Mg-Fe and trace element zonation in olivine phenocrysts of Kamchatka basalts : Implications for magma ascent and residence times

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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 resident olivine crystals and newly arrived 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 will apply this approach to a series of different arc basalt lavas from Kamchatka to better 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. In our preliminary studies from olivine-bearing basalts from Kamchatka have shown that mixing, crystal growth and residence in magma chambers may be more complex. Therefore we will analyse and model zonation profiles for a range of elements with different diffusivities (e.g. Mg-Fe, Ca, Ni and other trace elements), find common model solutions and assess the role of variable diffusivities as a function of Mg/Fe in the olivines and temperature. A large sample collection already is available to us from earlier collaboration with our Russian colleagues (Tatiana Churikova and Boris Gordeychik) and most of these samples have previously been geochemically analysed (see our publications). We will specifically select samples that cover different volcanological settings from basalts erupted from the central vent of large-volume stratovolcanoes, small-volume monogenetic cinder cones from areas surrounding the stratocones as well as from extensive plateau-basalt occurrences (e.g. Klyuchevskoy, Tolbachik, Shiveluch, Kharchinsky, Zarechny, Ploskie Sopki, Sredny and cinder cones from the Kostakan area, Mount Peschanaya near Bakening volcano). With this large selection of olivine-bearing samples we cover the principal modes and regimes of magma evolution, ascent, mixing and eruption in Kamchatka. For samples representing these distinct magmatic regimes we expect different olivine zoning histories, which will constrain the chronology of processes of their formation and transport to the surface.