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U-Th ages and compositions of zircons in Dominica: constraints on a magma plumbing system

Holli M. Frey 1, Matthew R. Manon1, Sarah K. Brehm2, Sarah Hickernell3

  • Affiliations: 1Department of Geosciences, Union College, Schenectady, NY, USA; 2Department of Atmospheric and Geological Sciences, SUNY Oswego, Oswego, NY, USA; 3Department of Earth and Planetary Sciences, Stanford University, Stanford, CA, USA

  • Presentation type: Talk

  • Presentation time: Tuesday 14:45 - 15:00, Room R380

  • Programme No: 1.3.3

  • Theme 1 > Session 3

Abstract

Within the last 200 kyr, Dominica has experienced multiple voluminous eruptions across the island and continues to show signs of potential future activity, with active hydrothermal areas, including Boiling Lake, and periodic swarms of shallow earthquakes. Like other Caribbean islands, it has been proposed that Dominica has a trans-crustal magmatic system, deeply sourced and wet, leading to multiple explosive eruptions, preserved as ignimbrites within valleys and coastal exposures. In this study, we present >200 U-Th ages and compositions of zircon rims, which help to refine the eruption age(s), crystallization histories, and sources of six ignimbrites from across the island. Several of the ignimbrites display polymodal distributions of ages, including eruption age zircons (relative to minimum 14C ages and (U-Th)/He eruption ages from previous studies). There is no discernible island-wide time progression of eruptions geographically, as previously proposed. Many of the units have significant antecrystic zircon population(s), punctuated by hiatuses of 10s of kyrs, suggesting sub-solidus storage conditions and intermittent crystallization prior to eruption. Overall, trace element concentrations in the analyzed zircons span broad ranges, with little correlation with respect to ignimbrite and/or zircon age. Uranium concentrations are low, with an average of 135 ppm, whereas Hf spans an extremely broad range from ~2200 to ~18,000 ppm. The distinct zircon crystallization histories from each ignimbrite deposit, coupled with the heterogeneous zircon compositions suggests that each explosive unit has a unique ascent path, and that ascending magma may entrain multiple discrete pockets of previously crystallized material en route to the surface.