720 km discontinuity

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The 720 km discontinuity is a seismic boundary layer just below the mantle transition zone . It is characterized by an increase in the propagation speed of seismic waves with depth. Their designation reflects the average depth of their occurrence.

root cause

The 720 km discontinuity is due to a phase transformation of the garnet , which, along with olivine, is one of the main components of the mantle rock. In the depths of the mantle transition zone, garnet is in the high-pressure modification majorite , which contains approx. 12 mol percent aluminum . Majorit transforms into the ilmenite structure at depths between 610 km and 660 km . Since this phase transformation occurs over a relatively large depth interval, it does not lead to any observable discontinuity .

The ilmenite phase, however, changes into aluminum-containing perovskite at a depth of about 710 to 720 km . If there are elevated temperatures at this depth, e.g. B. by an ascending plume , the majority structure remains stable down to greater depths and changes directly into aluminum perovskite. However, the phase transition is linked to a specific relationship between pressure and temperature , which is described by the Clapeyron slope . This is given for the 720 km discontinuity with approx. +1.3 MPa / K.

Since the Clapeyron slope of this phase transformation shows the opposite sign to the 660 km discontinuity , an increase in temperature can contribute to the convergence of the two boundary layers.

See also

literature

  • P. Vacher, A. Mocquet, C. Sotin: Computation of seismic profiles from mineral physics: the importance of the non-olivine components for explaining the 660 km depth discontinuity . In: Physics of the Earth and Planetary Interiors . tape 106 , no. 275-298 , 1998.
  • NA Simmons, H. Gurrola: Multiple seismic discontinuities near the base of the transition zone in the Earth's mantle . In: Nature . tape 405 , 2000, pp. 559-562 .
  • A. Kubo, M. Akaogi: Post-garnet transitions in the system Mg 4 Si 4 O 12 -Mg 3 Al 2 Si 3 O 12 up to 28 GPa: phase relations of garnet, ilmenite and perovskite . In: Physics of the Earth and Planetary Interiors . tape 121 , no. 1-2 , 2000, pp. 387-393 .
  • M. Akaogi, A. Tanaka, E. Ito: Garnet-ilmenite-perovskite transitions in the system Mg 4 Si 4 O 12 -Mg 3 Al 2 Si 3 O 12 at high pressures and high temperatures: phase equilibria, calorimetry and implications for mantle structure . In: Physics of the Earth and Planetary Interiors . tape 132 , no. 4 , 2002, p. 303-324 .