Abstract
Recent results (Wicks et al., 2010) suggest that a mixture of iron-enriched (Mg,Fe)O and ambient mantle is consistent with wavespeed reductions and density increases inferred for ultralow-velocity zones (ULVZs). We explore this hypothesis by simulating convection to deduce the stability and morphology of such chemically-distinct structures. The buoyancy number, or chemical density anomaly, largely dictates ULVZ shape, and the prescribed initial thickness (proxy for volume) of the chemically-distinct layer controls its size. We synthesize our dynamic results with a Voigt–Reuss–Hill mixing model to provide insight into the inherent seismic tradeoff between ULVZ thickness and wavespeed reduction. Seismic data are compatible with a solid-state origin for ULVZs, and a suite of these structures may scatter seismic energy to produce broadband PKP precursors.
Highlights
The large chemical, density, and dynamical contrasts associated with the juxtaposition of liquid iron-dominant alloy and solid silicates at the coremantle boundary (CMB) are associated with a rich range of complex seismological features
We are primarily concerned with the long-term stability and morphology of ultralow-velocity zones (ULVZs), during this time we make several observations
The chemical layer develops significant relief caused by the developing plumes, until the plumes merge and a large volume fraction of the layer is expelled from the CMB region (Fig. 1C)
Summary
The large chemical, density, and dynamical contrasts associated with the juxtaposition of liquid iron-dominant alloy and solid silicates at the coremantle boundary (CMB) are associated with a rich range of complex seismological features. Seismic heterogeneity at this boundary includes small patches of anomalously low sound velocities, called ultralow-velocity zones. Their small size (5 to 40 km thick) (e.g., Garnero and Helmberger , 1996) and depth (>2800 km) present unique challenges for seismic characterization. Recent thermochemical convection calculations lend support to these spatial correlations (McNamara et al, 2010)
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