Chemical geodynamics in a non-chondritic earth

Abstract

Over the past 30 years, chemical geodynamic models held that terrestrial silicate reservoirs differentiated from a primitive mantle (PM) with chondritic abundances of refractory lithophile elements. This basic assumption has had major consequences for our understanding of the mantle’s structure and composition. It is well established, for example, that in a chondritic Earth, the present-day continental mass does not contain a large enough budget of trace elements to balance depletion of the whole mantle. This result was taken as evidence that a hidden reservoir, either primitive or enriched in incompatible elements, segregated early in the history of the Earth and remained since then isolated from the convective system. This common view, however, is now being reconsidered, as 146Sm-142Nd studies of planetary and meteoritic material show that the bulk silicate Earth may in fact not have perfectly chondritic abundances of refractory lithophile elements, as has been assumed. This observation represents a challenge to compositional models of the Earth, which are systematicallyanchored on chondritic reference parameters. Chemical geodynamic models, based on mass balance relationships between the depleted, primitive, and enriched silicatereservoirs, would also need to be reconsidered. Here, I present a new set of reference parameters for the Sm-Nd, Lu-Hf, and Rb-Sr systems consistent with the presence of an 18 ppm 142Nd excess in the bulk silicate Earth. The trace element pattern obtained for the PM using this super-chondritic Earth model (SCHEM) suggests that the proto-Earth accreted from material depleted in incompatible elements compared to chondrites, most likely as a result of preferential impact erosion of shallow crustal reservoirs during the early stage of accretion. The isotopic and trace element signature of the (non-chondritic) PM matches that observed in most high 3He/4He oceanic island basalts, suggesting that pristine material may have been preserved in the interior of the Earth for the past 4.45Ga and is now sampled by modern plume magmatism. These primordial heterogeneities, however, are unlikely to represent a large reservoir, as most of the mantle appears to have been thoroughly degassed and depleted by crustal extraction. In a nonchondritic Earth, the compositional evolution of the “accessible” mantle appears to be essentially controlled by the continuous growth of the continental crust over the past 3.8 Ga and does not require the presence of hidden mantlereservoirs, either enriched or primitive.