Abstract

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.

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