Role of syn-eruptive plagioclase disequilibrium crystallization in basaltic magma ascent dynamics

Abstract

Timescales of magma ascent in conduit models are typically assumed to be much longer than crystallization and gas exsolution for basaltic eruptions. However, it is now recognized that basaltic magmas may rise fast enough for disequilibrium processes to play a key role on the ascent dynamics. The quantification of the characteristic times for crystallization and exsolution processes are fundamental to our understanding of such disequilibria and ascent dynamics. Here we use observations from Mount Etna's 2001 eruption and a magma ascent model to constrain timescales for crystallization and exsolution processes. Our results show that plagioclase reaches equilibrium in 1–2 h, whereas ascent times were <1 h. Using these new constraints on disequilibrium plagioclase crystallization we also reproduce observed crystal abundances for different basaltic eruptions. The strong relation between magma ascent rate and disequilibrium crystallization and exsolution plays a key role in controlling eruption dynamics in basaltic volcanism.

Timescales of water accumulation in magmas and implications for short warning times of explosive eruptions

“…The model proposed by Annen et al 41 successfully describes the system at physical conditions close to the thermodynamical equilibrium. However, the estimated rising velocities and those reported in the literature for natural systems suggest that the ascending magmas could not be at equilibrium conditions 52 . In particular, La Spina et al 52 indicated that basaltic magmas ascending from ~10 km depth need approximately 2 h to reach the equilibrium crystal volume content.…”

“…However, the estimated rising velocities and those reported in the literature for natural systems suggest that the ascending magmas could not be at equilibrium conditions 52 . In particular, La Spina et al 52 indicated that basaltic magmas ascending from ~10 km depth need approximately 2 h to reach the equilibrium crystal volume content. Therefore, when the average ascent velocity is higher than ~1 m s −1 , the system does not have enough time to reach the equilibrium condition, resulting in a delayed crystallization and leading to a lower viscosity of the mixture.…”

Mechanisms of Ash Generation at Basaltic Volcanoes: The Case of Mount Etna, Italy

“…O 10 −1 − 10 0 , and u z,avg = O 10 0 (La Spina et al, 2016), and laminar flow characterises nearly the entire conduit. Thus, assuming that the flow is fully developed and introducing the normalised radiusr = r R and the normalised velocityū = u z (r) u z,max (where u z,max is the maximum velocity), we can write:…”

“…The first mechanism concerns ash accompanying lava fountain activity and generated by fragmentation of fast-rising (order of 10 to 40 m/s, La Spina et al, 2016;Giuffrida et al, 2018), vesiculating magma. Previous studies have well documented that lava fountaining from SEC at Etna is driven by the superposition of two distinct outgassing mechanisms, gas foam collapse and syn-eruptive vesiculation in the glassy pockets between bubble walls (Figure 7) (Allard et al, 2005;Polacci et al, 2006Polacci et al, , 2009Vergniolle and Ripepe, 2008).…”

Diode laser-based gas analyser for the simultaneous measurement of CO&lt;sub&gt;2&lt;/sub&gt; and HF in volcanic plumes

“…The major volatile species, H 2 O, CO 2 , SO 2 , HCl and HF, all have pressure-dependent solubility profiles, producing significant variability in gas composition as a function of pressure in the magmatic system (Lesne et al, 2012) which is then further modulated by the degassing style and magma dynamics (La Spina et al, 2016). The viscosity of magma controls its flow dynamics, and this is in turn controlled by the dissolved water contents of the magma and vesicularity, producing complex non-linear relationships between magma ascent and degassing (Gonnermann and Manga, 2003).…”