A Mercury-like component of early Earth yields uranium in the core and high mantle 142Nd.

• Autores: Anke Wohlers, Bernard Wood
• Localización: Nature: International weekly journal of science, ISSN 0028-0836, Vol. 520, Nº 7547, 2015, págs. 341-344
• Idioma: inglés
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• Resumen

  • Recent 142Nd isotope data indicate that the silicate Earth (its crust plus the mantle) has a samarium to neodymium elemental ratio (Sm/Nd) that is greater than that of the supposed chondritic building blocks of the planet. This elevated Sm/Nd has been ascribed either to a 'hidden' reservoir in the Earth 1,2 or to loss of an early-formed terrestrial crust by impact ablation 3. Since removal of crust by ablation would also remove the heat-producing elements-potassium, uranium and thorium-such removal would make it extremely difficult to balance terrestrial heat production with the observed heat flow 3. In the 'hidden' reservoir alternative, a complementary low-Sm/Nd layer is usually considered to reside unobserved in the silicate lower mantle. We have previously shown, however, that the core is a likely reservoir for some lithophile elements such as niobium 4. We therefore address the question of whether core formation could have fractionated Nd from Sm and also acted as a sink for heat-producing elements. We show here that addition of a reduced Mercury-like body (or, alternatively, an enstatite-chondrite-like body) rich in sulfur to the early Earth would generate a superchondritic Sm/Nd in the mantle and an 142Nd/144Nd anomaly of approximately +14 parts per million relative to chondrite. In addition, the sulfur-rich core would partition uranium strongly and thorium slightly, supplying a substantial part of the 'missing' heat source for the geodynamo.