scholarly journals Seismic array detection of subducted oceanic crust in the lower mantle

2008 ◽  
Vol 113 (B6) ◽  
Author(s):  
Sebastian Rost ◽  
Edward J. Garnero ◽  
Quentin Williams
Keyword(s):  
Oceanic Crust ◽  
Lower Mantle ◽  
Seismic Array ◽  
Subducted Oceanic Crust ◽  
Array Detection

scholarly journals Seismic scatterers in the lower mantle near subduction zones

2019 ◽  
Vol 219 (Supplement_1) ◽  
pp. S2-S20 ◽  
Author(s):  
Satoshi Kaneshima
Keyword(s):  
Oceanic Crust ◽  
Lower Mantle ◽  
Subduction Zones ◽  
P Waves ◽  
Waveform Data ◽  
Basaltic Rocks ◽  
Short Period ◽  
Subducted Oceanic Crust ◽  
Conversion Depth ◽  
Seismic Networks

SUMMARY We investigate the global distribution of S-to-P scatterers in the shallow to mid-lower mantle beneath subduction zones, where deep seismicity extends down to the bottom of the upper mantle. By array processing broadband and short period waveform data obtained at seismic networks, we seek anomalous later phases in the P coda within about 15–150 s after direct P waves. The later phases usually arrive along off-great circle paths and significantly later than S-to-P conversion from the ‘660 km’ discontinuity, often show positive slowness anomalies relative to direct P, and do not show a conversion depth that is consistent among nearby events. They are thus adequately regarded as scattered waves, rather than conversion at a global horizontal discontinuity. The S-to-P scattered waves often show amplitudes comparable to ‘S660P’ waves, which indicates that a spatial change in elastic properties by several percent occurs at the scatterers as abruptly as the post-spinel transformation and should arise from compositional heterogeneity. We locate prominent S-to-P scatterers beneath Pacific subduction zones and beneath southern Spain. Nearly half of 137 S-to-P scatterers located in this study and previous studies by the authors are shallower than 1000 km, and the number of scatterers decreases with depth. Scatterers deeper than 1800 km are rare and mostly weak. We examine relations between the locations of the scatterers and recently subducted slabs inferred from seismic tomography. The scatterers of mid-mantle depths, deeper than about 1000 km, are located distant from tomographic slabs. On the other hand, the majority of shallower scatterers are located beneath the slabs rather than near their fastest portions, which would indicate that chemically heterogeneous materials are not extensively entrained within thickened and folded slabs when the slabs impinge on the lower mantle. We also find scatterers near the locations where basaltic rocks of recently subducted oceanic crust are expected to exist, which suggests that oceanic crust is not delaminating when slabs impinge on the lower mantle.

scholarly journals Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Wenzhong Wang ◽  
Yinhan Xu ◽  
Daoyuan Sun ◽  
Sidao Ni ◽  
Renata Wentzcovitch ◽  
...  
Keyword(s):  
Phase Transition ◽  
Oceanic Crust ◽  
Seismic Tomography ◽  
Lower Mantle ◽  
Shear Velocity ◽  
Calcium Ferrite ◽  
Velocity Anomaly ◽  
Mantle Heterogeneities ◽  
Subducted Oceanic Crust ◽  
Low Shear

AbstractSeismic heterogeneities detected in the lower mantle were proposed to be related to subducted oceanic crust. However, the velocity and density of subducted oceanic crust at lower-mantle conditions remain unknown. Here, we report ab initio results for the elastic properties of calcium ferrite‐type phases and determine the velocities and density of oceanic crust along different mantle geotherms. We find that the subducted oceanic crust shows a large negative shear velocity anomaly at the phase boundary between stishovite and CaCl2-type silica, which is highly consistent with the feature of mid-mantle scatterers. After this phase transition in silica, subducted oceanic crust will be visible as high-velocity heterogeneities as imaged by seismic tomography. This study suggests that the presence of subducted oceanic crust could provide good explanations for some lower-mantle seismic heterogeneities with different length scales except large low shear velocity provinces (LLSVPs).

scholarly journals Author Correction: Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Wenzhong Wang ◽  
Yinhan Xu ◽  
Daoyuan Sun ◽  
Sidao Ni ◽  
Renata Wentzcovitch ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Lower Mantle ◽  
Mantle Heterogeneities ◽  
Subducted Oceanic Crust

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 Subducted oceanic crust as the origin of seismically slow lower-mantle structures

2020 ◽  
Vol 7 (1) ◽  
Author(s):  
Timothy D. Jones ◽  
Ross R. Maguire ◽  
Peter E. van Keken ◽  
Jeroen Ritsema ◽  
Paula Koelemeijer
Keyword(s):  
Oceanic Crust ◽  
Lower Mantle ◽  
Subducted Oceanic Crust

Models of mantle convection: one or several layers

1989 ◽  
Vol 328 (1599) ◽  
pp. 417-424 ◽  
Keyword(s):  
Phase Transition ◽  
Oceanic Crust ◽  
Lower Mantle ◽  
Chemical Change ◽  
Model Calculations ◽  
Low Viscosity ◽  
Subducted Oceanic Crust ◽  
Limited Mass ◽  
Seismic Discontinuity ◽  
Material Exchange

Numerical model calculations are used to determine if convection in the Earth’s mantle could be organized in two or more layers with only limited mass exchange in between. The seismic discontinuity at 670 km depth and the top of the D"-layer at the bottom of the mantle are considered as candidates for internal boundaries. If the 670 km discontinuity is caused by an isochemical phase transition, it has to have a Clapeyron slope of dp/dT ⩽ — 6 MPa k -1 to prevent convection currents from crossing; this value is improbably low. If the discontinuity represents a chemical boundary, the intrinsic density difference has to exceed 3 % to prevent subducted lithospheric slabs from penetrating deeply into the lower mantle; also the condition is possibly hard to meet. The least improbable mechanism for a mid-mantle barrier for convection currents would be a combination of endothermic phase transition and chemical change. The boundary between upper and lower mantle would show considerable topography, and a limited material exchange is to be expected at any rate. The possibility of a downward segregation of former oceanic crust, transformed to dense eclogite, is studied in a further model series. It requires a region of low viscosity, as the Delayer probably is, and is faciliated by the decrease of the thermal expansion coefficient with pressure. About 20% of subducted oceanic crust could accumulate at the core-mantle boundary. The dense material would concentrate underneath rising therm al plumes, and some of it is entrained into the plumes, possibly affecting their geochemical signature.

scholarly journals Subducted oceanic crust as the origin of seismically slow lower-mantle structures

2020 ◽  
Author(s):  
Timothy Jones ◽  
Ross Maguire ◽  
Peter van Keken ◽  
Jeroen Ritsema ◽  
Paula Koelemeijer
Keyword(s):  
Oceanic Crust ◽  
Lower Mantle ◽  
Subducted Oceanic Crust
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