Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa
Sciences de la Terre, Université du Québec À Chicoutimi, Chicoutimi, Que. G7H 2B1, Canada; Department of Geology, University of Pretoria, Pretoria 0002, South Africa; South African Nuclear Energy Corporation, Pretoria 0001, South Africa; School of Geosciences, University of the Witwatersrand, Wits 2050, South Africa; School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF10 3YE, United Kingdom; Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, United Kingdom; Scripps Institution of Oceanography, University of California, San Diego, San Diego, CA 92093, United States; Department of Geological Sciences, Indiana University, Bloomington, IN 47405-7000, United States; Ithemba LABS - Gauteng, Wits 2050, South Africa
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 1. Large objects are widely believed to be completely melted or vaporized during high-angle impact with the Earth2,3. Consequently, identification of large impactors relies on indirect chemical tracers, notably the platinum-group elements4. Here we report the discovery of a large (25-cm), unaltered, fossil meteorite, and several smaller fragments within the impact melt of the giant (>70 km diameter), 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 the meteorite suggests that the Morokweng asteroid incorporated part of the LL chondrite parent body not represented by objects at present reaching the Earth. © 2006 Nature Publishing Group.
Earth (planet); Melting; Meteorites; Palladium; Radioactive tracers; Silicates; Vaporization; Chondritic chromium isotope ratios; Iron-rich silicates; Morokweng impact crater; Platinum-group element ratios; Solar system; chromium; iron; metal; platinum; silicate; sulfide; asteroid; astrobleme; chondrite; meteorite; platinum group element; solar system; article; astronomy; geochemistry; mineralogy; priority journal; South Africa; vaporization; velocity; Africa; South Africa; Southern Africa; Sub-Saharan Africa