Three-dimensional microstructure and mineralogy of a cosmic symplectite in the Acfer 094 carbonaceous chondrite: Implication for its origin

Megumi Matsumoto, Akira Tsuchiyama, Akira Miyake, Motoo Ito, Junya Matsuno, Kentaro Uesugi, Akihisa Takeuchi, Yu Kodama, Masahiro Yasutake, Epifanio Vaccaro

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)220-241
Number of pages22
JournalGeochimica et Cosmochimica Acta
Volume323
DOIs
Publication statusPublished - 2022 Apr 15

Keywords

  • Carbonaceous chondrite
  • Cosmic symplectite
  • Hot corrosion
  • NanoSIMS
  • TEM
  • X-ray nanotomography

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