Origin of phenocrysts and eruption mechanism -in case of Unzen Volcano-.

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Dacite lavas of the 1991-95 eruption at Unzen contain phenocrysts all indicating their disequilibrium with melt, and include basaltic to andesitic enclaves. The origin of the disequilibrium and the existence of enclaves can be explained by the process associated during cooling of a magma chamber, rather than by magma mixing prior to or during eruption. The magma chamber consisted of hot convecting rhyodacite melt in the interior and the boundary layr against the country rock. The boundary layr had the crystal-dominant ("chilled") margin outside and the crystal mush inside, increasing proportion of rhyolite melt inward. Phenocrysts are either the crystals which had precipitated once at the margin when conductive flux in the country rock was large initially, and, then, were disaggregated by melting when heat flux from magma exceeded the conductive flux, or the restites of the wallrock which survived melting. All crystals incorporated into the convecting interior melt were resorbed before eruption. Enclaves represent pieces of either the chilled margin or crystal mush in the boundary layr. Overgrowth rims were formed on phenocrysts by degassing accompanied with eruption. It is unlikely that the idea of magma mixing prior to or during eruption is supported always by disequilibrium phenocryst assemblage/texture or the existence of mafic enclaves in porphyritic lavas.