Chemical and Isotopic Gradients around Bubbles in Volcanic Feeder Systems

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Volcanic eruptions are among the most spectacular, and potentially devastating, geological events on Earth?s surface. The explosivity of volcanic eruptions, and the hazards they pose, depend on how volcanoes outgas, or specifically, the amount and rate of gas bubble formation and efficiency of gas removal from rising magma. Although methods exist to estimate the total gas budget of parental and erupted lava, it is more difficult to get information about the timing and rates of bubble growth and gas loss as magma moves towards the Earth?s surface. This study offers a new perspective by focusing on small-scale textures and chemical gradients preserved around bubbles in volcanic glass. The research is based on the realistic assumption that slow diffusion may inhibit bubble-magma equilibrium, leading to H2O and CO2 gradients in the melt. The sign, magnitude, and length scale of these gradients can be measured using modern analytical techniques and they provide new clues for understanding the processes responsible for separating volatile species and their isotopes during bubble growth, resorption and removal from magma. Samples that will be investigated include bubbles in low- to high-silica volcanic glass and bubbles grown within experimental capsules. The study will focus largely on the behavior of H2O and CO2, two important components of volcanic gas that differ in their chemical diffusivities. Since H2O has a high diffusivity, its concentration profiles are likely to record changes in equilibrium solubility due to pressure, temperature, and chemical changes in volcanic feeder systems. Since CO2 has a low diffusivity, its concentration profiles are likely to contain information about rates of bubble growth or resorption and fracture healing. The study also includes a novel approach for investigating the relationship between chemical speciation and diffusion in the melt phase. Different dissolved species (e.g., OH and H2Om) have different masses and should therefore exhibit different degrees of mass discrimination or isotope separation during diffusion. The plan for testing this hypothesis includes diffusion experiments and examination of isotopic gradients around individual bubbles. The project as a whole is a combination of field effort, experiments, and theory and will push the limits of analytical and experimental capabilities. Although not stated in the objectives, the proposed measurements will help inform the next generation of volcanic conduit models and may refine techniques used for estimating the fluxes and isotopic compositions of volcanic gas species released to the Earth?s atmosphere.