|Abstract or Summary
- Several different petrological techniques have been applied to lava flows between
200 to 475,000 years old from Mount Hood, Oregon. Mount Hood is unusual, in comparison to nearby Mount St. Helens and Mount Jefferson, in that it has produced relatively homogeneous lava compositions over 475,000 years. Erupted lavas are mostly crystal rich andesites and in total vary between 53 and 63 weight percent silica, with ~85% of the lavas having silica between 58-62 wt. %. The most evolved lavas have only erupted within the past 15,000 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. 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).
Results show that the lavas at Mount Hood are derived from mixing of two end member magmas. Each of the end members contribute a plagioclase crystal population to the final erupted lava that is distinguishable in the lavas by CSD studies and trace element compositions. The first crystal population belongs to a relatively undepleted mafic end member that has only been crystallizing plagioclase for approximately 3.9 – 5.5 years, assuming a crystal growth rate of 10[superscript -9] mm/sec, while the other is from a more
silicic end member that has been crystallizing plagioclase for 177 – 227 years, assuming a crystal growth rate of 10[superscript -10] mm/sec. Textural disequilibrium textures are found near the rim of plagioclase phenocrysts from the second populations of crystals and correlate to
increases in anorthite, MgO, FeO, Ti, and Sr and decreases in Ba and Ce indicating that
the plagioclase have recorded a mixing event between the host and a relatively more
mafic magma during the later stages of crystallization, which may have triggered
eruptions. Eruptions of these mixed magmas occur approximately 3 – 0.2 years after the
mixing event takes place.