MINERALOGY, PETROGRAPHY AND OXYGEN-ISOTOPIC COMPOSITIONS OF Ca, Al-RICH INCLUSIONS AND AMOEBOID OLIVINE AGGREGATES IN THE CR CARBONACEOUS CHONDRITES

Introduction: Most CAIs from CV and CO chondrites are characterized by large variations in O-isotopic compositions of primary minerals, with spinel, hibonite, and pyroxene being ~40‰ more O-rich than melilite and anorthite [1]. The reasons for this extreme isotope heterogeneity are not well understood, but must be related to multiple thermal events experienced by the CAIs either in different nebular regions (e.g., characterized by isotopically distinct gaseous reservoirs) or, possibly, in an asteroidal environment [2-8]. In order to better understand the possible origins of the O anomaly and its heterogeneous distribution in chondritic materials, we have been studying CAIs in various types of primitive meteorites, including CH and the ungrouped metal-rich chondrites Hammadah al Hamra 237 (HH 237) and QUE 94411. Contrary to the patterns in CV and CO CAIs, those in HH 237 and QUE 94411 have uniformly Opoor compositions [9], whereas CH chondrites contain populations of homogeneous CAIs that are either Opoor or O-rich [10]. In this work, we have systematically surveyed the mineralogy, petrography and Oisotopic compositions of 28 CAIs and amoeboid olivine aggregates (AOAs) from 8 CR carbonaceous chondrites. We find a striking correlation of CAI texture with isotopic composition, and confirm the close genetic relationship of CAIs and AOAs. Samples and Analytical Techniques: Thirty polished thin sections of 16 CR chondrites (Acfer 087, 139, El Djouf 001, EET87730, 87747, 87770, 92041, 92042, 92147, GRA95229, MAC87320, MET00426, PCA91082, QUE99177, Renazzo, Temple Bay) were studied by optical microscopy, BSE imaging using Zeiss DSM-962 and JEOL JSM-5900LV SEMs, X-ray elemental mapping, and EPMA using a Cameca SX-50 microprobe. Oxygen-isotopic compositions in 28 CAIs from 8 CR chondrites were measured in situ with the UCLA Cameca IMS 1270 ion microprobe in multicollection mode. The O and O ion beams were measured at low Mass Resolving Power (MRP ~ 2000) using a Faraday cup (FC) and an electron multiplier (EM), respectively, and O was analyzed with the axial EM at MRP > 6000. Instrumental mass fractionation was corrected by analyzing olivine and spinel standards; error estimates were obtained from the dispersion of the standard analyses (error ellipses are shown in the figures). Mineralogy and Petrography: Refractory inclusions in CR chondrites are rare (< 1 vol%), small (<500 μm) and irregularly-shaped; most CAIs are fragmented. The CR CAIs can be divided into different types based on dominant mineralogy: grossite±hibonite-rich, melilite-rich (Type A), pyroxene-anorthite-rich, spinelmelilite-pyroxene aggregates, and AOAs. Type A CAIs and AOAs dominate and other CAI types are rare. Secondary minerals (phyllosilicates, carbonates, nepheline?, sodalite?) were found only in 3 CAIs, all Type A. A grossite-hibonite-rich CAI consists of a hibonite core surrounded by a grossite mantle and a melilitepyroxene rim; grossite contains abundant inclusions of perovskite. Another grossite-rich CAI has a massive grossite core surrounded by spinel and melilite layers. Melilite-rich (Type A) CAIs consist of melilite, spinel ±anorthite; pyroxene and perovskite are minor. The spinel grains in most samples have anhedral morphology, although one CAI fragment contains euhedral spinel grains, probably indicating crystallization from melt. Anorthite is a secondary mineral that replaces melilite. The CAIs are typically surrounded by monomineralic pyroxene rims; pyroxene-forsterite rims are less common. Pyroxene-anorthite-rich CAIs consist of irregularlyshaped anhedral pyroxene and anorthite grains; spinel and melilite are minor. Anorthite replaces melilite. Four fragmented pyroxene-anorthite-rich CAIs have igneous textures and can be classified as Type B CAIs. They consist of pyroxene, melilite and lath-shaped anorthite with all minerals poikilitically enclosing euhedral spinel grains. Spinel-melilite-pyroxene aggregates consist of multiple spinel-perovskite±hibonite cores surrounded by the melilite and pyroxene layers. AOAs consist of anhedral forsterite grains, FeNimetal nodules, and small, incorporated refractory inclusions composed of spinel, pyroxene, and anorthite. Mineral chemistry: Hibonite in CR CAIs is MgOpoor (0.2-2.6 wt%) and TiO2-rich (2.3-4.4 wt%). Perovskite, grossite, and anorthite are compositionally pure. Spinel is FeO-poor [Fe/(Fe+Mg) < 0.01] and Cr2O3-poor (<0.5 wt%). Spinels in Type B CAIs contain up to 1 wt% Cr2O3. Melilite is generally Na-free and akermanitepoor (Ak5-40). Melilite grains in individual CAIs typically have narrow compositional ranges (<15 mol% Ak). Melilite in a Type B CAI is the only exception; it shows large compositional variations in Ak content (17-77 mol%) and contains detectable concentrations of Na2O (0.25 wt%). Pyroxenes show large compositional variations in Al2O3 (0.7-18 wt%) and TiO2 (0.1-9.5 wt%); these elements are positively correlated. Most pyroxenes are MnO-free and Cr2O3-poor (<0.2 wt%); pyroxenes in the Type B CAIs contain up to 1 wt% Cr2O3. Olivine grains are MgO-rich (Fa<3.5) and contain low concentrations of MnO (<0.4 wt%) and Cr2O3 (<0.4 wt%).