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| Titel |
Consequences on lower-mantle plume dynamics with the post-perovskite phase change and strongly depth dependent thermodynamic and transport properties |
| VerfasserIn |
Nicola Tosi, David Yuen, Ondrej Cadek |
| 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 |
250036943
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| Zusammenfassung |
| We have carried out numerical simulations of 2-D mantle convection specifically with the
deep phase change from perovskite (pv) to post-perovskite (ppv). Using the extended
Boussinesq approximation for a fluid with both temperature and pressure dependent viscosity,
we have performed an extensive sensitivity analysis of the post-perovskite phase parameters
and investigated their effects on the convective planform, heat transport and mean
temperature profiles. Since the rheology of ppv is expected to be relatively weak
with respect to pv (Ohta et al., 2008; Hunt et al., 2009) and to have a large thermal
conductivity (Hofmeister, 2007), we assume that the transition from pv to this ppv phase is
accompanied by both a reduction in viscosity by 1 to 2 orders of magnitude and by an
increase in thermal conductivity by a factor of 2. Furthermore, we investigate the
combined effects of decreasing pressure-dependent thermal expansivity, by considering
the most recent findings by Katsura et al. (2009), and steeply increasing thermal
conductivity (Hofmeister, 2008, Tang and Dong, 2010). As long as the thermal
expansivity and conductivity are constant, ppv exerts a small but measurable effect on
mantle convection: it destabilizes the D" layer and causes focusing of the heat-flux
peaks and an increase of the average mantle temperature and also of the temporal
and spatial frequency of upwellings. When depth dependent thermal expansivity
and the latest thermal conductivity models are introduced, the effects of ppv are
dramatic. On the one hand, without ppv, we obtain a very sluggish convective regime
characterized by a relatively cool mantle dominated by large downwellings that tend to
stagnate beneath the transition zone. With ppv, on the other hand, we observe an
extremely significant increase of the average mantle temperature due to the formation of
large sized and vigorous upwellings that in some cases tend to cluster, thus forming
superplumes. If a very large thermal conductivity at the core-mantle boundary is assumed
(k ~ 20 W/Km) we obtain a quasi-steady state regime characterized by large and very
stable plumes with long lifetimes. The combination of strongly depth dependent
expansivity and conductivity is a viable mechanism for the formation of long-wavelength,
long-lived structures in the deep mantle (Torsvik et al., 2008;Dziewonski et al.,
2010). |
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