’s interior. Previously, we reported results of sulfur 2-isotope (34S/32S) analyses of pyrite, basaltic glass, and barite with an ion microprobe, CAMECA IMS 1280-HR at Kochi Institute, JAMSTEC [2]. Recently, we installed the modified detector slits and achieved capability of the sulfur 4-isotope (34S/32S. 33S/32S/ 36S/32S) analysis using four Faraday cup (FC) detectors (c.f., [3]). We also explored sulfur 2-isotope analysis using ~2 µm in diameter primary beam and applied this analytical protocol to the study on the seafloor hydrothermal deposits [4]. In this presentation, we will report the results of test analyses of sulfur 4-isotope analyses and the results of small beam sulfur 2-isotope analyses of fine-grained pyrites in the seafloor hydrothermal deposits.
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 [4].
[1] Kita N. T. et al. (2009) Chem. Geol. 264, 43-57.
[2] Ushikubo T. and Shimizu K. (2017) JpGU abstract #SGC54-04
[3] Ushikubo T. et al. (2014) Chem Geol. 383, 86-99.
[4] Nozaki T. et al. (2020) this conference.
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 [4].
[1] Kita N. T. et al. (2009) Chem. Geol. 264, 43-57.
[2] Ushikubo T. and Shimizu K. (2017) JpGU abstract #SGC54-04
[3] Ushikubo T. et al. (2014) Chem Geol. 383, 86-99.
[4] Nozaki T. et al. (2020) this conference.