A model for episodic degassing of an andesitic magma intrusion

Abstract

[1] Episodic magmatic degassing has been observed at numerous volcanoes, especially those of intermediate composition. It can span timescales from years to decades. Here we propose a physical model for the degassing of a shallow magma intrusion to explain this phenomenon. The magma cools by convection, which leads to melt crystallization, volatile exsolution, and magma overpressure. When the pressure reaches a critical value, wall rocks fracture and the exsolved gas escapes. The intrusion then returns to the initial lithostatic pressure and a new cooling-crystallization-degassing cycle occurs. A series of such cycles leads to episodic degassing. The trend and timescale of the degassing process are mainly governed by magma cooling. Two degassing regimes are exhibited: an early phase with a high frequency of gas pulses and a later phase with a lower gas pulse frequency. The transition between these two regimes is caused by the viscosity increase when the magma crystallinity exceeds the crystal percolation threshold. We find that the time to this transition is dependent on magma volume, to a first approximation. Where observations are available from sustained geochemical surveillance, the model provides constraints on key aspects of the subsurface magmatic system, with estimation of the volume of an intrusion and tensile strength of the surrounding rocks. It therefore represents a relevant tool for volcanic surveillance and hazard assessment.

Density muon radiography of La Soufrière of Guadeloupe volcano: comparison with geological, electrical resistivity and gravity data

“…1). The last 1976-1977 eruption has been interpreted as a stillborn or failed magmatic eruption (Feuillard et al 1983;Komorowski et al 2005;Villemant et al 2005;Boichu et al 2008Boichu et al , 2011 linked to the intrusion of a small volume of viscous andesitic magma that stopped within a few kilometres of the surface triggering pressurization of the hydrothermal system, phreatic explosions and continuing episodic chlorine degassing into the hydrothermal system (Villemant et al 2005).…”

The Influence of Episodic Shallow Magma Degassing on Heat and Chemical Transport in Volcanic Hydrothermal Systems

“…While the physical model of the cycle of magma cooling, crystallization, and degassing has been previously developed and is correlated to the time series of Cl observed at springs, the transport process of the volatiles between the starting point, magma, and the ending point, spring, is less well known (Boichu et al, 2008;Boichu et al, 2011;Villemant et al, 2005). The chloride time series records episodic degassing events, with lag times consistent with measured spring flow rates.…”

“…Within the eruptive conduit ( Figure 1c), high-temperature gas and vapor emission occurs in pulses and is related to a two-step magma degassing process characterized by the initial rapid release of HCl-rich H 2 O vapor during emplacement of new magma, followed by protracted release of gases and vapor at diminishing rates during crystallization of the newly emplaced magma (Boichu et al, 2008). Within the eruptive conduit ( Figure 1c), high-temperature gas and vapor emission occurs in pulses and is related to a two-step magma degassing process characterized by the initial rapid release of HCl-rich H 2 O vapor during emplacement of new magma, followed by protracted release of gases and vapor at diminishing rates during crystallization of the newly emplaced magma (Boichu et al, 2008).…”

“…Fluid discharged from the spring is a combination of meteoric water and magmatic gases. Within the eruptive conduit ( Figure 1c), high-temperature gas and vapor emission occurs in pulses and is related to a two-step magma degassing process characterized by the initial rapid release of HCl-rich H 2 O vapor during emplacement of new magma, followed by protracted release of gases and vapor at diminishing rates during crystallization of the newly emplaced magma (Boichu et al, 2008). This pulsatory magma degassing has previously been conceptualized as a series of instantaneous pulses that release hot gas and vapors (including HCl) to the overlying aquifer(s), causing observed thermal and geochemical anomalies, including a conservative Cl pulse (Villemant et al, 2005).…”

Textural and geochemical constraints on eruptive style of the 79 ad eruption at Vesuvius

“…The surrounding rock failure criteria also depend on the exsolved fluid composition (Tait et al 1989). The occurrence of a two-phase fluid in Vesuvius reservoir may significantly increase its value (Boichu et al 2008). …”