Morokweng meteorite, South Africa

Chondrule in Morokweng meteorite

Meteorites provide a sample of Solar System bodies and so constrain the types of objects that have collided with Earth over time. Meteorites analysed to date, however, are unlikely to be representative of the entire population and it is also possible that changes in their nature have occurred with time. Large objects are widely believed to be completely melted or vaporized during high-angle impact with the Earth. Consequently, identification of large impactors relies on indirect chemical tracers, notably the platinum-group elements. Here we report the discovery of a large (25-cm), unaltered, fossilmeteorite, and several smaller fragmentswithin the impact melt of the giant (>70 kmdiameter), 145-Myr-old Morokweng crater, South Africa. The large fragment (clast) resembles an LL6 chondrite breccia, but contains anomalously iron-rich silicates, Fe-Ni sulphides, and no troilite or metal. It has chondritic chromium isotope ratios and identical platinum-group element ratios to the bulk impact melt. These features allow the unambiguous characterization of an impactor at a large crater. Furthermore, the unusual composition of themeteorite suggests that theMorokweng asteroid incorporated part of the LL chondrite parent body not represented by objects at present reaching the Earth.

Maier W.D, Andreoli M.A.G., McDonald I. Higgins M.D., Boyce A.J., Shukolyukov A., Lugmair G.W., Ashwal L.D., Graser P., Ripley E.M., Hart R.J., 2006, Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa. Nature 441 (7090): 203-206. doi:10.1038/nature04751