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

用MA-8型高压装置研究了CaO-MgO-Al_2O_3-SiO_2系地幔物质的熔融关系。在上述实验的基础上，估算了在3~8 GPa压力范围内与橄榄石、辉石和石榴石共存的熔体的组成。与上述四种相共存的熔体的CaO/Al_2O_3值在4 GPa时基本一致，并随压力的升高而增大。这一结果表明，CaO/Al_2O_3比值大于1的科马提岩岩浆可能是由部分熔融形成的，而橄榄岩或球粒陨石物质是地球的主要物质，其CaO/Al_2O_3比值为单位。为了将上述结果应用于自然系统，有必要了解其他组分对熔体和主要矿物相之间元素分配的影响。从这一角度出发，对天然玄武岩体系进行了熔融实验。主量元素在石榴石和熔体之间的分配没有改变，即使石榴石的组成随着压力的增加而变化。然而，像Na这样的次要元素的分配随着压力的增加而急剧变化。这可能表明，除了上述简单的体系外，还应在Nabering体系中进行熔融实验。

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.