Magma ascent and eruption in the Aleutian arc
DESCRIPTION: The Aleutian arc is one of the most volcanically active regions of the world. Frequent eruptions of tall ash clouds by Aleutian volcanoes present a significant hazard to local communities and to travelers flying from North America, Europe, and Asia. In order to mitigate this hazard, Alaskan volcanoes are extensively monitored by the Alaska Volcano Observatory (AVO), using seismic data, geodetic data and satellite imagery. These techniques are largely successful in enabling the prediction of volcanic eruptions; however, the links between precursor signals of an eruption (as inferred from seismic and geodetic data) and eruptive style are poorly understood. Factors that are thought to play a role in controlling the vigor of a volcanic eruption include the volatile content of the magma, the ascent rate of the magma in the volcanic conduit, and the temperature of the magma. These parameters are difficult to measure directly, but they can be inferred a posteriori from the analysis of erupted materials. The goal of the proposed work is to measure gradients of water and other chemical species in erupted crystals and glasses from three Aleutian arc volcanoes. These chemical gradients are signatures of magma ascent, degassing, and cooling. Recently developed techniques for interpreting such chemical gradients will be used to constrain ascent rates and temperatures of magmas in the minutes to hours prior to their eruption. Such measurements will further our understanding of processes operating in the volcanic conduit in the build-up to an eruption, and will provide a physical framework in which to place real-time volcano monitoring data. The proposed study integrates recently developed petrologic methods for determining the ascent rates and syneruptive thermal histories of magmas erupted at three Aleutian arc volcanoes (Okmok, Seguam, and Pavlof) with contrasting explosivities (Volcanic Explosivity Index, VEI, of 1 ? 3). Magma ascent rates for each of the three eruptions will be calculated using three independent techniques: (1) water loss from olivine-hosted melt inclusions of different sizes; (2) water zonation in olivine; and (3) water zonation in clinopyroxene. Magma cooling histories during ascent and eruption will be calculated using MgO concentration gradients inside olivine-hosted melt inclusions. Integration of these techniques on samples from volcanoes with different eruptive styles will allow identification of relationships between magma volatile concentrations, ascent rate, thermal history, and eruptive style. The proposed analyses will enhance our understanding of the behavior of water and other volatile elements in olivine-hosted melt inclusions and in arc volcanoes more broadly, and they will enable more reliable and accurate determination of pre-eruptive volatile concentrations from olivine-hosted melt inclusions.