AVO Staff Directory:

We performed three sets of decompression experiments on a hydrated K-phonolite melt at 880 °C in order to study the exsolution and degassing behavior of this lowviscosity melt. The range of decompression rates studied was 0.01MPa/s to 0.25MPa/s.

Mongrain, J., Larsen, J.F., and King, P.L., 2008, Rapid water exsolution, degassing, and bubble collapse observed experimentally in K-phonolite melts: Journal of Volcanology and Geothermal Research, v. 173, n. 3-4, p. 178-184.

Okmok volcano is situated on oceanic crust in the central Aleutian arc and experienced large (15 km3) caldera-forming eruptions at 12 000 years BP and 2050 years BP. Each caldera-forming eruption began with a small Plinian rhyodacite event followed by the emplacement of a dominantly andesitic ash-flow unit, whereas effusive interand post-calderalavashavebeenmorebasaltic.

Finney, Benjamin, Turner, Simon, Hawkesworth, Chris, Larsen, Jessica, Nye, Chris, George, Rhiannon, Bindeman, Ilya, and Eichelberger, John, 2008, Magmatic differentiation at an island-arc caldera: Okmok Volcano, Aleutian Islands, Alaska: Journal of Petrology, v. 00, n. 0, p. 1-28, doi:10.1093/petrology/egn008 .

Larsen, J.F., 2008, Heterogeneous bubble nucleation and disequilibrium H2O exsolution in Vesuvius K-phonolite melts: Journal of Volcanology and Geothermal Research, v. 175, p. 278-288, doi:10.1016/j.jvolgeores.2008.03.015 .

Almberg, LD., Larsen, J.F., Eichelberger, J.C., Vogel, T.A., and Patino, LC., 2008, Comparison of eruptive and intrusive samples from Unzen Volcano, Japan: Effects of contrasting pressure-temperature-time paths: Journal of Volcanology and Geothermal Research, v. 175, p. 60-70, doi:10.1016/j.jvolgeores.2008.03.020 .

Finney, Benjamin, Turner, Simon, Hawkesworth, Chris, Larsen, Jessica, Nye, Chris, George, Rhiannon, Bindeman, Ilya, and Eichelberger, John, 2008, Magmatic differentiation at an island-arc caldera: Okmok Volcano, Aleutian Islands, Alaska: Journal of Petrology, doi:10.1093/petrology/egn008 .

Okmok volcano, in the central Aleutian arc, Alaska, produced two caldera-forming eruptions within the last ~12,000 years. This study describes the stratigraphy, composition, and petrology of those two eruptions. Both eruptions initially produced small volumes of felsic magmas, followed by voluminous andesite and basaltic andesite. The Okmok I eruption produced >30 km3 DRE of material on Umnak Island, and Okmok II ~15 km3.

Larsen, J. F., Neal, Christina, Schaefer, Janet, Beget, Jim, and Nye, Chris, 2007, Late Pleistocene and Holocene caldera-forming eruptions of Okmok Caldera, Aleutian Islands, Alaska, in Eichelberger, John, Gordeev, Evgenii, Izbekov, Pavel, Kasahara, Minoru, and Lees, Jonathan, eds., Volcanism and Subduction: The Kamchatka Region: Geophysical Monograph 172, American Geophysical Union, p. 343-364.

This study presents a pre-eruptive magma storage model for the rhyodacite and andesite magmas erupted during the 3430 yBP caldera-forming eruption of Aniakchak volcano, Alaska, derived from phase equilibria experiments and petrological data. The compositions of Fe-Ti oxide pairs from the early erupted Plinian rhyodacite pumice yield core temperatures of 871-900C, with rims up to ~942C, and fO2 from -10.6 to -11.8 log units. Melt inclusions entrapped in plagioclase phenocrysts have H2O contents between 3 and 5 wt%, estimated by FTIR and electron microprobe volatiles by difference methods, with no detectable CO2. Assuming water saturation, this corresponds to entrapment pressures between ~65 and 150 MPa.

