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Posted by sanik on December 23, 2005, 11:18 am
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George =E5=86=99=E9=81=93=EF=BC=9A
> http://www.eurekalert.org/pub_releases/2005-12/ci-2sb121905.php
>
> Contact: Dr. Richard Carlson
> Graphics can be viewed at link, above.
>
> Washington DC-- Earth's future was determined at birth. Using refined
> techniques to study rocks, researchers at the Carnegie Institution's
> Department of Terrestrial Magnetism (DTM) found that Earth's mantle--the
> layer between the core and the crust--separated into chemically distinct
> layers faster and earlier than previously believed. The layering happened
> within 30 million years of the solar system's formation, instead of
> occurring gradually over more than 4 billion years, as the standard model
> suggests. The new work was recognized by Science magazine, in its December
> 23 issue, as one of the science breakthroughs for 2005.
> Carnegie scientists Maud Boyet and Richard Carlson analyzed isotopes--ato=
ms
> of an element with the same number of protons, but a different number of
> neutrons--of elements in rock samples for their work. As Carlson explains,
> "Isotopes exist naturally in different proportions and are used to
> determine conditions under which rock forms. Radioactive isotopes are
> particularly handy because they decay at a predictable rate and can reveal
> a sample's age and when its chemical composition was established."
>
> In the standard model of the geochemical evolution of the Earth, the
> Earth's mantle has been evolving gradually over Earth's 4.567-billion-year
> history primarily through the formation of the chemically distinct
> continental crust. Shortly after solid material began condensing from the
> hot gas of the cooling early solar system, the object that would become
> Earth grew by the collision and accretion of smaller rocky bodies. The
> chemical composition of these building blocks is preserved today in
> primitive meteorites called chondrites.
>
> In the 1980s, scientists analyzed the ratio of isotopes of the rare earth
> element neodymium in chondrites and various terrestrial rocks collected at
> or near the Earth's surface and found that the samples shared a common
> composition. Researchers believed that this ratio remained constant from
> the beginning of Earth formation. Using new-generation equipment, Boyet a=
nd
> Carlson found, surprisingly, that the terrestrial samples did not have the
> same ratio as the meteorites. Compared to chondrites, all terrestrial roc=
ks
> measured have an excess of the mass 142 isotope of neodymium (142Nd), whi=
ch
> is the decay product of a now-extinct radioactive isotope of samarium of
> mass 146 (146Sm) that was present at the birth of the solar system but
> decayed away shortly thereafter. The excess in 142Nd allowed the
> researchers to determine when the composition of the Earth diverged from
> that of the meteorites--within the first 30 million years after solar
> system formation, which is less than 1% of the age of our planet.
>
> To explain the excess of 142Nd found in the terrestrial samples, the
> Carnegie scientists believe that the Earth was largely molten during its
> formation and that rapid crystallization of Earth's early magma ocean
> caused the mantle to separate into chemically distinct layers, one
> containing a high ratio of Sm to Nd similar to that observed today in the
> mantle source of the volcanism along ocean ridges. The complementary
> reservoir, with low 142Nd abundance, has never been sampled at the surface
> and hence could now be deeply buried in the so-called D" layer at the very
> base of the mantle, above the core. This "missing" layer should be rich in
> the elements uranium, thorium, and potassium, whose long-lived radioactive
> decay heats Earth's interior and causes our planet to remain geologically
> active. This hot layer above the core could help to keep the outer core
> molten so that circulation of liquid iron can produce Earth's magnetic
> field, and it could instigate the hot plumes of upwelling mantle material
> that give rise to volcanically active islands, such as Hawaii.
>
> Measurements by Boyet and Carlson also show that lunar rocks have the same
> abundance of 142Nd as the terrestrial samples, a finding that adds to the
> evidence that the Moon formed from the Earth. Since Mars also experienced
> early melting, as indicated by the chemical and isotopic composition of
> Martian meteorites, the new results now link the early evolution of Earth,
> Moon, and Mars and highlights the importance of early events in determini=
ng
> the chemical characteristics of the terrestrial planets.
>
> "The work of Boyet and Carlson, when added to what has already been
> determined for the Moon and Mars, shows that the earliest days of the inn=
er
> planets were violent times in solar system history," adds DTM director Se=
an
> Solomon. "Theoretical work by Carnegie scientist George Wetherill had
> pointed to this result, but now we have a clear chemical signature of this
> episode of Earth history."
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