Needless to say, knowledge of phase relations of constituent materials is the most essential to understand the Earth’s structure. Because of high reliability of multi-anvil high-pressure experiments, phase relations of the upper mantle minerals are already well-known. In contrast, those of the lower mantle minerals are poorly known because of the limited pressure range of the multi-anvil apparatus. We plan the following project to determine the lower-mantle phase relations by technical development of multi-anvil experiments.
Origin of extremely sharp 660-km discontinuity
Seismological studies suggest that the thickness of the 660-km seismic discontinuity is less than 2 km. Since mantle minerals are essentially binary ferromagnesian minerals, the 660-km discontinuity, which is considered to be caused by the dissociation of (Mg,Fe)2SiO4 to (Mg,Fe)SiO3 bridgmanite plus (Mg,Fe)O ferropericlase (the post-spinel transition), should have finite thickness. Therefore it is important to examine whether the pressure interval of the post-spinel transition can explain the extreme sharpness of the 660-km discontinuity. If it cannot explain, our view of the structure of the deep mantle should be radically changed.
Although some old studies suggested the small pressure interval of the post-spinel transition is very small, these results are found to be not robust because of sluggish kinetics and imprecise pressure determination. In order to solve this problem, we are planning highly precise phase relation studies using the experimental station being constructed in PETRA III in DESY, explained in the page of Developement of multi-anvil technology.