Vesiculated basaltic enclave flotation as a mechanism of fluid pressure increase in magma chambers during silicic-basaltic magma mixing

We present undated results of a numerical model of pressure increase in a quasi-isochoric magma system due to the flotation of bubble-crystal clusters (vesicular mafic enclaves), that are common in silicic rocks that experience magma chamber refill by water and CO2-saturated basaltic magma. The model accounts for water solubility and diffusion and utilizes measured size distributions of mafic enclaves in volcanic and plutonic rocks around the world. These mafic enclaves have sizes ranging from (~1 to >30 cm), lognormal size distribution that we explain by flotation. They have lower densities (density difference =10-30%) than silicic hosts due to bubbles. The principal results of the model are: 1) enclaves are capable of rapid (days-months) flotation leading to pressurization of shallow magma chambers over cracking limit (100 MPa) using fluid rise mechanisms below; 2) The dynamics of pressure increase is strongly non-linear and is determined by the initial size distribution of enclaves, and other parameters; 3) Diffusion of water out of enclaves is slower than the rise time for most realistic viscosities of the silicic host melt. We consider cases when overpressuring causes density reversal due to solubility increase, and consider the role of convection. We present examples of high concentrations of enclaves within silicic domes that we explain by pre-eruption accumulation by flotation to the roof 

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