Reconciling magma‐ocean crystallization models with the present‐day structure of the Earth's mantle

Abstract

Terrestrial planets are thought to experience episode(s) of large‐scale melting early in their history. Fractionation during magma‐ocean freezing leads to unstable stratification within the related cumulate layers due to progressive iron enrichment upward, but the effects of incremental cumulate overturns during MO crystallization remain to be explored. Here, we use geodynamic models with a moving‐boundary approach to study convection and mixing within the growing cumulate layer, and thereafter within the fully crystallized mantle. For fractional crystallization, cumulates are efficiently stirred due to subsequent incremental overturns, except for strongly iron‐enriched late‐stage cumulates, which persist as a stably stratified layer at the base of the mantle for billions of years. Less extreme crystallization scenarios can lead to somewhat more subtle stratification. In any case, the long‐term preservation of at least a thin layer of extremely enriched cumulates with Fe# > 0.4, as predicted by all our models, is inconsistent with seismic constraints. Based on scaling relationships, however, we infer that final‐stage Fe‐rich magma‐ocean cumulates originally formed near the surface should have overturned as small diapirs, and hence undergone melting and reaction with the host rock during sinking. The resulting moderately iron‐enriched metasomatized/hybrid rock assemblages should have accumulated at the base of the mantle, potentially fed an intermittent basal magma ocean, and be preserved through the present‐day. Such moderately iron‐enriched rock assemblages can reconcile the physical properties of the large low shear‐wave velocity provinces in the present‐day lower mantle. Thus, we reveal Hadean melting and rock‐reaction processes by integrating magma‐ocean crystallization models with the seismic‐tomography snapshot.

Lateral Mixing Processes in the Hadean

“…A similar effect might be induced by a localized (e.g., hemispheric) subduction event. Other possibilities for the origin of these anomalies may include postmagma ocean heterogeneity (e.g., Ballmer, Lourenço, et al, ; Boyet & Carlson, ; Maurice et al, ), early degree‐1 overturn events (e.g., Debaille et al, ), or, conceivably, a legacy of the Moon‐forming impact (Canup, ).…”

“…However, the extreme sensitivity of mixing models to the proportion of the weak phase means that compositional variations within this range could give rise to domains in the lower mantle of extreme viscosity (bridgmanite‐enriched ancient mantle—or BEAMS). Ballmer, Lourenço, et al () demonstrate that these BEAMS may be preserved in a convecting mantle over the lifetime of a planet.…”

Core‐Exsolved SiO2 Dispersal in the Earth's Mantle

“…To determine the time scale for the rate of growth of a Rayleigh-Taylor instability at the CMB, we use a modified form of the theory of Ballmer et al (2017). The layer thickness is governed by the time that it takes for the layer to become buoyantly unstable.…”

“…In their earlier analysis, Ballmer et al (2017) used a 2-D Cartesian geometry (Turcotte & Schubert, 2002), but here we use a 3-D treatment to account for the two-dimensionality of the CMB. Ribe (1998) developed a theory of the Rayleigh-Taylor instability that describes the rate of growth of diapirs forming on a 2-D surface when denser material overlies less dense material.…”

“…The layer thickness is governed by the time that it takes for the layer to become buoyantly unstable. To determine the time scale for the rate of growth of a Rayleigh-Taylor instability at the CMB, we use a modified form of the theory of Ballmer et al (2017). In it, the critical layer thickness, b crit is given by…”