Final stages of magma evolution in Klyuchevskoy Volcano, Kamchatka: Evidence from melt inclusions in minerals of high-alumina basalts

Magmatic inclusions were studied in phenocrysts from the high-alumina basal ts of Klyuchevskoy Volcano, with special emphasis on melt inclusions in oli vine (Fo(71-79))r clinopyroxene (Mg# = 70-81), and plagioclase (An(47-84)) phenocrysts. Glassy melt inclusions were analyzed for major elements by an electron microprobe, and water content was estimated by various calculation methods. The data obtained allowed us to estimate the physicochemical cond itions of melt crystallization. Variation in water content was the major fa ctor controlling liquidus phase relationships of the melts. Water accumulat ion during olivine and clinopyroxene crystallization resulted in delay of p lagioclase crystallization and melt enrichment in Al2O3 producing magmas ch emically similar to high-alumina basalt. The subsequent evolution of melt c omposition was controlled by plagioclase crystallization from the magma, wh ich resulted in melt depletion in Al2O3. The phenocrysts of olivine (Fo(71- 79)), clinopyroxene (Mg# = 70-81), and plagioclase (An(47-84)) of high-alum ina basalts crystallized from compositionally diverse melts at temperatures 1145-1030 degreesC and pressures no higher than 3.0-3.5 kbar (1.5 kbar, on the average). The earliest of these phenocrysts correspond to melts contai ning similar to5-6 wt % MgO and varying amounts of Al2O3 (16-20 wt %) and H 2O (0-5 wt %). A model is proposed for the final stage of the Klyuchevskoy Volcano magma evolution. According to this model, the crystallization of mi nerals of the high-alumina basalts occurred under conditions of rapid decom pression during magma ascent toward the surface and was accompanied by a de crease in H2O content owing to degassing of the magma. It was shown that th e formation of dacite melt trapped by the plagioclase and clinopyroxene phe nocrysts was probably related to the processes of extensive crystal fractio nation of initial basaltic magmas. The formation of more silicic melts can he explained by the specific mechanism of melt inclusion entrapment by plag ioclase crystals.