660 km discontinuity

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The 660 km discontinuity defines the lower limit of the mantle transition zone and thus the beginning of the Earth's lower mantle . It is characterized by an increase in seismic velocities with depth. Their designation is based on the average global depth of their occurrence according to the reference earth model IASP91 , but can vary among subduction zones . In other speed models of the earth (e.g. PREM , Preliminary Reference Earth Model) the depth can also be specified as 670 km.

The 660 km discontinuity is caused by the phase transformation of γ-olivine (“spinel”). There, γ-olivine breaks down into perovskite (Mg, Fe) SiO 3 and ferropericlase (Mg, Fe) O (often imprecisely referred to as magnesiowustite).

γ- (Mg, Fe) 2 [SiO 4 ] → (Mg, Fe) [SiO 3 ] + (Mg, Fe) O
γ-olivine → (Mg, Fe) -perovskite + ferropericlase

The occurrence of the phase change is linked to a specific relationship between pressure and temperature . These can be represented by Clapeyron slopes . The corresponding Clapeyron slope for the 660-km discontinuity -2 MPa / K indicated. The average depth of 660 km corresponds to a pressure of approximately 23.5 G Pa . Due to the negative Clapeyron slope, an increase in the temperature in the earth's mantle means that lower pressure must prevail for the conversion of γ-olivine. The temperature increase can be caused by a rising plume .

See also

credentials

  1. Peter M. Shearer, T. Guy Masters: Global mapping of topography on the 660-km discontinuity Archived from the original on March 13, 2014. Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. In: Nature . 355, Feb. 27, 1992, pp. 791-796. Retrieved February 20, 2013. @1@ 2Template: Webachiv / IABot / mahi.ucsd.edu
  2. Eji Ito, Eiichi Takahashi Postspinel transformations in the system Mg2SiO4-Fe2SiO4 and some geophysical implications , Journal of Geophysical Research, Volume 94, 1989, 10637-10646, abstract
  3. Reinhard Boehler , L. Chudinovskikh: High-pressure polymorphs of olivine and the 660-km seismic discontinuity , Letters of Nature 411, 574-577 (2001)
  4. SH Shim, TS Duffy, G Shen: The post-spinel transformation in Mg2SiO4 and its relation to the 660-km seismic discontinuity . In: Nature . 411, May 31, 2001, pp. 571-574. doi : 10.1038 / 35079053 .
  5. Navrotsky, A. (1980). Lower mantle phase transitions may generally have negative pressure-temperature slopes. Geophysical Research Letters 7 (9), ISSN  0094-8276 .
  6. Ito, E., H. Yamada, Stability relations of silicate spinels, ilmenites and perovskites, Adv. Earth Planet. Sci., 12, 405-419, 1982.