Primitive arc magmatism - experimental constraints and implications for subduction zone thermal structure

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Volcanic arcs are sites of explosive volcanism, seismicity, and the formation of new continental crust. Understanding the linked processes that lead to these phenomena has implications not only for natural hazard assessment but also for the fundamental understanding of our planet. Despite several decades of focussed research on arc volcanism, our knowledge of the links between magmatism and the origin of primary melts remains remarkably limited. There is, for instance, no agreement on what causes the chemical diversity of mantle-derived (primitive) basalts and what processes lead to the conversion of these magmas to crustal rocks with a predominantly andesitic composition. It is also unclear, what role the mantle beneath an arc plays in this process and how subduction-derived fluids and melts influence these processes. We aim to address this uncertainty through a combined field, petrological and modelling study, using the Kuril-Kamchatka volcanic arc as our natural laboratory.Kuril-Kamchatka arc is one of the most active arcs in the world, comprising 30 active stratovolcanoes and nearly 200 monogenetic vents. The Klyuchevskoy volcano, part of the Klyuchevskoy volcanic group, on it own has produced an average of 63,000,000 tons of magma per year over the last 5,000 years. Klyuchevskoy releases 400,000 tons of sulfur and 600,000 tons of CO2 per year into the atmosphere, accounting for up to 8% of the global volcanic discharge. In terms of current arc volcanism Kuril-Kamchatka is a major player.The Klyuchevskoy volcanic group is known for its variety of primitive basaltic magmas, which are derived from the mantle wedge and believed to be parental to the rest of the igneous suite. Primitive arc magmatism is very uncommon worldwide but vital for understanding the formation and evolution of arc magmas. Another important feature of the Klyuchevskoy volcanic group is the occurrence of mantle xenoliths, which are fragments of the mantle carried up to the surface by the magmas. These offer a rare opportunity to glance into the Earth's deeper interior and are a source for understanding the origin of melts. Like primitive magmas, mantle xenoliths in arc settings are not very commonly found.The appearance of diverse primitive magmas and mantle xenoliths in the same volcanic setting makes Kamchatka an ideal place to answer three fundamental questions; what controls the variability of primitive arc magmas? at what conditions are these magmas generated? what role does heterogeneity of the mantle play in generating the diversity of primitive magmas?This study will:1) Collect and analyse natural samples (magmas and mantle xenoliths) from the Klyuchevskoy and Tolbachik volcanoes;2) Conduct high-pressure and high-temperature experiments on two primitive basalts from the Klyuchevskoy and Tolbachik volcanoes to constrain the pressure and temperature conditions of magma generation, magma evolution and thermal state of the mantle beneath these volcanoes;3) Combine experimental results with observations on natural samples to provide the answers to posed questions. This investigation of the Klyuchevskoy volcanic group will increase our understanding of the nature of arc volcanism and subduction zone geodynamics.