NSF Award
2426075
Subduction zone volcanoes occur where one tectonic plate goes beneath another. Many millions of people live near subduction volcanoes. This means that understanding subduction volcanoes and the hazards they present is important. Magmatic activity at a volcano is usually studied using methods from geophysics. One such method is monitoring how volcanoes change shape (volcano deformation) over time. Geologists can also study igneous rocks, which form from magmas, to learn about volcanoes and magmas. Extrusive igneous rocks form from magmas that erupt from a volcano. Intrusive igneous rocks form from magmas that crystallize beneath the Earth's surface. For this project, the research team will study an ancient subduction zone volcano in Washington where they find both types of igneous rocks. They will reconstruct the record of volcanic eruptions and subvolcanic intrusive activity. To do this, they will study the geochemistry, geochronology, and petrology of the rocks. They will use these data to understand three things. First, they will determine when the magmas formed and if the erupted magmas and intrusive magmas existed at the same time. Second, they will determine how the composition of the magmas changed through time. Third, they will determine how deep the magmas were beneath the Earth's surface. The research team will also make videos about the motivation, importance, and results of their research. They will work with Professor Nick Zentner (at Central Washington University) to make these videos. The research team will also work with the Indiana School of the Deaf to produce new lab exercises for their high school science courses.<br/><br/>The ancient volcano that will be studied lies just to the north of Mount St. Helens and includes the upper crustal Oligocene Spirit Lake Pluton and surrounding volcanic rocks. It represents a deeply eroded portion of the ancestral Cascade Arc and was the focus of detailed 1:24,000-scale geologic mapping by the USGS in the 1980s and 1990s. Existing geo- and thermochronology constrains the duration of pluton emplacement to <1.5 Myr and demonstrates that eruptions of the surrounding volcanic rocks pre-dated, were coeval with, and post-dated the pluton. Existing whole rock geochemical data show a compositional range from quartz diorite to granite in the pluton, and a range from basalt to high-silica rhyolite in the volcanics. This research aims to produce a detailed chronological and geochemical record of pluton construction and associated volcanism to better understand when the intrusive rocks were emplaced, if intrusive activity affected the rate and/or style of volcanic eruptions, and if any of the erupted magmas were derived from the pluton. This project will produce a detailed timeline of events using high-precision U-Pb zircon geochronology for 15 samples each from the pluton and the associated volcanic section. These data, along with geochemical and textural observations, will allow the team to answer questions such as: 1) Was there a long-lived magmatic mush within the now solidified plutonic complex? and 2) Did emplacement of the pluton lead to changes in eruption style, composition, or rate? Geobarometric data will allow the team to directly test whether any volcanic eruptions were sourced from the same depth as the currently exposed pluton. Taken together these geochronologic, geochemical, and geobarometric datasets will offer a holistic record of how the magmatic system evolved over the lifespan of a single arc volcano.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
