Effects of Magma Ocean Crystallization and Overturn on the Development of Nd-142 and W-182 Isotopic Heterogeneities in the Primordial Mantle

One possible mechanism to explain the observed variability of the short- lived ¹⁴⁶Sm → ¹⁴²Nd and ¹⁸²Hf → ¹⁸²W systems recorded in some early Earth rocks is crystal-liquid fractionation and overturn in an early magma ocean. This process could also potentially explain the deviation between the ¹⁴²Nd isotopic composition of the accessible Earth and the chondritic average. To examine these effects, the magma ocean solidification code of Elkins-Tanton (2008) and a modified Monte Carlo algorithm, designed to randomly choose physically reasonable trace element partition coefficients in crystallizing mantle phases, are used to model the isotopic evolution of early Earth reservoirs. This model, also constrained by the ¹⁴³Nd composition of the accessible Earth, explores the effects of changing the amount of interstitial liquid trapped in cumulates, the half-life of ¹⁴⁶Sm, the magnitude of late accretion, and the simplified model of post-overturn reservoir mixing. Regardless of the parameters used, our results indicate the generation of early mantle reservoirs with isotopic characteristics consistent with observed anomalies is a likely outcome of magma ocean crystallization and overturn of shallow, enriched, and dense (i.e., gravitationally unstable) cumulates. The high-iron composition and density of a hypothesized, early-formed enriched mantle reservoir is compatible with seismic observations indicating large, low-shear velocity provinces (LLSVPs) (e.g., Trampert et al., 2004) present in the mantle today. Later melts of an enriched reservoir are likely to have remained isolated deep within the mantle (e.g., Thomas et al., 2012), consistent with the possibility that the presently observed LLSVPs could be partially or fully composed of remnants of an early enriched reservoir.