Amoeboid Olivine Aggregates in CR Chondrites

Introduction: Amoeboid Olivine Aggregates (AOAs) are irregular-shaped, fine-grained aggregates of olivine and refractory minerals and are important components of C chondrites. The AOAs in CV chondrites have been interpreted to be primary nebular condensates [1] and therefore, can provide constraints on models for nebular condensation. AOAs are intermediate in composition, and may be a link, between the older, refractory calcium-aluminum-rich inclusions (CAIs) and the younger, moderately volatile ferromagnesian chondrules [2, 3], and can provide information on the astrophysical settings of CAI and chondrule formation [e.g., 4]. Additionally, AOAs may be among the precursor solid materials that accreted and were melted to form chondrules [5]. In the highly primitive CR chondrites, AOAs are rare but intriguing components. Some of the AOAs in CRs contain FeNi metal [6] and Mn-enriched forsterite, in which Fe and Mn are decoupled with Fe/Mn ratios less than one [7,8]. Similar Mn-enriched olivine has been described in interplanetary dust particles (IDPs) and chondrite matrices [9]. Herein we present the preliminary results of our re-examination of the AOAs in the CR chondrites, which include detailed petrologic analyses and a SEM-cathodoluminescence study. Our goals are to complete a comprehensive petrologic study of the AOAs in CR chondrites, understand their petrogenesis and their relationship to other chondritic components in the CRs and to AOAs and chondritic components in other chondrite groups. Results: We are studying 19 AOAs from 5 CR chondrites. Petrography. The AOAs in the CR chondrites are irregular-shaped aggregates of olivine and, in many cases, the olivine surrounds nodules of refractory-rich phases that include Ti-, Al-rich Ca-pyroxene, anorthite and in some cases submicron-size spinel and perovskite (Fig. 1, 2). In some AOAs, Ca-pyroxene appears to be an overgrowth on the olivine. Some AOAs contain tiny blebs of FeNi metal. The AOAs lack the spherical (melt-droplet) form and interstitial glassy mesostasis that is characteristic of many chondrules. Texturally, the AOAs in the CR chondrites are very similar to AOA's in the Allende CV chondrite described by Grossman and Steele [1]. The AOAs in the CRs range in size from about 70 μm to 1.2 mm, whereas in CV chondrites they are larger, up to 5mm in size. The larger AOAs in the CRs appear to be composed of numerous nodules consisting of refractory phases and each nodule is surrounded by fine olivine (Fig. 1). Some of the smaller AOAs appear to be fragments from larger aggregates and others may be isolated nodules that never accreted into larger aggregates. Some of the smaller aggregates are located in dark inclusions (DIs) within the CR chondrites and their small sizes (<100μm) are compatible with the other fine scale objects found in the DIs. Individual mineral grains in the AOAs are generally very fine ranging from <1 to 7 μm in size. Metal blebs range in size from ~0.2 to 7 μm.