Degassing during magma ascent in the Mule Creek vent (USA)

Abstract

The structures and textures of the rhyolite in the Mule Creek vent (New Mexico, USA) indicate mechanisms by which volatiles escape from silicic magma during eruption. The vent outcrop is a 300-m-high canyon wall comprising a section through the top of a feeder conduit, vent and the base of an extrusive lava dome. Field relations show that eruption began with an explosive phase and ended with lava extrusion. Analyses of glass inclusions in quartz phenocrysts from the lava indicate that the magma had a pre-eruptive dissolved water content of 2.5-3.0 wt% and, during eruption, the magma would have been water-saturated over the vertical extent of the present outcrop. However, the vesicularity of the rhyolite is substantially lower than that predicted from closed-system models of vesiculation under equilibrium conditions. At a given elevation in the vent, the volume fraction of primary vesicles in the rhyolite increases from zero close to the vent margin to values of 20-40 vol.% in the central part. In the centre the vesicularity increases upward from approximately 20 vol.% at 300 m below the canyon rim to approximately 40 vol.% at 200 m, above which it shows little increase. To account for the discrepancy between observed vesicularity and measured water content, we conclude that gas escaped during ascent, probably beginning at depths greater than exposed, by flow through the vesicular magma. Gas escape was most efficient near the vent margin, and we postulate that this is due both to the slow ascent of magma there, giving the most time for gas to escape, and to shear, favouring bubble coalescence. Such shear-related permeability in erupting magma is supported by the preserved distribution of textures and vesicularity in the rhyolite: Vesicles are flattened and overlapping near the dense margins and become progressively more isolated and less deformed toward the porous centre. Local zones have textures which suggest the coalescence of bubbles to form permeable, collapsing foams, implying the former existence of channels for gas migration. Local channelling of gas into the country rocks is suggested by the presence of sub-horizontal syn-eruptive rhyolitic tuffisite veins which depart from the vent margin and invade the adjacent country rock. In the central part of the vent, similar local channelling of gas is indicated by steep syn-eruption tuffisite veins which cut the rhyolite itself. We conclude that the suppression of explosive eruption resulted from gas separation from the ascending magma and vent structure by shear-related porous flow and channelling of gas through tuffisite veins. These mechanisms of gas loss may be responsible for the commonly observed transition from explosive to effusive behaviour during the eruption of silicic magma.

Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

“…We also see evidence for outgassing via shear-induced fracture and healing of the melt, also observed during shearing of analogue silicic magma samples (Okumura et al, 2010;Shields et al, 2014) and in natural samples (e.g., (Stasiuk et al, 1996;Tuffen et al, 2003;Castro et al, 2012;Cabrera et al, n.d.). Fig.…”

“…Early work by (Eichelberger et al, 1986) suggested that silicic magma rises as a highly permeable foam which, upon extrusion, loses gas via development of bubble connectivity, enabling outgassing to the conduit walls (Jaupart & Allegre, 1991). Another body of work describes shear-induced fracturing of magma, leading to permeable pathways through which volatiles can escape (Stasiuk et al, 1996;Gonnerman & Manga, 2003;Okumura et al, 2009;Cabrera et al, 2010;Castro et al, 2012). There is widespread evidence for this non-explosive magma fragmentation, in the form of brittle fracture and healing.…”

“…There is widespread evidence for this non-explosive magma fragmentation, in the form of brittle fracture and healing. Observations of transient pyroclastic channels, or tuffisites, have been made at numerous volcanoes: Mule Creek Vent, USA (Stasiuk et al, 1996), Torfajökull, Iceland (Tuffen et al, 2003), Chaitén, Chile as well as explosive products from the Monte Pilato-Rocche Rosse eruption, Lipari (Cabrera et al, n.d.). These veins of welded fragmental material describe shear fracture and healing of silicic magma (Tuffen et al, 2003) and may result in dense bubble-free obsidian bands (Castro et al, 2005a;Tuffen & Dingwell, 2005;Gonnerman & Manga, 2005;Cabrera et al, n.d.).…”

Spherulite crystallization induces Fe-redox redistribution in silicic melt

“…1). They occur in obsidian domes, vitrophyric ash-flow tuffs (e.g., Smith et al 2001), largevolume rhyolite flows such as those at Yellowstone (e.g., Wright, 1915), and in shallow volcanic conduits (e.g., Stasiuk et al, 1993;Tuffen and Castro, 2009). Spherulites nucleate and grow in response to large undercoolings (>200°C) rapidly imposed on the magma by its degassing and quenching (e.g., Swanson et al, 1989).…”

Ascent and decompression of viscous vesicular magma in a volcanic conduit

“…This may be responsible for the peculiar crease structures which characterize such phases of dome growth [Anderson and Fink, 1990]. In a dome, depending on its thickness, [Stasiuk et al, 1996]. $tasiuk et al [1996] proposed that vesicular magma was permeable and able to leak gas to the surrounding country rock, but there are several difficulties with this explanation, as discussed by Jaupart [1998].…”