Siderophile-rich inclusions from the Morokweng impact melt sheet, South Africa: possible fragments of a chondritic meteorite

An 870 m thick (diameter ∼30 km) melt sheet associated with the Morokweng impact structure, South Africa, contains Cr-rich silicate inclusions (up to 30 mm in diameter) as well as disseminated Ni-rich sulphides and oxides that commonly occur in mutual intergrowths. Although the silicate inclusions are highly altered, the mineral chemistry and petrographic evidence, along with broadly chondritic Re–Os systematics and platinum-group element (PGE) signatures, all support the view that the silicate inclusions represent relict fragments of an impactor with compositions similar to ordinary chondrites. The Ni-rich sulphides and oxides are significantly enriched (up to 35× chondrites) in PGE, Ni and Cr. Although the bulk PGE signature of the melt sheet is chondritic, the sulphides and oxides display mm and sub-mm scale fractionation of PGE into different inclusion components. A model is presented whereby pieces of the projectile react with the relatively more oxidising impact melt into which they fell, resulting in oxidation and fractionation of the metal–silicate fragments to form the inclusion assemblages. This may be the first reported occurrence of a large melt sheet (300 km3) contaminated by swarms of discrete pebble-sized fragments of asteroid material. The high proportion of meteorite fragments in the Morokweng melt sheet suggests either that the structure was formed by a relatively low velocity (<18 km/s) impact event, or that the angle of impact was optimum for retaining maximum amounts of meteoritic material within the crater.