Chemical and seismological constraints on mantle heterogeneity.
Recent seismological studies that use scattered waves to detect heterogeneities in the mantle reveal the presence of a small, distributed elastic heterogeneity in the lower mantle which does not appear to be thermal in nature. The characteristic size of these heterogeneities appears to be ca. 8 km, suggesting that they represent subducted recycled oceanic crust. With this stimulus, old ideas that the mantle is heterogeneous in structure, rather than stratified, are reinterpreted and a simple, end-member model for the heterogeneity structure is proposed. The volumetrically largest components in the model are recycled oceanic crust, which contains the heat-producing elements, and mantle depleted of these and other incompatible trace elements. About 10% of the mantle's mass is made up of recycled oceanic crust, which is associated with the observed small-scale seismic heterogeneity. The way this heterogeneity is distributed is in convectively stretched and thinned bodies ranging downwards in size from 8 km. With the present techniques to detect small bodies through scattering, only ca. 55% of the mantle's small-scale heterogeneities are detectable seismically.
Helffrich
George
G
Department of Earth Sciences, Wills Memorial Building, University of Bristol, Queen's Road, UK.
D003198:Computer Simulation; D004190:Disasters; D018534:Earth, Planet; D019418:Evolution, Chemical; D019419:Evolution, Planetary; D019015:Geologic Sediments / Q000032:analysis* / Q000737:chemistry; D005844:Geology / Q000379:methods; D021621:Imaging, Three-Dimensional / Q000379:methods; D008962:Models, Theoretical; D012212:Rheology / Q000379:methods; D013314:Stress, Mechanical; D014054:Tomography / Q000379:methods*; D014732:Vibration