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Abstract:
Nitrogen and noble gases were measured in a bulk sample and in acid-resistant carbon-rich residues of the ureilite Allan Hills (ALH) 78019 which has experienced low shock and is free of diamond. A small amount of amorphous carbon combusting at less than or equal to500 T carries most of the noble gases, while the major carbon phase consisting of large crystals of graphite combusts at greater than or equal to800 T, and is almost noble- gas free. Nitrogen on the other hand is present in both amorphous carbon and graphite, with different delta(15)N signatures of -21parts per thousand and + 19parts per thousand, respectively, distinctly different from the very light nitrogen (about -100parts per thousand) of ureilite diamond. Amorphous carbon in ALH 78019 behaves similar to phase Q of chondrites with respect to noble gas release pattern, behavior towards oxidizing acids as well as nitrogen isotopic composition. In situ conversion of amorphous carbon or graphite to diamond through shock would require an isotopic fractionation of 8 to 12% for nitrogen favoring the light isotope, an unlikely proposition, posing a severe problem for the widely accepted shock origin of ureilite diamond.
