Invalid 4.01-Gyr model U–Pb ‘age’ of the Nakhla meteorite

Abstract

NAKHLA, an achondrite composed dominantly of the minerals ‘diopside’ and olivine¹, is possibly a unique meteorite. It has a ³⁹Ar–⁴⁰Ar age of about 1.3 Gyr (refs 2 and 3) and recently its Rb–Sr age was shown by Papanastassiou and Wasserburg⁴ to be similar. In their discussion, these authors⁴ use the concept of model Rb–Sr ‘ages’ as part of their argument that Nakhla originated in a differentiated planetary body not unlike the Earth, and in particular to argue that this parent body suffered gross differentiation between 3.6 and 1.37 Gyr ago. We show that the U–Pb systematics of Nakhla are inconsistent with one or more of the assumptions required for the calculation of a model U–Pb ‘age’, thus indicating that model ‘ages’ for other radioactive decay schemes may be generally invalid.