A careful assessment of SRFs shows that the continental lithospheric mantle generally contains numerous positive and negative velocity discontinuities and is spatially heterogeneous. However, some xenolith sections contain no amphibole, suggesting this may not be a universal explanation. Amphibole-rich layers are likely to form at MLD depths in metasomatised regions, making amphibole a possible cause for the MLD. Low-velocity minerals can cause sharp and large decreases in seismic velocity. Some geometries of radial anisotropy can produce consistent negative phases but such geometries are not observed universally and are hard to explain tectonically. However, azimuthal anisotropy will not produce consistently negative phases independent of back-azimuth. Radial and azimuthal seismic anisotropy are both capable of producing negative SRF phases. We find that changes in mantle iron content (Mg#) and elastically-accommodated grain-boundary sliding are unlikely to result in sufficiently large velocity decreases to produce an observable SRF response, while partial melt will generally only exist at greater depths within stable lithosphere.

In this manuscript, we summarise the current observations and assess the main mechanisms that could produce such a feature. A significant velocity drop at such depths is unexpected and its cause is unknown. However, in tectonically stable areas it occurs within the continental lithospheric mantle and has been termed the mid-lithosphere discontinuity (MLD). In young, actively tectonic areas with thin lithosphere, this feature is generally interpreted as the lithosphere–asthenosphere boundary. SRF data frequently contain negative phases at depths between ∼80 and 100 km within the continental lithosphere, indicative of large and sharp velocity drops at these depths. The study, published in Earth and Planetary Science Letters on April 14, was supported by the National Natural Science Foundation of China, PIFI from Chinese Academy of Sciences (CAS) and NERC Grants.Seismic S-wave receiver functions (SRF) are a uniquely powerful tool for imaging velocity discontinuities within the upper mantle. The width of the phase loop was negligible for water contents less than about 1000 ppm, but then grew rapidly to become about 10 km wide, before shrinking again as the water content approaches ringwoodite saturation. This variation on depth was relatively small compared to seismically observed variations in the 660 km discontinuity of around 35 km and so water alone could not account for the observed 660 km discontinuity topography.įurthermore, this study demonstrated that the addition of water caused a large broadening of the three-phase loop (ringwoodite + bridgmanite + MgO) through the development of post-spinel transition. 1 wt.% water increased the depth of phase transition onset by about 8 km. On the other hand, water had a moderate effect on its depth. The results indicated that the water only had a very small effect on the Clapeyron slope of the post-spinel transition. John Brodholt from University College London (UCL), investigated the effect of water on the post-spinel transition at high temperatures and pressures. ZHANG Feiwu from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS), in collaboration with Prof.

Joshua Muir, the postdoctoral researcher, and the team leader Prof. According to recent studies, the transition zone may contain significant amounts of water, and thus understanding the effect of water on the post-spinel transition is important to determine its properties.ĭr. One important parameter that can potentially affect the post-spinel transition is hydration.

It is considered to be caused by the transformation of ringwoodite to bridgmanite and MgO (post-spinel transition). The seismic discontinuity that occurs at an average depth of 647-654 km, usually called the 660 km seismic discontinuity, is the boundary between the transition zone and the lower mantle.

Seismac discontinuity