Highly N-enriched Chondritic Clasts in the Cb/ch-like Isheyevo Meteorite

L. Bonal*, G.R. Huss, A.N. Krot, and K. Nagashima, HIGP/SOEST, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA. *[email protected] Metal-rich (CB, CH, CB/CH-like) carbonaceous chondrites exhibit several anomalous characteristics not observed in other chondrite groups, including (i) a nearly complete absence of interchondrule fine-grained matrix, and (ii) the highest whole-rock enrichments in N (δN up to +1500‰) [e.g., 1, 2]. The fine-grained material occurs mainly as chondritic clasts, which represent foreign material having experienced some secondary processes on their initial parent body and subsequently incorporated into or accreted with the CH and CB high-temperature components [e.g., 1, 3]. In order to constrain the origin of the lithic clasts and to look for the pristine carrier of heavy nitrogen, we initiated a detailed study of the clasts in the CB/CHlike chondrite, Isheyevo. Based on petrography, chemical composition of altered minerals, the structural degree of the polyaromatic organic matter, and the oxygen isotopes of carbonates, we concluded that 1) the lithic clasts in Isheyevo are diverse and 2) they may originate from several parent bodies, some of them unsampled in our collections of cosmomaterials [3]. Three main groups of clasts were defined by [3]. Group I clasts have a mineralogy indicating of a high degree of aqueous alteration and they are the most common. Group II clasts contain variable abundances of anhydrous silicates; carbonates, magnetite and sulfide are rare. Group III clasts are characterized by the absence of magnetite and the presence of Fe,Ni-metal. They experienced thermal processing comparable to type 3.0 chondrites, as attested to by the structural order of aromatic organic matter. Groups I and II are characterized, with only a few exceptions, by a poorly ordered organic matter, comparable with CI and CM. The present work is focused on the rare group II clasts. We report their petrographic characteristics, the O-isotopic composition of the anhydrous silicates, and their Cand N-isotopic compositions. Their origin and implication for the metal-rich chondrites are discussed. Analytical procedures: Petrography: A systematic survey allowed us to identify 6 group II clasts with large anhydrous silicates. The clasts were mapped in Na, O, Mg, Al, Si, S, Ca, Mn, and Fe X-rays using a JEOL 5900LV SEM. The anhydrous silicates, microCAIs, and micro-chondrules were imaged by BackScattered Electrons (BSE) and their compositions were estimated from SEM X-ray. The opaque minerals were studied in reflected light and in BSE images. Isotope measurements: Isotope measurements were carried with the University of Hawai‘i Cameca ims 1280 microprobe. Analytical technique for oxygen isotope measurements is described by [4]. Isotope ratio maps for C and N were collected by scanning ion imaging. A Cs primary beam of <10pA was rastered over areas of 50×50 μm. A mass resolving power of ~ 6000 was used and signals of C, C, O, CN, CN and Si were measured with the monocollector electron multiplier. A polished thin section of Orgueil (CI) was used as a standard, assuming bulk C and N values from [5]. The accuracy of the ratios inferred from image processing (L’image, L. Nittler) was checked by measuring the ratios in normal monocollection mode. Good agreement was obtained.