Connections between the bulk composition, geodynamics and habitability of Earth

Abstract

NATURE GEOSCIENCE | ADVANCE ONLINE PUBLICATION | www.nature.com/naturegeoscience 1 T he growth, internal differentiation, and geodynamic evolution of our planet can be partly constrained by the average composition of the silicate part of Earth (bulk silicate Earth). This basic constraint on Earth's history is difficult to infer, however, because geochemists cannot adequately sample a planet composed of oceanic and continental crust, a compositionally heterogeneous mantle, and a core. Owing to the heterogeneous make-up of Earth and necessarily incomplete sampling of the accessible parts of Earth, one approach to predict the bulk silicate Earth composition is to use models for Earth's formation. A long-standing conceptual picture for Earth's composition assumes that it formed by accretion of chondritic precursors, which are the most primitive, undifferentiated objects in the solar system (Box 1). All classes of chondrites have relatively constant ratios of elements that are both refractory (elements with high condensation temperatures) and lithophile (elements that remain in the silicate portion of Earth, including the crust and mantle), such as Samarium (Sm) and Neodymium (Nd). These elements are not fractionated during condensation from a hot nebular gas 1-4 or during separation of a metallic core with relatively low sulphur concentrations 5 . Therefore, ratios of the refractory lithophile elements in the silicate Earth have long been assumed to be the same as in chondrites, and the bulk silicate Earth has chondritic Sm/Nd ratios (ref. 2). The 146 Sm-142 Nd decay scheme ( 146 Sm half-life = 68 to 103 million years; ref. 6) provides a way to test this model: if the bulk silicate Earth and the chondrite reservoir have the same Sm/Nd, the 142 Nd/ 144 Nd of Earth and ordinary chondrites should be the same. The discovery that the accessible, modern terrestrial mantle has 142Nd/ 144 Nd that is 18 ± 5 ppm (0.018 ± 0.005‰) higher than ordinary chondrites, which exhibit isotopic compositions representative of planetary building material 4,7,8 , challenges the assumption that Earth has chondritic Sm/Nd ratios (refs 9-12). While there are other models for the origin of the superchondritic 142 Nd/ 144 Nd in the bulk silicate Earth (Box 1), we explore the hypothesis that the higher 142 Nd/ 144 Nd in the modern terrestrial mantle relative to ordinaryThe bulk composition of the silicate part of Earth has long been linked to chondritic meteorites. Ordinary chondrites -the most abundant meteorite class -are thought to represent planetary building materials. However, a landmark discovery showed that the 142 Nd/ 144 Nd ratio of the accessible parts of the modern terrestrial mantle on Earth is greater than that of ordinary chondrites. If Earth was derived from these precursors, mass balance requires that a missing reservoir with 142 Nd/ 144 Nd lower than ordinary chondrites was isolated from the accessible mantle within 20 to 30 million years of accretion. This reservoir would host the equivalent of the modern continents' budget of ...

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