1. The Link Between Amphibole Aluminum Content and Reaction Rim Mineralogy Ross Ellingwood Oregon State University College of Earth, Ocean and Atmospheric Sciences Methods Conclusion In this study samples from Mount Hood’s two most recent eruptions were examined. Mount Hood has a shallow (~3-6 km) and deep (~10- 15 km) magma storage chamber that erupts hornblendes with high (10-13 wt.%) and low (6-9 wt.%) Al 2 O 3 content (Koleszar 2011). Hornblende contains ~2 wt.% H 2 O in its crystal structure and crystallizes at depth in Mount Hood’s magma chambers. As magma ascends decompression causes the concentration of dissolved water to decrease leading to unstable conditions for amphibole formation. Amphiboles breakdown from the rim inward as anhydrous minerals (pyroxene, plagioclase, Fe-Ti oxides) crystallize from the dissolved amphibole content. This study examines if there is a connection between the Al content of hornblende and the mineralogy and texture of the reaction rim. IntroductionResults Fig. 3 (A,C) image of hornblende crystals under transmitted light at 10x magnification. (B,D) back-scatter electron images (BSE) of hornblende crystals (A,C). Petrographic microscope using transmitted light to measure hornblende size and reaction rim thickness. Cameca SX-100 Electron Microprobe at Oregon State University used for chemical analysis and imaging of amphiboles. References Fig. 1,2 illustrations of Mount Hood magma chambers and amphibole stability region. Fig. 5, graph of Al 2 O 3 wt.% compared with measured hornblende reaction rim thickness (low Al in blue, high Al in red). Fig. 4, graph of Al 2 O 3 wt.% compared to calculated aspect ratio of hornblende crystals (low Al in blue, high Al in red). Fig. 6, classification of the calcic amphiboles in which (Na+K) A ≥ 0.50, Ca B ≥ 1.50. Classification from Leake et. al (1997). Low Al in blue, high Al in red. Analysis of hornblende crystals shows no apparent relationship between amphibole Al content and mineralogy and texture of the reaction rim. As shown in Fig. 4 there seems to be no preferential aspect ratio for high or low Al content. Fig. 5 indicates that Al content of the hornblende crystal makes no difference in the reaction rim thickness. Examination of BSE images showed that hornblendes with high and low Al content had similar mineralogy and texture. As shown in Fig. 7, hornblende reaction rim mineralogy consisted of orthopyroxene (OPX), apatite, and Fe-Ti oxides in both high and low Al hornblendes. Fig. 7, BSE images of hornblende crystals. (A,C) Hornblende crystal with reaction rim texture shown (B,D). (E,G) High Al hornblende (Al 2 O 3 wt.% marked by x) compared to low Al (F,H). OPX, orthopyroxene. -Koleszar, Alison M., Adam JR Kent, Paul J. Wallace, and William E. Scott. "Controls on Long-term Explosivity at Andesitic Arc Volcanoes: Insights from Mount Hood, Oregon.”Journal of Volcanology and Geothermal Research (2012). -Leake, Bernard E. "Nomenclature of Amphiboles: Report of the Subcommittee on Amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names." The Canadian Mineralogist 35 (1997): 219-46. -Special thanks to Adam Kent for providing the research topic and guidance on this project. ActinoliteMagnesiohornblende Tschermakite pargasite Ferro- actinolite FerrohornblendeFerrotschermakite