The role of iron partitioning in mantle composition, evolution, and scale of convection

摘要: The effect on composition and evolution of the mantle recently-observed strong concentration iron in (Mg, Fe)O-magnesiowustite (mw) at expense Fe)SiO_3-perovskite (pv) structure is studied by calculating a temperature- pressure-dependent partitioning coefficient for lower mantle. value standard entropy MgSiO_3-perovskite found to be 69.4±10.3 J/mole deg from recently determined phase diagram forsterite. Iron remains concentrated Fe)O throughout entire if account taken an FeO change, with (x^(pv)_(Fe)/x^(pv)_(Mg))/(x^(mw)_(Fe)/x^(mw)_(Mg)) increasing 0.04 0.8 between 670 Km core-mantle boundary. Partitioning has negligible gross density elastic properties Of By using recent shock wave static compression results MgSiO_3-perovskite, we find that more pyroxene-rich than upper as iron-rich, or somewhat less so, Mg/(Mg + Fe) = 0.93–0.95 compared 0.85–0.90 uppermost Mg/Si ratio closer chondritic values (0.99± 0.03) (≈1.5 peridotite px/ol 0.4(molar)), thus supporting idea chemically layered implications style convection. While no significant density, perovskite components have essentially same densities. Mantles higher bulk contents denser component; magnesium mole fraction of, 0.80, difference 0.7–0.8 g/cm^3. We investigate feasibility Mao, Bell, Yagi gravitational separation hypothesis which iron-rich present loses through sinking O, conclude process cannot successfully compete solid state convection unless implausibly large grain sizes unacceptably low viscosities are invoked. A likely explanation removal initially rich upward extraction FeO-enriched basalts picrites garnets during accretion earth subsequent passing partial melt zone. Thus was depleted relative both mantles small terrestrial planets satellites, do not pressures sufficient form perovskite-structure silicates, had accretional temperatures extensive melting. On this basis, Venus would expected similarly iron.