Nitrogen in diamond-free ureilite Allan Hills 78019: clues to the origin of diamond in ureilites

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 ≤500 °C carries most of the noble gases, while the major carbon phase consisting of large crystals of graphite combusts at ≥800°C, and is almost noble-gas free. Nitrogen on the other hand is present in both amorphous carbon and graphite, with different δ¹⁵N signatures of -21%_o and +19%_o, respectively, distinctly different from the very light nitrogen (about -100%_o) 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.