ACFER-182 - SEARCH FOR THE LOCATION OF N-15-ENRICHED NITROGEN IN AN UNUSUAL CHONDRITE

Acfer 182, an unusual chondrite recovered from the Sahara in 1990, is highly enriched in N-15 (bulk deltaN-15 ca. +600 parts per thousand; deltaN-15(max) = + 1584 parts per thousand at 900-degrees-C). reinforcing conclusions that it is related to the very unusual chondrite ALH 85085. Stepped combustion of whole-rock Acfer 182 releases N-15 over a wide temperature range, excluding a single carrier phase for the N-15-enriched nitrogen. The highest relative abundance of N-15 is found in phase ''N(C)'', so far unidentified mineralogically, with a C/N of ca. 8, which releases its nitrogen on combustion of the whole rock at 850-950-degrees-C. Attempts to isolate N(C) by physical means proved unsuccessful. Hence a chemical treatment was tried, with preparation of an HF/HCl-resistant residue. Approximately two-thirds of the original amount of nitrogen in the sample was lost on dissolution. Almost all of component N(A), the carbonaceous component, was removed, without significant change in deltaN-15 (+950 parts per thousand) or C/N ratio (75). N(B), originally thought to be nitrogen in metal, was unchanged in abundance, deltaN-15 (+500 parts per thousand) and C/N ratio (ca. 20), indicating that it could not be present in metal, unless as an insoluble nitride; it might even be an isotopically light entity mixing with residual N(A) and N(C). This last component now combusted at a lower temperature than in the whole rock, and has a lower deltaN-15 value (+ 1274 parts per thousand). A fourth, minor component N(D) is revealed on combustion of the residue, which burns above 675-degrees-C, with deltaN-15 ca. + 1000 parts per thousand, and C/N ratio ca. 50. The N-15-enriched nitrogen in almost all constituents of Acfer 182 is interpreted as emanating from a single source, i.e. interstellar grains, which were incorporated within the Acfer 182 parent body during aggregation. Vaporisation. condensation and mixing of components caused redistribution of N-15-enriched nitrogen throughout the patent. with nitrogen either trapped or implanted into silicates and metal.