Melting Relations in the System, CaO-MgO-Al_2O_3-SiO_2, at Ultra-high Pressure

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Melting relations of the mantle materials in the system, CaO-MgO-Al_2O_3-SiO_2, have been investigated with high pressure apparatus of MA-8 type. The compositions of melt coexisting with olivine, pyroxenes and garnet were estimated in the pressure range from 3 to 8 GPa based on the above experiments. CaO/Al_2O_3 ratio of the melt coexisting with the above four phases is almost unity up to 4 GPa and increases with increasing of pressure. This result indicates that komatiite magmas with CaO/Al_2O_3 ratio larger than unity can be formed by partial melting of peridotite or chondritic materials which are supposed to be primary materials for Earth and have CaO/Al_2O_3 ratio of unity.In order to apply the above results to the natural system, it is necessary to know the effect of other components on the partitioning of elements between melt and major mineral phases. From this view point, melting experiments on natural basalt system were also performed. The partitioning of major elements between garnet and melt does not change even the composition of garnet changes with increasing pressure. The partitioning of minor elements such as Na, however, changes drastically with increasing pressure. This may indicate that melting experiments should be done in the Nabering system addition to the above simple system.