Origins of Chondrules in the Metal-rich Carbonaceous Chondrites

Introduction: The metal-rich CH and CB chondrites are among the most primitive (unmetamorphosed, unaltered), but controversial meteorites: both nebular and asteroidal models have been proposed to explain the origins of their components [1-14]. In addition to porphyritic chondrules and rare refractory inclusions, the CHs contain a high abunbance (~20 vol%) of metal and dominant cryptocrystalline (CC) chondrules of small sizes (20-70 µm). Based on these unusual characteristics, it was proposed that CHs are modified "subchondritic" meteorites containing some chondrules and metal grains produced during a highly energetic asteroidal collision [4]. Based on the condensation origin of zoned metal grains [6,7] and 16 O-enriched isotopic compositions of chondrules and refractory inclusions [15-18], it has been subsequently concluded that refractory inclusions, chondrules, and zoned metal grains in CHs are in fact pristine nebular products. The CBs contain much higher abundance of metal (~70 vol%) and bigger chondrules (0.1-7 mm) than CHs [19]; most chondrules have non-porphyritic [CC and skeletal olivine (SO)] textures. Similarly to CHs, the CB b chondrites (Hammadah al Hamra 237, QUE94411) contain chemically-zoned metal condensates [8,11,18] and rare refrac-tory inclusions [20]. Based on the young chondrule ages (4562.7±0.5 Myr), lack of evidence for the recycling of chon-drules and metal condensates, and lack of interchondrule, fine-grained matrix material, Krot et al. [21] concluded that the CB chondrules and metal formed from a vapor-melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated. The recently discovered metal-rich chondrite Isheyevo contains the metal-rich (>70 vol%) and the metal-poor (<20 vol%) lithologies, which show mineralogical, chemical and isotopic similarities to both CHs and CBs [22]. Since this meteorite can potentially provide a clue for a genetic relationship between the CHs and CBs and for the origin of metal-rich chondrites in general, we initiated detailed mineralogical and isotopic studies of its components. Here, we describe the mineralogy and petrography of chondrules in both lithologies. Results: Chondrules in Isheyevo can be divided into ferro-magnesian and Al-rich (>10 wt% Al 2 O 3). The ferromagnesian chondrules have non-porphyritic [CC, SO, and radial pyroxene (RP)] and porphyritic [olivine (PO), olivine-pyroxene (POP), and pyroxene (PP)] textures (Fig. 1); the Al-rich chondrules are mainly porphyritic. SO chondrules are Mg-rich and consist of forsteritic olivine overgrown for Al-rich low-Ca pyroxene and high-Ca pyroxene, and glassy mesostasis (Fig. 1b). CC chon-drules can be magnesian or ferrous; the former are dominant (Fig. 1a) …