Arc Magmas from Slab to Eruption: The Case of Kliuchevskoy Volcano

Arc magmas are generated by a number of mantle and crustal processes. Our multidisciplinary, long-term research is aimed at deciphering these processes for a single arc volcano, Kliuchevskoy volcano in Kamchatka. Some key results of the study follow: 1) Modeling of trace element and H2O contents in melt inclusions suggests that the primary magmas originate via hydrous flux-melting of the mantle wedge at temperatures close to the dry peridotite solidus. The role of decompression melting is minor or absent at Kliuchevskoy and other arc volcanoes built on relatively thick crust. 2) Geochemistry of high-Mg olivine suggests that primary Kliuchevskoy magmas have substantial contribution from olivine-free pyroxenite (up to 30 %), which could be formed by reaction of slab melts (or supercritical fluids) with mantle wedge peridotite. 3) Parental Kliuchevskoy melts start to crystallize as deep as the Moho boundary, and the erupted magmas reflect multistage and complex processes of crystallization, magma mixing and crustal assimilation. None of the Kliuchevskoy rocks analyzed thus far represent true primary melt compositions. 4) The Kliuchevskoy Holocene eruptive history is not steady-state in terms of eruption rate and geochemistry. There are two millenial cycles with major and trace element and OSr- Nd-Pb and U-series isotope compositions of the magmas changing gradually from more to less affected by crustal (?) assimilation. The onset of the cycles correlates with periods of enhanced volcanic activity in Kamchatka, suggesting that the extent of magma-crust interaction is inversely related to magma production rate and thus magma flux from the mantle

Mantle melting conditions under the eastern volcanic front of Kamchatka estimated from melt inclusions in olivine

Here we present new data on the composition of olivine phenocrysts, melt inclusions and inclusions of chromium spinel in olivine from high Mg# basalts of the Eastern Volcanic Front in Kamchatka

Geochemistry of the Late Holocene rocks from the Tolbachik volcanic field, Kamchatka: Quantitative modelling of subduction-related open magmatic systems

We present new major and trace element, high-precision Sr-Nd-Pb (double spike), and Oisotope data for the whole range of rocks from the Holocene Tolbachik volcanic field in the Central Kamchatka Depression (CKD). The Tolbachik rocks range from high-Mg basalts to low-Mg basaltic trachyandesites. The rocks considered in this paper represent mostly Late Holocene eruptions (using tephrochronological dating), including historic ones in 1941, 1975-1976 and 2012-2013. Major compositional features of the Tolbachik volcanic rocks include the prolonged predominance of one erupted magma type, close association of middle-K primitive and high-K evolved rocks, large variations in incompatible element abundances and ratios but narrow range in isotopic composition. We quantify the conditions of the Tolbachik magma origin and evolution and revise previously proposed models. We conclude that all Tolbachik rocks are genetically related by crystal fractionation of medium-K primary magmas with only a small range in trace element and isotope composition. The primary Tolbachik magmas contain ~14 wt% MgO and ~4% wt% H2O and originated by partial melting (~6%) of moderately depleted mantle peridotite with Indian-MORB-type isotopic composition at temperature of ~1250oC and pressure of ~2 GPa. The melting of the mantle wedge was triggered by slab-derived hydrous melts formed at ~2.8 GPa and ~725oC from a mixture of sediments and MORB- and Meiji- type altered oceanic crust. The primary magmas experienced a complex open-system evolution termed Recharge-Evacuation-Fractional Crystallization (REFC). First the original primary magmas underwent open-system crystal fractionation combined with periodic recharge of the magma chamber with more primitive magma, followed by mixing of both magma types, further fractionation and finally eruption. Evolved high-K basalts, which predominate in the Tolbachik field, and basaltic trachyandesites erupted in 2012-2013 approach steady-state REFC liquid compositions at different eruption or replenishment rates. Intermediate rocks, including high-K, high-Mg basalts, are formed by mixing of the evolved and primitive magmas. Evolution of Tolbachik magmas is associated with large fractionation between incompatible trace elements (e.g., Rb/Ba, La/Nb, Ba/Th) and is strongly controlled by the relative difference in partitioning between crystal and liquid phases. The Tolbachik volcanic field shows that open-system scenarios provide more plausible and precise descriptions of long-lived arc magmatic systems than simpler, but often geologically unrealistic, closed-system models