Effect of Water on the Composition of Magmas Formed at High Pressures

Portions of the system MgO-CaO-Na2O-Al2O3-SiO2-H2O have been studied in the pressure range 13–35 kb at near-liquidus temperatures. The liquidus field of forsterite relative to that of orthopyroxene is considerably wider under anhydrous than under anhydrous conditions and it covers part of the plane of silica-saturation in a wide pressure range. Partial melting of simple garnet lherzolite (= forsterite+orthopyroxene+clinopyroxene+garnet) with water produces quartz-normative liquids at pressures up to at least 25 kb regardless of water content. Hydrous minerals are not encountered at or near the solidus temperatures except in a Na-rich part of the system. Microprobe analysis of the run products in this synthetic system shows that the liquid (glass) in equilibrium with the lherzolite mineral assemblage is silica and alumina-rich at 20 kb under vapor-present conditions. With increasing degree of partial melting, the liquid changes its composition, passing into a ‘vapour-absent region’ and becoming less silicic. Fractional crystallization of olivine tholeiitic magma under hydrous conditions also produces silica-rich magmas at high pressures. If the system is open to water, and water pressure is less than total pressure, the composition of the liquid varies from quartz-normative to olivine (±nepheline)-normative depending on water pressure. It is suggested that in the presence of water, silica-rich magmas such as those of calc-alkalic andesite or dacite may be formed by direct partial melting of the peridotitic upper mantle at depths down to about 80 km. A large degree of partial melting of lherzolite under hydrous conditions would produce SiO2 and MgO-rich magmas. The clinoenstatite rock from Cape Vogel, Papua, may have been formed by such a process. Peridotites with low CaAl2SiO5/jadeite ratios in the clinopyroxene could produce nepheline-normative magma by small degree of partial melting and tholeiitic magma by large degree of partial melting under hydrous conditions.