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| D″ layer | |
|---|---|
| Name | D″ layer |
| Caption | Artist's impression of the lowermost mantle above the outer core |
| Depth | 200–400 km above the Core–mantle boundary |
| Composition | perovskite, ferropericlase, post-perovskite (variable) |
| Discovery | mid-20th century seismic studies |
D″ layer The D″ layer is the lowermost region of the Earth's mantle immediately above the Core–mantle boundary and beneath the Lower mantle. It is characterized by abrupt seismic heterogeneity, sharp gradients in physical properties, and complex interactions with the outer core and inner core. Studies link the D″ layer to phenomena observed in Plate tectonics, Mantle plume emergence, and variations in Geomagnetic reversal behavior.
The D″ layer sits at the interface between the Lower mantle and the outer core, forming a transitional zone implicated in Mantle convection, hotspot feeding, and chemical exchange with the Iron–nickel core. Seismic studies following investigations from research groups at institutions such as the Scripps Institution of Oceanography, Lamont–Doherty Earth Observatory, and the Bayerisches Geoinstitut revealed the layer's heterogeneity. Its existence influences interpretations of data from the International Seismological Centre, Incorporated Research Institutions for Seismology, and observatories like USGS facilities.
Physical characteristics of the D″ region include strong lateral variations in Seismic wave velocity, both for P waves and S waves, and anomalous seismic attenuation reported by teams at California Institute of Technology, Massachusetts Institute of Technology, and the University of Cambridge. Heat flux across the D″ is constrained by studies from Woods Hole Oceanographic Institution and models tested at the Princeton University geodynamics group. Laboratory investigations at Lawrence Livermore National Laboratory and ETH Zurich measure density, sound speed, and thermal expansivity under Diamond anvil cell conditions representative of D″ pressures and temperatures.
Mineralogical models propose that D″ contains high-pressure phases such as bridgmanite (formerly perovskite-structured silicates), Ferropericlase, and post-perovskite, with compositions influenced by Basaltic accumulations and Subducted slab material originating at convergent margins like the Ring of Fire. Experimental results from groups at Carnegie Institution and Tokyo Institute of Technology suggest iron partitioning and element segregation, including Lithium and Oxygen behavior, modify mineral stability. Geochemical fingerprints compared with isotopic reservoirs sampled by the Loihi Seamount and Hawaii hotspot lavas point to recycled components derived from Mid-ocean Ridge basalt and Ocean island basalt sources.
Seismic imaging of D″ uses techniques developed by researchers at California Institute of Technology, Stanford University, and the National Earthquake Information Center. Methods include waveform inversion, seismic tomography, and shear-wave splitting analyses incorporating data from networks like the Global Seismographic Network and regional arrays deployed by IRIS. Observations reveal structures such as ultra-low velocity zones mapped near African Plate and Pacific Plate lowermost mantle regions, and discontinuities associated with transition zones first suggested after studies of the 1960 Valdivia earthquake. Advanced imaging links D″ heterogeneities with surface mantle signatures identified by the African superswell and Ontong Java Plateau.
The D″ region affects the style and vigor of mantle convection simulated by groups at University of Oxford, ETH Zurich, and University of Texas at Austin using numerical codes benchmarked against laboratory Rayleigh–Bénard convection experiments. It contributes to the anchoring of Mantle plume conduits beneath hotspots like Hawaii and Iceland, and may store dense recycled lithosphere from subduction beneath trenches related to events like the Cretaceous Normal Superchron. Thermal and compositional heterogeneities in D″ modulate plume buoyancy and couple to lithospheric dynamics impacting continental rifting events such as the East African Rift.
Processes at the D″–core interface influence Geomagnetism and Core dynamics investigated by teams at University of Leeds, NCAR, and Los Alamos National Laboratory. Heat flow variations across D″ affect outer core convection patterns that drive the Geodynamo and secular variation recorded by observatories like British Geological Survey magnetometer arrays. Chemical reactions and iron alloying at the boundary are implicated in Inner core anisotropy observed in seismic studies by the Seismological Society of America community. Episodes of core–mantle interaction may tie to abrupt geomagnetic phenomena recorded during events like the Laschamp event.
Research combines high-pressure experiments at facilities such as Argonne National Laboratory and Institut de Physique du Globe de Paris, seismic data processing at ETH Zurich and University of Cambridge, and global mantle convection modeling by groups at Caltech and Princeton University. Open questions include the exact mineralogical assemblage, the scale and origin of lateral heterogeneity, the role of D″ in long-term chemical reservoirs, and the coupling between D″ structure and Geomagnetic reversal frequency. Future work will integrate data from ocean-bottom seismometer deployments by institutions like Woods Hole Oceanographic Institution and enhanced geodynamo simulations developed at University College London.
Category:Earth structure