Compositional zonation in olivine phenocrysts and diffusion modelling have been used in the last ten years to estimate magma residence times and the duration of magma ascent. The fundamental assumption is that mixing with newly injected magma into a reservoir triggers diffusional exchange between mafic olivine crystals and more evolved magma and that this magma mixing eventually triggers eruption. If depth of mixing is known, this translates to ascent rates of magmas to the surface. We applied this approach to a series of different arc basalt lavas from Kamchatka to constrain the rates of magma ascent and magma resident in what is one of the most active subduction zones in the world that is also dominated by an abundance of unusually mafic magmas. Our sample collection cover the principal modes of arc magmatism in Kamchatka: from different volcanic complexes (stratovolcano, dikes, summit eruptions, monogenetic cones), of different age (from Late-Pleistocene to Holocene and recent eruptions), from different magmatic regimes (long-lived volcanoes vs. monogenetic eruptions) and different major element composition (from basalt to basaltic andesite of different geochemical character including LILE enrichments). We analyzed and modelled zonation profiles for a range of elements with different diffusivities (e.g. Mg-Fe, Ca, Ni, Mn, Cr) to assess the role of variable diffusivities as a function of major and trace elements in the olivines from different P-T conditions. First data were obtained on samples from the Klyuchevskoy, Shiveluch and Tolbachik, including recent most eruption in 2012/2013. These data show that for some samples the zonation patterns are much more complex than is usually observed: high-Mg olivines at different volcanoes have very different zonation patterns, including normally, reversely zoned grains or even show highly complex repetitive zonation that indicate large compositional changes in the surrounding magma at very short time scales (years). Thus in some Kamchatka basalts, we observe unusual Mg-Fe zonations that are linked to complex mixing, possibly resorption and subsequent crystal growth processes that are generally not preserved due to fast diffusion of Mg-Fe. Based on a first assessment of our measured profiles, the values for diffusion times in Fo-rich olivines (88 to 92% Fo) vary from only a few months to years and thus magma ascent from deep magma sources must have been fast.

Ebeko volcano at the northern part of Paramushir Island in the Kuril island arc produces frequent phreatic eruptions and relatively strong fumarolic activity at the summit area ~ 1000 m above sea level (asl). The fumaroles are characterized by low-temperature, HCl- and S-rich gas and numerous hyper-acid pools (pH < 1) without drains. At ~ 550 m asl, in the Yurieva stream canyon, many hot (up to 87 °C) springs discharge ultra-acidic (pH 1–2) SO4–Cl water into the stream and finally into the Sea of Okhotsk. During quiescent stages of degassing, these fumaroles emit 1000–2000 t/d of water vapor, < 20 t/d of SO2 and < 5 t/d of HCl. The measurement of acidic hot Yurieva springs shows that the flux of Cl and S, 60–80 t/d each, is independent on the volcanic activity in the last two decades. Such high flux of Cl is among the highest ever measured in a volcano–hydrothermal system. Oxygen and hydrogen isotopic composition of water and Cl concentration for Yurieva springs show an excellent positive correlation, indicating a mixing between meteoric water and magmatic vapor. In contrast, volcanic gas condensates of Ebeko fumaroles do not show a simple mixing trend but rather a complicated data suggesting evaporation of the acidic brine. Temperatures calculated from gas compositions and isotope data are similar, ranging from 150 to 250 °C, which is consistent with the presence of a liquid aquifer below the Ebeko fumarolic fields. Saturation indices of non-silicate minerals suggest temperatures ranging from 150 to 200 °C for Yurieva springs. Trace elements (including REE) and Sr isotope composition suggest congruent dissolution of the Ebeko volcanic rocks by acidic waters. Waters of Yurieva springs and waters of the summit thermal fields (including volcanic gas condensates) are different in Cl/SO4 ratios and isotopic compositions, suggesting complicated boiling–condensation–mixing processes.

Abstract—This study is concerned with the petrographic, mineralogic, and geochemical features in the K-high basaltic trachyandesites that were discharged by the 2012–2013 parasitic eruption on Ploskii Tolbachik Volcano. These K-high basaltic trachyandesites exhibit some obvious characteristics that testify to their suprasubduction origin. They are deeply differentiated rocks with strongly fractionated plagioclase.A study of the Sr, Nd, and Pb radiogenic isotope ratios in the K-high basaltic trachyandesites provided evidence of their mantle origin and of the fact that the crust has exerted no influence on their compositions. We performed a comparative analysis of the ratios of the concentrations for some incoherent elements in the K-high basaltic trachyandesites, as well as in intraplate, riftogenic, and island-arc moderate potassium basalts and basaltic andesites in relation to the concentrations of these elements in the primitive mantle. The geochemical features of these K-high basaltic trachyandesites classify them as belonging to the suprasubduction subalkaline formation of the potassium series.

