The effect of bulk composition and temperature on mantle seismic structure

被引:327
作者
Xu, Wenbo [2 ]
Lithgow-Bertelloni, Carolina [1 ]
Stixrude, Lars [1 ]
Ritsema, Jeroen [2 ]
机构
[1] UCL, Dept Earth Sci, London WC1E 6BT, England
[2] Univ Michigan, Dept Geol Sci, Ann Arbor, MI 48109 USA
关键词
mantle composition; mechanical mixture; equilibrium assemblage; mantle discontinuity; transition zone; shear velocity;
D O I
10.1016/j.epsl.2008.08.012
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
We use a self-consistent thermodynamic formalism to compute the phase equilibria and physical properties of model mantle compositions, demonstrating that the seismological properties of a mantle comprised of (1) an equilibrium assemblage (EA) of pyrolitic composition and (2) a mechanical mixture (MM) of basalt and harzburgite with identical bulk composition differ significantly. We calculate the density, compressional wave velocity (V,,), and shear wave velocity (Vs) of EA and MM with basalt fraction varying from 0% to 100% and along adiabats with potential temperatures ranging from 1000 K to 2000 K. For MM, Vs in the transition zone is greater, increases more rapidly with depth, and is insensitive to basalt fraction, while for EA Vs decreases by 2.5% with increasing basalt fraction for fractions < 70%. The magnitude and sharpness of the 520-km discontinuity depends strongly on temperature in both EA and MM, which may explain lateral variations in its seismic detection. Both MM and EA feature complex structure in the depth range 640750 km due to the transformations of akimotoite, ringwoodite, and garnet to denser assemblages. MM is faster than most seismological models in the upper mantle, and slower in the lower mantle, suggesting an increase of basalt fraction with depth in the mantle. (c) 2008 Elsevier B.V. All rights reserved.
引用
收藏
页码:70 / 79
页数:10
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