Two rare meteorites found in Antarctica two years ago are from a previously unknown, ancient asteroid with an outer layer or crust similar in composition to the crust of Earth's continents, reports a research team primarily composed of geochemists from the University of Maryland.

Reported in the January 8 issue of the journal Nature, this is the first ever finding of material from an asteroid with a crust like Earth's. The discovery also represents the oldest example of rock with this composition ever found.

These meteorites point "to previously unrecognized diversity" of materials formed early in the history of the Solar System, write authors James Day, Richard Ash, Jeremy Bellucci, William McDonough and Richard Walker of the University of Maryland; Yang Liu and Lawrence Taylor of the University of Tennessee and Douglas Rumble III of the Carnegie Institution for Science.

Prof. James Day looking at a portion of the meteorite in the University of Maryland's isotope geochemistry lab. In the background is a mass spectrometer used to analyze the meteorite samples.

"What is most unusual about these rocks is that they have compositions similar to Earth's andesite continental crust -- what the rock beneath our feet is made of," said first author Day, who is a research scientist in Maryland's department of geology. "No meteorites like this have ever been seen before".

Day explained that his team focused their investigations on how such different Solar System bodies could have crusts with such similar compositions. "We show that this occurred because of limited melting of the asteroid, and thus illustrate that the formation of andesite crust has occurred in our solar system by processes other than plate tectonics, which is the generally accepted process that created the crust of Earth".

The two meteorites (numbered GRA 06128 and GRA 06129) were discovered in the Graves Nunatak Icefield during the US Antarctic Search for Meteorites (ANSMET) 2006/2007 field season. Day and colleagues immediately recognized that these meteorites were unusual because of elevated contents of a light-colored feldspar mineral called oligoclase. "Our age results point to these rocks being over 4.52 billion years old and that they formed during the birth of the Solar System. Combined with the oxygen isotope data, this age points to their origin from an asteroid rather than a planet," he said.


Posted by: Sean    Source

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