Mineralogical and Chemical Models of Earth's Lower Mantle: Where are We Heading to?

HR: 0830h
AN: S21E-0372 [PDF]
TI: Mineralogical and Chemical Models of Earth's Lower Mantle: Where are We Heading to?
AU: * Fiquet, G
EM: fiquet@lmcp.jussieu.fr
AF: Laboratoire de Min\'eralogie-Cristallographie UMR CNRS 7590 Universit\'e Paris 6 Institut de Physique du Globe de Paris, 4 Place Jussieu, Paris cedex 05, 75252 France
AU: Badro, J
EM: James.Badro@lmcp.jussieu.fr
AF: Laboratoire de Min\'eralogie-Cristallographie UMR CNRS 7590 Universit\'e Paris 6 Institut de Physique du Globe de Paris, 4 Place Jussieu, Paris cedex 05, 75252 France
AU: Guyot, F
EM: guyot@lmcp.jussieu.fr
AF: Laboratoire de Min\'eralogie-Cristallographie UMR CNRS 7590 Universit\'e Paris 6 Institut de Physique du Globe de Paris, 4 Place Jussieu, Paris cedex 05, 75252 France
AB: (Mg,Fe,Al)(Al,Si)O$_3$ perovskite and (Mg,Fe)O magnesiow\"ustite are the most important phases of the Earth's lower mantle. Knowledge of their elastic properties is essential to understand the mineralogy, chemical composition, and thermal structure of the lower mantle. A wealth of data available for the MgSiO$_3$ perovskite end-member and periclase MgO end-member provides a good starting point for understanding the mantle. However, in the mantle the chemistry of those compounds is certainly far more complex, and this may greatly affect the physical properties. Inferences concerning Earth's lower mantle will depend upon how accurately we can address the chemical composition dependency of perovskite's elasticity. At present, the properties of perovskite with realistic mantle compositions are highly uncertain. For example, it has been shown in recent years that knowing properties of aluminium-bearing silicate perovskite was essential for constraining the chemical and physical state of the deep Earth. To our knowledge, there is no information on the single-crystal elastic properties or aggregate sound velocities for Al and Fe-bearing (Mg,Fe,Al)(Al,Si)O$_3$-Pv. Most of what we know about the effects of Al and Fe in perovskite comes primarily from compression studies, and existing results are contradictory. In the same manner, recent experiments evidenced a spin transition in iron in magnesiow\"ustite (Mg$_{0.83}$Fe$_{0.17}$)O occurring in the 60-70 gigapascal pressure range, corresponding to depths of 2000 km in the Earth's lower mantle, and greatly affecting iron chemistry in magnesiow\"ustite and magnesium silicate perovskite and showing that composition itself is susceptible to drastically change with depth. This observation is compatible with seismically observed heterogeneities in the lower mantle. While bulk modulus (K) of perovskite or magnesiow《tite can be constrained by static compression experiments, this property constrains only the density and bulk sound velocity in the lower mantle. Information on the shear moduli (G) and acoustic velocities for these phases would allow a better interpretation of the seismic properties of the lower mantle, including average longitudinal and shear velocity structure, anisotropy, and heterogeneity, provided these properties can be measured as a function of composition. We will show that such data are within the reach of mineral physics, and offer fresh perspectives in our attempt to formulate a comprehensive theory of deep-Earth dynamics and composition.
DE: 0350 Pressure, density, and temperature
DE: 1025 Composition of the mantle
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
SC: Seismology [S]
MN: 2003 Fall Meeting