Concentrations and emission rates of major gas species were measured in August 2015 at Ebeko volcano, a quiescently degassing andesitic volcano on Paramushir Island, Northern Kuriles. Using mobile and scanning DOAS measurements we estimated SO 2 emission from the active crater of the volcano at 100 +36/-15 t/d. Based on the comparison of plume areas of individual fumaroles, ca. 90% of the total gas emission from Ebeko in 2015 was provided by a single powerful vent (" Active Funnel " fumarole) and the rest was shared among low-temperature fumaroles. At the time of measurements, gases from the main fumarole had temperature from 420 to 490 °C and composition close to the average arc gas [1], as shown in Table. Gas species CO2 SO2 H2S HCl H2O T, °C mmol/mol Main fumarole 27.9 23.5 6.1 5.6 936 420 Low-temp. jets 92.2 2.62 0.68 1.6 902 <120 Low-temperature fumaroles (<120 °C) emitted gas enriched in CO 2 (up to 28 mol%, 9.2 mol% on average). Such CO 2 enrichment together with depletion in HCl and sulfur species can be explained by scrubbing of soluble gas species by a well-developed hydrothermal system which discharges ultra-acid SO 4-Cl waters [2]. A weighted-average estimate of the total gas+vapor emission from the Ebeko summit provided 1470 t/d, which includes ~ 101 t/d SO2, ~ 110 t/d CO2, ~ 14 t/d H2S and HCl, and 1230 t/d of water vapour with > 50% of the magmatic component. The gas fluxes measured in August 2015 using DOAS fall into the range of previous measurements made from 1960 to 2012 that used direct methods [2] and correspond to the moderate degassing rate of the volcano.

The northeast Kamchatka Peninsula is characterized by unique tectonic regimes: (i) the triple junction ~30 km off the east coast [1], (ii) subduction of the Emperor Seamount Chain [2], and (iii) possible asthenospheric flow between the mantle wedge and the sub-slab mantle via the edge of subducted Pacific slab [3]. Within this area, a monogenetic volcanic group occurs along the east coast, including high-Mg andesitic rocks and relatively primitive basalts (East Cones, EC [4]). We have conducted geochemical studies of the EC lavas, with bulk rock major and trace elements, Sr-Nd isotopic compositions, and K-Ar and Ar-Ar ages, based on which a possible contribution of subducted seamounts and its relation to the tectonic setting are discussed.
The elemental and isotopic compositions indicate that the lavas from individual cones have distinct mantle sources with different amounts and/or compositions of slab-derived fluids. Based on mass balance, water content and melting phase relations, we estimate the melting P-T conditions to be ~1200 ℃ at 1.5 GPa, while the slab surface temperature is 620 – 730 ℃ (at 50-80 km depth). The Sr-Nd isotopic compositions is close to Late Cretaceous Emperor Seamount Chain, especially Detroit [5]. The K-Ar and Ar-Ar ages of the Middle to Late Pleistocene are consistent with the present tectonic setting after 2 Ma [6].
These results suggest that the EC lavas including high-Mg andesite and basalt were generated by mantle flux-melting induced by dehydration of a subducted seamount inheriting a local thermal anomaly [7, 8]

Arc magmatism is a product of subduction factory, involving thermal and chemical interactions
between a subducted slab as a material input and mantle wedge as a processing factory. In turn, the
compositions of arc magma provide invaluable information concerning the material input and the
interactions. The northeast Kamchatka Peninsula is an ideal field to examine such interactions and
relationships, being characterized by (1) subduction of the Emperor Seamount Chain (Davaille and
Lees, 2004), and (2) possible material and thermal interaction among the subducted slab, the
overlying mantle wedge and the sub-slab mantle via the edge of subducted Pacific slab (Portnyagin
and Manea, 2008). Within this area, a monogenetic volcanic group occurs along the east coast,
including high-Mg andesitic rocks and relatively primitive basalts (East Cones, EC (Fedorenko,
1969)). We have conducted geochemical studies of the EC lavas, with bulk rock major and trace
elements, and K-Ar and Ar-Ar ages, based on which a possible contribution of subducted seamounts
and its relation to the tectonic setting are discussed.
The elemental compositions indicate that the lavas from individual cones have distinct mantle
sources with different amounts and/or compositions of slab-derived fluids. Based on mass balance,
water content and melting phase relations, we estimate the melting P-T conditions to bet ~1200 ℃
at 1.5 GPa, while the slab surface temperature is 620 –730 ℃ (at 50-80 km depth). Compared with
the southern part of Kamchatka, the slab surface temperature beneath EC seems to be high due to the
thinner Pacific slab associated with the seamount chain and/or the plate rejuvenation from a mantle
plume impact (Davaille and Lees, 2004; Manea and Manea, 2007).
The K-Ar and Ar-Ar ages of the Middle Pleistocene are consistent with the tephrochronological
study (Uspensky and Shapiro, 1984) and the present tectonic setting after 2 Ma (Lander and Shapiro,
2007). The high-Mg andesite with the highest SiO2 content in the EC lavas shows the oldest age
(0.73 ±0.06 Ma) within not only EC but also the northeast part of Kamchatka (e.g., Churikova et
al., 2015, IAVCEI). On the other hand, the rest of EC lava samples show relatively younger ages to
0.18 ±0.07 Ma. These results suggest that the EC lavas including high-Mg andesite and basalt were
generated by mantle flux-melting induced by dehydration of a subducted seamount inheriting a local
thermal anomaly (Nishizawa et al., 2014, JpGU; 2015, JpGU).