Magmatic conditions and processes in the storage zone of the 2004-2006 Mount St. Helens dacite

The 2004-6 eruption of Mount St. Helens produced
dacite that contains 40-50 volume percent phenocrysts of
plagioclase, amphibole, low-Ca pyroxene, magnetite, and
ilmenite in a groundmass that is nearly totally crystallized.
Phenocrysts of amphibole and pyroxene range from 3 to 5
mm long and are cyclically zoned, with one to three alternations of Fe- and Al-rich to Mg- and Si-rich layers showing
little indication of phenocryst dissolution between zones.
Similar-size plagioclase phenocrysts also contain several
cyclic zones ranging between ~An₆₈ and An_45-35. Textural evidence indicates that amphibole, pyroxene, and ilmenite began
to crystallize before the most An-rich plagioclase. Magnetite
and ilmenite phenocrysts are small (less than 100 μm), vary
somewhat in composition from grain to grain, and are sporadically zoned. Magnetite-ilmenite pairs yield temperatures
of equilibration ranging from 820°C to 890°C and f
_O2
values
of NNO +1 log unit. Magnetite compositions suggest that the
2004-6 magma was formed by mingling of magmas less than
5-8 weeks before eruption and that the magma last equilibrated within this temperature range. The amphibole phenocryst zoning involves approximately equal amounts of a
pressure-sensitive Al-Tschermak molecular substitution and
a temperature-sensitive edenite substitution in one cycle of
growth. Hydrothermal experiments done on the natural dacite
show that crystallization of the Fe- and Al-rich amphibole
end member requires pressures of 200-300 MPa at temperatures of 900°C, conditions approaching the upper temperature limit of amphibole stability. The dacitic magma crystallizes the An₆₈ plagioclase when the pressure drops to 200
MPa at 900°C. The magma must cool at this depth to produce
a complete An₆₈-An₄₀ plagioclase zone and a Mg-rich layer on the amphiboles before the magma is cycled back to a high
pressure, when a new layer of Fe-rich amphibole is acquired.
The amphibole crystallizing in the dacite experiments at less
than 200 MPa is lower in aluminum than any compositions
in the natural cyclically zoned phenocrysts. The outer rim on
some 2004-6 amphibole phenocrysts appears to have formed
in the 100-200 MPa range, as do some phenocrysts in the
May 1980 dacite pumice. Plagioclase rims of An₃₅ in the
2004-6 magmas indicate that phenocryst growth continued
until the pressure decreased to 130 MPa and that ascent was
slow until this depth. Magma then entered the conduit for a
relatively rapid ascent to the surface as indicated by the very
thin (less than 5 μm) decompression-induced rims on the
amphibole phenocrysts.