Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.226g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 226
Mathematics
Logic
2
Scientific paper
Acfer 182 is an unusual chondrite, with abundant small chondrules and CAIs (mean diameter ca. 100 micrometers), and rich in metal (ca. 15 vol%). It is closely related to ALH 85085, and, like that meteorite, is highly enriched in ^15N (bulk delta^15N ca. +600o/oo; delta^15N(sub)max = +1584o/oo at 900 degrees C; ref. 1). Stepped combustion of Acfer 182 (see figure) releases ^15N over a wide temperature range, indicating that its carriers must be dispersed throughout the meteorite, possibly occurring in carbonaceous material, fine-grained matrix, clasts, and metal. The highest relative abundance of ^15N is found in phase "N(sub)C", so far unidentified mineralogically, with a C/N ca. 10, which releases its nitrogen on combustion of the whole rock at 850-950 degrees C. N(sub)C is more apparent in Acfer 182 than ALH 85085, accounting for ca. 8 ppm of the total nitrogen inventory of 85.4 ppm. An attempt to isolate NC by physical means proved unsuccessful [1], therefore chemical treatments were tried: an HF/HCl-resistant residue was prepared from 9 g of fragments. Examination of the remaining material confirmed that it was dominantly composed of Mg-Al spinels, chromite, hibonite, and Cr-rich sulphides. Approximately two thirds of the original amount of nitrogen in the sample has been lost on dissolution (see figure), including any associated with Fe-Ni metal. There has been a reduction of over 50% of the nitrogen that was released up to 500 degrees C and presumed present in a carbonaceous component, without significant change in delta^15N value or C/N ratio. The most visible difference between results from the whole-rock and HF/HCl-resistant residue is that the combustion temperature of NC has decreased to 550-700 degrees C, with a concomitant drop in delta^15N from +1584o/oo to +1274o/oo It is unlikely that a minor (even heavier) sub-fraction of the ^15N-rich material has been removed; now that N(sub)C combusts at a temperature closer to the more abundant "organic" nitrogen, it is probable that mixing lowers the observed delta^15N. N(sub)C itself does not seem to be totally immune from the acid attack: only 3 ppm remain from the original concentration of 8 ppm in the whole-rock meteorite. N(sub)C has still not been identified, but is further operationally defined. The combustion temperature of N(sub)C in the acid-resistant residue constrains its identification: nitrogen present as Si3N4 or substituted in SiC does not seem likely, unless the minerals are extremely fine-grained or exceptionally radiation-damaged. It is possible that N(sub)C is hosted by a mineral not previously suggested as a presolar grain, assuming that the ^15N-enrichments described are indicative of such an origin. The more complex problem of how isotopically heavy nitrogen is so completely distributed throughout the meteorite may become more easy to address once the most likely source of the ^15N-enrichment is positively identified. One possibility is that parent-body processing, including shock, degassed a component of interstellar origin, which subsequently exchanged with or nitrified other phases. Reference: [1] Grady, M. M. and Pillinger, C. T. (1992) Earth Planet. Sci. Lett. (submitted)
Arden John W.
Grady Michael
Pillinger Colin T.
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