Lunar and Planetary Science XXXIII (2002)
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MINERALOGY, PETROGRAPHY AND OXYGEN-ISOTOPIC COMPOSITIONS OF Ca, Al-RICH INCLUSIONS AND AMOEBOID OLIVINE AGGREGATES IN THE CR CARBONACEOUS CHONDRITES. Alexander N. Krot1, Jérôme Aléon2, and Kevin D. McKeegan2, 1Hawaii Institute of Geophysics & Planetology, School of Ocean & Earth Science & Technology, University of Hawaii at Manoa, Honolulu HI 96822, USA,
[email protected]; 2University of California, Los Angeles, CA 90095, USA. 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 16O-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 16O 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 16Opoor compositions [9], whereas CH chondrites contain populations of homogeneous CAIs that are either 16Opoor or 16O-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 16O and 18O ion beams were measured at low Mass Resolving Power (MRP ~ 2000) using a Faraday cup (FC) and an electron multiplier (EM), respectively, and 17O 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 (
