We determined phase relations of harzburgite and basalt (mid‐ocean ridge basalt, MORB) at 12–28 GPa and 1600–2200 °C with a large number of experiments using a multianvil high‐pressure apparatus. These phase relations were precisely compared with those of pyrolite simultaneously determined by multisample cell technique. The post‐spinel (pSp) transition of harzburgite occurs at 23 GPa and 1600 °C with the boundary slope of −3 ± 1 MPa/°C. The post‐garnet (pGt) transition boundary of MORB, defined as the beginning of the majorite garnet‐bridgmanite transition, is located at 25 GPa and 1600 °C, with a slope of 1.5 ± 1 MPa/°C. The pSp transition of harzburgite occurs at higher pressure by 0.5 GPa at 1600 °C and has the more negative slope than that of pyrolite (−1 ± 1 MPa/°C). The pGt transition of MORB occurs at higher pressure by 3 GPa than the pSp transition of harzburgite. At 1600 °C, the density of pyrolite is smaller than those of harzburgite and MORB before the pSp transition of pyrolite (pyrolite–harzburgite/MORB = −0.17 g/cm³). After the pSp transition of pyrolite, harzburgite has lower densities than that of pyrolite at 22–27 GPa (pyrolite–harzburgite = 0.31 and 0.08 g/cm³ before/after the pSp transition in harzburgite, respectively). On the other hand, the density of MORB becomes the highest among the three rocks above 25 GPa after the pGt transition of MORB (pyrolite–MORB = −0.18 g/cm³). The present density contrasts suggest that harzburgite and MORB may stagnate and accumulate in the transition zone by slab subduction, resulting in that the pyrolitic part of slabs is subducted into the lower mantle.

We have generated over 40 GPa pressures, namely, 43 and 44 GPa, at ambient temperature and 2000 K, respectively, using Kawai-type multi-anvil presses (KMAP) with tungsten carbide anvils for the first time. These high-pressure generations were achieved by combining the following pressure-generation techniques: (1) precisely aligned guide block systems, (2) high hardness of tungsten carbide, (3) tapering of second-stage anvil faces, (4) materials with high bulk modulus in a high-pressure cell, and (5) high heating efficiency.