Degassing during magma ascent in the Mule Creek vent (USA) |
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| Authors: | Mark V Stasiuk Jenni Barclay Michael R Carroll Claude Jaupart James C Ratté R Stephen J Sparks Stephen R Tait |
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| Institution: | (1) Université Paris 7 et Institut de Physique du Globe, 4, place Jussieu, F-5252 Paris Cedex 05, France, FR;(2) Department of Geology, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK, GB;(3) United States Geological Survey, Denver Federal Center, Denver, Colorado 80225, USA, US |
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| 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.
Received: 24 May 1995 / Accepted: 13 March 1996 |
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| Keywords: | Rhyolite Volatiles Vent Eruption transitions Shear Permeable Tuffisite |
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