Dominica Pumiceous Eruptions: Reconstruction of Dynamics and Timescales of Magma Chamber Processes from Crystal Record and Diffusion Modelling.

Abstract:

In the last 50ky, Dominica Island (Lesser Antilles Arc) has experienced three major pumiceous ignimbritic eruptions: Layou (~50ka), Roseau (~30ka), and Rosalie (~20ka). These eruptions emitted magma volumes one order of magnitude larger (tens of km³ DRE/eruption) than those of the neighboring islands of Martinique and Guadeloupe (<1 km³/eruption). Reservoir processes and pre-eruptive magma dynamics of these eruptions are still poorly constrained.

Our study focuses on the basal Plinian fallout deposit of these three eruptions. We studied the crystal assemblage and performed a detailed analysis of chemical zoning patterns, textures and composition variations on orthopyroxene crystals (12-16% total crystal content). A system analysis approach is conducted to unravel crystals’ remobilization processes in the reservoir before eruption. Timescales of these processes have been calculated performing Fe²⁺-Mg interdiffusion modelling on selected orthopyroxenes by intercalibration of high resolution BSE images with EPMA analyses.

Results suggest the existence of a main magmatic environment, containing approximately 80-85% of the total orthopyroxene amount. The remnant 15-20% orthopyroxenes exhibit clear zoning with normal, reverse and double zoning for Layou and Rosalie eruptions and almost reverse and double zoning for Roseau. They display cyclic interaction with smaller magmatic environments of an either more or less evolved composition. Mobilization of crystals in between these different environments causes the simple or double normal and reverse zoning in crystals.

In order to constrain timescales of reservoir processes, diffusion has been modelled along the a- and b-axis on zoned orthopyroxenes of Layou and Roseau eruptions at 850°C. For both eruptions, dating of diffusion kinetics on crystal zoning provides comparable timescale distributions of ~3-4 years before each eruption. This suggests magma remobilization of about 3 years prior each eruption, likely acting as a final eruption trigger.