Abstract The Kasatochi volcano, one of the Andreanof Islands in western Aleutians, erupted explosively with little warning on August 7, For two days the eruption sent ash clouds to the altitude of nearly ft asl. Within two weeks, immediately after the activity at Kasatochi decreased, the island was visited by AVO scientist Chris Waythomas, who was able to collect a suite of samples from pyroclastic flow deposits formed during the climatic phase of the eruption. Pumiceous juvenile blocks appear to be one of the predominant lithologies in the pyroclastic flows. The whole rock composition of magmas erupted during the climatic phase shows little variation, e.g wt. % SiO 2, wt. % CaO, and wt.% K 2 O. The erupted andesite is crystalline-rich, with phenocryst content of nearly 40 vol. %. The mineral assemblage includes plagioclase, ortho- and clinopyroxenes, hornblende, and Ti- magnetite. The matrix glass is clear, compositionally uniform (68.0±0.8 wt.% SiO 2 ) and contains elongated microlites of plagioclase, pyroxenes, and amphibole. Most mineral phases appear to be chemically and texturally homogeneous with little or no signs of disequilibrium. Hornblende phenocrysts have no reaction rims that form in response to syn-eruptive ascent and decompression of magmas. Our ongoing petrological investigation will use compositions of mineral and glass phases in the erupted products to constrain pre- and syn-eruptive magma conditions during the 2008 Kasatochi event.

Location Russia United States Japan Russia United States Japan Kasatochi volcano (52.177°N, °W) is small, 3-km-diameter island volcano, which summit is truncated by 750-m-wide crater occupied by a crater lake. © Game McGimsey, 2008

Eruption chronology  Due to the remote location of Kasatochi, there are no confirmed records of historical eruptions. Prior to 2008 eruptions possibly occurred in 1760, , and  On August 7, after at least several weeks of increased seismic activity, a M5.6 earthquake occurred within a few kilometers from Kasatochi. One hour later the first eruption cloud was observed in satellite imagery.  Three distinct explosions occurred with intervals of ca. 3 hours in-between.  The height of the eruptive cloud reached the altitude of 45,000-50,000 feet above sea level.

Samples and analytical techniques  Samples of the 2008 Kasatochi eruption have been collected by Chris Waythomas two weeks after the end of explosive phase.  Whole-rock compositions were analyzed by X-ray fluorescence analysis using Axios Panalytical instrument at the University of Alaska Fairbanks.  Analyses of glasses and minerals for major elements were completed at the University of Alaska Fairbanks using a Cameca SX-50 electron microprobe, which is equipped with four wavelength-dispersive and one energy-dispersive spectrometers. A 15 keV, 10 nA, 10- µm-diameter beam was used for glasses, whereas 15 keV, 10 nA, 1-3-µm focused electron beam was used for minerals. Major elements in wt. %, trace elements in ppm, n.d. – not determined 08CW002- 1C 08CW002- 1B/A 08CW002- 1B/B SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K2OK2O P2O5P2O Total Ni 977 Cr 23n.d. V Ba Rb Sr Zr Y Cu Zn

Mineral assemblage and crystallinity Sample: 08CW002- 1C 08CW002- 1B/A 08CW002- 1B/B Orthopyroxene (OPx) Clinopyroxene (CPx) Hornblende (Hb) 0.1 Plagioclase (Pl) Magnetite (Mt) Glass mm Hb Pl CPx OPx Hb Pl CPx OPx Mt Abundances of minerals and glasses based on mass balance calculations (wt. %): The mineral assemblage of the 2008 Kasatochi andesite is represented by plagioclase, two pyroxenes, magnetite, and hornblende with trace amounts of apatite and Fe-sulfides. Vesicularity varies from 20 to 45 vol. %. Matrix glass is clear and contains rare microlites of plagiocalse, pyroxenes, and hornblende.

Whole rock and glass compositions

Plagioclase compositions and textures Phenocrysts Microlites and phenocryst rims Oscillatory zoned plagioclases are most abundant, whereas dusty-zoned plagioclases are not common. Xenocrystic An 90 cores are rare. Composition of microlites is the same as the compositional plateau in oscillatory-zoned phenocrysts.

Compositions of ortho- and clinopyroxenes Fs Wo En CPx OPx CPx Pl Orthopyroxene phenocryst Clinopyroxene phenocryst Pyroxenes show remarkable compositional homogeneity. Microlites and phenocrysts have the same composition.

Composition of amphiboles and magnetites Mt Hb Magnetite is the only Fe-Ti-oxide. Ilmenite is absent. Hornblende is compositionally homogeneous and euhedral. No reaction rims!

Pre-eruptive conditions Kasatochi Average Mas-12 SiO TiO Al 2 O Fe 2 O MnO 0.22 n.a. MgO CaO Na 2 O K2OK2O P2O5P2O Total The deficiency of totals in melt inclusions have a mode at 7±1 wt.%. Assuming that CO 2 content is negligible, this may correspond to a pre-eruptive pressure of MPa. Solubility of water in matrix glass of Kasatochi andesite based on the calibration of Papale P, Moretti R, Barbato D (2006) Phase diagram for the andesitic composition Mas-12 at water- saturated conditions from Moore and Carmichael (1998) If Kasatochi andesite resided at water-saturated conditions, the pre-eruptive PT-range narrows further to temperatures below 925°C. Interestingly, the Hb-Pl geothermometer by Blundy and Holland (1990) yields a temperature of 800±75°C.

Thoughts on findings and future directions of study  The available samples of the 2008 Kasatochi eruption are represented by a relatively homogeneous, crystal-rich andesite, which likely originated from a stagnant magma reservoir.  High deficiency of totals in melt inclusions suggest that the pre-eruptive pressure in magma reservoir was ca. 3 kbar or higher, although this number will likely be refined further by ion probe analyses.  The apparent paucity of disequilibrium features is really intriguing. Is it possible that the eruption was triggered tectonically, not by a mafic injection? Alternatively, the signs of the replenishment could be present only in earliest products of the eruptions and yet to be found during the planned field works next summer.  The experimental results of Moore and Carmichael (1998) for a similar composition, as well as Pl-Hb geothermometer suggest that the andesite was last equilibrated at temperatures below °C. An experimental study using the Kasatochi andesite as a starting material is necessary to further constrain pre-eruptive pressure and temperature of the 2008 Kasatochi magma. Contact information: Pavel Izbekov