Larsen, J.F., 2006, Rhyodacite magma storage conditions prior to the 3430 yBP caldera-forming eruption of Aniakchak volcano, Alaska: Contributions to Mineralogy and Petrology, v. 152, n. 4, p. 523-540, doi: 10.1007/s00410-006-0121-4.

Szramek, Lindsay, Gardner, J.E., and Larsen, Jessica, 2006, Degassing and microlite crystallization of basaltic andesite magma erupting at Arenal Volcano, Costa Rica: Journal of Volcanology and Geothermal Research, v. 157, p. 182-201, doi:10.1016/j.jvolgeores.2006.03.039 .

"The purpose of this study was to replicate experimentally the growth of new rims around highly anorthitic plagioclase “core” phenocrysts, analogous to the incorporation of xenocrysts into a silicic magma body through magma mixing. Aniakchak rhyodacite forms the bulk starting composition, and phase-equilibria experiments constrained the pre-eruption magma conditions to be ~110 MPa and 870–880 °C. The experimental runs were seeded with Great Sitkin anorthite (An91–95) crystals. New rim growth of An28–38 plagioclase occurred at rates between 3.5 (±0.3) × 10–10 to 60.6 (±20.0) × 10–10 cm/s at pressures and temperatures from 50 to 150 MPa and 825 to 880 °C."

Larsen, J. F., 2005, Experimental study of plagioclase rim growth around anorthite seed crystals in rhyodacitic melt: American Mineralogist, v. 90, p. 417-427.

"Okmok Volcano is an active volcano located on the northeast end of Umnak Island, about 100 kilometers southwest of Dutch Harbor. The volcano is capped by a large, circular caldera, 10 kilometers in diameter and 500 to 800 meters deep, that formed during catastrophic eruptions about 12,000 and 2,000 years ago. Since then, more than a dozen separate vents within the caldera have erupted, some of them multiple times, to produce lava flows and widespread tephra (volcanic ash) deposits, with the most recent eruption occurring in 1997. Very large floods and lahars also have accompanied past eruptions, one of which destroyed an Aleut village in 1817. Multiple eruptions of lava and ash also have occurred from vents on the volcano's flanks. The volcano's long-term pattern of frequent eruptions, and dramatic intra-caldera deformation and accompanying seismicity since 1997 show that it remains active, and almost certainly will be the site of explosive and nonexplosive eruptions in the future. Understanding the variety of potential hazards associated with different types of volcanic processes typical of Okmok Volcano can help minimize the impacts of future eruptions."

Beget, J.E., Larsen, J.F., Neal, C.A., Nye, C.J., and Schaefer, J.R., 2005, Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2004-3, 32 p., 1 sheet, scale 1:150,000.
full text of report : 3.4 MB
map sheet : 53.2 MB
link to all files for this report on DGGS website

Aniakchak caldera, Alaska, produced a compositionally heterogeneous ignimbrite 3400 years ago, which changes from rhyodacitic at the base to andesitic at the top of the eruptive sequence. Interpretations of compositionally heterogeneous ignimbrites typically include either in situ fractional crystallization of mafic magma and generation of a stratified magma body or replenishment of a silicic magma chamber by mafic inputs. Another possibility, silicic replenishment of a more mafic chamber, exists. Geochemical characteristics of the caldera-forming rhyodacite and several late precaldera rhyodacites indicate independent origins for each, within a maximum of
5000 years prior to caldera formation. Isotopic considerations preclude derivation of the caldera-forming rhyodacite from the caldera-forming andesite. However, the caldera-forming rhyodacite can be explained as the residual liquid of a mostly crystallized basalt, with addition of crustal material. The Aniakchak andesite probably formed in a shallow chamber by successive mixing events involving small volumes of basalt and rhyodacite, together with contamination. The pre-caldera rhyodacites represent erupted portions of intruding silicic magma, whereas another portion homogenized with the resident mafic magma. The caldera-forming event reflects a large influx of rhyodacite, which erupted before significant mixing occurred and also triggered draining of much of the andesitic magma from the chamber. KEY WORDS: Aniakchak; caldera-forming eruption;

Dreher, S.T., Eichelberger, J.C., and Larsen, J.F., 2005, The petrology and geochemistry of the Aniakchak caldera-forming ignimbrite, Aleutian Arc, Alaska: Journal of Petrology, v. 46, n. 9, p. 1747-1768, doi: 10.1093/petrology/egi032.

