Magma degassing: Defusing volcanic eruptions?

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At depth, magma contains dissolved volatiles, such as H2O and CO2. As magma rises to the surface, these volatiles exsolve to form gas bubbles. Unless this gas escapes from the melt, pressure builds up during magma ascent and this can lead to an explosive volcanic eruption. In principle, gas escape thus has the potential to defuse explosive volcanic eruptions. However, in reality, the link is more complex and the efficiency and timescales over which this gas escapes compared to the magma ascent rate are crucial in determining the actual eruption style. The main aim of this study is to quantify the timescales and efficiency of the processes by which gas escapes and their contribution to volcanic emissions. When the main volatile components exsolve, trace elements with an affinity for the gas phase will also partition into this. An example is radon, a noble gas and short-lived radionuclide in the Uranium-238 decay chain. Over time, continuous loss of radon from the magma through gas escape results in deficits of its daughter nuclide 210Pb relative to its parent 226Ra in degassed magma, and in excesses of 210Pb in the magma through which the gas is transported. Such disequilibria suggest that gas escape is fast and magma degassing occurs over periods of years to decades prior to an eruption. This requires efficient gas escape mechanisms even in viscous, silicic magma. Growth and expansion of bubbles can lead to gas escape through coalescence and fracturing. These pathways are transient with typical lifespans from seconds, in the case of foam collapse, to hours for fractures, and years to decades for bubble ascent in silicic melt. The efficiency of these pathways, and their contribution to volcanic emissions is very different. Elements will be continuously redistributed between gas and melt during magma ascent, but in a dynamic system there may not be sufficient time to attain equilibrium partitioning for all elements. This implies that the gas composition changes not only with depth, but also with the relative ascent rates of gas and melt. Whereas gas is typically not or poorly preserved, the complementary quenched melts, volcanic glass, do preserve a record of gas-melt interaction in their chemical heterogeneity. Theses glasses thus provide a record of the mechanisms and kinetics of magma ascent and degassing. In this research program we will develop the tools to read this record. As part of this project chemical and isotopic variation in volcanic rocks will be mapped on a scale of meters (in the field) to micro-meters (in the lab). Particular focus will be on the distribution of the metals and 210Pb-226Ra. To retrieve timescale information from this chemical heterogeneity, experiments will be conducted on the behaviour and mobility of different elements during gas-melt interaction. These fundamental parameters will be used to set up a reactive transport model that simulates the evolving melt and gas chemistry during magma ascent. Such a model would tie surface emissions to magma ascent and gas escape rates, which is critical for accurate hazard assessment at volcanoes and can help identify potentially explosive events ahead of time. However the mobility of metals has many applications beyond volcanology, including the release and concentration of metals into potential ore-forming fluids, glass industry, and analogous industrial degassing processes such as smelting and waste incineration. An understanding of metal mobility and its dependence on glass and melt properties can be used to retain, or segregate metals in industrial processes and limit their emission.