UA GEOS 474A - Research Paper (6 pages)

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Research Paper



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Research Paper

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Pages:
6
School:
University of Arizona
Course:
Geos 474a - Geochronology and Thermochronology

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RESEARCH ARTICLES 142Nd Evidence for Early 94 53 Ga Global Differentiation of the Silicate Earth M Boyet and R W Carlson New high precision samarium neodymium isotopic data for chondritic meteorites show that their 142Nd 144Nd ratio is 20 parts per million lower than that of most terrestrial rocks This difference indicates that most 70 to 95 of Earth s mantle is compositionally similar to the incompatible element depleted source of mid ocean ridge basalts possibly as a result of a global differentiation 4 53 billion years ago Ga within 30 million years of Earth s formation The complementary enriched reservoir has never been sampled and is probably located at the base of the mantle These data influence models of Earth s compositional structure and require revision of the timing of global differentiation on Earth s Moon and Mars Radiogenic isotope tracers are often used to understand the chemical evolution of planetary bodies In one system Sm decays to Nd via two radioactive decay schemes 146Sm 142Nd Ehalf life T1 2 0 103 million years My and 147Sm 143Nd ET 1 2 0 106 billion years Gy Both Sm and Nd are refractory lithophile prefer silicates over metal elements whose relative abundances should not be affected by either volatile loss or core formation The longlived 147Sm 143Nd system has been widely used to trace planetary scale processes such as the evolution of the bulk silicate Earth BSE defined as all the Earth except for its metallic core and its chemical differentiation into crust and mantle over Earth s history The early epic of Earth s differentiation is better investigated with the short lived chronometer 146Sm 142Nd because of the lack of available samples from Earth s first 500 My of existence and the sensitivity of most long lived radioactive systems to resetting by events occurring later in Earth s history Recent studies have reported small 142Nd 144Nd excesses in a small number of samples from 3 8 Ga from the Isua Supracrustal Belt Greenland 1 2 which is



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