¹Megumi Matsumoto,^2,3,4Akira Tsuchiyama,⁵Akira Miyake,⁶Motoo Ito,²Junya Matsuno,⁷Kentaro Uesugi,⁷Akihisa Takeuchi,⁸Yu Kodama,⁷Masahiro Yasutake,⁹Epifanio Vaccaro
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.02.024]
¹Department of Earth and Planetary Materials Science, Tohoku University, Miyagi 980-8578, Japan
²Research Organization of Science and Technology, Ritsumeikan University, Shiga 525-8577, Japan
³CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), Guangzhou 510640, China
⁴CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
⁵Division of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan
⁶Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kochi 783-0093, Japan
⁷Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan
⁸Marine Works Japan Ltd., Kanagawa 237-0063, Japan
⁹Department of Earth Sciences, The Natural History Museum, London SW7 5BD, U.K
Copyright Elsevier

Cosmic symplectites (COSes), consisting mainly of nanoscaled symplectic intergrowths of magnetite and Fe-Ni sulfides, have extremely heavy oxygen isotopic compositions and are considered tracers of 16O-poor primordial ice in the early solar system. We examined the three-dimensional microstructure and mineralogy of one COS particle, COS#1, in the Acfer 094 carbonaceous chondrite and investigated its origin. Synchrotron-radiation based X-ray computed nanotomography revealed a presence of micro-inclusions inside COS#1. The largest inclusion consists mainly of high-temperature phases of anhydrous sodium sulfate (Na2SO4) and elemental sulfur, which seem to have been formed from a Na2SO4-S eutectic melt. COS#1 showed a trilayered structure surrounding the large inclusion: the innermost coarse-grained layer consisting mainly of 100–200 nm-sized magnetite and Fe-sulfide, the symplectite layer consisting mainly of nanoscaled symplectic intergrowths of magnetite and Fe-Ni sulfides, and the outermost Fe-oxide layer. The symplectite layer comprises the major volume of COS#1 and shows the pseudomorphic structure of precursor Fe-Ni metal grains. The coarse-grained layer seems to have been formed via metal–salt interaction (hot corrosion) at high temperatures, where the precursor Fe-Ni metals contacted with the Na2SO4-S melt. The symplectite formed simultaneously with the coarse-grained layer due to high-speed diffusion of sulfur and oxygen inside the metal grains. The high-temperature metal–salt interactions should have occurred before the incorporation of COS#1 into the meteorite parent body. The precursor of COS#1 should have consisted of Fe-Ni metals and O-Na-S-rich material. The two reductive and oxidative components seem to have formed separately and got together by some mechanical mixing processes in nebula. The COS#1 precursor was heated in a short period and the O-Na-S-rich material melted. The melt induced the hot corrosion of the Fe-Ni metals and was subsequently cooled and solidified. Subsequently, it was incorporated into the meteorite parent body as COS#1. In the parent body, aqueous alteration occurred and formed the outermost Fe-oxide layer on the COS#1 surface.