Geochemical Insights Into the Post-Caldera Architecture of the Yellowstone Magma Reservoir

University Of California-Davis, Davis CA

Investigators

Abstract

Volcanic systems such as Yellowstone in Wyoming produce very large, highly explosive eruptions that form immense craters called calderas. The volcanic system at Yellowstone has had three of these caldera-forming eruptions over the past two million years. In between these very large eruptions the system has also produced many smaller eruptions that were less explosive, which fed large lava flows that partially filled the caldera. As a result, the potential hazards and destructiveness of an eruption at volcanoes such as Yellowstone can vary widely, ranging from nation-wide or even global effects to more local hazards. An open question in volcano science is what factors control whether a volcano produces a very large explosive eruption or a smaller lava flow in a particular eruption. This probably depends partly on how the “plumbing system” beneath volcanoes changes over time, and whether large bodies of mostly-liquid magma are present continuously, whether they build up slowly over long periods of time, or whether they are only present just before an eruption. For example, does generating a very large eruption require a longer time to build a big pool of magma? Does it require more magma bodies to be present than a smaller eruption, or are there the same number of magma bodies present at all times but more of them get triggered to produce a very large eruption? In this project researchers will use chemical “fingerprints” of different magma bodies to identify how many different magma bodies have been tapped in the most recent series of smaller eruptions at Yellowstone, and will compare this to the number of magma bodies that have been identified as part of the most recent caldera-forming eruption to help answer these questions. Understanding how different kinds of eruptions are produced will contribute to volcanic hazard forecasting and risk management. The project will also train graduate and undergraduate students, contributing to the scientific workforce, and public outreach about the results will help contribute to scientific understanding for the general public. Large silicic systems produce a wide range of eruption styles and eruptive hazards, but the connections between the architecture of the magma storage region and the type of eruption produced are not well understood. For example, does a caldera-forming eruption require a different distribution of melts within the shallow reservoir, and/or a different triggering mechanism, than a smaller eruption? As a contribution to answering these broad questions, this team proposes to compare the architecture of the magma system during intra-caldera eruptions to existing data for caldera-forming eruptions at Yellowstone Caldera, Wyoming. New high-precision 40Ar/39Ar dating of the most recent intracaldera eruptions at Yellowstone (the Central Plateau Member, CPM) shows that they were erupted in five episodes, each of which spanned at most 1-2 kyr. This provides an opportunity to examine the number and distribution of magma bodies during each of five snapshots of the magma system, and to compare this with published work on the Yellowstone caldera-forming eruptions. They will investigate compositional diversity recorded in sanidine and zircon crystals both within and between individual eruptions and eruptive episodes using 238U-230Th age data coupled with geochemical data for both sanidine and zircon. Based on previous work, the sanidine and zircon surface compositional data will provide insights into the number of distinct magma bodies present during the period immediately prior to CPM eruptive episodes, as well as constraining the time scale of assembly and storage of different magma bodies prior to eruption. In contrast, zircon interiors are typically older with ages spanning 10s of kyr prior to eruption, which will provide a measure of compositional diversity present on a longer time scale, between eruptive episodes. The team will test the hypothesis that the basic architecture of the magma reservoir consists of multiple compositionally distinct magma bodies within a more heterogeneous crystal mush, and that the state of the system is similar before CFE and recent intracaldera eruptions. The data collected will also allow them to assess whether there are systematic changes in the average composition of erupted crystals over time, which may reflect secular variations in the proportions of mantle-derived and crustal-derived melts contributing to the system. Synthesizing their new data with data for Yellowstone CFE and data for other silicic systems will provide additional insights into the development and evolution of large silicic magma reservoirs. Broader impacts will include outreach by the PI to increase scientific literacy and public engagement, training and mentoring of diverse graduate and undergraduate students, strengthening partnerships between academia and the USGS, and contributing information that will be used to inform public policy and volcanic hazard mitigation. 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.