TY - JOUR
T1 - Effects of magma ocean crystallization and overturn on the development of 142Nd and 182W isotopic heterogeneities in the primordial mantle
AU - Brown, Stephanie M.
AU - Elkins-Tanton, Linda
AU - Walker, Richard J.
N1 - Funding Information:
This work was supported by NSF CSEDI grants EAR1265169 to RJW, EAR1160656 to LTET, and NASA grant NNX12AH80G to TLG. We extend our gratitude to Timothy Grove, Alexandra Andrews, Ann Bauer, and Benjamin Mandler for helpful discussion and to Christopher Reeder for helpful discussion about algorithms. We graciously thank Maud Boyet and two anonymous reviewers for their thoughtful comments and questions, which improved this paper.
Publisher Copyright:
© 2014 Elsevier B.V.
PY - 2014/12/5
Y1 - 2014/12/5
N2 - One possible mechanism to explain the observed variability of the short-lived Sm146→Nd142 and Hf182→W182 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.
AB - One possible mechanism to explain the observed variability of the short-lived Sm146→Nd142 and Hf182→W182 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.
KW - Early Earth
KW - Hidden reservoir
KW - LLSVPs
KW - Magma ocean
KW - Nd
KW - W
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U2 - 10.1016/j.epsl.2014.10.025
DO - 10.1016/j.epsl.2014.10.025
M3 - Article
AN - SCOPUS:84910098303
SN - 0012-821X
VL - 408
SP - 319
EP - 330
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -