Geochemical Journal, Vol. 51 (No. 1), pp. 3-15, 2017
Kenichi Abe,1* Naoya Sakamoto,2 Alexander N. Krot3 and Hisayoshi Yurimoto1,2,4
1Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
2Isotope Imaging Laboratory, Creative Research Institution Sousei, Hokkaido University, Sapporo 001-0021, Japan
3Hawai'i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, Honolulu, Hawai'i 96822, U.S.A.
4ISAS/JAXA, Sagamihara 252-5210, Japan
(Received June 16, 2016; Accepted October 24, 2016)
We describe the petrographic occurrences, abundances, and compositional variations of symplectically intergrown Fe,Ni-sulfides and 17,18O-rich magnetite (Δ17O ~ 90‰), named cosmic symplectites (COS; Sakamoto et al., 2007), from the Acfer 094 (C3.0) ungrouped carbonaceous chondrite. A total of 314 COS studied in two polished sections of this meteorite are uniformly distributed in its matrix with ~600 ppm surface area abundance. No COS have been identified in the Acfer 094 dark inclusions (chondritic lithic clasts) which appear to have experienced extensive aqueous alteration prior to incorporation into the host meteorite. The structure of COS can be arranged in a hierarchy of four categories (from finer to coarser): (1) symplectite structure composed of nanocrystalline magnetite and Fe,Ni-sulfides, (2) submicron-sized wormy structure composed of nanocrystalline symplectites, (3) micrometer-sized irregular rope-like structure composed of wormy structure, and (4) aggregates of the rope-like structure. COS typically associate with fractured Fe,Ni-sulfides and lack Fe,Ni-metal. Most COS studied have smooth surfaces; four grains contain abundant pores. In a single COS, the pore-rich regions are depleted in sulfur and nickel relative to the pore-free regions, indicating that the former are depleted in Fe,Ni-sulfides. Most COS studied contain similar abundances of Fe,Ni-sulfides and magnetite, whereas Ni/(Fe+Ni) atomic ratio in Fe,Ni-sulfides ranges from 0 to 0.4. The lack of Fe,Ni-metal associated with COS supports the formation process proposed by Seto et al. (2008), i.e., oxidation of Fe,Ni-metal and sulfides by 17,18O-rich water vapor in the outer part of the protoplanetary disk. The similar abundance ratio for Fe,Ni-sulfide and magnetite cannot be simply explained by oxidation processes of Fe,Ni-sulfide precursors after sulfurization of Fe,Ni-metal precursors for COS formation as proposed by Seto et al. (2008).
Key words: cosmic symplectite, magnetite, Fe,Ni-sulfides, Acfer 094 carbonaceous chondrite