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Rader, E.L., 2010

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Rader, E.L., 2010, Mineral stability in H2O undersaturated magmas: Experiments on basaltic andesite from Westdahl Volcano: University of Alaska Fairbanks, M.S. thesis, 91 p.

Abstract

Magma differentiation in the Aleutians occurs along two main geochemical pathways, calc-alkaline (defined by alkali-enrichment) and tholeiitic (defined by iron enrichment). Experimental petrology was used to investigate the impact of changing XH2O on magma differentiation trends (the liquid line of descent), using natural basaltic andesite from the 1991-1992 eruption of Westdahl volcano, and a similar basaltic andesite from Okmok volcano as a proxy. Experimental conditions were held at NNO between temperatures of 900-1200°C and pressures of 50-180 MPa for between 4 and 126 hours. Phases stable included Fe-Ti-oxides, clinopyroxene, plagioclase, amphibole, and orthopyroxene. Water decreases the stability field of plagioclase and amphibole, resulting in Fe-Ti-oxides defining the liquidus and producing calk-alkaline evolution. Water also increased the partition coefficient of plagioclase and the Mg# of clinopyroxene. Olivine was not stable in the experiments indicating the fO2 at Westdahl is > NNO. The change from tholeiitic to calc-alkaline evolution with the addition of water indicates Westdahl volcano is a dry system. Magma evolution in the Aleutian arc is predominantly controlled by the tectonics of the region, however, water content and oxygen fugacity could also be affecting the volcanoes along the arc at a local scale.

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