Temperature and discontinuities in the transition layer within the Earth's mantle: Geophysical application of the olivine-spinel transition in the Mg₂SiO₄-Fe₂SiO₄System

The nature of the transition layer (the C layer) is considered with the homogeneous chemical composition, model. The physical properties of the transition layer are determined essentially by the mineral assemblage and the Fe/Mg ratio of mantle materials. The upper mantle is believed to be composed mainly of olivine and pyroxene, and the content of olivine possibly amounts to about 80% of the whole. The plausible values for the Fe/Mg ratio of mantle minerals lie between 1:9 and 2:8, and from the petrological evidences the magnesium-rich side in this range seems more probable. The olivine-spinel transition accounts for the sharp discontinuity that starts at the depth of about 370 km in the mantle and whose thickness is of the order of several tens of kilometers, according to the latest velocity models. The temperature at this depth is estimated to lie in the range between 1150° and 1530°C, using the Fe/Mg ratio of 1:9. The thickness of the transition region is directly related to the temperature distribution in it. The 50-km thickness requires a constant temperature distribution, but, to spread the thickness over a 70-km interval, the temperature must be raised by 100°C within this region, for a Fe/Mg ratio of 1:9. The location and the sharpness of discontinuities in the transition layer expected from the recent progress of high-pressure mineralogy show a substantial agreement with the latest seismological evidences. Contrary to the traditional interpretation, the actual transition region might not spread over the entire C layer, and there is a growing possibility that there exist several discontinuities in the transition layer that arise from successive polymorphic transitions in mantle minerals. The mode of sequence of seismic discontinuities in the transition layer varies with the ratio of olivine to pyroxene. If the content of coexisting pyroxene in the upper mantle increases, another seismic discontinuity corresponding to the pyroxene breakdown may become observable at the depth between olivine-spinel and post-spinel transitions.