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| Titel |
The effect of iron spin transition on convective dynamics, slab dynamics and the geoid |
| VerfasserIn |
Michael Jacobs, Arie van den Berg, Wim Spakman, Ondrej Cadek, Hana Cizkova, Ctirad Matyska |
| 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 |
250075780
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| Zusammenfassung |
| Iron bearing minerals in the Earths lower mantle show a transition from high-spin to low-spin
in the iron constituent. This has been observed in particular for ferropericlase both
experimentally (Fei et al, 2007, Lin et al. 2005) and in first principles calculations (Wu et al,
2009). The situation is less unambiguous for perovskite. Umemoto et al (2010) showed that
the effect on volume is small compared to experimental uncertainty. Therefore we only
considered the spin effects in ferropericlase in our models.
The spin transition is characterized by a high valued positive Clapeyron slope
γ = 19MPa-K while the smoothness of the transition increases with temperature. Fei et al.
(2007) showed that at room temperature the spin transition pressure for iron richer
composition occurs at higher values, e.g 40 GPa at 20 mol% FeO, 60 GPa at 40 mol%
FeO.
In order to get a full thermodynamic description of mantle material that includes the
effects of spin transitions in ferropericlase we developed a model based on the
multi-Einstein vibrational model approach of Jacobs et al. (2013). This model represents
volume-pressure data of Lin et al. (2005), spin fraction data predicted by Wu et al. (2009)
and it also includes the observed composition dependence of the spin transition
pressure.
Our new model further includes the thermodynamic description of Jacobs and de Jong
(2007) that has been extended to describe thermodynamic properties of iron bearing
(Mg,Fe)SiO3 perovskite. Because the spin transition pressure is composition
dependent, the spin transition results in the formation of miscibility gap regions
separating compositions enriched in high spin and compositions enriched in low-spin
state.
The spin transition affects thermodynamic properties, density, thermal expansivity, bulk
modulus and heat capacity which in turn impact the convection dynamics of the Earth mantle.
For instance, due to the high positive Clapeyron-slope of the transition convective
mixing becomes more vigorous as observed in Boussinesq type modelling results of
Bower et al, 2009, Shanas et al, 2011. Negative buoyancy of lithospheric slabs in the
deep mantle is enhanced by the increase of thermal expansivity induced by the spin
transition. Therefore the sinking rate of slabs are affected by the presence of the
spin transition. Therefore the effects of the transition must be included in mantle
convection modelling, done in order to bracket mantle viscosity values (Cizkova et al.,
2012).
Here we investigate the impact of the iron spin transition on the convective dynamics of the
mantle and the distribution of material properties. As the spin transition related variations of
material properties (e.g. thermal expansivity) are significant especially at lower temperatures,
we concentrate mainly on the consequences for slab dynamics. To this end we use a
compressible convection model based on a self consistent formulation of the thermo-physical
material properties density, thermal expansivity and specific heat at constant pressure as
described in (Jacobs and van den Berg, 2011). Finally, we evaluate the consequences of
spin induced density contrasts in cold downwellings for the interpretation of the
geoid.
Bower et al. (2009) Geophys Res Lett, 36, L10306
Cizkova et al. (2012) Phys Earth Planet Inter 200, 56-62
Fei et al. (2007) Geophy res Lett, 34, L17307, 1-5
Jacobs and de Jong (2007) Geochim Cosmochim Acta, 71, 3630-3655
Jacobs and van den Berg (2011) Phys Earth Planet Inter, 186, 36-48
Jacobs et al. (2013) Phys Chem Minerals, in press
Lin et al. (2005) Nature 436, 377-380
Shahnas et al (2011) J Geophys Res 116, B08205, 1-16
Umemoto et al (2010) Phys Earth Planet Int, 180, 209-214
Wu et al (2009) Phys Rev B 80, 014409, 1-8 |
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