X discontinuity

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The X discontinuity (also: 300 km discontinuity ) was first reported in the early 1990s. So far, it has presented itself as a regionally very limited seismological boundary layer, which is defined by a sudden change in seismic speeds . It has been observed at depths between 250 and 350 km.

To explain their occurrence, various mineralogical backgrounds have been discussed in the specialist literature, such as the phase transformation to a dense magnesium silicate with a high water content ("hydrous phase A") or a conversion from orthorhombic to monoclinic pyroxene . According to recent laboratory measurements, both proposals appear to be unrealistic: the water-containing phase only proves to be stable up to approx. 1,100 ° C and should therefore only be observable in subduction zones , while the pyroxene conversion generates too little impedance contrast and thus cannot explain the observed change in speed .

The phase transition of SiO 2 from coesite to stishovite is gaining importance as an alternative explanation . According to laboratory tests, this conversion occurs at pressures between 8.5 and 11 G Pa , which corresponds to a depth range of about 265 to 310 km in the earth's mantle at temperatures in a possible range of fluctuation from 1,050–1,500 ° C. The phase transformation is able to explain the seismological observations with a high increase in seismic velocities of more than 30%.

The discontinuity, which was initially very controversial, has now been discovered in several studies in different regions of the world, some of which are exposed to very different tectonic conditions. A direct connection to certain tectonic framework conditions is largely excluded.

Individual evidence

  1. ↑ a b Revenaugh, J. & Jordan, TH, 1991: Mantle layering from ScS reverbarations 3. The upper mantle. In: Journal of Geophysical Research , Vol. 96, pp. 19781-19810
  2. ↑ Woodland, AB, 1998: The orthorhombic to high-P monoclinic phase transition in Mg-Fe pyroxenes: Can it produce a seismic discontinuity? In: Geophysical Research Letters , Vol. 25, pp. 1241-1244
  3. ^ A b Wiliams, Q. & Revenaugh, J., 2005: Ancient subduction, mantle eclogite, and the 300 km seismic discontinuity. In: Geology , Vol. 33, pp. 1-4
  4. ↑ Liu, J., Topor, L., Zhang J., Navrotsky, A. & Liebermann RC, 1996: Calorimetric study of coesite-stishovite transformation and calculation of the phase boundary. In: Physics and Chemistry of Minerals , Vol. 23, pp. 11-16

Further literature

  • Revenaugh, J. & Sipkin, SA, 1994: Mantle discontinuity structure beneath China. In: Journal of Geophysical Research , Vol. 99, pp. 21911–21927 (English)
  • Deuss, A. & Woodhouse, JH, 2002: A systematic search for mantle discontinuties using SS-precursors. In: Geophysical Research Letters , Vol. 29, No. 8, doi : 10.1029 / 2002GL014768 (English)
  • Stixrude, L. & Lithgow-Bertollini, C., 2005: Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone. In: Journal of Geophysical Research , Vol. 110, B03204, doi : 10.1029 / 2004JB002965