Evidence for Post-impact Plume Origin of Porphyritic Chondrules in Isheyevo (ch/cb) and Ch Carbonaceous Chondrites

It is suggested metal-rich carbonaceous chondrites (CCs), CB, CH and Isheyevo, contain various proportions of chondritic components (CAIs, chondrules, metal) formed (i) by melting, evaporation, and condensation processes in an impact plume resulted from a planetary-scale collision, and (ii) by thermal processing of dust aggregates in the protoplanetary disk (PPD) [1−4]. The impact-plume products include magnesian non-porphyritic chondrules, zoned metal grains, and metal-sulfide nodules. The nebular components include ferromagnesian and Al-rich chon-drules with porphyritic textures and 16 O-rich CAIs (Δ 17 O ~ −22±2‰) dominated by hibonite-and grossite-rich types [4−10]. Based on the young absolute ages of the magnesian non-porphyritic chondrules in CB chondrites and the lack of inter-chondrule fine-grained matrix material, it was hypothesized that dust in the PPD largely dissipated by the time of formation of the CB chondrite components, and, therefore, these ages can be used to constrain lifetime of the PPD [11]. However, the presence of abundant nebular components in Isheyevo and CH chondrites [9,10], and the inferred genetic relationship between these meteorites and CB chondrites [4] contradict this hypothesis. CBs, CHs and Isheyevo contain a common population of ig-neous, isotopically uniform and 16 O-depleted (Δ 17 O ~ −7±3‰) CAIs [4,12,13]. It is suggested these CAIs formed by remelting of pre-existing, probably 16 O-rich refractory inclusions during formation of magnesian non-porphyritic chondrules in the impact plume [4]. Krot et al. [14] reported abundant relict CAIs in por-phyritic chondrules from Isheyevo and CH chondrites; about 50% of these CAIs are grossite-and hibonite-rich. Because these CAI types are common only in CHs and Isheyevo [5−7,10,15], they suggested porphyritic chondrules in metal-rich CCs formed at a different time or in a different place than chondrules from other groups of CCs. This hypothesis is consistent with O-isotope compositions of the Isheyevo porphyritic chondrules: contrary to typical chondrules from CCs, on a three-isotope oxygen diagram, most porphyritic chondrules in Isheyevo plot at and above the terrestrial fractionation line [16]. In situ O-isotope measurements of relict CAIs inside porphyritic chondrules from Isheyevo and Acfer 182/214 (CH) show the presence of two isotopically distinct CAI populations: 16 O-enriched and 16 O-depleted (Δ 17 O ~ −20‰ and ~ −7‰, respectively). We suggest (i) at least some of the porphyritic chondrules in metal-rich CCs postdate planetary-scale collision invoked for the origin of the 16 O-depleted CAIs and magnesian non-porphyritic chondrules [4,11,16], and (ii) the PPD must have been in existence at the time …