The chemical compositions of primary magmas of olivine tholeiite (OTB), high‐alumina basalt (HAB), and alkali olivine basalt (AOB) are obtained by the olivine maximum fractionation model for Quaternary magnesian basalts from the Northeastern Japan arc. These basalts are assumed to have fractionated only olivine crystals before eruption. The melting phase relations for three primary basalt compositions have been determined under both anhydrous and water‐undersaturated conditions. The AOB melt coexists with olivine, orthopyroxene, and clinopyroxene at 17 kbar and 1360°C under anhydrous conditions and at 23kbar and 1320°C in the presence of 3wt % water. The HAB melt also coexists with the above three phases at 15 kbar and 1340°C under anhydrous conditions and at 17 kbar and 1325°C in the presence of 1.5 wt % water. The OTB melt, on the other hand, coexists with olivine and orthopyroxene at 11 kbar and 1320°C under anhydrous conditions. The water contents in arc basalt magmas are estimated to be about 3, 1.5, and nearly O wt % for the AOB, HAB, and OTB, respectively, on the basis of the solubility limit of water in silicate melts. Based on these estimates and the experimental results, the AOB, HAB, and OTB magmas are suggested to segregate from the mantle at about 1320°C and at 23, 17, and 11 kbar, respectively. As the temperatures at the segregation of the magmas given above appear to be too high for a stable mantle geotherm, the mantle diapir is the most probable mechanism for the magma production in a subduction zone. Considering the heat of formation of melt in the diapir, the region with temperatures higher than 1400°C has to be present in the mantle wedge.

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