Experimental studies of the Earth's mantle

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Diamonds form in the Earth's mantle at depths of > ~130 km and temperatures > ~ 900C. But there remain many questions about how and why they form, and what they tell us about the mantle in which they form. My research aims to address these questions, and addresses the behaviour of carbon in the mantle more broadly, because we need to understand the conditions in which carbon will be stable as diamond rather than as crystalline carbonate or dissolved in a fluid or melt. By reproducing the pressures and temperatures in the Earth's mantle at diamond-stable conditions in the laboratory, we can explore these questions in a way complementary to the observations that other researchers make on natural diamonds and their fluid and mineral inclusions.***We proposed recently a model for diamond formation in which the key element is how a water-rich fluid interacts with the different rock types that make up the Earth's upper mantle at diamond-stable conditions. In some rock types, this fluid would trigger melting, and diamonds would have to form in the presence of that melt. In the most common rock type associated with diamonds, however, melting would not occur, and the fluid itself could precipitate diamond by cooling or ascending. This model is a fundamental shift in thinking about how diamond may form in the mantle, but we need to know more about how diamond behaves in these fluids and melts, and even what kind of fluids and melts could be present in the mantle at these conditions. ***Where do these fluids come from? One likely source is altered oceanic crust and mantle that is recycled into the mantle in subduction zones and gives off fluids as it subducts, down to 200 km depth or more. These fluids would add H2O, CO2, Cl, and other volatiles back into the mantle. The first two have been studied extensively, but not Cl and others. ***We want to study what happens when these fluids interact with the different rock types of the Earth's upper mantle. When will they trigger melting? How does the presence of Cl change the melting behaviour? We will also study how diamond dissolves in melts that are produced by the interaction of these fluids with mantle rocks, and how that solubility changes with pressure and temperature. This will tell us how effective these melts would be in forming diamonds. ***Finally, some diamonds contain up to 5000 ppm nitrogen. Does this mean that there is a lot or only a little nitrogen in some diamond-forming fluids or melts? We will grow diamond in N-bearing fluids and melts to understand how nitrogen partitions between those media and the growing diamond.***This research is important both from an academic perspective helping us to understand how diamond forms and what sort of fluids and melts have modified the Earth's mantle but also from a more practical perspective. Diamonds are a key economic driver in Canada's north, and if we can develop a better understanding of how diamonds form, that will inform models to help find new deposits.