Compositions of magmas formed by partial zone melting of the Earth's upper mantle

Abstract

The upper mantle of the earth is probably composed of peridotites consisting mainly of olivine, orthopyroxene, clinopyroxene, and garnet or spinel similar to those found as inclusions in kimberlite and basaltic rocks. The compositions of the magmas produced by the partial melting of the mantle peridotites are closely dependent on the position of the isobaric invariant point where olivine and orthopyroxene are involved in systems including components of minerals of the mantle peridotites. The positions of such invariant points have been investigated in the systems Mg2SiO4-SiO2-X, where X represents CaMgSiO4, CaAl2O4, NaAlSiO4, and MgAl2O4, in the pressure range 7 to 40 kb. It was found that the liquidus boundary between forsterite and enstatite solid solution and the invariant point where forsterite and enstatite solid solution are involved shift from silica-rich compositions toward silica-poor compositions with increasing pressure up to at least 40 kb in all the systems examined. The experimental results suggest that the compositions of the magmas produced by the partial melting of the mantle peridotites would become more silica-deficient with increasing pressure. It is also suggested that magmas consisting essentially of pyroxene and garnet components are formed by partial melting of the mantle peridotites at pressures less than 30 kb. Magmas formed in the upper mantle by partial melting would ascend by the mechanism of ‘partial zone melting’ in the levels where temperature is close to the beginning of melting of the mantle peridotites. Compositional change in the magmas that ascend by the partial zone melting is dependent on the isobaric invariant point where olivine and orthopyroxene are involved and can be estimated on the basis of the present experiments.