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| Titel |
Thermo-chemical convection and the nature of the deep mantle chemical heterogeneities |
| VerfasserIn |
Frédéric Deschamps, Takashi Nakagawa, Paul J. Tackley |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036029
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| Zusammenfassung |
| In the past decade, the combination of seismological observations and mineral physics data
has revealed the presence of large thermo-chemical heterogeneities in the deep mantle.
However, the nature of the chemical heterogeneities remains a matter of debate, and two main
origins have been explored, in particular by numerical models of thermo-chemical
convection. First, chemical heterogeneities may result from the interaction between mantle
convection and an initial reservoir of dense material, e.g. enriched in iron. The presence of a
primitive, undepleted and isolated reservoir(s) hidden in the deep mantle has long been
advocated by geochemists to be one of the sources of ocean island basalt. Recent models and
geochemical data suggest that such reservoirs may result from the early partial
differentiation of the Earth’s mantle, or the recycling on an early crust. Our recent models of
thermo-chemical convection, which consider the evolution of an initial layer of dense
material, indicate that strong thermal viscosity contrasts play a major role in maintaining
disconnected pools of dense material in the deep mantle, and that a negative Clapeyron
slope at 660-km strongly reduces the flux of dense material from the lower to the
upper mantle. The second main source of chemical heterogeneity is the recycling
of oceanic crust entrained by slabs in the deep mantle. Models that include the
production and recycling of MORBs indicate that the formation and shape of pools or
dense material in the deep mantle depend on the buoyancy ratio of MORBs, on the
Clapeyron slope of the post-perovskite phase transition, and on the core properties.
When self-consistent mineralogy is included in models with spherical geometry,
the MORBs segregate in the bottom of the system, and form a continuous layer
around the core-mantle boundary that is locally disrupted by downwelling slabs.
Individually, pools of primitive material and recycled MORBs do not fully satisfy the
seismological observations. Thermo-chemical calculations that model the evolution of
primitive reservoirs indicate that the present day distribution of the dense is perfectly
correlated with that of the hot material, in contradiction with the thermal and chemical
density anomalies seen by normal modes. On the other hand, high-pressure MORBs
are seismically faster than the average pyrolitic mantle, and would therefore not
explain the large low shear-wave velocity provinces observed at the bottom of the
mantle. Very likely, the seismic velocity and density anomalies observed in the deep
mantle originate from a combination of thermal anomalies and two or more chemical
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. The next step is therefore to build thermo-chemical
models of convection that combine two sources of chemical heterogeneity – primitive
reservoirs and recycled MORBs – and account for the post-perovskite phase transition. |
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