Refractory inclusions in the CH/CB‐like carbonaceous chondrite Isheyevo: I. Mineralogy and petrography

Abstract

Abstract-The CH/CB-like chondrite Isheyevo consists of metal-rich (70-90 vol% Fe,Ni-metal) and metal-poor (7-20 vol% Fe,Ni-metal) lithologies which differ in size and relative abundance of Fe,Nimetal and chondrules, as well as proportions of porphyritic versus non-porphyritic chondrules. Here, we describe the mineralogy and petrography of Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in these lithologies. Based on mineralogy, refractory inclusions can be divided into hibonite-rich (39%), grossite-rich (16%), melilite-rich (19%), spinel-rich (14%), pyroxeneanorthite-rich (8%), fine-grained spinel-rich CAIs (1%), and AOAs (4%). There are no systematic differences in the inclusion types or their relative abundances between the lithologies. About 55% of the Isheyevo CAIs are very refractory (hibonite-rich and grossite-rich) objects, 20-240 μm in size, which appear to have crystallized from rapidly cooling melts. These inclusions are texturally and mineralogically similar to the majority of CAIs in CH and CB chondrites. They are distinctly different from CAIs in other carbonaceous chondrite groups dominated by the spinel-pyroxene ± melilite CAIs and AOAs. The remaining 45% of inclusions are less refractory objects (melilite-, spinel-and pyroxene-rich CAIs and AOAs), 40-300 μm in size, which are texturally and mineralogically similar to those in other chondrite groups. Both types of CAIs are found as relict objects inside porphyritic chondrules indicating recycling during chondrule formation.We infer that there are at least two populations of CAIs in Isheyevo which appear to have experienced different thermal histories. All of the Isheyevo CAIs apparently formed at an early stage, prior to chondrule formation and prior to a hypothesized planetary impact that produced magnesian cryptocrystalline and skeletal chondrules and metal grains in CB, and possibly CH chondrites. However, some of the CAIs appear to have undergone melting during chondrule formation and possibly during a major impact event. We suggest that Isheyevo, as well as CH and CB chondrites, consist of variable proportions of materials produced by different processes in different settings: 1) by evaporation, condensation, and melting of dust in the protoplanetary disk (porphyritic chondrules and refractory inclusions), 2) by melting, evaporation and condensation in an impact generated plume (magnesian cryptocrystalline and skeletal chondrules and metal grains; some igneous CAIs could have been melted during this event), and 3) by aqueous alteration of pre-existing planetesimals (heavily hydrated lithic clasts). The Isheyevo lithologies formed by size sorting of similar components during accretion in the Isheyevo parent body; they do not represent fragments of CH and CB chondrites.

Multiple Generations of Refractory Inclusions in the Metal‐Rich Carbonaceous Chondrites Acfer 182/214 and Isheyevo

“…These observations may indicate that these CAIs experienced limited interaction with the nebular gas below 1350 K, suggesting either a short residence time in the CAI-forming region or the rapid cooling of the CAI-forming region itself. These CAIs are texturally and mineralogically similar to the majority of CAIs in the previously studied CH chondrites and to a lesser degree to those in the CB chondrites ( Krot et al 2006b and references therein). The mineralogy, textures, and sizes of these CAIs are quite distinct from those of CAIs in other groups (e.g., CM, CO, CR, CV) of carbonaceous chondrites (MacPherson et al 2005;Lin et al 2006 and references therein).…”

“…The mineralogy and petrography of the Isheyevo CAIs are reported by Krot et al (2006b) and are only briefly summarized here.…”

“…We have recently reported that the metal-rich (CH and CH/CBlike) carbonaceous chondrites contain at least two mineralogically distinct populations of CAIs (Krot et al 2002(Krot et al , 2006a(Krot et al , 2006b. We suggested that CH CAIs formed under variable physicochemical conditions, possibly at different times.…”

Oxygen isotopic compositions of chondrules from the metal-rich chondrites Isheyevo (CH/CBb), MAC 02675 (CBb) and QUE 94627 (CBb)

“…This interpretation is consistent with the common presence of the mineralogically (grossite-and hibonite-rich) and isotopically ( 26 Al-poor) unique refractory inclusions inside some of the Isheyevo porphyritic chondrules (Krot et al, 2007). Similar inclusions are almost exclusively found in the metal-rich carbonaceous chondrites-Isheyevo and CHs (e.g., Kimura et al, 1993;Weber et al, 1995;Krot et al, 2006aKrot et al, , 2008b. None of the Isheyevo Type I chondrules analyzed show evidence for oxygen isotopic heterogeneity, which may reflect the small number of olivine and pyroxene grains analyzed within an individual chondrule and total number of chondrules analyzed.…”

Isotopically uniform, 16O-depleted calcium, aluminum-rich inclusions in CH and CB carbonaceous chondrites

“…The mineralogy and petrography of CAIs from CBs, CHs and Isheyevo have been previously reported in several publications (Kimura et al, 1993;Weber et al, 1995;Krot et al, 2001aKrot et al, , 2006Krot et al, , 2008dGounelle et al, 2009). Here we describe mineralogy and petrography only of the inclusions measured for oxygen isotopes during this study.…”