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

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences

Abstract

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.

Planned Impact

The aim of this study is better understanding of subduction-related volcanism and thermal structure of sub arc mantle. Therefore, various UK-based and international groups could benefit from this project: academics, volcano observatories, resource industries and the general public. We anticipate that our data and thermal model will eventually be applicable to other arcs that erupt similar primitive magmas and mantle xenolith. Consequently our data will be helpful for volcanologists to assess volcanic activities and seismic hazard. The association of porphyry copper deposits with subduction zones, and the well-known occurrence of copper-rich fumarole condensates on Tolbachik, provides an obvious economic geology link, enhanced by our close involvement with BHP Billiton through their sponsorship of a major porprhyry copper research initiative at Bristol.

The specific impact-related activities we plan are as follows:

Film. To interest the general public in the geological processes that shape our planet we are planning to produce a small, 8-10 minutes film about volcanism of Kamchatka. This film will be suitable for GCSE and A-level pupils. Some footage for the film was already gained during our pilot expedition last summer. The Bristol-based video production company "Back to the Planet" specialising in environmental and social issues will support us with advice on achieving high-quality, high-interest footage for relatively low cost with minimal environmental impact. We have been working with this film production for the last 12 months, producing short films aimed at PhD student recruitment.

Scientific Workshops. On an international level our principle impact activities consist organising two workshops; at Moscow State University (MSU) and at Institute of Volcanology and Seismology of Petropavlovsk-Kamchatsky. The workshop in Moscow is primarily aimed at an academic audience and is designed to strengthen collaborations between Russia and Bristol. The workshop in Petropavlovsk-Kamchatsky will be aimed on sharing acquired knowledge and data with scientists who deal with Kamchatka's volcanic activities on daily bases. We hope that our findings will be valuable for building models, which will help volcanologist and seismologist to assets volcanic hazards.

Public Engagement. Geology is well suited to engagement with school students. We intend to exploit this opportunity through Moscow State University's unique School of Geoscience. The School is aimed at GCSE and A-level school children that are interested in geology and natural sciences in general. It was established in 1946 and since than proved to be a highly successful organisation for engaging school children in geology. The majority of the children who went through the school continue their education in Department of Geology (or other natural sciences departments) at MSU or other Russian universities. The School is a charitable organisation run by stuff and students of MSU. Project Partner Vasily Shcherbakov is currently the Director of the school. We know that it is a major challenge in the UK to engaging school children in the natural scientist and geology particularly. By organising workshop at the School of Geoscience we are hopping to make the contribution to the charitable organisation and learn about the structure of the school and main principals, so we can use this knowledge, in the future, in engaging school children in the UK.

Publications

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Description We carried out fieldwork in the Vanuatu island arc in 2016. We discovered a number of cumulate xenoliths, similar to those collected from the Lesser Antilles. These xenoliths have informed our experimental strategy for magma generation above subduction zones and provided insights into the structure of the arc crust beneath Vanuatu and subduction zones more generally.
Exploitation Route The discovery of cumulate xenoliths in Vanuatu is useful for understanding the structure of volcanic arcs
Sectors Environment