Abstract
Abstract Under anhydrous conditions magnesian olivine and not orthopyroxene is on the liquidus in the primary basalt magmas after “magma separation” within the upper mantle which probably consists of olivine-rich lherzolite. The experiments carried out by Green and Ringwood (1967) on some primary basalt compositions in the pressure range in which orthopyroxene is on the liquidus would not be applicable to the fractional crystallization of the primary basalt magmas after “magma separation”. Under hydrous conditions, the field of orthopyroxene crystallization relative to that of magnesian olivine is reduced at higher pressures when the bulk chemical composition is kept constant. The possibility that silica-saturated magmas are formed increases under hydrous conditions, even if the water pressure is less than the total pressure. Green's suggestion, based on experiments with unsealed capsules, that the generation of olivine nephelinite and of olivine melilite nephelinite magmas under hydrous conditions would not be valid when the bulk chemical composition is kept constant.
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