斑状ボニナイトの起源 : 固液境界層の破壊とマグマ混合

抄録

We propose a model of the boninite magma chamber beneath Chichijima, Bonin Islands on the basis of texture and mineral chemistry of a porphyritic boninite. The boninite-series volcanic rocks differentiate from boninite through andesite and dacite up to quartz-phyric rhyolite. Whole-rock compositions and mineralogical evidence indicate that both boninite and andesite are formed by mixing of magmas, while andesite, dacite and rhyolite are products of crystallization differentiation. This view is supported by the fact that both boninite and andesite lavas are intercalated with each other but that dacite and rhyolite lavas are underlain by boninite and andesite, and are superposed one after another in ascending order. Density and viscosity contrasts, which largely affect the ability of mixing of two magmas, are small between boninite and andesite magmas, and enabled mixing of these two. Detail examination of texture and zoning of orthopyroxene phenccrysts in CH501, which is a representalive of pophyritic boninites, has shown that 80% of the orthopyroxene phenocrysts (Type I) have cores with uniform compositions with magnesian rims. The homogeneous core compositions differ from grain to grain and range in Mg# from 90 to 78, but have the highest frequency at Mg#86. Another 20% of phenocrysts (Type II) have cores with multiply repeated resorption and reverse zoning in Mg, Fe and Ca, which are formed by repeated injection of hot, primitive magmas. In order to explain the population, texture and chemistry of these phenocrysts,a flat, sheet-like magma chamber is proposed. Magma in the chamber is cooled from above and crystallizes against the roof of the chamber. The flat roof prohibits segregation of residual melt from crystals and thus forms mushy layers growing downward. New input of hot, primitive magmas form fountains rising up to the roof of the chamber and destruct the mushy layers and incorporate cumulus crystals into hybrid magmas. Some parts of the mushy layers resisted against the flush of the fountains are resorbed and reequilibrated with the hybrid magmas, and are overgrown with new crystals and melts. Type II phenocrysts are such long-lived cumulus crystals which experienced repeated magma input. Type I phenocrysts are derived from cumulus crystals in the newly formed part of the mushy layers.