Highly 15N-enriched chondritic clasts in the CB/CH-like meteorite Isheyevo

The metal-rich carbonaceous chondrites (CB and CH) have the highest whole-rock N-15-enrichments (delta N-15 up to 1500 parts per thousand) among planetary materials. They are also characterized by the absence of interchondrule fine-grained matrix. The only fine-grained material is present as lithic clasts, which experienced extensive aqueous alteration in contrast to the surrounding high-temperature components (chondrules, refractory inclusions, metal grains). Hence, the clasts are foreign objects that were incorporated at a late stage into the final parent body of Isheyevo. Their origin is poorly constrained. Based on mineralogy, petrography, and thermal processing of the aromatic carbonaceous component, different types of clasts have been previously identified in the CB/CH-like chondrite Isheyevo. Here, we focus on the rare lithic clasts characterized by the presence of anhydrous silicates (chondrules, chondrule fragments, and CAIs). Their mineralogy and oxygen isotopic compositions reveal them to be micro-chondrules, fragments of chondrules, and refractory inclusions related to those in the Isheyevo host, suggesting accretion in the same region. In contrast to previously studied IDPs or primitive chondritic matrices, the fine-grained material in the clasts we studied is highly and rather uniformly enriched in heavy nitrogen, with bulk delta N-15 values ranging between 1000 parts per thousand and 1300 parts per thousand. It is also characterized by the presence of numerous N-15 hotspots (delta N-15 ranging from 1400 parts per thousand to 4000 parts per thousand). No bulk (delta D <-240 parts per thousand) or localized deuterium enrichments were observed. These clasts have the highest bulk enrichment in heavy nitrogen measured to date in a fine-grained material. They represent a unique material, of asteroidal or cometary origin, in our collection of cosmomaterials. We show that they were N-15-enriched before their incorporation in the final parent body of Isheyevo. They experienced an extensive aqueous alteration that most likely played a role in redistributing N-15 over the whole fine-grained material and may have significantly modified its initial hydrogen isotopic composition. Based on a review of isotopic fractionation models, we conclude that the nitrogen isotopic fractionation process, its timing, and its location are still poorly constrained. The N-15-rich clasts may represent the surviving original carrier of the N-15 anomaly in Isheyevo whole-rock. (C) 2010 Elsevier Ltd. All rights reserved.