V003-0009
New insights into the construction of the 25.4 ka Oruanui, New Zealand, magma reservoir from a comparison of whole-rock and plagioclase-hosted zircon records

Monday, 7 December 2020
Poster
Tyler Schlieder, University of California Davis, Earth and Planetary Sciences, Davis, CA, United States, Kari M Cooper, UC Davis, Davis, CA, United States, Adelicia Johnson, University of California Davis, Davis, CA, United States, Adam JR Kent, Oregon State University, College of Earth, Ocean and Atmospheric Sciences, Corvallis, OR, United States, Darren McClurg Gravley, University of Canterbury, Christchurch, New Zealand and Chad Daniel Deering, Michigan Technological University, Department of Geological and Mining Engineering & Sciences, Houghton, MI, United States
Abstract:
Silicic magmatic systems are responsible for the most catastrophic volcanic eruptions in Earth’s history, yet many aspects of these systems remain poorly understood. Previous analysis of zircon crystal interiors from the world’s youngest super eruption, the 25.4 ka Oruanui eruption, have provided unique insight into the evolution of large silicic magma bodies(1,2). Additional information about the magma system can be gained by analyzing the latest stages of zircon crystallization (via analysis of unpolished surfaces) and by understanding the petrologic context of analyzed zircon. We report new 238U-230Th ages and trace-element data for zircons separated from both whole-rock (WR) samples and plagioclase separates from the Oruanui eruption, New Zealand, which provide a more complete record of reservoir processes.

Whole-rock zircon surface ages yield a single eruption age peak (25.7 ka +2.7 ka/-2.6 ka), whereas the WR zircon interior age spectrum is more complex. WR zircon surfaces are generally more restricted in composition than WR interiors and display two clear compositional groups in some trace-element ratios (e.g. Y/P) while WR interiors record no such bimodality. In order to preserve this bimodality without additional surface crystallization recording equilibrium with the erupted glass, the two populations must have been incorporated into the erupted Oruanui magma body within ~2 kyrs of eruption.

Plagioclase-hosted zircon surface and interior age spectra are both statistically indistinguishable from WR zircon interiors. Systematic variations in Eu/Eu*, Hf, and Sc with Ti-in-zircon temperature and U-Th age recorded in plagioclase-hosted zircon surfaces indicate that the portion of the Oruanui magmatic system that hosted the zircon-bearing plagioclase experienced a shift to higher temperatures and less evolved compositions at ~45 ka. The preservation of compositionally distinct old (>45 ka) plagioclase-hosted zircon surfaces requires that some component of Oruanui plagioclase crystallized >~18 kyrs prior to eruption. These data provide a more complete record of zircon crystallization and demonstrate that plagioclase-hosted zircons record processes obscured in WR zircon data.

  1. Barker SJ et al (2014). J Petrol 55:1511–1533.
  2. Charlier BLA et al (2005). J Petrol 46:3–32.
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