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| Titel |
Reservoirs of Undegassed Material in the Deep Mantle and the Origin of Mantle Plumes |
| VerfasserIn |
Frédéric Deschamps, Paul Tackley, Laura Cobden, Edouard Kaminski |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250081425
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| Zusammenfassung |
| The large scattering in the isotopic Helium ratio (4He/3He) observed in Ocean
Island Basalts (OIB) suggests that the plumes at the origin of OIB sample several
reservoirs. The low values (< 30000) of the Helium ratio indicates that OIB sample
an undegassed reservoir. Its lowest value, around 15000, imposes a constraint on
the entrainment of primitive material by plumes, which should not exceed 10%.
Numerical experiments of thermo-chemical convection in 3D-Cartesian and spherical
geometries showed that reservoirs of primordial material can be maintained at the
bottom of the system, the shape and stability of these reservoirs depending on the
chemical density contrast and on the thermal viscosity contrast. In addition, plumes are
generated at the top of these reservoirs, entraining small fraction of primordial
material up to the surface. Numerical experiments showed that this entrainment
quantitatively agrees with OIB data, with values around 9%. The location of the undegassed
reservoirs is still a matter of debate. Images of slabs penetrating in the deep mantle
indicate that the lower mantle itself is not isolated. The undegassed reservoirs may
instead consist of pools of chemically distinct material located in the lowermost
mantle. Possible candidates for these pools are the low shear-wave velocity provinces
(LLSVP) observed by seismic tomography. Additional observations, including the
anti-correlation between shear- and bulk-sound velocity anomalies, show that these
structures are caused by large scale thermo-chemical anomalies. The exact nature of the
chemical component of these anomalies is still unclear, two end-members hypotheses
(namely the recycling of MORB by subduction, and the survival of primordial deep
reservoirs) being usually advocated. The combination of mineral physics data and global
tomographic models shows that LLSVP are better explained by material enriched in
iron and silicates than by high pressure MORB, unless these LLVSP are hotter
than the average mantle by about 1500 K. Slabs may however reach the bottom
of the mantle and numerical models of thermo-chemical convection have shown
that they can accumulate there, in which case OIB plumes may also sample high
pressure MORB. A full explanation of lowermost mantle tomography and OIB
signatures thus requires two chemical sources, recycled MORB and primordial
reservoirs.
Deschamps, F., E. Kaminski and P. J. Tackley (2011) The deep origin for the primitive
signature of ocean island basalt, Nature Geoscience 4, 879-882, doi:10.1038/ngeo1295.
Deschamps, F., L. Cobden and P. J. Tackley (2012) The primitive nature of large low
shear-wave velocity provinces, Earth Planet. Sci. Lett. 349-350, 198-208 |
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