Service Communautaire d'Information sur la Recherche et le Développement - CORDIS

Final Activity Report Summary - C2C (Crust to core: the fate of subducted material)

Subduction of oceanic crust at convergent plate boundaries is arguably the central process in the solid Earth and occurs in the Pacific ring of fire, the Carribean, in some regions of the Mediterranean Sea, and some other geographic locations. The down-going colder, more rigid and "wet" (hydrated) plate induces a large variety of phenomena at the Earth's surface, ranging from volcanism in volcanic arcs behind the subduction front (e.g. in the Aleuteans, Japan, Central and South America, Caribbean, Mediterranean) to the deepest and most destructive earthquakes. The processes that lead to deep earthquakes and the exact location and character of volcanic eruptions are not understood to date, and answers will require many more years of research in the Earth sciences, especially in source mechanisms for earthquakes, the transport of fluids through the mantle above the down-going plate, and the extraction of magma from the Earth's interior.

Understanding the controlling mechanisms of these phenomena requires a good characterisation of the physical properties of the minerals that comprise the slab and the environment of the Earth's mantle with which it interacts. These properties are also of critical importance as input parameters in studying the dynamics of the plate as it sinks into the mantle. Beyond the aspects of geo-hazards, such research is critical in answering basic questions on the chemical and thermal history of our planet, e.g. how continents developed on Earth.

Over the past four years the project has advanced research in this direction through scientific projects at eleven partner institutions all over Europe. During the course of this research the network has contributed significantly in training young researchers in their doctoral studies and their initial post-doctoral career. In the following presentation of results from the Network we focus on one aspect of the research performed. In arc volcanism volatiles, such as water and CO2, play an essential role as melting temperatures are decreased in the presence of these molecules.

While there has been considerable work on the stability of water bound in minerals previously; the role of CO2 has not been investigated extensively. In the c2c Marie Curie Network the role of CO2 and carbon in general has been looked at from different angles:
- In experiments at ETH Zürich a postdoctoral fellow, Dr Ute Mann, has looked at the melting behaviour of carbon-bearing rocks, mostly sediments, and found that melting in the presence of CO2 occurs at much higher temperature than in H2O-bearing systems. Consequently CO2 is retained to much greater depths, i.e. farther away from the trench where subduction is initiated.
- The stability of carbonates (MgCO3-FeCO3-CaCO3) at high pressure has been studied by two groups in the network. A doctoral student at ETH Zürich, Ettore Franzolin has looked at the phase stability in this mineralogical system. A postdoctoral researcher at the Universite Pierre et Marie Curie in Paris, Dr Marta Marocchi, has investigated the dissolution of the Fe-carbonate FeCO3 (siderite) in aqueous solution and found a redox-reaction for the Fe2+ In the system to Fe3+ plus metallic iron, with the simultaneous formation of organic molecules. Such abiotic formation of organic material could play an important role in the origin of life on Earth.
- The composition of fluids released in subduction zones has been the subject of a joint study between Bayerisches Geoinstitut, Germany, and Universita degli Studii di Milano. Exhumed samples from the extinct subduction zone in the Western Gneiss region of Norway revealed oxidising conditions at fluid-mantle interaction, emphasizing the role of H2O over CO2 in the composition of the fluid released from the slab even at great depths.

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