Magma storage constrains by compositional zoning of plagioclase from dacites of the caldera forming eruptions of Vetrovoy Isthmus and Lvinaya Past' Bay (Iturup Island, Kurile Islands)

Abstract

The Vetrovoy Isthmus and the Lvinaya Past' Bay on the Iturup island (Kuril island arc) are the results of large Plinian eruptions of compositionally similar dacitic magmas. This study is devoted to a comparative analysis of the storage and crystallization conditions for magma reservoirs, which were a source of large-scale explosive eruptions. The plagioclase is most informative mineral in studying of the melt evolution. The studied plagioclases possess a complex zoning patterns, which are not typical for silicic rocks in island-arc systems. It was shown that increase of Ca in the plagioclase up to unusually high An95 is related to increase of H2O pressure in both volcanic magma chambers. The study revealed that minerals of the Vetrovoy Isthmus and Lvinaya Past' crystallized from compositionally similar melts. Despite the compositional similarity of the melts, the phenocryst assemblage of the Lvinaya Past' differs from the Vetrovoy Isthmus by the presence of the amphibole, which indicates that the pressure in the magmatic chamber exceeded 1-2 kbar at a 4-6 wt. % of H2O in the melt. The rocks of the Vetrovoy Isthmus do not contain amphibole phenocrysts, but melt and fluid inclusions assemblages in plagioclase demonstrate that the magma degassed in the course of evolution. This is an indication that the pressure did not exceed significantly 1-2 kbar.