Melting of a peridotite nodule at high pressures and high water pressures

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.