Abstract

To understand the dynamics of magmatic systems, one must first seek to characterize the time-dependent behavior of magma storage and ascent. Herein, we do this through a combination of the Crystal System Approach and careful study of Fe-Mg interdiffusion in orthopyroxene. This allows us to trace the pre-eruptive dynamics of magma plumbing systems, both in space and time. We apply this novel approach on two large silicic eruptions (about 3–5 km3 DRE/eruption) that occurred in the central part of Dominica Island (Lesser Antilles Arc): the eruptions of Layou (∼51 ka) from Morne Diablotins, and Roseau (∼33 ka) from Morne Trois Pitons-Micotrin. For the Roseau eruption, two magmatic environments (MEs) are identified on the basis of orthopyroxene composition, with a dominant reverse-zoning pattern from 50 to 54 to 54–59 mol% enstatite (En), indicating interaction with hotter magma. For the Layou eruption, three MEs are observed as represented by three populations of pyroxenes: En47-51, En51-53 and En53-58. The normal-zoning pathway from En51-53 to En47-51 is significantly registered by crystals, interpreted as convective mixing in a zoned reservoir. The reverse-zoning pathway from En47-51 to En51-53 and also En53-58 is also significantly present, supporting the mixing within the zoned reservoir but also suggesting mixing with a hotter magma, possibly stored in another part of a sub-volcanic mush. The crystal and glass compositions (melt inclusion and matrix glass) from both studied eruptions suggest heating and mixing between different magma pockets located within the mush that were the dominant process for mobilizing eruptible magma. In parallel, we constrain the associated pre-eruptive timescales by modeling the diffusive relaxation of Fe-Mg chemical gradients that originated within the zonation of the same orthopyroxene crystals. Diffusion modeling was considered along the b-axis of 66 zoned orthopyroxene crystals for these two eruptions, at a magmatic temperature of 850 ± 25°C. In light of these results, we propose that the Layou and the Roseau magma reservoirs were rejuvenated and heated by ∼25–50°C about 10 years prior to eruption by the injection of an underplating, hotter magma, creating the observed dominant reverse-zoning patterns of the erupted orthopyroxenes. We thus have evidence that silicic mush can be re-mobilized over timescales of decades prior to eruption, as previously suggested for Santorini and Taupo volcanoes.

Highlights

  • How magma is stored at depth and how magma bodies assemble prior to eruption are two questions that are still widely debated

  • We present an original study using orthopyroxenes that combine a variant of the Crystal System Analysis method and intracrystalline diffusion modeling

  • A total of 1,452 opx were analyzed for the two eruptions (701 for Roseau, 751 for Layou), including their textural features and chemical compositions (Figure 3; Table 1; Supplementary Tables S2, S3)

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Summary

Introduction

How magma is stored at depth and how magma bodies assemble prior to eruption are two questions that are still widely debated. In the process of sequential accretion of sills by which large crustal magma reservoirs form (Annen, 2009; Bachmann and Huber, 2016; Blundy and Annen, 2016), the resident magma can cool and partially crystallize in between successive magma pulses to become a crystal mush (Marsh, 2006). Crystallinity stays in the 50–60% range and the solidus is not reached quickly because of the release of latent heat during crystallization (Marsh, 1989). This type of highly-crystallized crustal reservoir raises questions regarding magma mobility and the timescale over which magma can be remobilized through the rejuvenation of the mush to become eruptible melt. Estimated timescales suggest that the reactivation timescale is much longer than the following homogenization timescale (Huber et al, 2012)

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