ИСТИНА

Magmas erupted from volcanoes of Central KamchatkaDepression can be represented as a mixture of 1) magmas of subduction origin 2) adakitic products of the lower crust melting1 and 3) alkaline potassic magma. Existence of the thirdcomponent is proven by occurrence of absarokite dyke atHarchnsky volcano2, rare mellilitic xenolyths and melanitic garnet inclusions in olivine. Periodically high potassiumbasalts were erupted in Tolbachik and Plosky volcanoes in Holocene. We present experimental data demonstrating thatmelting mediated by the dry reduced carbonic fluid leads to the enrichment of the melt with K2O, Rb2O and LREE. In our experiments fluid was generated at the decomposition of natural siderite placed in the small capsule within welded largeone with the source basalt. CO2-CO fluid effectively transfers Al2O3, K2O and other elements to form Ol-Spl aggregate with alkaline melt replacing oxides remaining after siderite decomposition in the small capsule. Melt show highly fractionated REE patterns La/Yb is up to 76. Potassium contentattains up to 10 wt.% of K2O. Potassium and sodium separation factor (K/Namelt)/(K/Nasource) = 25-30. Our results support concept of the strong link between potassiumand carbon in magmas3. Experiments were performed at P=2-5 kbar and T=900-1000oC, we suggest that this mechanism will work at the mantle PT parameters too. It is expected that mantle wedge under CKD was locallycarbonatized during accretion of the Kronotsky paleoarc about5 Myrs ago. At the overstepping of islands by subduction zonelarge blocks of silicified carbonates and coals can be draggedunder mantle wedge as envisaged by numerical modeling4. Large masses of sedimentary rocks presumably had ascendedunder Central Kamchatka ridge and are moving eastward in theslow wedge counterflow. Silicified carbonates rising to approximately 100 km depth react to form CO2 and pyroxene.Reduced carbonic fluid is involved in the potassium magmaformation. Work was supported by RFBR grants #13-05-00397 and13-05-00994. [1] Gorbach and Portnyagin, 2011, Petrology, 19(2):134. [2] Volynets et al., 1999, Volcanol. Seismol., 1: 31(in Russian) [3] Gupta, 2015, Springer. 4 Simakin, 2014, Terra-Nova, 26(1):22 .