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| Titel |
The nature and dynamics of deep mantle thermo-chemical reservoirs |
| VerfasserIn |
Frédéric Deschamps, Paul Tackley, Yang Li, Takashi Nakagawa |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056692
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| Zusammenfassung |
| During the past decade, increasing seismological evidences have credited the existence of
large scale thermo-chemical heterogeneities in the lowermost (> 2000 km) mantle. However,
the origin and nature of chemical anomalies is still debated. Two end-members hypothesis,
the subduction and stacking of MORB, and the survival of primitive reservoirs of dense
material, are usually advocated.
Slabs sinking in the deep mantle have been detected by seismic tomography. Furthermore,
numerical models of thermo-chemical convection that include the production and recycling of
oceanic crust have shown that MORB can segregate in stable pools at the bottom of the
mantle. Our recent models, which include self-consistent mineralogy and spherical geometry,
pointed out that the exact shape of these reservoirs is very sensitive to the composition of
MORB in major oxides. It is important to note that high-pressure MORB are seismically
faster than the average pyrolitic mantle. Thus, to explain the large low shear-wave velocity
provinces observed at the bottom of the mantle, reservoirs of recycled MORB should be
hotter than average.
The presence of a primitive undegassed reservoir located in the deep mantle is needed to
explain the geochemistry of ocean island basalt, in particular the low values of the 4He/3He
ratio. Primitive reservoirs may result from early partial differentiation of the Earth’s
mantle, or the recycling on an early crust. Experimental and numerical models of
thermo-chemical convection that consider the evolution of an initial layer of dense
material (including our recent models in 3D-Cartesian geometry), indicate that
reservoirs of primitive material can survive convection for periods of time larger
than the age of the Earth. Assuming that primitive reservoirs are enriched in iron
and silicate, as suggested by probabilistic tomography and by enstatite chondrite
models of Earth’s composition, the amplitude and RMS of seismic velocity anomalies
predicted by our 3D-Cartesian models are consistent with those observed by seismic
tomography.
Very likely, thus, the seismic velocity and density anomalies observed in the deep mantle
originate from a combination of thermal anomalies and two (or more) sources of chemical
heterogeneities. In addition, the post-perovskite may play a significant role. In cold regions
(e.g., slab graveyards) perovskite may transform to post-perovskite at relatively shallow
depths, whereas in hot regions (e.g., plume sources) it may not transform at all. We
continue our exploration of the model space of thermo-chemical convection, including
models with primitive reservoirs in spherical geometry, and models that combine two
sources of chemical heterogeneity, i.e. recycled MORB and primitive reservoirs. |
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