Phase relations in pyrolite were studied at pressures of 24 and 30 GPa and temperatures up to 2200¿C. At 24 GPa and 1600¿C, an assemblage of Mg-perovskite, magnesiow¿stite, Ca-perovskite, and garnet was observed, with the proportion of garnet increasing continuously with temperature. At 30 GPa, only the two perovskites and magnesiow¿stite were observed, and no significant changes in mineral proportions with temperature were observed. Iron was almost equally partitioned between Mg-perovskite and magnesiow¿stite at temperatures below about 1800¿C at 24 and 30 GPa. With increasing temperature, however, iron was concentrated more in magnesiow¿stite at both of these pressures. The decrease of Al content in Mg-perovskite with temperature, which is caused by an increase in the proportion of garnet, is likely to be responsible for the iron partitioning behavior at 24 GPa, while the minor elements (Na, Al, and Cr) in magnesiow¿stite may play an important role in Fe partitioning at 30 GPa. The density difference between ascending hot plumes (ΔT = 200 and 400¿C) and the surrounding mantle was calculated using the present experimental data. The density difference (ΔT = 400¿C) at 30 GPa (806-km depth) is 0.06 g/cm3 but increases to 0.22 g/cm3 at 24 GPa (670-km depth). The large increase in the density difference that occurs in a hot plume at the uppermost part of the lower mantle originates from an increase of the proportion of garnet and may be a trigger for the plume's passage through the boundary between the lower mantle and the mantle transition zone.