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

Watt, J. Peter and Ahrens, Thomas J. (1982) The role of iron partitioning in mantle composition, evolution, and scale of convection. Journal of Geophysical Research B, 87 (B7). pp. 5631-5644. ISSN 0148-0227. http://resolver.caltech.edu/CaltechAUTHORS:20141103-151200534

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Abstract

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


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Related URLs:
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http://dx.doi.org/10.1029/JB087iB07p05631DOIArticle
http://onlinelibrary.wiley.com/doi/10.1029/JB087iB07p05631/abstractPublisherArticle
Additional Information:Copyright 1982 by the American Geophysical Union. (Received June 10, 1981; revised January 11, 1982; accepted January 15, 1982.) Paper number 2B0088. We appreciate discussions with D. L. Anderson, A. Navrotsky, D. J. Stevenson, J. M. Vizgirda, and E. B. Watson. E. Ito kindly made available a preprint. D. L. Anderson offered detailed comments on the manuscript; B. H. Hager and an anonymous reviewer provided helpful suggestions. Supported by NSF grants EAR 79-06766 and EAR 79-26384. Contribution 3647, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.
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Funding AgencyGrant Number
NSFEAR 79-06766
NSFEAR 79-26384
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Caltech Division of Geological and Planetary Sciences3647
Record Number:CaltechAUTHORS:20141103-151200534
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20141103-151200534
Official Citation:Watt, J. P., and T. J. Ahrens (1982), The role of iron partitioning in mantle composition, evolution, and scale of convection, J. Geophys. Res., 87(B7), 5631–5644, doi:10.1029/JB087iB07p05631.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:51198
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:03 Nov 2014 23:46
Last Modified:03 Nov 2014 23:46

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