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Press release
The young Earth's early history | |||
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Paris, May 22, 2003 |
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from the "Laboratoire de physique et chimie des interactions géologiques"
(Institut de Physique du Globe de Paris* - CNRS and Université
Denis-Diderot Paris 7), in conjunction with the University of Oxford, have
developed a new method for the analysis of neodymium (Nd) in order to date
the formation of the Earth's primitive crust, which was extracted from the
mantle approximately 4.46 billion years ago. It formed on the surface of
the Earth shortly after the formation of the core and the terrestrial atmosphere,
about 100 million years after the beginning of the formation of the solar
system. These results were published in the May 22 issue of the journal
Nature. To retrace the evolution of the Earth during its first hundreds of millions of years, geochemists use radioactive isotopes as tracers and references. The isotopic tracer analyzed here (neodymium 142) comes from the radioactive decrease of samarium 146, a chemical element produced in the stars by synthesis reactions of the core; today it is no longer found on Earth. Guillaume Caro, Bernard Bourdon, Jean-Louis Birck ("Laboratoire de physique et chimie des interactions géologiques")** and Stephen Moorbath (University of Oxford) have analyzed certain isotopes of neodymium contained in extremely small amounts in rocks from the Isua greenstone belt of West Greenland, which are dated to be 3.75 billion years old. The oldest terrestrial rocks date to approximately 3.7 to 4 billion years ago; they are found especially in the shield of Greenland. They are old metamorphosed sedimentary rocks from the erosion of basalt rocks, of which they have retained the geochemical signature. The lack of rocks older than 4 billion years obscures the very early history of the Earth's crust. The Isua rocks present a very low neodymium 142 anomaly compared to modern terrestrial rocks: an enrichment of 15 millionths. This anomaly comes not only from the basalt rocks from which the subsequent rocks originated, but also from the Earth's crust, from which the basalts were extracted. Basalts are formed by the fusion of rocks in the mantle and thus inherit their geochemical characteristics. The researchers compared these traces of neodymium in the rocks by determining their quantity as compared to that found in meteorites, which serve as a reference for the global composition of the Earth and the solar system. It took the scientists nearly two years to develop a method of analysis using a new generation mass spectrometer, which separates the elements and increases fivefold the precision of the measurement of neodymium142. These measurements show that a primitive crust had been extracted from the mantle approximately 4.46 billion years ago. This age seems to coincide with the end of the accretion period of the planetary bodies which formed the Earth 4.55 billion years ago. At the time, the Earth was probably already very similar to the Earth as we know it, in terms of its dynamics and internal structure in concentric layers, with a core composed essentially of iron in the center; a mantle made of iron- and magnesium-rich silicated rocks; and a crust that is richer in silicium and aluminum than the mantle. According to certain theories, the energy of these giant impacts must have been sufficient to cause the fusion of the mantle on a planetary scale, leading to the formation of a magma ocean. The extraction of a primitive crust could thus have followed the cooling and crystallization of a terrestrial magma ocean, approximately 100 million years after the beginning of the formation of the solar system and shortly after the formation of the core (formed in 30 million years) and the terrestrial atmosphere (in 100 million years). It therefore appears that the Earth with its core, mantle, crust and atmosphere developed extremely rapidly. * http://www.ipgp.jussieu.fr ** Geochemistry and cosmochemistry team: http://www.ipgp.jussieu.fr/francais/rub-recherche/eq05geochimie-et-cosmochimie/acc05.html Researcher
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