Crystal-rich an
desites that erupte
d from Re
doubt Volcano in 2009 range from 57.5 to 62.5 wt. % SiO
2 an
d have phenocryst an
d phenocryst-melt relations consistent with staging in the upper crust. Early explosive pro
ducts are low-silica an
desites (LSA, < 58 wt. % SiO
2) that ascen
de
d from
deeper crustal levels
during or before the 6 months of precursory activity, but a broa
d subsequent succession to more evolve
d an
d cooler pro
ducts, an
d pre
dominantly effusive
dome growth, are interprete
d to result from progressive mobilization an
d mixing with
differentiate
d magmas tappe
d from pre-2009 Re
doubt intrusions at ~ 3-6 km
depth. Initial explosions on March 23-28 ejecte
d pre
dominantly LSA with a uniform phenocryst assemblage of high-Al amphibole, ~ An
70 plagioclase, ortho- an
d clinopyroxene, FeTi oxi
des (890 to 960 ¡ãC), an
d traces of magmatic sulfi
de. Melt in the
dominant microlite-poor LSA was compositionally uniform
dacite (67-68 wt. % SiO
2) but range
d to rhyolite with greater microlite growth. Minor amounts of interme
diate- to high-silica an
desite (ISA, HSA; 59-62.5 wt. % SiO
2) also erupte
d during the early explosions an
d most carrie
d rhyolitic melt (72-74 wt. % SiO
2). A lava
dome grew following the initial tephra-pro
ducing events but was
destroye
d by an explosion on April 4. Ejecta from the April 4 explosion consists entirely of ISA an
d HSA, as
does a subsequent lava
dome that grew April 4-July 1; LSA was absent. An
desites from the April 4 event an
d from the final
dome ha
d pre-eruptive temperatures of 725-840 ¡ãC (FeTi oxi
des) an
d highly evolve
d matrix liqui
ds (77-80 wt. % SiO
2), inclu
ding in rare microlite-free pyroclasts. ISA has mixe
d populations of phenocrysts suggesting it is a hybri
d between HSA an
d LSA. The last lavas from the 2009 eruption, effuse
d May 1-July 1, are
distinctly
deplete
d in P
2O
5, consistent with low temperatures an
d high
degrees of crystallization inclu
ding apatite.
Plagioclase-melt hygrometry and comparison to phase equilibrium experiments are consistent with pre-eruptive storage of all three magma types at 100-160 MPa (4-6 km depth), if they were close to H2O-saturation, coincident with the locus of shallow syn-eruptive seismicity. Deeper storage would be indicated if the magmas were CO2-rich. Relatively coarse-grained clinopyroxene-rich reaction rims on many LSA amphibole phenocrysts may result from slow ascent to, or storage at, depths shallow enough for the onset of appreciable H2O exsolution, consistent with pre-eruptive staging in the uppermost crust. We interpret that the 2009 LSA ascended from depth during the 8 or more months prior to the first eruption, but that the magma stalled and accumulated in the upper crust where its phenocryst rim and melt compositions were established. Ascent of LSA through stagnant mushy intrusions residual from earlier Redoubt activity mobilized differentiated magma pockets and interstitial liquids represented by HSA, and as LSA-HSA hybrids represented by ISA, that fed the subsequently erupted lava domes.