A nitrogen isotope study of bencubbinites

Abstract

Nitrogen-isotopic compositions of the bencubbinites-Bencubbin, Hammadah al Hamra (HH) 237, and Queen Alexandra Range (QUE) 94411-and in a petrographically similar chondrite Grosvenor Mountains (GRO) 95551 were measured by stepped-combustion static mass spectrometry. Hammadah al Hamra 237 and QUE 94411 contain isotopically heavy N, but not as heavy as that in Bencubbin or Weatherford. Grosvenor Mountains 95551 contains isotopically near-normal N and light N and, hence, it is not related to the bencubbinites, which is also indicated by its O-isotopic composition. The N carriers in these meteorites were investigated using secondary ion mass spectrometry. In the bencubbinites, N is mostly located around sulfide in metal clasts and in impact-melt areas. The N carriers in the former are taenite, carbide, or both; whereas those in the latter are molten metal, tiny graphitic carbon in metal, oxi-nitride glass, or both. In the various N carriers, N is isotopically equilibrated, and therefore the carriers are not pristine presolar grains. Isotopically near-normal N in GRO 95551 is located in graphite. The carrier of isotopically light N in GRO 95551 has not been found.