¹⁵N-enriched nitrogen in polymict ureilites and its bearing on their formation

Abstract

Three polymict ureilites have been identified as a subunit within the ureilite group of achondrites¹: North Haig, Nilpena and the Antarctic meteorite EET 83309. They are characterized by their brecciated nature and the presence of lithic clasts and mineral fragments of rock types different from main-group ureilites (MGUs). Here we present analysis of the nitrogen isotope compositions of these three ureilites, which show them to be highly enriched in ¹⁵N; measured δ¹⁵N values reach +540‰, relative to air, but mass-balance calculations suggest a component of at least +600‰. Nitrogen in unbrecciated MGUs does not exhibit such unusual isotopic compositions. At least two nitrogen components exist, and analogies may be drawn with the Bencubbin meteorite (also an achondritic breccia), but no definite connection can be made. We favour a hypothesis that calls for the introduction of isotopically heavy nitrogen into polymict ureilites during a brecciation event caused by a more primitive, possibly carbonaceous chondrite-like projectile.