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Abstract:
One dimensional, cylindrical, entry of silicate melt into a mineral phenocrysts, is called an embayment. It is commonly found in volcanic rocks, and profiles of volatile element concentration, caused by diffusive loss of volatile components can be measured along the embayments. These record the last minutes to seconds of ascent dynamics of the magma. By fitting a diffusion model over the profile, previous studies found magma ascent rates ranging from 0.02 MPa.s-1 to 3 MPa.s-1. As more embayment studies are engaged on various volcanoes of the world, there are discoveries of profiles that do not have a monotonic decrease of concentration expected from a degassing driven diffusion. On the contrary, it may show an increase, or even oscillatory profiles. Furthermore, there are concentration profiles of H2O and CO2 that cannot be derived from a simple degassing path (closed or opened). To understand such complexity found in natural samples, we have modified a pre-existing diffusion degassing model and ran several forward models to explore various scenarios. Our preliminary tests indicate that the quantity of exsolved gas in magma and disequilibrium degassing explain typical deviations from a simple diffusive degassing. However, an oscillatory profile requires a magma ascent scenario with convection, magma mixing, or both (see accompanying presentation by Georgeais et al.).
