Other
Scientific paper
Oct 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005psrd.repte..98t&link_type=abstract
Planetary Science Research Discoveries
Other
Chondrule, Cb Chondrites, Lead-Lead, Isotopic Dating, Impact, Planet Formation
Scientific paper
Alexander (Sasha) Krot (University of Hawaii), Yuri Amelin (University of Toronto), Pat Cassen (SETI Institute), and Anders Meibom (Museum National d'Histoire Naturelle, Paris) studied and then extracted frozen droplets of molten silicate (chondrules) from unusual meteorites rich in metallic iron-nickel. Called CB (Bencubbin-like) chondrites, these rare but fascinating meteorites contain chondrules with different properties than those in other types of chondrites. Most notably, the chondrules contain very small concentrations of volatile elements and variable concentrations of refractory elements. (Volatile elements condense from a gas at a relatively low temperature, or are boiled out of solids or liquids at relatively low temperature. Refractory elements are the opposite.) Some of the metal grains in CB chondrites are chemically zoned, indicating that they formed by condensation in a vapor cloud.
The most intriguing feature of chondrules in CB chondrites is their relatively young age. Lead-lead isotopic dating of chondrules separated from two CB chondrites show that they formed 5 million years after formation of the first solids in the solar system (calcium-aluminum-rich inclusions), which is about at least two million years after formation of other chondrules, and after energetic events in the solar nebula stopped. Krot and his colleagues suggest that the CB chondrules formed as the result of an impact between Moon- to Mars-sized protoplanets. Such impacts were so energetic that huge amounts of material were vaporized and then condensed as chondrules or chemically zoned metal grains. This event enriched refractory elements and depleted volatile elements. Such large impacts appear to play important roles in planet formation, including the formation of the Moon.
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