Searching study for a new indicator of partial melting of high-grade metamorphics

本研究的目的是建立在远高于其最低熔融温度下形成的高变质岩部分熔融的新指标。首先，我们寻找了在火成岩和变质岩中常见的造岩矿物的结构和组成特征，发现如下：1。石榴石卟卟母细胞中常见的自面体和钙斜长石包裹体。包体斜长石比基质斜长石含钙量高得多。石榴石和其他共存的硅酸盐矿物对磷和重稀土元素的化学分带。石榴石卟卟母细胞通常由贫磷岩心和富磷边缘组成，贫磷岩心有自面体和钙斜长石包裹体，富磷边缘有磷矿物包裹体。此外，石榴石中重稀土元素带化常保持自面体模式，而主要元素带化模式为同心圆和扩散控制。这清楚地表明磷矿物参与了部分石榴石形成（部分熔融）反应。因此，这些特征可以很好地解释为以下连续而不一致的熔融反应：硅线石+黑云母+钠质斜长石+石英+磷矿物<正负>H_2O=熔体+石榴石+钙质斜长石+金红石（或钛铁矿）。锆石的数量、形态、内部结构和组成，以及磷灰石、独一石、xenotime等磷矿物，在SHRIMP定年中可能具有重要的地球化学模拟意义。这些特征和见解的一部分已被再现，并检查了高温熔融实验与HIP设备。我们还试图通过实验产生斑晶矿物，精确估计九州Aira火山喷发出的大体积长英质岩浆的温度、压力和含水量。

The purpose of this study is to establish new indicators of partial melting of high-grade metamorphic rocks which formed at temperatures much higher than those of their minimum melting. First, we looked for textures and compositional features of rock-forming minerals that occur commonly in both igneous and metamorphic rocks, and have found the followings.1.The common occurrence of euhedral and calcic plagioclase inclusions in garnet porphyroblasts. Inclusion plagioclase is much more calcic compared to the matrix plagioclase.2.The chemical zoning of garnet and other coexisting cilicate minerals for phosphorus and heavy rare earth elements. Garnet porphyroblasts often consist of phosphorus-poor cores with euhedral and calcic plagioclase inclusions and phosphorus-rich rim with phosphorus mineral inclusions. In addition, garnet often preserves euhedral patterns for heavy rare earth element zonation while major elements zonation patterns are concentric and diffusion controlled. These clearly indicate that phosphorus minerals takes in part of the garnet-forming (partial melting) reaction.Thus, these features are well explained by the following continuous and incongruent melting reaction :sillimanite+biotite+sodic plagioclase+quartz+phosphorous minerals<plus-minus>H_2O=melt+garnet+calcic plagioclase+rutile (or ilmenite)The amount, morphology, internal structure and compositions of zircon in addition to phosphorus minerals such as apatite, monazite, xenotime and may be significant in geochemical modeling, as shown by the SHRIMP dating.Part of these features and insights have been reproduced and examined by high-temperature melting experiments with a HIP apparatus.We also have tried to precisely estimate the temperature, pressure and water content of large-volume felsic magmas erupted from the Aira volcano in Kyushu by experimentally producing phenocryst minerals in them.