HR: 17:45h
AN: T34A-06    [Abstracts]
TI: The Impact of Superplasticity and the Iron Spin Transition on Mantle Dynamics
AU: * Shahnas, H
EM: shahnas@atmosp.physics.utoronto.ca
AF: University of Toronto, Department of Physics, 60 St. George Street, Toronto, M5S 1A7, Canada
AU: Peltier, W
EM: peltier@atmosp.physics.utoronto.ca
AF: University of Toronto, Department of Physics, 60 St. George Street, Toronto, M5S 1A7, Canada
AB: Our understanding of mantle dynamical processes depends critically upon knowledge of the physical properties of mantle minerals at high pressure and temperature. The Earth's lower mantle is believed to consist mainly of iron bearing aluminous silicate perovskite [Al-(Mg,Fe)SiO3] and ferropericlase [(Mg,Fe)O] together with a small amount of calcium silicate perovskite (CaSiO3) and Al2O3 (eg. Ringwood, 1982). In light of recent experimental measurements it is now generally accepted that the dominant mineral of the lowermost mantle is a "post-perovskite phase" (Murakami, 2004). The exothermic perovskite-post-perovskite (pv-ppv) phase transition may accommodate a higher rate of heat extraction from the core. On the other hand the electronic spin and valence states of iron-bearing minerals may also strongly influence the properties of mantle material and in turn the dynamics of Earth's interior. Most recent experimental results suggest that the electronic iron spin transition is associated with significant changes in density, compressibility, sound velocities, radiative thermal conductivity and electrical conductivity in ferropericlase (Lin et al., 2008). These studies also show that both perovskite and post- perovskite accommodate the same intermediate-spin Fe2+ state and that their opacity and absorption behavior at infrared wavelengths are very similar in high P-T experiments. Therefore corresponding changes in the radiative thermal conductivity, electrical conductivity and iron partitioning would occur due to the crystal structural transition, rather than due to the electronic spin transition, from perovskite to post-perovskite (Lin et al., 2008). Furthermore, dislocation creep which is believed to be the dominant deformation mechanism in the post-perovskite phase would be associated with lower viscosity than in the viscosity of perovskite at the same P-T conditions (Cizkova et al., 2008). These results may have significant implications for the dynamics of Earth's lower mantle and for the extent to which the convective circulation is layered, an issue that remains outstanding. In the work to be reported in this paper we have studied the impact of the iron spin transition on the style of convective mixing. Although the density increase with depth may slightly lower the mantle mean temperature, the higher thermal conductivity due to the increase of the radiative contribution below the D" layer enhances the influence of the lower viscosity by increasing the mean mantle temperature. A further issue that we address in this paper concerns the extent to which superplastic behavior associated with the endothermic phase transition at 660 km depth may influence mantle layering. During a solid-solid phase transition a polycrystalline material may deform superplastically due to the reduction in grain size that occurs when material changes phase (e.g. Sammis & Dein, 1974; Paterson, 1983; Ranalli, 1991). Our numerical models show that the inclusion of such a superplastic layer at the level of the 660 km transition enhances the degree of mantle layering and the regularity of the avalanches of material that episodically occur across this important interface that separates the upper mantle from the lower mantle. Keywords: layered mantle convection, iron spin transition, perovskite-post perovskite phase transition, superplasticity
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 3924 High-pressure behavior
DE: 4255 Numerical modeling (0545, 0560)
DE: 8180 Tomography (6982, 7270)
SC: Tectonophysics [T]
MN: 2009 Joint Assembly