No abstract

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.

The viscosities of melts of NaA1Si:O6 (jadeite) and Na:SiaO7 compositions have been determined at pressures between 5 and 24 kbar, using the falling sphere method and graphite capsules 10 mm long in solid media piston-cylinder apparatus. The viscosity of NaAISi:O6 melt decreases from 3.4 X 10 • P at 5 kbar to 5.3 X 10 a P at 24 kbar at 1350øC, whereas that of Na:SiaO7 melt decreases by a factor of less than 3 from I atm to 20 kbar at 1175øC. These results indicate that the remarkable decrease in viscosity of NaA1Si:Oo melt with increasing pressure is largely due to the presence of A1 in the melt. It is most probable that a part of A1 in the melt changes from four-to six-fold coordination at high pressures. The density of glass of NaA1Si:Oo composition increases with increasing pressure of quenching from 2.42 g/c a at 5 kbar to 2.58 at 21 kbar, indicating that the melt also changes to a denser structure with the coordination change of Al at high pressures. It is suggested that most magmas undergo similar structural changes in the upper mantle and have higher density and lower viscosity at greater depths.

Melting of a natural peridotite (spinel‐bearing lherzolite) which occurs as a nodule in the tuff of Salt Lake, Hawaii, has been studied at pressures between 1 atm and 50 kb under anhydrous conditions and at pressures between 20 and 60 kb under hydrous conditions with the tetrahedral‐anvil type of high‐pressure apparatus. Under anhydrous conditions the lherzolite begins to melt near the liquidus of some olivine tholeiites. Garnet is stable near the solidus at pressures higher than at least 30 kb. Under hydrous conditions, when sealed capsules are used, the solidus of the lherzolite is at about 1000°C at 26 kb and about 1150°C at 60 kb. It is 400–700°C lower than the solidus under anhydrous conditions. When unsealed capsules are used, the solidus is raised by 200–400°C from the solidus determined by using sealed capsules. From the present experiments it appears that under anhydrous conditions magmas of olivine tholeiite composition can be formed from lherzolite, but those of quartz‐tholeiite composition cannot be formed by partial melting, at least in the pressure range 10–30 kb. Quartz‐tholeiite magma, however, can be formed within a much larger pressure range under hydrous conditions. The solidus under hydrous conditions (water pressure is equal to total pressure) would give a possible lowest temperature of beginning of melting of the upper mantle. It is also suggested that the partial melting of the hydrous upper mantle may play an important part in the formation of the low‐velocity zone.