http://media-newswire.com/release_1083611.html
(Media-Newswire.com) - COLLEGE PARK, Md. - 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.
Published 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.
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. 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 his 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.
There are a number of asteroids in the asteroid belt that may have properties
like the GRA 06128 and GRA 06129 meteorites including the asteroid (2867)
Steins, which was studied by the European Space Agency's Rosetta spacecraft
during a flyby this past September. These so-called E-type asteroids reflect
the Sun's light very brightly, as would be predicted for a body with a crust
made of feldspar.
According to Day and his colleagues, finding pieces of meteorites with andesite
compositions is important because they not only point to a previously
unrecognized diversity of Solar System materials, but also to a new mechanism
to generate andesite crust. On the present-day Earth, this occurs dominantly
through plates colliding and subduction - where one plate slides beneath
another. Subduction forces water back into the mantle aiding melting and
generating arc volcanoes, such as the Pacific Rim of Fire - in this way new
crust is formed.
"Our studies of the GRA meteorites suggest similar crust compositions may be
formed via melting of materials in planets that are initially volatile- and
possibly water-rich, like the Earth probably was when if first formed" said
Day." A major uncertainty is how evolved crust formed in the early Solar System
and these meteorites are a piece in the puzzle to understanding these
processes."
This research was funded by the NASA cosmochemistry program.
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