"We have experimentally studied degassing of phonolite melts between 50 and 200 MPa and 825 and 850 jC. Our results indicate that decompressed phonolite melts contain equilibrium water contents regardless of decompression rate or final pressure. In addition, the total porosities of our experiments agree with predicted equilibrium values, regardless of final pressure or decompression rate. Combined observations of mean bubble diameters, porosities, and arrangement of bubbles in our experiments indicate that degassing occurs preferentially in the large bubble fraction of the population. Because of this, retardation of growth in the small bubble population occurs."

Larsen, J. F., and Gardner, J. E., 2004, Experimental study of water degassing from phonolite melts: implications for volatile oversaturation during magmatic ascent: Journal of Volcanology and Geothermal Research, v. 134, n. 1-2, p. 109-124.

"We have experimentally studied the process of bubble coalescence in rhyolite and phonolite melts of natural composition. The experiments involved decompression of water-saturated melts equilibrated at pressures and temperatures from 100 to 150 MPa and 775 to 840 °C in vertically oriented, rapid-quench capable, cold seal pressure vessels. One type of experiments (rhyolite MCR-100, 120, 150 and phonolite LSP-120 series') approximates a "static" bubble coalescence case, where we held the decompressed samples for 5 seconds to 4320 minutes (3 days) before quenching. The second type (rhyolite LPC-100 series) replicates an "expanding" bubble coalescence environment, where we continually decompressed the experiments at a rate of 0.5 MPa/s, examining samples quenched at ending pressures between 10 and 80 MPa. Our "static" case (MCR-100, 120, and 150, and LSP-120) results show significant increases in the modal bubble sizes and in the sizes of the largest bubbles, corresponding to measurable broadening in the size distributions."

Larsen, J. F., Denis, M. H., and Gardner, J. E., 2004, Experimental study of bubble coalescence in rhyolitic and phonolitic melts: Geochimica et Cosmochimica Acta, v. 68, p. 333-344.

"The purpose of this study is to investigate differences in dome mineralogy and alteration using Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) satellite imagery, field observations, and laboratory petrologic and spectrographic analyses of the Black Peak volcano complex, Alaska. Black Peak is a 3.5km-wide-caldera on the Alaska Peninsula. The caldera formed ~4,600 years ago by an eruption that deposited <20km3 of ash-flow tuffs which filled the valleys to the west and north. The floor of the ice-free caldera consists of approximately a dozen overlapping, mainly andesitic lava domes and coulees."

Adleman, J.N., Larsen, J.F., Ramsey, M.S., McGimsey, R.G., and Neal, C.A., 2004, Analysis of composition and chronology of dome emplacement at Black Peak, Alaska utilizing ASTER remote sensing data and field-based studies [abs.]: in Abstract Volume, IAVCEI General Assembly, November 2004, Pucon, Chile, s08c_pf_165.
full-text PDF : 60 KB

"The purpose of this study is to investigate differences in dome mineralogy and alteration using Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) satellite imagery, field observations, and laboratory petrologic and spectrographic analyses of the Black Peak volcano complex, Alaska. Black Peak is a 3.5km-wide-caldera on the Alaska Peninsula. The caldera formed ~4,600 years ago by an eruption that deposited <20kmof ash-flow tuffs which filled the valleys to the west and north. The floor of the ice-free caldera consists of approximately a dozen overlapping, mainly andesitic lava domes and coulees."

Adleman, J.N., Larsen, J.F., Ramsey, M.S., McGimsey, R.G., and Neal, C.A., 2004, Analysis of composition and chronology of dome emplacement at Black Peak, Alaska utilizing ASTER remote sensing data and field-based studies [abs.]: in Abstract Volume, IV International Biennial Workshop on Subduction Processes emphasizing the Japan-Kurile-Aleutain Arcs, August 2004, unpaged.
full-text PDF : 16 KB