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Refractory Inclusions and Aluminum-rich Chondrules in the CB/CH-like Carbonaceous Chondrite Isheyevo

Introduction: The recently discovered metal-rich chondrite Isheyevo [1] consists of at least two lithologies composed of the mineralogically similar components (Fe,Ni-metal, chondrules, refractory inclusions, and heavily-hydrated matrix lumps); the lithologies, however , differ in size, relative proportions, and textures of chondrules [2]. The coarser-grained, metal-poor (<20 vol%) lithologies (CH-like) contain higher abundance of porphyritic chondrules than the finer-grained, metal-rich (~70 vol%) lithology (CB b-like), which is dominated by cryptocrystalline and skeletal olivine chondrules [2]. Here, we describe the mineralogy and petrography of the refractory inclusions and Al-rich (>10 wt% bulk Al 2 O 3) chondrules in both lithologies and compare them with those in other chondrite groups. Results: Based on the major mineralogy, 37 refrac-tory inclusions found in about 5 cm 2 polished sections of Isheyevo can be divided into grossite-rich (35%), meli-lite-rich (24%), hibonite-melilite ±pyroxene ±spinel (16%), spinel-rich (14%), and pyroxene-hibonite (8%) Ca,Al-rich inclusions (CAIs), rare isolated coarse spinel grains, and an amoeboid olivine aggregate (AOA) (3%); small perovskite grains occur in most CAI types (Figs. 1a-d). The metal-rich lithology contains higher abundance of CAIs than the metal-poor lithology (86% vs. 14%); however, no systematic differences in the CAI types were found between the lithologies. Most CAIs are compact, rounded or ellipsoidal objects, 40 to 180 μm in apparent diameter (average = 75 μm), which appear to have igneous textures. They are typically surrounded by a thin single-or double-layered rim composed of melilite and/or Al-diopside; occasionally, a discontinuous layer of spinel is also observed. An AOA consists of forsterite, Al-diopside, and anorthite (Fig. 1d). The Al-rich chondrules are divided into Al-diopside-rich and plagioclase-rich (Figs. 1e,f). The Al-diopside-rich chondrules consist of euhedral forsterite and spinel grains surrounded by Al-diopside. The isolated coarse spinel grains could be the fragments of such chondrules. The plagioclase-rich chondrules are composed of low-Ca pyroxene, high-Ca pyroxene, forsterite, anorthitic pla-gioclase, spinel, Fe,Ni-metal, and glassy mesostasis; they are mineralogically similar to the Al-rich chondrules in other carbonaceous chondrites [3,4]. Neither chondrules nor refractory inclusions show evidence for Fe-alkali metasomatic or aqueous alteration. Discussion: The Isheyevo CAIs are dominated by very refractory minerals, such as hibonite, grossite, perovskite, and gehlenitic melilite; AOAs are very rare. Some of the CAIs may have experienced high-temperature alteration reactions, including replacement of grossite by melilite, and of melilite by diopside and anorthite; secondary anorthite, however, is very minor, suggesting a short residence time of the CAIs in the