About 470 million years ago – in a time period called Ordovician – the parent asteroid of one of the L chondrites, one of the most common meteorite types, was disrupted in a collision with another body. This event led to a subsequent bombardment of Earth with collisional debris for at least 10 million years. This finding is reported in a recent study by Field Museum scientists Dr. Birger Schmitz (Research Associate), Robert A. Pritzker Assistant Curator of Meteoritics and Polar Studies Dr. Philipp Heck, and an international team of coauthors. Their paper was published in the journal Earth and Planetary Science Letters and focuses on rocks from the marine, 460 million year old 7-km-wide Swedish Lockne impact crater. Schmitz and his student isolated extraterrestrial minerals (chromites) from the crater rocks. Heck and colleagues used the electron and ion microprobes at the University of Wisconsin, Madison to analyze the elemental and oxygen isotopic composition of these chromites. Their analysis shows that the impactor was an ordinary (L) chondrite. A recent hypothesis by researchers from Berlin and Brussels that claims the impactor was an iron asteroid based on platinum group elements (PGE) in the crater can now be clearly rejected. This is supported by PGE analyses by a collaborator at Woods Hole Oceanographic Institution, which shows that PGEs cannot identify the impactor type in this case. Noble gas analyses of the chromites performed by colleagues at ETH Zurich in Switzerland revealed that the grains traveled to Earth inside an asteroid and not as meteorites or micrometeorites. The authors conclude that a fragment of the L chondrite parent body caused the impact. The new study defines the period of increased bombardment of Earth with extraterrestrial material during the mid-Ordovician to at least 10 million years. The paper, “Determining the impactor of the Ordovician Lockne crater: Oxygen and neon isotopes in chromite versus sedimentary PGE signatures,” can be accessed at doi:10.1016/j.epsl.2011.04.028.
|Artist's impression of an asteroid impact into a shallow sea. Credit: Don Davis and NASA.|