Highly N-enriched Chondritic Clasts in the Isheyevo (ch/cb) Meteorite: Tem Analyses

Introduction: Isheyevo, a metal-rich CH/CB-like meteorite, shows whole-rock δ 15 N enrichment up to ~+1500‰ [1]. Bonal et al. discovered rare chondritic clasts in Isheyevo containing mafic silicates, large bulk 15 N enrichments (δ 15 N from +1000 to 1300‰) and µm-sized " hotspots " with δ 15 N approaching +5000‰ [2,3]. The source of these high 15 N enrichments, whether due to molecular cloud, nebular and/or parent body processing, is presently unknown. Because the clast matrix material is fine-grained, we are carrying out FIB/TEM petrographic studies to be coordinated with NanoSIMS mapping to identify in situ the carrier(s) of their bulk and hotspot 15 N enrichments and to establish the possible origin(s) of these clasts. Methods: Regions of lithic clasts with anhydrous silicates were isotope-mapped (Cameca ims1280 ion microprobe) [2]. Sample outgassing of the polished section prevented direct NanoSIMS isotope imaging at higher spatial resolution. Cameca ims1280 mapped areas were imaged by high-resolution secondary electron microscopy (JEOL JSM7401F FE-SEM) to correlate δ 15 N and 28 Si maps with local mineralogy and textures. Fo-cused ion beam thin sections were extracted (FEI Nova600 NanoLab FIB) to include bulk and hotspot materials for transmission electron microscopy analysis (FEI 200kV Tecnai and 300kV Titan TEMs). FIB sections will be mapped by NanoSIMS. Results and Discussion: The rare Isheyevo chondritic clasts containing anhydrous silicates are, in fact, extensively aqueously altered. Bulk regions contain disordered ferrihydrite and extremely fine-grained (typically 1-2 layers) smectite-group phyl-losilicates finely intermixed and pseudomorphically replacing anhydrous silicates. Magnesian olivines and pyroxenes up to several µm in diameter dominate the remnant silicates, all of which display corroded surfaces. Disordered ferrihydrite is present between silicate regions, and several-µm-sized domains of poorly ordered ferrihydrite intergrown with serpentine-group phyllosili-cates are rimmed by framboidal magnetite and goethite. 15 N-hotspot regions contain larger quantities of these Fe-rich hydrous materials and a few small silicates, 50-500 nm olivine, low-Ca and high-Ca pyroxene. Banded features typical of shock are evident in a pyroxene. Most interesting is the presence of a semi-continuous network of amorphous organic material embedded in the hydrous phases in the vein including globule-like objects a few hundred nm in diameter. These differ in morphology from the organic globules reported in other meteorites [e.g. 4] and may represent modifications or processing post-formation. The extreme 15 N anomalies corresponding to locations of veins containing an organic network in heavily hydrated matrix suggests that 1) the organic material …