The morphology, evolution and seismic visibility of partial melt at the core-mantle boundary: Implications for ULVZs

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Authors

Juliane Dannberg , Robert Myhill , Rene Gassmoeller , Sanne Cottaar 

Abstract

Seismic observations indicate that the lowermost mantle above the core-mantle boundary is strongly heterogeneous. Body waves reveal a variety of ultra-low velocity zones (ULVZs), which extend not more than 100 km above the core-mantle boundary and have shear velocity reductions of up to 30%. While the nature and origin of these ULVZs remain uncertain, some have suggested they are evidence of partial melting at the base of mantle plumes.

Here we use coupled geodynamic/thermodynamic modelling to explore the hypothesis that present-day deep mantle melting creates ULVZs and introduces compositional heterogeneity in the mantle. Our models explore the generation and migration of melt in a deforming and compacting host rock at the base of a plume in the lowermost mantle. We test whether the balance of gravitational and viscous forces can generate partially molten zones that are consistent with the seismic observations.

We find that for a wide range of plausible melt densities, permeabilities and viscosities, lower mantle melt is too dense to be stirred into convective flow and instead sinks down to form a completely molten layer, which is inconsistent with observations of ULVZs. Only if melt is less dense or at most ca. 1% more dense than the solid, or if melt pockets are trapped within the solid, can melt remain suspended in the partial melt zone. In these cases, seismic velocities would be reduced in a cone at the base of the plume. Generally, we find partial melt alone does not explain the observed ULVZ morphologies and solid-state compositional variation is required to explain the anomalies.
Our findings provide a framework for testing whether seismically observed ULVZ shapes are consistent with a partial melt origin, which is an important step towards constraining the nature of the heterogeneities in the lowermost mantle and their influence on the thermal, compositional, and dynamical evolution of the Earth.

DOI

https://doi.org/10.31223/X5532R

Subjects

Geophysics and Seismology

Keywords

Structure of the Earth, Composition and structure of the mantle, Numerical modelling, Magma genesis and partial melting, Mantle processes, Composition and structure of the mantle, Magma genesis and partial melting, Mantle processes

Dates

Published: 2021-04-14 11:33

License

CC BY Attribution 4.0 International

Additional Metadata

Data Availability (Reason not available):
All software and data used to generate the results and figures in this manuscript are freely available. The specific files and instructions are published as a Zenodo data package (Gassmoeller et al., 2021) and include the source code and links to the GitHub versions of both BurnMan and ASPECT as well as the input files to compute the models and plotting scripts to generate the figures.

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