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What do zircon age distributions tell us about the magma storage conditions of large ignimbrite eruptions?

Benjamin Z. Klein1, Michael P. Eddy2, Othmar Müntener1

  • Affiliations: 1Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland; 2Department of Earth, Atmospheric and Planetary Sciences, Purdue University, Indiana, USA

  • Presentation type: Talk

  • Presentation time: Tuesday 14:30 - 14:45, Room R380

  • Programme No: 1.3.2

  • Theme 1 > Session 3

Abstract

In the last decade, high precision U-Pb zircon geochronology studies have been conducted on numerous large-volume felsic eruptions. Many of these studies have yielded single zircon ages with <10 kyrs uncertainties. This unprecedented resolution has allowed investigation of pre-eruption storage timescales, but the interpretation of these datasets remains contentious due to the potential for whole grain dissolution to average multiple zircon age domains. This averaging has the potential to either decrease age dispersion within a zircon population by averaging multiple autocrystic growth domains, or to increase age dispersion within a zircon population by integrating inherited or antecrystic zircon cores. To provide new insights into this issue, we compiled CA-ID-TIMS zircon U-Pb datasets combined with petrographic and geochemical data for 31 ignimbrites. These data reveal that the age dispersion in crystal-poor ignimbrite is unresolvable or <100 kyrs, while crystal-rich systems frequently record age dispersion >200 kyrs. These relationships suggest that the high-precision datasets faithfully document zircon crystallization timescales. Further, we evaluate two mechanisms to produce short duration zircon crystallization intervals in crystal-poor systems: 1) late zircon saturation immediately preceding eruption; or 2) extraction of a zircon-saturated melt from a crystal mush immediately preceding eruption without remobilizing existing zircon. While the first mechanism may be possible in exceptional scenarios, it requires generation of a parental magma by re-melting of zircon-free cumulates. If the second mechanism is instead the dominant process for creating these systems, it places strong constraints on the maximum melt velocities possible during melt extraction from the crystalline mush.