Magma Chamber Processes Over the Past 475,000 Years at Mount Hood, Oregon: Insights From Crystal Zoning and Crystal Size Distribution Studies

We have studied the relationship between lava composition and magma chamber processes at Mount Hood, Oregon for the last 475,000 years. Mount Hood is unusual, in comparison to nearby Mount St. Helens and Mount Jefferson, in that it has produced relatively homogeneous lava compositions over this time period. Erupted lavas are mostly crystal rich andesites and in total vary between 55 and 63 weight percent silica, with ~80% of the lavas having silica between 58-62 weight percent. The most evolved lavas have only erupted within the past 250 years. Despite this homogeneity, and as with many other silicic volcanoes, petrographic features such as mineral zoning, sieved textures and dissolution surfaces, suggest a more complex magmatic history. To further investigate this issue we have applied a number of different petrological techniques to lava flows between 200 to 475,000 years old from Mount Hood. Crystal Size Distribution (CSD) studies have been used to identify different plagioclase crystal populations, to recognize mixing of these populations and to estimate crystal residence times within the magma chamber. The major and trace element compositions of crystal populations have also been determined using Electron Microprobe Analysis (EMPA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Initial results suggests that the overall control on the bulk chemistry of lavas, illustrated by a linear relationships between major elements and silica, is simple two component mixing plus crystal fractionation. CSD studies further show that the more evolved lavas are mixtures of two or more different crystal populations, while more primitive lavas are composed of just one crystal population. Calculated crystal residence times range between 7 to 65 years. Observed zoning within plagioclase phenocrysts represent both changes in temperature and pressure within the chamber and the addition of a more mafic magma; the later being characterized by an increase in anorthite composition that corresponds with spongy dissolution surfaces and increased trace element abundances. In some lavas recharge by mafic magma occurs late in crystal growth histories and thus may be related to the onset of eruption.