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

An 870 m thick (diameter similar to 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 x 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 km) 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 as 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. (C) 2002 Elsevier Science B.V. All rights reserved.