Can nanolites enhance eruption explosivity?

Abstract

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.

The role of oxides in the shallow vesiculation of ascending magmas

“…It is thus not accidental that the critical ascent rates at which transitional effusive and explosive activities occur overlap those at which new oxides are nucleated in the conduit. Oxide microlites are a primer for explosive behavior (Cáceres et al, 2020) because they considerably lower the threshold of pressure changes needed for heterogeneous bubble nucleation (Shea et al, 2010). This strongly supports the conjecture that the OND has to be above a given threshold for the decompression front (section 4.2) to nucleate bubbles.…”

“…J o u r n a l P r e -p r o o f (2008) and Cáceres et al (2020), it is thus reasonable to posit that a magma feeding a dome eruption and bearing large amounts of magnetite microlites would be more prone to nucleate gas bubbles in response to small pressure changes during an eruption than a magma bereft of magnetite. Once nucleated in the conduit, bubbles respond fast to pressure changes (e.g., Lensky et al, 2004;Giachetti et al, 2010) and the ensuing volume changes favor rapid expansion, fragmentation, and explosive behavior.…”

“…There are more OND data for decompression experiments of water-saturated magmas (e.g., Martel and Schmidt, 2003;Shea et al, 2010), some of them specifically addressing heterogeneous bubble nucleation in the presence of oxides (Hurwitz and Navon, 1994;Gardner and Denis, 2004;Gardner, 2007;Cluzel et al, 2008;Cáceres et al, 2020). That oxide volume fraction is small has been used as a common argument to minimize its role in experimental work addressing bubble growth J o u r n a l P r e -p r o o f (Burgisser and Gardner, 2004;Cichy et al, 2011;Fiege and Cichy, 2015).…”

Rheological change and degassing during a trachytic Vulcanian eruption at Kilian Volcano, Chaîne des Puys, France

“…Nanolite crystallization has been attributed to late stage crystallization in shallow regions, and cooling and oxidation as proposed by Mujin and Nakamura (2014) for Shinmoedake volcano (Japan). We note that these nanolites likely formed substrates for heterogeneous bubble nucleation, causing very high bubble number densities in some dome pyroclasts (Colombier et al 2017a;Shea 2017;Cáceres et al 2020).…”

In situ observation of nanolite growth in volcanic melt: A driving force for explosive eruptions

“…In particular, the formation of nanolites can induce the nucleation of bubbles and cause them to remain coupled to the rising magma. Such distinctive vesicle patterns could be used to fingerprint the influence of syn-eruptive nanolite formation (50), even if the nanolites are transient and subsequently reabsorbed or "ripened" to give microlites. The in situ XRD techniques used in this study are difficult to adapt to a pressurized system that would allow controlled undercooling by degassing.…”