THE ORIGIN AND COMPOSITION OF PRIMITIVE MELTS OF KLYUCHEVSKOY VOLCANO, KAMCHATKA - INSIGHT FROM MELT INCLUSIONS STUDY

Klyuchevskoy volcano is one of the largest and most productive volcanoes in the world. It is located in the Central Kamchatka Depression on Kamchatka peninsula a part of Kurile-Kamchatka island arc. Age of volcano is about 7 thousand years. Volcanic products are represented by series of high magnesia to high alumina basalts. To obtain information about the composition of primitive melts and conditions of their generation, there were studied melt inclusions in olivine phenocrysts (Fo91-89) from different Klyuchevskoy basalts (high magnesian basalts of Bulochka and Luchitskii cones, magnesian basalts of Ochki and Tuyla flows and high alumina basalt of Apakhonchich flow) erupted in the last 4000 years. Most of melt inclusions were partially crystallized so they were experimentally reheated. For this purpose heating stage with visual control of Sobolev-Slutsky construction [1] was used. The inclusions were partially homogenized (melt + fluid phase) and quenched. Small part of studied inclusions was represented by naturally quenched inclusions, consisting of glass and gas bubble. For major elements all glasses were analysed by microprobe, for trace elements and water by SIMS. Analysed compositions were corrected using method [2] considering re-equilibration of inclusions with their host. Initial FeO content in melts was assumed 8.5 wt.% corresponding to average FeO content of Klyuchevskoy rocks. The pressure of crystallization was determined using temperature difference for Ol-Cpx cotectic [3], temperature-using Ol-melt thermometer [4]. Temperature was corrected for pressure (5 °С/kbar) and for water content (2-2.5 wt. % dT=100 °С [5]). Melt oxidation state was estimated using composition of Cr-spinels trapped in olivines as crystal inclusions [6], [7]. The most primitive melts start to crystallize at pressure 1.5 GPa, temperature 1300 °C and oxygen fugacity close to NNO buffer. Average composition of the melts is (in wt.%): SiO2=47.9, TiO2=0.8, Al2O3=15.5, FeOtot=8.5, MgO=10.7, CaO=12.8, Na2O=2.8, K2O=0.6, P2O5=0.13, S=0.2, Cl=0.1, F=0.034. Wide variations were found for H2O content (0-2.5 wt.%). Minimum CO2 content was estimated to be 0.4 wt.% from the density of CO2 – rich fluid inclusions in olivine Fo88. Despite general similarity, the melts trapped in high-Mg olivines show variable SiO2, Al2O3, CaO, Na2O и H2O content. All primitive melts were divided by water content into two groups: low water (LW) with H2O content less then 1 wt.% and high water melts (HW) H2O content more then 1 wt.% and were calculated to equilibrium with Fo91. Negative (SiO2-CaO, SiO2-Al2O3) and positive (Na2O-Al2O3) correlations are observed between SiO2, Al2O3, CaO and Na2O content so that H2O-rich melt inclusions tend to show higher SiO2 and lower Al2O3, CaO and Na2O compared with H2O-poor inclusions (fig.1).