520 km discontinuity

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The 520 km discontinuity is a seismic boundary layer within the mantle transition zone and is characterized by an increase in seismic velocities with depth. Their designation is based on the average global depth of their occurrence.

The 520 km discontinuity is caused by a phase transformation of one of the main components of the mantle rock. In the process, olivine of the β- spinel structure (also known as wadsleyite ) changes into the γ-spinel structure (also: ringwoodite ). These mineral phases are identical in their chemical composition, but differ in their lattice structure and thus in their physical density . The occurrence of the phase change is linked to a specific relationship between pressure and temperature . The average depth of 520 km corresponds to a pressure of approx. 18 G Pa . An increase in the temperature in the earth's mantle (for example by an ascending plume ) requires a higher pressure for the phase transformation. The corresponding Clapeyron slope is given with values ​​between +4.0 and +5.3 MPa / K.

There is, however, another mineral phase transformation of another component of the mantle. Here grenade into a calcium-rich perovskite converted -Gestein. At an average temperature of the jacket, both transformations take place at a comparable depth and therefore overlap. Because of the opposite sign of their Clapeyron slopes, an increase in temperature can lead to both transformations shifting to significantly different depths, which can be observed in seismological studies as a splitting of the 520 km discontinuity.

Strictly speaking, the phase transformation is not a discontinuity in the sense of the word, since the transformation takes place over an extensive depth range in which both mineral phases coexist. Since the seismic velocities in this transition area increase slowly and continuously ( gradually ), the detection of the transition is often difficult, which is reinforced by the superimposition of the garnet transformation. While the global existence of the phase change is now generally recognized, corresponding seismological observations are often controversial.

See also

Individual evidence

  1. PM Shearer: Transition zone velocity gradients and the 520-km-discontinuity , Journal of Geophysical Research, Vol. 101, pp. 3053-3066, 1996
  2. a b P. R. Cummins et al .: The 520 Km Discontinuity? , Bulletin of the Seismological Society of America, Vol. 82, pp. 323-336, 1992
  3. G. Helffrich: Topography of the transition zone seismic discontinuities , Reviews of Geophysics, Vol. 38, pp. 141-158, 2000
  4. a b A. Deuss & J. Woodhouse: Seismic observation of splitting of the mid-transition zone discontinuity in Earth's mantle , Science, Vol. 294, pp. 354-357, 2001
  5. ^ A. Saikia, DJ Frost & DC Rubie: Splitting of the 520-km seismic discontinuity and chemical heterogeneity in the mantle , Science, Vol. 319, pp. 1515-1518, 2008
  6. G. Bock: Synthetic seismogramm images of upper mantle structure: No evidence for a 520-km-discontinuity , Journal of Geophysical Research, Vol. 99, pp. 15843-15851, 1994