Bubble accumulation and its role in the evolution of magma reservoirs in the upper crust

Here, the authors model the fluid dynamics that controls the transport of the magmatic volatile phase (MVP) in crystal-rich and crystal-poor magmas; they find that the MVP tends to migrate efficiently in crystal-rich parts of a magma reservoir but to accumulate in crystal-poor parts—possibly explaining why crystal-poor silicic magmas are particularly prone to erupting. The flow of volatiles in crustal magma reservoirs Substantial volumes of crystal-poor, silicic magma can be stored at shallow depths in the Earth's crust. Through volcanic eruption, these magmas could potentially release large amounts of gas into the atmosphere, including sulfur, a possible agent of global cooling. Andrea Parmigiani et al . have modelled the fluid dynamics controlling the transport of magmatic volatile phase in crystal-rich and crystal-poor magmas. They show how the interplay between capillary stresses and the viscosity contrast between the magmatic volatile phase and the host melt results in counterintuitive dynamics whereby volatiles tend to migrate efficiently in crystal-rich parts of a magma reservoir and accumulate in crystal-poor horizons. The accumulation of low-density bubbles of volatiles in crystal-poor magmas could explain the observation that crystal-poor silicic magmas are relatively prone to eruption, and also explain the source of excess sulfur released during explosive eruptions. Volcanic eruptions transfer huge amounts of gas to the atmosphere 1 , 2 . In particular, the sulfur released during large silicic explosive eruptions can induce global cooling 3 . A fundamental goal in volcanology, therefore, is to assess the potential for eruption of the large volumes of crystal-poor, silicic magma that are stored at shallow depths in the crust, and to obtain theoretical bounds for the amount of volatiles that can be released during these eruptions. It is puzzling that highly evolved, crystal-poor silicic magmas are more likely to generate volcanic rocks than plutonic rocks 4 , 5 . This observation suggests that such magmas are more prone to erupting than are their crystal-rich counterparts. Moreover, well studied examples of largely crystal-poor eruptions (for example, Katmai 6 , Taupo 7 and Minoan 8 ) often exhibit a release of sulfur that is 10 to 20 times higher than the amount of sulfur estimated to be stored in the melt. Here we argue that these two observations rest on how the magmatic volatile phase (MVP) behaves as it rises buoyantly in zoned magma reservoi