The density of anhydrous basalt magmas measured with the falling-sphere method increases almost linearly with an average rate of 0.014 (±0.004) g/cc per 0.1 GPa in the pressure range from 1 atm to 1.2 GPa. Based on this compressibility, the density change of magma during crystallization of an anhydrous tholeiitic magma at 0.8 GPa is calculated. Under such conditions, even calcic plagioclase floats in the fractionating magma and would be carried upward with the host magma. Density filtering must be efficient for anhydrous tholeiitic magmas. Plateau basalt magmas can only be erupted on the surface when fissures extend to the middle-lower continental crust. In the presence of 3 wt. % H2O, the density of basalt magmas becomes smaller than that of intermediate plagioclase and plagioclase would sink even in the lower crust, forming hornblende gabbro cumulates, which are often observed as xenoliths in island arc volcanic rocks. The chemical compositions of melts formed by partial melting of a peridotite under hydrous conditions have been determined with the sandwich method at pressures between 1.2 and 2.0 GPa. Based on the results of these experiments, the composition of melts formed in the mantle wedge of NE Honshu is estimated; it ranges from olivine tholeiite in the hottest part of the mantle wedge, through high-alumina basalt, to magnesian andesite in the shallowest part of the partially melted region. Such a variation may also exist in a narrow region from the center of a hot diapir to the surrounding mantle at shallow level in the mantle wedge.
