decompression-driven crystallization
Geology, Blundy & Cashman, Abstract
Geology:
Vol. 33, No. 10, pp. 793–796, 2005.
Rapid decompression-driven
crystallization recorded by melt inclusions from Mount St. Helens
volcano
Jon Blundy
Department of Earth Sciences, University
of Bristol, Wills
Memorial Building,
Queens Road, Bristol BS8
1RJ, UK
Kathy Cashman
Department of Geological Sciences,
University of Oregon,
Eugene, Oregon
97403-1272, USA
Manuscript Received by the Society 1 March 2005
Revised Manuscript Received 13 June 2005
Manuscript Accepted 16 June 2005
ABSTRACT
Crystals
in hydrous magmas can form in response to falling temperature (magma cooling)
or degassing (magma decompression). It remains unclear which process dominates
beneath explosive silicic volcanoes. Because decompression and cooling operate
on very different time scales, resolving the driving force behind
crystallization is of fundamental importance for determining magma dynamics and
eruption hazard. Here we use ion-microprobe measurements of dissolved H2O
in phenocryst-hosted melt inclusions from pumices erupted between May and
October 1980 at Mount St.
Helens volcano to show that
all microlites and a significant proportion of phenocrysts were formed by near
isothermal decompression. Magmas erupted after 18 May show evidence for
subsequent crystallization of both phenocrysts and microlites, indicating that
the time scales of crystal nucleation and growth are on the order of months or
less.
Keywords: Mount
St. Helens, melt inclusions,
volcano, ion probe, decompression crystallization.
