Formation and fractionation of basic magmas at high pressures

Abstract

Experiments were conducted at 30 kilobars on dry synthetic mixtures in the join diopside (CaMgSi₂O₆)-pyrope (Mg₃Al₂Si₃O₁₂), on a small number of compositions in adjacent parts of the plane CaSiO₃-MgSiO₃-Al₂O₃ and one composition in the plane diopside-pyrope-forsterite. In addition, experiments were made at the same pressure on the joins chrome diopside-enstatite, chrome diopside-pyrope, and chrome diopside-pyrope-olivine using the separated analysed minerals of a natural garnet-peridotite nodule in kimberlite, and the join garnet-omphacite using the separated analysed minerals of an eclogite nodule in kimberlite. A few experiments were made on the whole-rock sample of some eclogite nodules in kimberlite. The results have an important bearing on the range ofcomposition of clinopyroxene solid solutions, the conditions of equilibration and original formation of eclogite and kimberlite, and the existence of an orthopyroxene-liquid reaction relationship in olivine and hypersthene-normative liquids at 30 kilobars pressure. They lead to the construction of a new fractionation scheme at high pressure in which a hypersthene-normative picrite or olivine tholeiite magma is the primary liquid, formed by partial melting of a garnet-peridotite mantle. Precipitation of garnets and clinopyroxenes (eclogite accumulates) leads to me development of alkaline and silica-poor mafic magmas from this primary liquid.