Document Type
Thesis
Date of Degree Completion
Summer 2012
Degree Name
Master of Science (MS)
Department
Geological Sciences
Committee Chair
Wendy Bohrson
Second Committee Member
Carey Gazis
Third Committee Member
Chris Mattinson
Abstract
Mount Mazama is the volcanic edifice that cataclysmically erupted ~503 km of relatively homogeneous rhyodacite lava ~ 7,700 years ago, forming the caldera known as Crater Lake. Within a few hundred years, andesitic eruptions built three distinct volcanic edifices on the floor of Crater Lake; ~ 3000 years later, rhyodacite eruptions formed a dome (Bacon et al., 2002). How magmatic systems evolve following a shallow, relatively large silicic eruption is the focus of this study. In situ geochemical analysis coupled with high-resolution textural images of plagioclase crystals in the four postcaldera volcanic edifices were used to identify distinct crystal populations and identify magmatic processes involved in their formation. Five distinct crystal populations were identified within the postcaldera rocks: LSr antecrysts and Bytownite antecrysts originated in the climactic magma chamber; granitoid antecrysts were part of the magma chamber wall rock; and andesite phenocrysts and basalt phenocrysts represent new magma or melt ± crystals in the postcaldera magma system.
The postcaldera magma system of Mount Mazama’s climactic eruption is an example of how large silicic magma chambers evolve following a large silicic eruption. The results show that assimilation, and fractional crystallization, magma mixing with new mafic magma occurred in the postcaldera magma chamber. These results establish that a large silicic eruption may be followed by imputs of new magma. Combining in situ chemical data with high resolution textural imaging is an effective way to identify distinct crystal populations and discern magmatic processes in a large silicic magma chamber and may be applicable to other magma systems.
Recommended Citation
Tebbe, Michelle Leanna, "Petrogenetic Relationship of the Postcaldera Eruptions of Mount Mazama, Crater Lake, Oregon; Evolution of a Sub-volcanic Magma Chamber Following a Large Silicic Eruption" (2012). All Master's Theses. 1454.
https://digitalcommons.cwu.edu/etd/1454
Appendix A: Thin Section Photos
B_NDIC_and_BSE_images.pdf (40126 kB)
Appendix B: NDIC and BSE Images
D_EMP_data.xls (117 kB)
Appendix D: EMP Data
E_Combined_Data.xls (403 kB)
Appendix E: Combined Data
071409_Bohrson_CD-185-5_plag_calc.xls (299 kB)
Appendix C: part 1
071409_Bohrson_MC1172_calc.xls (314 kB)
Appendix C: part 2
071409_Bohrson_stds_calc.xls (190 kB)
Appendix C: part 3
071509_Bohrson_80c_516_plag_calc.xls (356 kB)
Appendix C: part 4
071509_Bohrson_81C_673_plag_calc.xls (331 kB)
Appendix C: part 5
071509_Bohrson_185_plag_calc.xls (180 kB)
Appendix C: part 6
071509_Bohrson_CD175_2_plag_calc.xls (408 kB)
Appendix C: part 7