Magmatic Differentiation at an Island-arc Caldera: Okmok Volcano, Aleutian Islands, Alaska |
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Authors: | Finney Benjamin; Turner Simon; Hawkesworth Chris; Larsen Jessica; Nye Chris; George Rhiannon; Bindeman Ilya; Eichelberger John |
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Institution: | 1Department of Earth Sciences, Wills Memorial Building, University of Bristol, Bristol BS8 1RJ, UK
2Gemoc, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
3Alaska Volcano Observatory, Geophysical Institute, University of Alaska, Fairbanks, AK 901775-7320, USA
4Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, USA |
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Abstract: | Okmok volcano is situated on oceanic crust in the central Aleutianarc and experienced large (15 km3) caldera-forming eruptionsat 12 000 years BP and 2050 years BP. Each caldera-forming eruptionbegan with a small Plinian rhyodacite event followed by theemplacement of a dominantly andesitic ash-flow unit, whereaseffusive inter- and post-caldera lavas have been more basaltic.Phenocryst assemblages are composed of olivine + pyroxene +plagioclase ± Fe–Ti oxides and indicate crystallizationat 1000–1100°C at 0·1–0·2 GPain the presence of 0–4% H2O. The erupted products followa tholeiitic evolutionary trend and calculated liquid compositionsrange from 52 to 68 wt % SiO2 with 0·8–3·3wt % K2O. Major and trace element models suggest that the moreevolved magmas were produced by 50–60% in situ fractionalcrystallization around the margins of the shallow magma chamber.Oxygen and strontium isotope data (18O 4·4–4·9,87Sr/ 86Sr 0·7032–0·7034) indicate interactionwith a hydrothermally altered crustal component, which led toelevated thorium isotope ratios in some caldera-forming magmas.This compromises the use of uranium–thorium disequilibria(230Th/ 238U) = 0·849–0·964] to constrainthe time scales of magma differentiation but instead suggeststhat the age of the hydrothermal system is 100 ka. Modellingof the diffusion of strontium in plagioclase indicates thatmany evolved crystal rims formed less than 200 years prior toeruption. This addition of rim material probably reflects theremobilization of crystals from the chamber margins followingreplenishment. Basaltic recharge led to the expansion of themagma chamber, which was responsible for the most recent caldera-formingevent. KEY WORDS: Okmok; caldera; U-series isotopes; Sr-diffusion; time scales; Aleutian arc |
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Keywords: | : Okmok caldera U-series isotopes Sr-diffusion time scales Aleutian arc |
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