Experimental determination of the partition coefficient of water between phenocrysts and silicate melts in magma chambers.

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One of the most important factors in determining the way a volcano behaves is the amount of water dissolved in the magma at depth. Water influences the viscosity and density of the magma, both by affecting the properties of the silicate melt phase itself, and by suppressing the degree of crystallisation of the magma. During the eruption of the magma the reducing pressure causes the dissolved water to exsolve, forming bubbles which drive the magma to the surface. The interplay between the amount of water (and other dissolved gases) and chemical composition of the magma type causes the range of behaviour from slowly flowing lavas (which are not usually dangerous to humans), to rapidly moving pyroclastic flows which can have devastating consequences for populations around volcanoes. As a result, volcanologists are very interested in determining the amount of water in magma chambers of historic eruptions, to enable predictions of the behaviour of future eruptions to be made. There are a range of methods to analyse the products of volcanic eruptions to estimate pre-eruptive water concentrations, but all have important limitations. The aim of the present proposal is to develop a new method which relies on measuring the very small concentrations of water which are dissolved in the crystals of volcanic rocks, then to use these values to calculate the water concentration in the magma prior to eruption. We have already shown that it is possible to make measurements of the water concentrations of the crystals using infrared spectroscopy, we now wish to perform experiments to measure the partitioning of water between silicate minerals and melts. We will then be able to use our measured values of water in crystals to calculate the water concentration in the melt phase. If successful, this project will allow a much more detailed picture of the volatile evolution of magmas to be obtained.