HR: 10:50h
AN: MR22A-03 INVITED     [Abstracts]
TI: Electronic Transitions in (Mg,Fe)SiO$_3$ Perovskite and Post-perovskite phases: Implications for Deep-Earth Dynamics
AU: * Badro, J
EM: badro@lmcp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Laboratoire de Mineralogie Cristallographie (UMR CNRS 7590), 140 rue de Lourmel, Paris, 75015 France
AU: Fiquet, G
EM: fiquet@lmcp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Laboratoire de Mineralogie Cristallographie (UMR CNRS 7590), 140 rue de Lourmel, Paris, 75015 France
AU: Guyot, F
EM: guyot@lmcp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Laboratoire de Mineralogie Cristallographie (UMR CNRS 7590), 140 rue de Lourmel, Paris, 75015 France
AB: It was recently reported that perovskite undergoes a crystallographic phase transition at D'' conditions (Murakami {\em et al.}) above 120 GPa. We measured the spin state of iron in magnesium silicate perovskite (Mg$_{0.9}$ Fe$_{0.1}$)SiO$_3$ at high pressure and found two electronic transitions occurring at 70 GPa and at 120 GPa, corresponding to partial and full electron pairing in iron, respectively. The pressure range of the first transition (70 GPa) is consistent with the depth (1700 km) at which lower mantle chemical heterogeneities have been proposed, as well as with the transition pressure for iron in magnesiow\"ustite, the other main iron-bearing mineral of the lower mantle. The second transition pressure (120 GPa, corresponding to a depth of 2600 km), at which the HS-LS transition is completed in perovskite, is consistent with that of the D" layer; at higher pressure (or below this depth) all lower-mantle minerals consist of LS iron. An important characteristic of this assemblage would be an increased radiative conductivity. The transitions should have a strong dynamical signature as inferred from geodynamical modeling, because an increase in thermal conductivity will result in a decrease of the Rayleigh number, and hence may hinder convection and favor layering. The proportion of iron in the low spin state thus grows with depth, increasing the transparency of the mantle in the infrared region, with a maximum at pressures consistent with the D" layer above the core-mantle boundary. Concerning the second transition, one can conjecture here that it is linked to, or could even be a driving force for, the crystallographic phase transformation. This claim is actually strengthened by our finding of a Clapeyron slope of 120 K.GPa$^{-1}$ for the transition, in excellent agreement with the results from two recent and independent theoretical studies of the crystallographic transition.
DE: 5109 Magnetic and electrical properties
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
DE: 1025 Composition of the mantle
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting