COMPOSITION OF MAGMAS OF THE 1996 ERUPTION AT THE KARYMSKII VOLCANIC CENTER, KAMCHATKA: EVIDENCE FROM MELT INCLUSIONS

Abstract

Melt inclusions were examined in phenocrysts of basalt from the Akademiya Nauk caldera (Novogodnii Fissure, Karymskoe Lake) and andesite (1996 eruption) from Karymskii volcano. The inclusions were studied by homogenization, and their glasses (more than 50) were analyzed on an electron microprobe. The SiO2 concentrations of melt inclusions in plagioclase and olivine from the basalt range from 47.4 to 57.1 wt %, and these values for plagioclase from the andesite are 55.7-67.1 wt %. The basaltic melts are characterized by usual concentrations of FeO (11.4-6.8%), MgO (6.1-2.3%), CaO (10.8-6.7%), and TiO2 (1.3-0.7%) and are enriched in Na2O (7.4-2.9%, average 5.1%), with the highest Na2O concentrations (7.4-5.3%) detected in the least silicic melts (SiO2 = 47.4-52.0%). The K2O concentrations vary from 0.4 to 1.7%. The basaltic melt was determined to contain 0.09% Cl and 0.14% S. The andesitic melts are characterized by high concentrations of FeO (average 6.5%) and CaO (5.2%) and usual concentrations of Na2O (4.5%) and K2O (2.1%). The Cl content of the melt was 0.26%, and that of S was 0.07%. The comparison of andesites from Karymskii volcano with the previously studied andesitic melts from the Shiveluch and Bezymyannyi volcanoes revealed their significant differences. The former are less silicic, higher in Fe, Ti, Ca, Mg, P, and Na but significantly lower in Na2O. The Karymskii melts are most probably less differentiated than the melts of Bezymyannyi and Shiveluch volcanoes.