Repetti discontinuity

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The Repetti discontinuity is a postulated boundary layer between two layers of the lower mantle . It is defined by an increase in seismic velocities with depth. It was named after William C. Repetti , an American geophysicist who investigated this boundary layer using seismological methods as part of his doctoral thesis, which he completed in 1930 at St. Louis University .

It is not a zero order boundary layer (see also the article Discontinuity ), but a gradual transition. Seismological studies with short-period data indicate, however, that the transition area is relatively thin (≈ 10 km). The cause of the Repetti discontinuity is so far unclear and its occurrence has so far only been proven by relatively few observations. These investigations were mostly related to subduction zones and showed a seismic discontinuity at very different depths (between 900 and 1080 km). Only recently has such a discontinuity been found outside of a subduction area. It is therefore suspected that it could be a global discontinuity. According to the mean depth of its occurrence, it is sometimes referred to in the specialist literature as the 920 km discontinuity . Other studies, on the other hand, have provided no evidence of a global discontinuity in this depth range, which is therefore controversial.

So far, there are only speculative approaches to its cause. In one possible explanation, the discontinuity is considered to be the lower edge of a layer of garnet , a rock layer that contains a large proportion of garnet . This is caused by the transformation of material from the oceanic crust , which was subducted into the mantle . This explanation is likely to prove inadequate if the global existence of the boundary layer is confirmed. Another hypothesis explains the occurrence of the 920 km discontinuity through a change in the ratio of magnesiowustite (MgO) to quartz (SiO 2 ) in the earth's mantle and the associated change in density (see also the article 1200 km discontinuity ) . The quartz enrichment is explained by the fluid transport in subducted slabs. At depth, water-containing minerals become unstable and dehydrate , with the quartz being washed out.

Individual evidence

  1. ^ A b F. Niu, H. Kawakatsu: Depth variation of the mid-mantle seismic discontinuity
  2. ^ A b c H. Kawakatsu & F. Niu: Seismic evidence for a 920-km discontinuity in the mantle . In: Nature , Vol. 371, pp. 301-305, 1994
  3. ^ A b Y. Shen, CJ Wolfe & SC Solomon: Seismological evidence for a mid-mantle discontinuity beneath Hawaii and Iceland . In: Earth and Planetary Science Letters , Vol. 214, pp. 143-151, 2003
  4. ↑ JC Castle & RD van der Hilst: Searching for seismic scattering off mantle interfaces between 800 km and 2000 km . In: Journal of Geophysical Research , Vol. 108 (B2), 2095, doi : 10.1029 / 2001JB000286 , 2003