Early formation of evolved asteroidal crust

Mechanisms for the formation of crust on planetary bodies remain poorly understood(1). It is generally accepted that Earth's andesitic continental crust is the product of plate tectonics(1,2), whereas the Moon acquired its feldspar- rich crust by way of plagioclase flotation in a magma ocean(3,4). Basaltic meteorites provide evidence that, like the terrestrial planets, some asteroids generated crust and underwent large- scale differentiation processes(5). Until now, however, no evolved felsic asteroidal crust has been sampled or observed. Here we report age and compositional data for the newly discovered, paired and differentiated meteorites Graves Nunatak ( GRA) 06128 and GRA 06129. These meteorites are feldspar- rich, with andesite bulk compositions. Their age of 4.5+/-0.06 Gyr demonstrates formation early in Solar System history. The isotopic and elemental compositions, degree of metamorphic re-equilibration and sulphide- rich nature of the meteorites are most consistent with an origin as partial melts from a volatile- rich, oxidized asteroid. GRA 06128 and 06129 are the result of a newly recognized style of evolved crust formation, bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early- formed materials in the Solar System.