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Degassing dynamics play a crucial role in controlling the explosivity of magma at erupting volcanoes. Degassing of magmatic water typically involves bubble nucleation and growth, which drive magma ascent. Crystals suspended in magma may influence both nucleation and growth of bubbles. Micron- to centimeter-sized crystals can cause heterogeneous bubble nucleation and facilitate bubble coalescence. Nanometer-scale crystalline phases, so-called “nanolites”, are an underreported phenomenon in erupting magma and could exert a primary control on the eruptive style of silicic volcanoes. Yet the influence of nanolites on degassing processes remains wholly uninvestigated. In order to test the influence of nanolites on bubble nucleation and growth dynamics, we use an experimental approach to document how nanolites can increase the bubble number density and affect growth kinetics in a degassing nanolite-bearing silicic magma. We then examine a compilation of these values from natural volcanic rocks from explosive eruptions leading to the inference that some very high naturally occurring bubble number densities could be associated with the presence of magmatic nanolites. Finally, using a numerical magma ascent model, we show that for reasonable starting conditions for silicic eruptions, an increase in the resulting bubble number density associated with nanolites could push an eruption that would otherwise be effusive into the conditions required for explosive behavior.

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Accurate estimates of the atmospheric impacts of large eruptions on the environment are complicated by a paucity of models for gas-magma or gas-rock interactions that can occur in the subsurface. It is in these environments that high-temperature scavenging of magmatic gases, degassed during eruption, may play a major role, resulting in significant time-dependence of the bulk gas budget of the major erupted volatile species. Recent experimental work has identified the principal mechanisms involved in high-temperature scavenging in SO 2 and HCl-dominated gas environments, but has neglected the effect of humidity and in-dome gas-magma reactions. Here we present scaled experimental results for the scavenging potential in SO 2-H 2 O mixtures using glassy rhyolitic particles above the acid dew point at 200-800 ºC. First, we reproduce previous results for anhydrous SO 2 scavenging. Such scavenging is accommodated by the growth of CaSO 4 crystals on the ash surface and limited by the temperature-and ash-size dependent diffusion of Ca 2+ to the surface of the ash. Subordinate concentrations of Na 2 SO 4 and potassium-bearing salts also grow on the ash surfaces. Na + and K + diffusion appear to be the limiting mechanisms for the formation of these salts, but once the bulk glass network is relaxed and equilibrates oxidation state above ~600 ºC, the diffusion of these cations is inhibited by charge compensation with Fe 3+. In SO 2-H 2 O mixed-gas atmospheres, the diffusion of Ca 2+ appears unaffected by the activity of H 2 O on the ash surface. In contrast, the diffusion of Na + and K + toward the ash surface is enhanced. We speculate that this occurs by alkali exchange with inward diffusing H +. In hydrous atmospheres, the diffusion of Na + and K + is also markedly less at the threshold oxidation temperature. This oxidation temperature is reduced to lower values as the water activity at the surface is increased by increasing the partial pressure of H 2 O. Taken together, these results enable the exploration of scenarios for in-dome processes where either open-system or batch outgassing prevails through fractures filled with populations of welding volcanic ash. Such scenarios predict that a large fraction of the SO 2 flowing through the ash-filled fracture networks may be scavenged in permeable fractured domes, which act as scrubbing filters, potentially explaining some unexpectedly low SO 2 emissions from rhyolitic dome-forming eruptions.

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Many of the grand challenges in volcanic and magmatic research are focused on understanding the dynamics of highly heterogeneous systems and the critical conditions that enable magmas to move or eruptions to initiate. From the formation and development of magma reservoirs, through propagation and arrest of magma, to the conditions in the conduit, gas escape, eruption dynamics, and beyond into the environmental impacts of that eruption, we are trying to define how processes occur, their rates and timings, and their causes and consequences. However, we are usually unable to observe the processes directly. Here we give a short synopsis of the new capabilities and highlight the potential insights that in situ observation can provide. We present the XRheo and Pele furnace experimental apparatus and analytical toolkit for the in situ X-ray tomography-based quantification of magmatic microstructural evolution during rheological testing. We present the first 3D data showing the evolving textural heterogeneity within a shearing magma, highlighting the dynamic changes to microstructure that occur from the initiation of shear, and the variability of the microstructural response to that shear as deformation progresses. The particular shear experiments highlighted here focus on the effect of shear on bubble coalescence with a view to shedding light on both magma transport and fragmentation processes. The XRheo system is intended to help us understand the microstructural controls on the complex and non-Newtonian evolution of magma rheology, and is therefore