Towards consistent seismological models of the core-mantle boundary landscape

2020 ◽  
Author(s):  
Paula Koelemeijer
Keyword(s):  
Normal Mode ◽  
Lower Mantle ◽  
Seismic Velocity ◽  
Body Wave ◽  
P Wave ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Trade Offs ◽  
Multiple Data Sets

<p>The dynamic topography of the core-mantle boundary (CMB) provides important constraints on dynamic processes in the mantle and core. However, inferences on CMB topography are complicated by the uneven coverage of data with sensitivity to different length scales and strong heterogeneity in the lower mantle. Particularly, a trade-off exists with density variations, which ultimately drive mantle flow and are vital for determining the origin of mantle structures. Here, I review existing models of CMB topography and lower mantle density, focusing on seismological constraints (Koelemeijer, 2020). I develop average models and vote maps with the aim to find model consistencies and discuss what these may teach us about lower mantle structure and dynamics.</p><p>While most density models image two areas of dense anomalies beneath Africa and the Pacific, their exact location and relationship to seismic velocity structure differs between studies. CMB topography strongly influences the retrieved density structure, which partially helps to resolve differences between recent studies based on Stoneley modes and tidal measurements. CMB topography models vary both in pattern and amplitude and a discrepancy exists between models based on body-wave and normal-mode data. As existing models typically feature elevated topography below the Large-Low-Velocity Provinces (LLVPs), very dense compositional anomalies may be ruled out as possibility.</p><p>To achieve a similar consistency as observed in lower mantle models of S-wave and P-wave velocity, future studies should combine multiple data sets to break existing trade-offs between CMB topography and density. Important considerations in these studies should be the choice of theoretical approximation and parameterisation. Efforts to develop models of CMB topography consistent with both body-wave and normal-mode data should be intensified, which will aid in narrowing down possible explanations for the LLVPs and provide additional insights into mantle dynamics.</p><p><em>Koelemeijer, P. (2020), “Towards consistent seismological models of the core-mantle boundary landscape”. Book chapter in revision for AGU monograph "Mantle upwellings and their surface expressions", edited by Marquardt, Cottaar, Ballmer and Konter</em></p>

scholarly journals Hydrous SiO2 in subducted oceanic crust and water transport to the core-mantle boundary

2020 ◽  
Author(s):  
Yanhao Lin ◽  
Qingyang Hu ◽  
Jing Yang ◽  
Yue Meng ◽  
Yukai Zhuang ◽  
...  
Keyword(s):  
Partial Melting ◽  
Oceanic Crust ◽  
Lower Mantle ◽  
Oceanic Lithosphere ◽  
Low Velocity ◽  
Water Storage Capacity ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Subducted Oceanic Crust

Abstract Subduction of oceanic lithosphere transports surface water into the mantle where it can have remarkable effects, but how much can be cycled down into the deep mantle, and potentially to the core, remains ambiguous. Recent studies show that dense SiO2 in the form of stishovite, a major phase in subducted oceanic crust at depths greater than ~300 km, has the potential to host and carry water into the lower mantle. We investigate the hydration of stishovite and its higher-pressure polymorphs, CaCl2-type SiO2 and seifertite, in experiments at pressures of 44–152 GPa and temperatures of ~1380–3300 K. We quantify the water storage capacity of these dense SiO2 phases at high pressure and find that water stabilizes CaCl2-type SiO2 to pressures beyond the base of the mantle. We parametrize the P-T dependence of water capacity and model H2O storage in SiO2 along a lower mantle geotherm. Dehydration of slab mantle in cooler slabs in the transition zone can release fluids that hydrate stishovite in oceanic crust. Hydrous SiO2 phases are stable along a geotherm and progressively dehydrate with depth, potentially causing partial melting or silica enrichment in the lower mantle. Oceanic crust can transport ~0.2 wt% water to the core-mantle boundary region where, upon heating, it can initiate partial melting and react with the core to produce iron hydrides, providing plausible explanations for ultra-low velocity regions at the base of the mantle.

scholarly journals Properties of the Core-Mantle Boundary

1988 ◽  
Vol 129 ◽  
pp. 377-377
Author(s):  
Bradford H. Hager
Keyword(s):  
Mechanical Properties ◽  
Thermal Boundary Layer ◽  
Seismic Velocity ◽  
Velocity Versus ◽  
Compositional Gradient ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
The Earth ◽  
Thick Region

The core-mantle boundary (CMB), separating the molten metallic core from the overlying solid silicate mantle, marks the largest discontinuity in mechanical properties within the Earth. The ∼ 200 km thick region just above the CMB, named D″ by Bullen (1950), is characterized by an anomalous gradient in seismic velocity versus depth. D″ was originally interpreted as a region with a strong compositional gradient due to the accumulation of dense material at the base of the mantle. Subsequently, the anomalous gradient was interpreted as the result of a strong temperature gradient in a hot thermal boundary layer at the base of the mantle, an interpretation motivated by the requiremnet that heat involved in generating the geodynamo must be transported out of the core and through the mantle by convection.

The origin of short-period precursors to PKP

1975 ◽  
Vol 65 (3) ◽  
pp. 765-786
Author(s):  
C. Wright
Keyword(s):  
Wave Energy ◽  
Wave Train ◽  
Outer Core ◽  
Temporal Changes ◽  
P Wave ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Short Period ◽  
Lowermost Mantle

abstract An investigation of the origin of precursors to short-period PKP phases has been undertaken using 23 earthquakes recorded at the Yellowknife Array at distances between 123° and 143°. In particular, the pattern of slowness and azimuth changes with time has been examined for coherent bursts of energy occurring throughout the precursor wave train. These temporal changes demonstrate that the precursor energy is most satisfactorily explained by scattering from small inhomogeneities at the core-mantle boundary or in the lowermost mantle, both before P-wave energy enters the core and when it re-emerges into the mantle. Moreover, scattering before entry into the core seems to generate the larger amplitudes. The bulk of the data cannot be attributed to reflection or sharp upward refraction from velocity discontinuities within the lower part of the outer core, although there is some ambiguous evidence for a reflecting interface at a depth of about 4850 km.

Sign in / Sign up

Export Citation Format

Share Document