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

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

Apr 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002e%26psl.198...49h&link_type=abstract

Earth and Planetary Science Letters, Volume 198, Issue 1-2, p. 49-62.

Computer Science

11

Scientific paper

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.

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