Composition of the low seismic velocity E′ layer at the top of Earth's core

John Brodholt,James Badro

doi:10.1002/2017gl074261

Abstract

AbstractUsing ab initio simulations on Fe‐Ni‐S‐C‐O‐Si liquids, we constrain the origin and composition of the low‐velocity layer E′ at the top of Earth's outer core. We find that increasing the concentration of any light element always increases velocity and so a low‐velocity and low‐density layer (for stability) cannot be made by simply increasing light element concentration. This rules out barodiffusion or simple sedimentation of a light phase for its origin. However, exchanging elements can—depending on the elements exchanged—produce such a layer. We evaluate three possibilities. First, crystalization of a phase from a core may make such a layer, but only if the core contains more than one light element and only if crystalizing phase is very Fe rich. Second, the E′ layer may result from incomplete mixing of an early Earth core with a late impactor, depending on the light element compositions of the impactor and Earth's core. Third, using thermodynamic models for metal‐silicate partitioning, we show that a reaction between the core and an FeO‐rich basal magma ocean can result in a light and slow layer.

Highlights

It has long been recognized that the Earth’s outer core may have layers at both its top [Garnero et al, 1993] and bottom [Souriau and Poupinet, 1991]
4. a reaction with a pyrolitic mantle or with an FeO-enriched lowermost mantle, such as that expected from a residual magma ocean or from the accumulation of FeO rich cumulates at the CMB, can produce the characteristics of the E0 layer
It remains to be seen if a primordial layer dating back to the Moon-forming impact could survive to this day, and if a postimpact stratified outer core is consistent with thermodynamic constraints on the core [Nakajima and Stevenson, 2015] and geomagnetic constraints Buffett [2010], such as MAC waves [Buffett et al, 2016]

Summary

Introduction

It has long been recognized that the Earth’s outer core may have layers at both its top [Garnero et al, 1993] and bottom [Souriau and Poupinet, 1991]. A layer with both reduced velocities and densities is problematic This is because the simplest way of producing a light layer is to increase the concentration of a light element by some mechanism, whether by barodiffusion [Gubbins and Davies, 2013] or dissolving light element from the mantle [Buffett and Seagle, 2010]. The problem with this is that it is generally expected that the concentration of light elements affects the density more than the bulk modulus and so the layer should have increased velocities, not lower as observed.

Methods

Results

Conclusion