Effects of magma ocean crystallization and overturn on the development of 142Nd and 182W isotopic heterogeneities in the primordial mantle

Stephanie M. Brown, Linda Elkins-Tanton, Richard J. Walker

Research output: Contribution to journalArticle

15 Citations (Scopus)

Abstract

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.

Original languageEnglish (US)
Pages (from-to)319-330
Number of pages12
JournalEarth and Planetary Science Letters
Volume408
DOIs
StatePublished - Dec 5 2014

Fingerprint

overturn
Crystallization
magma
oceans
Earth mantle
crystallization
Earth (planet)
mantle
ocean
early Earth
Chemical analysis
cumulate
Liquid Crystals
Trace Elements
Fractionation
shear
Solidification
Quartz
liquid
Iron

Keywords

  • Nd
  • W
  • Early Earth
  • Hidden reservoir
  • LLSVPs
  • Magma ocean

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Cite this

Effects of magma ocean crystallization and overturn on the development of 142Nd and 182W isotopic heterogeneities in the primordial mantle. / Brown, Stephanie M.; Elkins-Tanton, Linda; Walker, Richard J.

In: Earth and Planetary Science Letters, Vol. 408, 05.12.2014, p. 319-330.

Research output: Contribution to journalArticle

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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.

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