Petrologic monitoring at Volcán de Fuego, Guatemala

Abstract

Paroxysmal activity represents an end-member in the common range of activity at mafic arc volcanoes, characterised by rapid transitions across the effusive-explosive interface and thus posing significant challenges to hazard assessment. Conceptual models to explain changes in the frequency and magnitude of these paroxysmal events are based either on magma recharge or an increase in gas flux, largely framed in the context of two-phase flow. Gas-and magmadriven models are both viable mechanisms to explain the varying styles of paroxysmal behaviour observed in mafic systems; however, each has different implications for future activity. We present time series petrologic data for ash and lava samples collected at Volcán de Fuego, Guatemala, during paroxysmal eruptions between 2011 and 2018. We show that a stepchange in glass composition occurred between 2015 and 2016, reflecting an increase in magma temperature and a reduction in pre-eruptive crystallisation, concurrent with an escalation in the frequency of paroxysmal activity. There was no change in the bulk or phase compositions during this period. To explain these observations, we propose that the increase in frequency of paroxysmal eruptions is modulated by the supply of exsolved volatiles from lower crustal degassing magmas, without invoking repeated transfer of new, primitive magma to a shallow reservoir. Protracted lava effusion, accompanied by more vigorous and more frequent Strombolian explosions and gas 'chugging', prior to the transition to sustained fountaining suggests that gas retention in crystal-rich magma may modulate the height of the magma column as gas supply increases.Slow decompression associated with effusion may determine the timing of effusive to explosive transitions in mafic arc systems more generally. A large paroxysmal eruption of Fuego on 3 June 2018, notable for the rapid escalation in eruptive intensity several hours into the eruption, produced ash with a range of textures and glass compositions consistent with magma evacuation over a range of depths and decompression rates. Given the protracted repose time between paroxysms before this event, we suggest that a shallow crystallised plug degraded, and ultimately failed, several hours into the eruption of 3 June 2018, triggering topdown decompression of magma in the conduit synchronous with the observed rapid acceleration in eruption rate. Ultimately, we propose that the frequency of paroxysms at Fuego is broadly proportional to the gas supply rate, whilst the range in glass compositions is related to the repose time prior to eruptive activity. Our data illustrate the potential of petrologic monitoring to distinguish between gas-and magma-driven paroxysm triggers and to anticipate future events, especially when interpreted in the context of geophysical observations and implemented within community-based ash collection initiatives.

Volcanic CO 2 tracks the incubation period of basaltic paroxysms

“…Notably, in contrast with some other larger-scale historical eruptions, little or no evidence was found for the addition of fresh basaltic magma to the LP magma storage zone, suggesting a dominant role for gas in eruption triggering ( 16 ). Elsewhere, petrological evidence suggests similarly that variations in the frequency and magnitude of recent paroxysmal eruptions at Volcan de Fuego, Guatemala—another open-vent magmatic system—can be explained by changes in the supply of deep-derived gas without the need to invoke the repeated transfer of new magma to shallow levels ( 55 ).…”

Open-vent volcanoes fuelled by depth-integrated magma degassing

“…Gas 'hold-up' (accumulation of gas within the magma) occurs when gas supply from depth is balanced by gas loss from the system and may be implicated as a triggering mechanism for paroxysmal eruptions more generally. For example, paroxysmal eruptions are often preceded by increases in the height of the magma column which may be caused by gas retention; the resulting lava effusion from either flank (Stromboli) or summit (Fuego) vents may then trigger decompression of the shallow conduit (Calvari et al 2011;Liu et al 2020b;Ripepe et al 2015). Similarly, correlations between lava lake surface elevations and gas flux at Villarrica (Johnson et al 2018), Erta Ale (Bouche et al 2010) and Masaya (Aiuppa et al 2018;Williams-Jones et al 2003), for example suggest that temporal fluctuations in deep (> 1-2 km) gas supply may be important in modulating surface activity at openvent volcanoes and in advecting heat to maintain an open state.…”

Downward-propagating eruption following vent unloading implies no direct magmatic trigger for the 2018 lateral collapse of Anak Krakatau

“…Variable degrees of isobaric, deep mafic-phase crystallisation are implied by the initial lateral shifts in MgO values (Liu et al, 2020). Modelled pathways subsequently follow relatively steep gradients in K2O, defining shallower isothermal decompression and decompression-induced plagioclase crystallisation (e.g., ~80 MPa at 1000 °C; Fig.…”

“…Only the model with 2.5 wt.% H2O content is shown in the main text (Fig. 8), as changes to initial water content or redox buffer do not affect the crystallisation paths considerably (Liu et al, 2020).…”