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An Extreme 16O-rich Chondrule from Acfer 214 CH Chondrite

Introduction: Oxygen isotope systematics between 16 O-rich and –poor reservoirs is well known in the solar system [1]. Since O is account for over 50 atomic % of terrestrial planets and also dominant in the solar nebular gas next to H and He [2], range and distribution of the systematics is an important tracer recording origin and global evolution of early solar system. The 16 O-rich endmenber, about 5% enrichment relative to the terrestrial mean value, has been observed only in refracto-ry inclusions that are <5 volume % constituents of chondrites [1,3]. Other 95 volume % of chondrite is relatively depleted in 16 O close to the terrestrial value. Therefore, it has been believed that 16 O-rich reservoirs existed at refractory inclusion forming event corresponding to the oldest dating in the solar system [4]. Here we report a chondrule consisting of about 8% enrichment of 16 O relative to the terrestrial value. This value corresponds to the lightest atomic weight of O in the solar materials ever found. The lightest oxygen found in abundant constituents of chondrites strongly suggests that 16 O-rich reservoir contained gaseous species with isotopically almost pure in 16 O or were universally distributed as dusts in the early solar system. In either case, O isotopic heterogeneities in the early solar system were at least two times greater than conventional thought. Analytical Methods: The chondrule was found in a polished thin section of Acfer 214 CH chondrite. Back scattered electron images collected using a JEOL JSM-5310LV scanning electron microscope equipped with an Oxford LINK-ISIS energy dispersive detector. Quantitative elemental analyses of minerals were collected using JEOL JXA-8800 electron microprobe. In situ oxygen isotope analyses were collected with a TiTech Cameca 1270 SIMS instrument under analytical conditions previously described [5]. Mineralogy and Textures: An extreme 16 O-rich chondrule of δ 17,18 OSMOW ≅-75‰, which we have designated a006, was found in a thin section of the Acfer 214 CH chondrite (Fig. 1). The a006 chondrule has spherical core-shell structure with ~130 µm across. The core is ~100 µm across and shows multiple extinction under polarized optical microscope, indicating a nonporphyritic cryptocrystal-line chondrule. The core is composed of submicrome-ter-size crystalline minerals (probably olivine and en-statite) and Ca-Al-containing glass enclosing small vesicles. Chromian spinel of <0.3 µm across appears on the surface as a minor phase. Unusual point from typical