Petrogenetic Relationship of the Postcaldera Eruptions of Mount Mazama, Crater Lake, Oregon; Evolution of a Sub-volcanic Magma Chamber Following a Large Silicic Eruption

Michelle Leanne Tebbe
August 2012


Mount Mazama is the volcanic edifice that cataclysmically erupted ~50 km3 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.

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