Abstract
Basaltic open vent volcanoes are major global sources of volcanic gases. Many of these volcanoes outgas via intermittent Strombolian-type explosions separated by periods of passive degassing. The gas emitted during the explosions has high molar CO2/SO2 and SO2/HCl ratios, while during the passive degassing these ratios are lower. We present new laboratory experiments in a model volcanic conduit, which suggest that these differences in gas geochemistry are a consequence of gas migration through crystal-rich magma. We show that gas may flow along channels through the particle-laden liquid and, at a critical depth, the gas may displace an overlying crystal-rich plug en masse, producing a growing slug of gas. Owing to the friction on the walls of the conduit, this plug becomes progressively sheared and weakened until gas enriched in the least soluble volatiles breaks through, causing an explosion at the surface. When the gas slug bursts, liquid is drawn up in its wake, which exsolves the more soluble volatile components, which then vent passively at the surface until the next explosive slug-bursting event.
Highlights
Basaltic open vent volcanoes are major global sources of volcanic gases
It is well known that volcanic gases emitted from volcanoes that exhibit Strombolian-type eruptions (e.g. Yasur, Stromboli, Villarrica, Erebus) are characterised by cyclic variations in volcanic gas flux and composition[1,2,3,4,5], with the bursting of large bubbles called slugs (‘active’ degassing) generating gases with relatively high molar C O2/ SO2 and S O2/HCl ratios, and degassing between explosions (‘passive’ degassing) generating gases with lower ratios (Fig. 1a–c)
It has been proposed that these cycles in observed volcanic gas composition are associated with separated bubble-magma flow coupled with either magma degassing at different depths in the c onduit[6] or with the intermittent release of gas slugs from a foam layer deep in the system if gas accumulates at the top of a s ill[7, 8]
Summary
To study the influence of a high crystal concentration on magma degassing, we develop an analogue experimental model in which we use a mixture of water, glycerol and particles (with a diameter of 0.002 m) in a 2 m long vertical tube, with a radius of 0.02 m (Table 1). With a thin particle zone, the particles mixed throughout this liquid layer to form a low-particle content liquid; with the higher gas fluxes, the particles did not mix to the top of the liquid layer, but remained as a high particle-content zone with a pure liquid layer above This liquid layer did not appear to influence the processes in the underlying particle layer, but allowed for easy observation of the gas leaving the top of the particle-laden layer. In the present experiments, we find that on reaching a depth of about 0.4 to 0.5 m below the top of particle layer (corresponding to particle volume fractions of 40 to 60%), a gas slug steadily grows below the overlying particle plug, which as a result, is displaced upwards This growing slug causes the overall level of the liquid-particle mixture to increase (Fig. 2b, panels i-iv). Some of the deeper liquid is drawn up in the wake of the gas, exchanging with the liquid originally above the slug (Fig. 2b, panel vii, 2c)
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