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Effect of Low-Spin Fe^(2+) on the Composition of the Lower Mantle

Gaffney, Edward S. and Anderson, Don L. (1973) Effect of Low-Spin Fe^(2+) on the Composition of the Lower Mantle. Journal of Geophysical Research, 78 (29). pp. 7005-7014. ISSN 0148-0227. doi:10.1029/JB078i029p07005.

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The effects of low-spin Fe^(2+) in the earth's lower mantle are investigated in considerable detail. The existence of low-spin Fe^(2+) permits the formation of an extra phase, since Mg^(2+) and low-spin Fe^(2+) probably do not form solid solutions. The elastic behavior of phases containing low-spin iron is predicted from bulk modulus systematics and is compared with available shock wave data. It is likely that the high pressure phases of several ferrous iron compounds involve low-spin Fe^(2+). Iron will be spin-paired in the mantle below 1700 km and perhaps at higher levels as well. The observed density and bulk modulus in the lower mantle are inconsistent with any combination of phases in a pyrolite bulk composition but can be fit quite well by a model with all Fe^(2+) spin-paired below 650 km. The lower mantle is richer in FeO and SiO_2 than the upper mantle. The spin-pairing of Fe^(2+) provides an excellent mechanism for both iron and silicon enrichment in the lower mantle by partial melting. Thus spin-pairing in iron provides support for previous arguments for chemical inhomogeneity of the mantle and also provides a mechanism for generating the inhomogeneity. This removes the motivation for reducing FeO and SiO_2 in the mantle to supply Fe and Si for the core.

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Additional Information:© 1973 by the American Geophysical Union. Received November 15, 1972, received May 1, 1973. Article first published online: 20 Sep 2012. Contribution 2233, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California. This study was supported by the National Science Foundation under grants GA 21396 and GA 12703. The authors thank G. F. Davies for permission to use the results of unpublished calculations; they have also benefited from discussions with W. B. Kamb. Roger Burns provided a useful critical review.
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NSFGA 21396
NSFGA 12703
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Caltech Division of Geological and Planetary Sciences2233
Issue or Number:29
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Official Citation:Gaffney, E. S., and D. L. Anderson (1973), Effect of low-spin Fe2+ on the composition of the lower mantle, J. Geophys. Res., 78(29), 7005–7014, doi:10.1029/JB078i029p07005.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:55033
Deposited By: Ruth Sustaita
Deposited On:19 Feb 2015 23:39
Last Modified:10 Nov 2021 20:40

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