Andesites of the 2009 eruption of Redoubt Volcano, Alaska Michelle L. Coombs a, ⁎, Thomas W. Sisson b , Heather A. Bleick b , Sarah M. Henton c , Chris J. Nye d , Allison L. Payne a , Cheryl E. Cameron d , Jessica F. Larsen c , Kristi L. Wallace a , Katharine F. Bull d a Alaska Volcano Observatory, Volcano Science Center, U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508, USA b Volcano Science Center, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA c Department of Geology and Geophysics, Geophysical Institute, Alaska Volcano Observatory, University of Alaska, Fairbanks, AK 99708 d Alaska Volcano Observatory, Alaska Division of Geological and Geophysical Surveys, Fairbanks, AK 99708, USA abstract article info Article history: Received 1 June 2011 Accepted 6 January 2012 Available online 18 January 2012 Keywords: Andesite Petrology Geothermometry Amphibole Magma storage Eruption Crystal-rich andesites that erupted from Redoubt Volcano in 2009 range from 57.5 to 62.5 wt.% SiO 2 and have phenocryst and phenocryst-melt relations consistent with staging in the upper crust. Early explosive products are low-silica andesites (LSA, b 58 wt.% SiO 2 ) that ascended from deeper crustal levels during or before the 6 months of precursory activity, but a broad subsequent succession to more evolved and cooler products, and predominantly effusive dome growth, are interpreted to result from progressive mobilization and mixing with differentiated magmas tapped from pre-2009 Redoubt intrusions at ~ 3–6 km depth. Initial explosions on March 23–28 ejected predominantly LSA with a uniform phenocryst assemblage of high-Al amphibole, ~ An 70 plagioclase, ortho- and clinopyroxene, FeTi oxides (890 to 960 °C), and traces of magmatic sulfide. Melt in the dominant microlite-poor LSA was compositionally uniform dacite (67–68 wt.% SiO 2 ) but ranged to rhyolite with greater microlite growth. Minor amounts of intermediate- to high-silica andesite (ISA, HSA; 59–62.5 wt.% SiO 2 ) also erupted during the early explosions and most carried rhyolitic melt (72–74 wt.% SiO 2 ). A lava dome grew following the initial tephra-producing events but was destroyed by an explosion on April 4. Ejecta from the April 4 explosion consists entirely of ISA and HSA, as does a subsequent lava dome that grew April 4–July 1; LSA was absent. Andesites from the April 4 event and from the final dome had pre-eruptive temperatures of 725–840 °C (FeTi oxides) and highly evolved matrix liquids (77–80 wt.% SiO 2 ), including in rare microlite-free pyroclasts. ISA has mixed populations of phenocrysts suggesting it is a hybrid between HSA and LSA. The last lavas from the 2009 eruption, effused May 1–July 1, are distinctly depleted in P 2 O 5 , consistent with low temperatures and high degrees of crystallization including 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 H 2 O-saturation, coincident with the locus of shallow syn-eruptive seismicity. Deeper storage would be indicated if the magmas were CO 2 -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 H 2 O 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. Published by Elsevier B.V. 1. Introduction Andesitic dome-building volcanoes are among the most active and hazardous worldwide due to the frequency with which they produce airborne ash clouds and flowage deposits that may travel tens to hundreds of kilometers down valley. Dome-building eruptive sequences may last weeks to years, making these volcanoes persistent threats. Real-time monitoring of restless and erupting volcanoes has dramatically improved our ability to forecast volcanic activity and understand sub- volcanic magmatic processes. Seismic, geodetic, and gas-emission measurements all provide critical yet limited views of a volcano's behavior. While often slightly more retrospective, petrologic study of the eruptive products can yield corroboration or revision of magmatic models developed using real-time data (Hammer and Rutherford, 2003; Pallister et al., 2008; Larsen et al., 2010; Samaniego et al., Journal of Volcanology and Geothermal Research 259 (2013) 349–372 ⁎ Corresponding author. Tel.: + 1 907 786 7403; fax: + 1 907 786 7425. E-mail address: mcoombs@usgs.gov (M.L. Coombs). 0377-0273/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jvolgeores.2012.01.002 Contents lists available at SciVerse ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores