1200 km discontinuity

from Wikipedia, the free encyclopedia

The 1200 km discontinuity is a postulated boundary layer between two layers of the lower mantle . It is defined by an increase in seismic velocities , especially the S wave, with depth. It is believed that it exists globally. However, so far it has only been observed in a few seismological surveys. Their designation is based on the average global depth of their occurrence, which can, however, vary widely. (see also the article discontinuity )

The cause of this boundary layer is so far unclear. According to one hypothesis, the increase in speed is caused by a phase transformation of the quartz (SiO 2 ). In laboratory tests under high pressure, a conversion of the high pressure structure of the quartz, the stishovite , into the CaCl 2 structure (which does not mean the mineral itself, but only the structure of its crystal lattice ) could be observed. The conversion takes place at around 50 G Pa , which corresponds to a depth of around 1200 km in the earth's mantle . With the phase transformation, a drastic increase in seismic velocities of 20% or 60% for the P or S wave has been determined, so that a low SiO 2 content of only approx. 2% in the lower mantle would be sufficient, to create a detectable seismic discontinuity at that depth.

SiO 2 could z. B. be enriched by fluid transport in subducted , so submerged in the mantle, lithospheric plates : the submerged plate contains, among other things, water-containing minerals, which can be transported as so-called fluid high-pressure phases to great depths. However, these water-rich mineral phases ultimately become unstable and the subducted material is drained, which can increase the SiO 2 content.

Individual evidence

  1. ↑ a b L. Vinnik, M. Kato & H. Kawakatsu, 2001: Search for seismic discontinuities in the lower mantle. In: Geophysical Journal International , Vol. 147, pp. 41-56
  2. ^ KJ Kingma, RE Cohen, RJ Hemley & H. Mao, 1995: Transformation of stishovite to a denser phase at lower-mantle pressures. In: Nature , Vol. 374, pp. 243-245
  3. ↑ BB Karki, L. Stixrude, J. Crain, 1997: Ab initio elasticity of three high-pressure polymorphs of silica. In: Geophysical Research Letters , Vol. 24, pp. 3269-3272
  4. ↑ BB Karki, L. Stixrude, RM Wentzcovitch, 2001: High-pressure elastic properties of major materials of earth's mantle from first principles. In: Reviews of Geophysics , Vol. 39 (4), pp. 507-534
  5. ^ E. Ohtani, M. Toma, K. Litasov, T. Kubo, A. Suzuki, 2001: Stability of dense hydrous magnesium silicate phases and water storage capacity in the transition zone and lower mantle. In: Physics of the Earth and Planetary Interiors , Vol. 124, pp. 105-117