Compaction and Melt Redistribution within UltraLow Velocity Zones (ULVZs)
Abstract
Geophysical evidences at the interface between the Earth's core-mantle boundary reveals a patchwork of low-amplitude structures of high density (8-10%) and possibly low viscosity materials. These patchy structures -UltraLow Velocity Zones (ULVZs), show up to 30% reduction in shear and 10% reduction in compressional wave speeds. Such anomalies are best explained by compositional heterogeneity, as supported by recent studies showing a presence of up-to 10 vol% of neutrally buoyant melt. Recent studies suggest strong coupling between the flow within ULVZs and large scale mantle structures, such as Large Low Shear Velocity Provinces (LLSVPs) and mantle plumes likely aided by the low viscosity of the ULVZs. There is, however, a lack of general understanding about how the melt migration within ULVZs respond to different kinds of mantle flow patterns. This study investigates such mechanical conditions, and their resultant flow dynamics, on the evolution of ULVZs. We employ a set of coupled conservation equations to solve the two-phase flow behaviors, by treating the high viscous part as `matrix' phase and lower viscosity `melt' phase occupying the interstices of the matrix. The finite-element simulations involve two types of boundary conditions, impervious vertical walls (no-slip conditions) and zero net material influx through the vertical walls (free-slip conditions) supplemented with shear and no-shear conditions at top boundary. Results suggest the choice of boundary conditions have significant implication for drainage of the dense melt fraction, which governs the evolution of ULVZs. Shearing inhibits down welling of the melt fraction and tends to store them at the top of ULVZs, in contrast to models with no-shear where the melt percolates globally.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMDI41B2625H
- Keywords:
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- 5430 Interiors;
- PLANETARY SCIENCES: SOLID SURFACE PLANETSDE: 8121 Dynamics: convection currents;
- and mantle plumes;
- TECTONOPHYSICSDE: 8124 Earth's interior: composition and state;
- TECTONOPHYSICSDE: 8147 Planetary interiors;
- TECTONOPHYSICS