Dr. Kari Cooper of UC Davis will give a talk at Speakers Club entitled "Magmas, mushes and mobility: chemical insights into the conditions of magma storage beneath volcanoes."

ABSTRACT:

The processes involved in formation and storage of magma within the Earth’s upper crust are of fundamental importance to volcanology. The thermal, and therefore physical, conditions of magma storage control many important aspects of processes occurring during magma storage, such as magma mixing, crystallization, recharge, and development of high-liquid-fraction eruptible bodies, yet these conditions remain a source of much debate. For example, crystal-rich magmas are often argued to represent mobilization of magma stored at near-solidus conditions, yet other studies have argued that accumulation of significant bodies of eruptible magma, particularly for larger eruption, requires extended storage at high temperatures. One aspect that has been lacking in this debate is clear observational evidence connecting the thermal conditions within a magma reservoir to the time scales of storage – i.e., thermal histories – and until now there have been no geochemical or petrological approaches that provide these constraints. We have developed a novel method of constraining the thermal history of magma storage using a combination of time scales derived from U-series dating, textural information (CSD’s) and trace-element zoning of crystals. I will present results for this approach applied to Mount Hood, Oregon, where we show that only a small fraction (most likely less than 1%) of the total time that magma is stored underground is spent at high enough temperatures that would allow magma to be easily mobilized and erupted. A data compilation for other arc systems suggests that this mode of behavior is common. Evidence from crystal records at silicic magma systems (e.g., Yellowstone, WY and Taupo Volcanic Zone, New Zealand) suggests that they also have reservoirs dominated by crystal mush zones, and that the time scales of accumulation and storage of the erupted magmas are short relative to the time scale of development and storage of a crystal mush. The apparent prevalence of mush zones over time in many magma reservoirs opens the door to new research directions combining observational evidence (geochemical and geophysical) with numerical modeling, in order to understand the primary controls on the thermal and physical conditions of magma storage in Earth’s crust.