The Role of Iron Partitioning in Mantle Composition, Evolution, and Scale of Convection

The effect on composition and evoconvection with the entire mantle passing through lution of the mantle of the recently-observed the partial melt zone. Thus the lower mantle was strong concentration of iron in (Mg, Fe)O-magnedepleted of iron relative to both the upper siow•stite (row) at the expense of (Mg, Fe)SiO•mantle and the mantles of the small terrestrial perovskite (pv) structure is studied by calcOplanets and satellites, which do not have mantle lating a temperatureand pressure-dependent iron pressures sufficient to form perovskite-structure partitioning coefficient for the lower mantle. silicates, or which had lower accretional temperThe value of the standard entropy for atures and less extensive melting. On this basis, MgSiO•-perovskite is found to be 69.4 + 10.3 Venus would be expected to have a mantle similarJ/mol• deg from the recently determined phase ly depleted in iron. diagram of forsterite. Iron remains concentrated in (Mg, Fe)O throughout the entire lower mantle Introduction if account is taken of an FeO phase change .... with pv pv mw mw the partitioning coefficient (x_ /x•. )/(x_ /•. ) One of the major issues in the study of the increasing from 0.04 to 0.8 be•wee• 670 •m a•d earth's interior is the question of the radial the core-mantle boundary. Partitioning has chemical homogeneity of the mantle, the solution negligible effect on gross density and elastic of which has important implications for the properties of the lower mantle. By using recent evolution of the earth and for the style of shock wave and static compression results for FeO mantle convection. A chemical boundary at 670 km, and MgSiO3-perovskite, w find that the lower say, would either restrict convection to the mantle is more pyroxene-rich than the upper upper mantle or require two-layer circulation. mantle and as iron-rich, or somewhat less so, The present state of the convection problem has than the upper mantle. Mg/(Mg + Fe) = 0.93-0.95 recently been reviewed by Stevenson and Turner for the lower mantle compared with 0.85-0.90 for [1979]. Arguments in favor of whole-mantle conthe uppermost mantle. The lower mantle Mg/Si vection have been presented by Ringwood [1975], ratio is closer to chondritic values (0.99 Davies [1977], O'Connell [1977], Sammis et al. + 0.03) than is the upper mantle (•1.5 for a [1977], Hager and O'Connell [1978], Sharpe and peridotite with px/ol = 0.4(molar)), thus supPeltlet [1979], and Davies [1981], among others. porting the idea of a chemically layered mantle The idea of layered convection has been supported with implications for the style of mantle by studies such as those of Richter and Johnson convection. While partitioning of iron has no [1974], Anderson [1979], Wasserburg and DePaolo significant effect on gross lower mantle density, [1979], Jeanloz and Richter [1979 ], Christensen we find that the (Mg, Fe)O and perovskite [1981], and Richter and McKenzie [1981], for components of the lower mantle have essentially example. the same densities. Mantles with higher bulk iron In this paper, we investigate the effects of contents have (Mg, Fe)O denser than the the high-pressure behavior of olivines and perovskite component; for a bulk magnesium mole pyroxenes on mantle composition, evolution, and fraction of 3 0.80, the density difference is scale of convection. 0.7-0.8 g/cm . We investigate the feasibility of Ringwood [1962] suggested that the 670 km the Mac, Bell, and Yagi gravitational separation seismic discontinuity is caused by a phase transhypothesis of mantle evolution in which a mantle formation to perovskite-structure silicates. In more iron-rich than present loses iron through recent years, both experimental evidence for gravitational sinking of the denser (Mg, Fe)O, mantle minerals-see, for example, Liu [1979] for and we conclude that the process cannot a review of high-pressure products of olivines, successfully compete with solid state convection pyroxenes, and garnets-and studies of analog unless iraplausibly large grain sizes or materials, summarized by Liebermann [ 1979], have unacceptably low viscosities are invoked. A given strong support to the perovskite (pv) likely explanation for removal of iron from an hypothesis. Although there is evidence for initially iron rich lower mantle is upward additional minor discontinuities below 670 km, extraction of FeO-enriched basalts or picrites the properties of the lower mantle from 670 km to and concentration of iron in upper mantle garnets the core-mantle boundary appear to be dominated during accretion of the earth or subsequent by the perovskite transformation. Anderson [1977] has argued that a pyrolite mantle transforming to 1 perovskite-structure silicates and (Mg, Fe)O Now at Department of Geology, Rensselaer cannot explain the 670 km discontinuity. It has Polytechnic Institute, Troy, New York 12181. been proposed that the lower mantle is mainly FeO-poor perovskites [Sawamoto, 1977; Butler and Copyright 1982 by the American Geophysical Union. Anderson, 1978]. Seismic data for the lower mantle appear to be consistent with a mainly Paper number 2B0088. pyroxene stoichiometry [Anderson, 1970; Anderson 0148-0227/82/002B-0088505.00 et al., 1971; Gaffhey and Anderson, 1973; Burdick