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Mineralogical characterization of a unique material having heavy oxygen isotope anomaly in matrix of the primitive carbonaceous chondrite Acfer 094 |
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| Authors: | Yusuke Seto Naoya Sakamoto Takashi Kaito Hisayoshi Yurimoto |
| Institution: | a Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan b Isotope Imaging Laboratory, Creative Research Initiative “Sousei”, Hokkaido University, Sapporo 001-0021, Japan c SII NanoTechnology Inc., Core Technology Development Department, 36-1 Takenoshita, Oyama-cho, Sunto-gun, Shizuoka 410-1393, Japan d JEOL Ltd., Electron Optics Division, 1-2 Musashino 3 Akishima, Tokyo 196-8558, Japan |
| Abstract: | We report the mineral compositions and micro-texture of the isotopically anomalous (δ17,18OSMOW ∼ +180‰) Fe-S-Ni-O material recently discovered in matrix of the primitive carbonaceous chondrite Acfer 094 Sakamoto N., Seto Y., Itoh S., Kuramoto K., Fujino K., Nagashima K., Krot A. N. and Yurimoto H. (2007) Oxygen isotope evidence for remnants of the early solar system primordial water. Science317, 231-233]. Synchrotron radiation X-ray diffraction and transmission electron microscopy studies indicate that this material consists of the symplectitically intergrown magnetite (Fe3O4) and pentlandite (Fe5.7Ni3.3S8) with magnetite/pentlandite volume ratio of ∼2.3. Magnetite forms column-shaped grains (10-30 nm in diameter and 100-200 nm in length); pentlandite occurs as worm-shaped grains or aggregates of grains 100-300 nm in size between magnetite crystals. Although both the X-ray diffraction and electron energy loss spectra support identification of iron oxide as magnetite, the electron diffraction patterns show that magnetite has a weak 3-fold superstructure, possibly due to ordering of vacancies. We infer that the isotopically anomalous symplectite formed by sulfurization and oxidization of metal grains either in the solar nebula or on an icy planetesimal. The intersite cation distribution of pentlandite suggests that timescale of oxidation was no longer than 1000 years. |
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