The 2012-2013 Tolbachik Fissure eruption started from lava gushing and effusion in the Menyailov vent on November 27 th , 2012; after three days the activity of this vent ceased and the eruption continued from the Naboko vent until its end in September 2013. The eruption produced about 0.7 km 3 of high-Al basaltic trachyandesites (Dvigalo et al., 2014). At Menyailov vent SiO2 concentrations were as high as 55.35 wt.% and K2O -2.67 wt.% -higher than in any previously erupted rocks in Tolbachik monogenetic area. From the beginning of December, SiO 2 concentration in lavas dropped by 2 wt.% and remained at this level until the end of eruption. MgO, TiO2, Mg# increased, K2O, Na2O decreased slightly. Most prominent change we observe in K2O/MgO ratio, which was about 0.7 in the Menyailov vent rocks and became 0.5 in the Naboko vent rocks. Details of the geochemical composition of the volcanic rocks produced by this eruption are reported elsewhere (Volynets et al., 2013, Volynets et al., 2014 a, b); here we present the results of the geochemical study of the main phenocrysts in the lavas – plagioclase. For the detailed geochemical study of the plagioclase we selected two samples from the Naboko vent (Pl-phyric lavas, erupted in February and August) and five crystal lapilli (two of them were erupted in December 2012, and three – during 2013, when the new cone has been already built). Plagioclases in these lavas are represented by two generations of labradorite and bytownite. Pl phenocrysts of the 1 st generation are large (up to 2 cm on the long axis) strongly resorbed at the edges and sometimes in the cores as well, containing lots of glass inclusions. Pl subphenocrysts of the 2 nd generation are smaller (less than 500 µm), usually nonresorbed and clean, having euhedral facets. Normal, reversed and patchy zoning are typical for all studied crystals (fig.1, I and II). Maximum concentrations of An (up to 83% at compositional variation between An50 and 74) has been measured in the patchy zones. Crystal lapilli are characterized by the oscillatory zoning with An fluctuations around An57-63 (fig. 1, III and IV). This kind of zoning is the result of the diffusion control of Pl growth at low growth rates (Sibley et al., 1976). The edges of lapilli are usually rich of glassy inclusions, tunnel-like dissolution structures, Ol, Px, Mt inclusions (fig. 1, III and IV). There are abundant resorption zones in lapilli, with plenty of glassy inclusions. These zones are characterized by the patchy zoning with An concentration jumps up to An74; usually these high-An areas are observed near the inclusions of glass. At the edges of lapilli there are zones with An gradual decrease towards the rim from An 74 to An61.

Plagioclase lapilli and phenocrysts in the lavas of the 2012-2013 Tolbachik Fissure eruption.

The geochemical variations of magmas across and along supra-subduction zones (SSZ) have been commonly attributed to profound changes in the phase and chemical compositions of the mantle source and subduction-derived melt and fluid fluxes, as well as the physical parameters (e.g. depth, temperature, oxygen fugacity etc) of slab dehydration, mineral breakdown and melting. Here we test the variability of the Late Quaternary primitive magmas in the southern and northern parts of the meridionally oriented Eastern Volcanic Belt (EVB) of Kamchatka, with a slab depth varying from 60 to 160 km. Eight high-Mg (Mg# > 60 mol%) basalts were characterized for major, trace and platinum-group element (PGE) abundances, as well as the compositions of olivine phenocrysts and olivine-hosted spinel inclusions. The basalts in our study are geochemically typical of SSZ magmas and contain similar liquidus assemblages of forsteritic olivine (Mg# 78–92 mol%), low-Ti Cr-spinel and clinopyroxene. Although the absolute abundances of major and trace elements, and their ratios, in the basalts fluctuate to some extent, the observed variability cannot be correlated with any of considered parameters in the geometry of the Kamchatka SSZ and conditions of melting. This unexpected result led to the evaluation of the platinum-group element (PGE) systematics against the lithophile and chalcophile trace element geochemistry and the compositions of phenocrysts. Total whole-rock PGE content varies from 2.3 to 11.7 ppb, whereas the normalized PGE concentration patterns are typical for supra-subduction zones magmas and broadly similar in all studied samples. They are enriched in Rh, Pd and Pt relative to mid-ocean ridge basalts (MORB) and have nearly identical concentrations of Ir-group PGE. The only parameter that correlates well with PGE contents is the average Mg# of olivine phenocrysts from 84 to 90.3 mol%. This is interpreted to result from minor cryptic fractionation of sulfide melt, together with primitive olivine, in low-to-mid crustal conditions. Negative Ru anomalies on chondrite-normalized diagrams correspond to the Fe2+/Fe3+ ratios in spinel (a proxy for magma redox conditions), which reflects a replacement of monosulfide solid solution by laurite in the mantle wedge during oxidation.

The 1996 short-lived subaqueous eruption at the Karymsky caldera lake suddenly changed the composition of the lake water. The lake, with a surface area of ∼10 km^2 and a volume of ∼0.5 km^3, became acidic, increased its salinity to ∼1000 mg/kg, and became dominated by SO4^2- and Ca^2+. Since the eruption, the lake chemistry has evolved in a predictable manner described by simple box model. As a result of dilution by incoming SO4-Ca-Mg-poor water, SO4, Ca, and Mg concentrations follow a simple exponential decrease with a characteristic time close to the residence time of the lake. Na, K, and Cl decrease relatively significantly slower, indicating a continuing input of these constituents into the lake that was initiated during the eruption. Thus, the dynamics of two groups of lake water solutes can be predicted by a simple box model for water and solute mass balance. Key Points Karymsky lake suddenly changed chemistry as a result of the 1996 eruption One-box dynamic model correctly describes the evolution of the lake chemistry The calculated fluxes of chemicals are in a good agreement with the field data

Abstract Interferometric synthetic aperture radar (InSAR) images reveal a pre-eruption deformation signal at Kizimen volcano, Kamchatka, Russia, where an ongoing eruption began in mid-November, 2010. The previous eruption of this basaltic andesite-to-dacite stratovolcano occurred in 1927–1928. InSAR images from both ascending and descending orbital passes of Envisat and ALOS PALSAR satellites show as much as 6 cm of line-of-sight shortening from September 2008 to September 2010 in a broad area centered at Kizimen. About 20 cm of opening of a nearly vertical dike provides an adequate fit to the surface deformation pattern. The model dike is approximately 14 km long, 10 km high, centered 13 km beneath Kizimen, and strikes NE–SW. Time-series analysis of multi-temporal interferograms indicates that (1) intrusion started sometime between late 2008 and July 2009, (2) continued at a nearly constant rate, and (3) resulted in a volume expansion of 3.2 × 107 m3 by September 2010, i.e., about two months before the onset of the 2010 eruption. Earthquakes located above the tip of the dike accompanied the intrusion. Eventually, magma pressure in the dike exceeded the confining strength of the host rock, triggering the 2010 eruption. Our results provide insight into the intrusion process that preceded an explosive eruption at a Pacific Rim stratovolcano following nearly a century of quiescence, and therefore have implications for monitoring and hazards assessment at similar volcanoes elsewhere.