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| Titel |
Branching of plumes at 660 km discontinuity and bounds on lateral viscosity contrast in the lower mantle |
| VerfasserIn |
Nicola Tosi, David Yuen |
| 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 |
250055972
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| Zusammenfassung |
| Recent high-resolution seismic imaging of the transition-zone thickness beneath the
Hawaiian hotspot (Cao et al., 2010) has evidenced a considerable uplift of the 660 km
discontinuity west of Hawaii without a correspondent depression of the 410 km discontinuity.
Such a structure is consistent with the geodynamical scenario of a deep-mantle plume first
deflected horizontally at 660 km depth and then reemerging, away from its lower mantle
source, as a secondary plume aligned with the present-day location of the hot-spot. Using a
cylindrical model of mantle convection featuring multiple phase transitions and
pressure-dependent thermodynamic properties according to recent mineral physics evidence,
we investigate the conditions under which such a peculiar plume morphology can be
realized. We focus on the magnitude ΔηT of the lateral viscosity contrast due to
temperature variations and show that this factor plays a first-order role on the dynamics of
plumes if pressure-dependent thermal expansivity and conductivity are taken into
account. For small values (ΔηT ~ 10), large-scale upwellings are generated at the
bottom thermal boundary layer that have enough buoyancy to pass undisturbed the
endothermic transition at 660 km depth in an essentially vertical fashion. For higher values
(ΔηT ~ 102 - 103) mantle layering becomes more pronounced, plumes are thinner and
weaker, still with enough buoyancy to reach the 660 km discontinuity but not to penetrate
it. Instead, they travel horizontally along the 660 km boundary following the top
part of lower mantle convection cells and rise again through the upper mantle at a
distance from their parent plume also controlled by ΔηT. Our findings argue for the
importance of using a temperature-dependent viscosity in numerical models that
feature pressure-dependent thermodynamic properties and on the possibility of
using plume dynamics to bound the temperature viscosity contrast in the lower
mantle.
Cao Q., R.D. van der Hilst, M.V. de Hoop, S. Shim. Complex plume dynamics in the
transition zone underneath the Hawaii hotspot: seismic imaging results. DI23C-02, AGU Fall
Meeting 2010, San Francisco. |
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