About 66 million years ago, 75 percent of all animal and plant species became extinct, including the dinosaurs. Since 1980, it has been known that the impact of an asteroid in what is now the Gulf of Mexico was responsible. Since then, scientists have been trying to find out more about the celestial body. An international team that includes impact researcher Christian Köberl (Department of Lithospheric Research, University of Vienna), has now discovered what kind of meteorite it was and that it originated in the outermost part of the asteroid belt.
Iridium and other platin-group-elements as impact tracers
The globally distributed iridium (Ir) anomaly at the Cretaceous–Paleogene (K–Pg) boundary (~66 Ma) has long been interpreted as definitive evidence for a large impact event; this was the result of a study published in 1980 in the journal „Science“ by Geologist Walter Alvarez, together with his Nobel--Prize-winning father Luis, a physicist, and a couple of colleagues. Not only Ir was found but also other platinum-group-elements (PGEs) at high abundances, all over the world, with the abundances being much higher than in normal terrestrial rocks, and the interelement ratios similar to those in (chondritic) meteorites. As Ir and the other PGEs are rare in the Earth’s crust but enriched in meteorites, such enrichments and elemental ratios are a sensitive impact tracer.
Not only this, but there are also some isotopic ratios of various, mostly metallic, elements, such as osmium or chromium, that are also showing extraterrestrial values in the boundary samples, which are ejecta of the giant impact. The large, almost 200-km-diameter Chicxulub impact crater was only discovered in the early 1990s, because it is covered by younger rocks and therefore not visible on the Earth’s surface.
Which kind of "chondrite"?
In addition to studying drill cores from the Chicxulub crater in Mexico, the ejecta layers around the world are preferably analyzed because higher concentrations of extraterrestrial material are found in these ejecta layers than in the crater itself. So while it was known already for some time that the impacting body had a composition similar to “chondrites", this did not narrow down the source much because most meteorites belong to this group. So more recently, not only elemental abundances but also isotope studies were used to narrow down the question of the type and origin of the impactor.
Two years ago, a team, which included Köberl, was able to show by using ruthenium isotopes that the asteroid was a "carbonaceous chondrite", which is a rare subgroup of chondrites, one which still has “primitive” composition, i.e., a composition that has not changed by any geological processes since the origin of the solar system about 4.6 billion years ago. Now a team with members from France, Austria, Belgium, and Canada has measured nickel isotopes in various ejecta layers around the world, as well as the composition of different carbonaceous chondrites. As they report in the journal "Science Advances," the "dinosaur-killer" asteroid had a composition similar to a so-called "CO chondrite."
Carbonaceous chondrites make up only five per cent of meteorites so far sampled on Earth. Carbonaceous chondrites of the Ornans class — CO chondrites — make up a tiny fraction of that group. They are some of the most primitive and untouched materials in the solar system. Many questions remain about the origins of the world-shattering meteorite. Potential sources include distant, debris-rich regions of the outer part of the asteroid belt near Jupiter.
