87Rb–87Sr chronology of the Binda howardite

Abstract

Differentiated meteorites have numerous similarities with igneous rocks from the Earth and the Moon. They are of primordial importance in our understanding of magmatic processes in the early Solar System. The howardites constitute a particular class of differentiated meteorite as they are mechanical mixtures1–7 (polymict breccia) of components from two other classes—eucrites and diogenites. The eucrites are representative of magmatic liquids and the diogenites are the corresponding cumulates3. This relationship is supported by oxygen isotopic data8. The 87Rb–87Sr chronology of eucrites9,10 shows that these objects result from a very early igneous activity near the surface of their parent body (<150 Myr after formation of the Solar System). As for howardites, within the same meteorite Kapoeta, basaltic clasts have been dated by the internal Rb–Sr isochron at 4,400 Myr and around 3,500 Myr (refs 11, 12). 39Ar–40Ar dating of these clasts provides evidence for their initial crystallisation 4,500 Myr ago, and for their independent history until the final lithification of Kapoeta, probably as late as 2,200 Myr ago13. Due to this complexity their initial age of crystallisation, and in particular, their 87Sr/86Sr initial ratio is not precisely known. We report here our investigation of the Binda howardite which seems to have had a simpler history; it is the only one which is a monomict breccia. The component representing the cumulate fraction seems to be in equilibrium with that which represents the eucritic melt3. Therefore one might expect to date the howardites formation by dating this meteorite, assuming that they all formed in the same parent body.