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Episodic crystallization in young explosive eruptions in Dominica, Lesser Antilles

Project Leads: 
Holli M. Frey, Matthew R.F. Manon, Sarah K. Brehm, and Rebecca N. Babiak

Episodic crystallization in young explosive eruptions in
Dominica, Lesser Antilles, revealed by U-Th dating of zircons

Holli M. Frey1, Matthew R.F. Manon1, Sarah K. Brehm2, and Rebecca N. Babiak3
1Department of Geology, Union College, Schenectady, New York 12308, USA
2Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
3Department of Geology, Memorial University of Newfoundland, St. John’s, Newfoundland and Labrador A1B 3X5 Canada


Understanding the storage conditions of silicic magmas prior to violent volcanic eruptions
is important as it places constraints on whether the presence of melt is indicative of volcanic
unrest. To address magma reservoir storage conditions in an active volcanic arc system,
ignimbrites from the Roseau Tuff in Dominica were characterized by geochronology and
thermometry. Sixty-five (65) new U-Th ages of zircon rims from ignimbrites spanning the
10-km-long Roseau Valley demonstrate that the deposits represent multiple distinct eruptive
events, including a significant Holocene eruption. The zircons from each ignimbrite show a
polymodal age distribution. The youngest zircons capture the eruption or near-eruption age
(ca. 3 to ca. 60 ka) of each ignimbrite, whereas a significant older, antecrystic zircon population
(80–200 ka) is found in most deposits. The zircon age distributions reflect discrete periods of
crystallization punctuated by hiatuses of tens of thousands of years, as opposed to continuous
or steady zircon crystallization, based on comparison with synthetic data sets and results
from mixing models. Iron-titanium (Fe-Ti) oxide thermometry suggests that the magmas were
significantly zircon undersaturated upon eruption and therefore a mafic rejuvenation event
driving the eruption was short lived (<2500 yr). The discontinuous nature of the zircon age
distribution in the ignimbrites contrasts with the more continuous zircon age distributions
in lava domes on Dominica, suggesting that explosive eruptions could be a consequence of
subsolidus storage conditions and intermittent rheologic lock-up.