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.

¹Cheikh Ahmadou Bamba Niang et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13788]
¹Département de Géologie, Université Cheikh Anta Diop, Dakar, Dakar, Senegal
²Institut Fondamental d’Afrique Noire Cheikh Anta Diop, Dakar, Senegal
³Aix-Marseille Univ, CNRS, IRD, INRAE, CEREGE, Aix-en-Provence, France
Published by arrangement with John Wiley & Sons

The 10.5-km-diameter, 1 Ma Bosumtwi impact structure in Ghana is one of the youngest, large impact structures known on Earth. The preservation of the morphology of its ejecta deposits, with an annular moat and outer ridge resembling those of rampart impact craters on Mars, makes Bosumtwi a remarkable impact structure on the African continent. An airborne radiometric survey of the southwestern part of Ghana reveals enigmatic circular feature enriched in potassium, coinciding with the crater rim and an outer ejecta ridge at Bosumtwi. The goal of this study is to investigate possible origins of these features, by impact processes (shock metamorphic effects, impact-induced hydrothermal systems) or postimpact surficial processes (erosion, weathering). The origin of these features is discussed here based on field observations, ground-based radiometric measurements, and first cosmogenic nuclide analyses (10Be). The data indicate that the rim and outer ridge were eroded more rapidly than the rest of the impact structure. Accordingly, the downward advance of the weathering fronts in the annular moat, after ejecta emplacement, are responsible for leaching of K from the lateritic residual observed at the surface. The Bosumtwi impact structure is, therefore, a valuable natural laboratory to investigate the factors controlling erosion and weathering processes in the Ashanti belt since impact 1 Ma ago. Simulations of vertical profiles of 10Be concentration further constrain local variations of the erosion rate. In light of this study, circular K anomalies in radiometric surveys might be indicative of potential impact structures in tropical regions.