Mineralogical characterization of a unique material having heavy oxygen isotope anomaly in matrix of the primitive carbonaceous chondrite Acfer 094 Yusuke Seto a, * , Naoya Sakamoto b , Kiyoshi Fujino a , Takashi Kaito c , Tetsuo Oikawa d , Hisayoshi Yurimoto a,b 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 Received 19 September 2007; accepted in revised form 20 March 2008; available online 31 March 2008 Abstract We report the mineral compositions and micro-texture of the isotopically anomalous (d 17,18 O SMOW  +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. Science 317, 231–233]. Synchrotron radiation X-ray diffraction and transmission elec- tron microscopy studies indicate that this material consists of the symplectitically intergrown magnetite (Fe 3 O 4 ) and pentland- ite (Fe 5.7 Ni 3.3 S 8 ) 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 time- scale of oxidation was no longer than 1000 years. Ó 2008 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Oxygen is cosmochemically a unique element. It is the most abundant element in solids formed in the solar system and has three isotopes (16, 17, 18) which exhibit mass- dependent and mass-independent fractionations, providing important constraints on the conditions during formation of solids in the early solar nebula (Clayton, 2006). The iso- tope fractionation scheme can be divided into two pro- cesses; mass-dependent and mass-independent. If the chemical reactions involving oxygen are controlled by mass-dependent kinetics and equilibrium, d 18 O variations are about two times larger than d 17 O variations, where d i O is [( i O/ 16 O) sample /( i O/ 16 O) reference  1]  1000 and the reference usually corresponds to the standard mean ocean water (SMOW). Most terrestrial materials follow this frac- tionation scheme, so that d 18 O and d 17 O constitute a line with slope of 0.5 on a three-isotope oxygen diagram (Fig. 1a), which is called the terrestrial fractionation (TF) line. In contrast, oxygen isotopic compositions of meteor- ites are typically displaced from the TF line (Fig. 1a). Cal- cium–aluminum-rich inclusions (CAIs) and some chondrules from primitive meteorites are highly 16 O-en- riched relative to the bulk meteorites, Earth, Moon, and 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.03.010 * Corresponding author. Fax: +81 11 706 4638. E-mail address: seto@mail.sci.hokudai.ac.jp (Y. Seto). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 2723–2734