U-Series Crystal Ages and Implications for Magma Dynamics

University Of California-Davis, Davis CA

Investigators

Abstract

Understanding the ways in which magmas beneath volcanoes evolve, and the time scales over which they evolve, is important both in terms of gaining fundamental insights into how volcanic systems work and in terms of improving our ability to forecast volcanic eruptions. Crystals in volcanic rocks erupted at the surface contain chemical records of their interactions with magmas beneath the surface, and dating of these crystals can therefore provide information about chemical changes within the system over time. However, ages of different types of crystals (for example, zircon and plagioclase) generally yield different characteristic ages. It is unclear whether this is due to the scale of analysis (individual crystals for zircon compared to aggregates of thousands of crystals for plagioclase), due to the fact that zircon and plagioclase have typically been analyzed in different types of volcanic systems (large, caldera-forming systems for zircon, smaller systems for plagioclase), or because the two types of crystals record different parts of a magma's history. This uncertainty makes it difficult to compare ages of different crystals and/or crystals from different volcanoes, hampering our ability to develop a general understanding of the timescales of magmatic processes. In this particular project, the investigators will address this problem by measuring ages of zircon and plagioclase in the same samples for two different systems: Tarawera volcano, New Zealand (part of a the Okataina caldera complex) and Devils Hills domes, South Sister volcano, Oregon (lava flows from a composite volcano). The research plan will include measuring in situ 238U-230Th ages in zircon and 226Ra-230Th and 238U-230Th ages in bulk separates of both plagioclase and zircon from samples derived from the two magmatic systems. These data will provide information about the temporal relationship between crystallization of major and trace phases in the same system and help delineate which parts of the magmatic history are recorded by each mineral. A secondary goal is to understand the dynamics of magma storage beneath Okataina and South Sister, as representatives of a large-volume silicic caldera system and small-volume silicic domes, respectively. Collectively, these data will provide an interpretive context for existing and future U-series crystal ages, and will also provide insights into the dynamics of magmas in two systems with disparate eruptive histories. This project will provide data that are fundamental to interpretation of U-series mineral ages, which will have broad scientific impact in use of these ages to understand magmatic systems. The Kaharoa project will foster international collaborations spanning three institutions (UC Davis, US; Univ. British Columbia, Canada; and Univ. Canterbury, New Zealand) and the results of the proposed work will therefore be integrated with a much larger body of data than would otherwise be possible.