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Some mineral physics constraints on the rheology and geothermal structure of Earth's lower mantle

Daisuke Yamazaki and Shun-ichiro Karato
Some mineral physics constraints on the rheology and geothermal structure of Earth's lower mantle
American Mineralogist (April 2001) 86 (4): 385-391

Abstract

We explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of Earth's lower mantle. We show that MgSiO (sub 3) perovskite is significantly stronger than MgO periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase, periclase. We calculate viscosities of the lower mantle for two cases: (1) where periclase occurs as isolated grains and (2) where periclase occurs as continuous films, using mineral physics data and models of two-phase rheology. We find that the effective viscosity for the former is about approximately 10-1000 times higher than the latter. We therefore suggest that the rheology of the lower mantle is structure-and hence strain-dependent, leading to weakening at large strains due to the formation of continuous films of periclase. Overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient. Both MgSiO (sub 3) perovskite and periclase have relatively small activation energies (E (super *) = gRT (sub m) with g = 10-14, where R is the gas constant and T (sub m) is melting temperature), and therefore the depth variation of viscosity is rather small, even for a nearly adiabatic temperature gradient. However, the geothermal gradients consistent with the geodynamical inference of nearly depth-independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood. A superadiabatic gradient of up to approximately 0.6 K/km is also consistent with mineral physics and geodynamical observations.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 86
Serial Issue: 4
Title: Some mineral physics constraints on the rheology and geothermal structure of Earth's lower mantle
Pages: 385-391
Published: 200104
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 49
Accession Number: 2001-049380
Categories: Solid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200115
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