Dynamics and timescales of mafic\u2013silicic magma interactions at Soufri\xe8re Hills Volcano, Montserrat

Rosa Didonna,Jenni Barclay,Simon Turner,Heather Handley,Fidel Costa

doi:10.1007/s00410-022-01891-z

Abstract

Mafic magma intrusions into silica-rich magmatic reservoirs are commonly proposed to drive the eruptions of andesitic hybrid magmas that characterise many arc volcanoes. However, interactions between contrasting magmas involve large gradients of physical and chemical properties that change over time, and the details of such processes have proven difficult to constrain. In this paper we investigate the dynamics of magma mingling and mixing using chemical and textural zoning patterns recorded in plagioclase crystals from the February 2010 eruption at Soufrière Hills Volcano, Montserrat. This eruption is considered a classic example of interactions between the andesite magma that constitutes the bulk of the erupted volume and the basaltic andesite that occurs as enclaves. We find that plagioclase crystals are characterised by two well-defined zones that record mafic–silicic magma interaction: a crystal interior, often identified by a patchy, dusty and oscillatory zoning, and an overgrowth zone (rim) of a different composition. We use the anorthite and Mg contents to track the thermal and compositional changes experienced by the crystals over time. Our results reveal that the crystal rims formed a few hours to days prior to eruption, during co-eruptive magmatic interactions. The interaction between the two magmas with contrasting rheology is likely increased by a narrowing conduit geometry towards the surface, which facilitates convection and additional interface contact of the two magmas. Our findings shed new light on the nature and timing of magmatic interactions driving the final eruptive phase at Soufrière Hills Volcano and help to propose an interpretative framework of the monitoring signals.

Highlights

The study of volcanic rocks and their relative composition is a window into the wide range of subvolcanic processes involved in their formation, including mixing of primitiveCommunicated by Timothy L
In this work we offer a novel insight into the final, Phase V, 2010 eruption of Soufriere Hills volcano (SHV) through diffusion modelling of Mg zoning patterns in plagioclase crystals
Despite the presence of enclaves in erupted deposits of each phase of the 1995-2010 SHV eruption (Phases I–V), this work supports a scenario during the final years of the eruption, the intrusion of new magma in the magma chamber ceased (e.g., Plail et al 2018; McGee et al 2019)

Summary

Introduction

The study of volcanic rocks and their relative composition is a window into the wide range of subvolcanic processes involved in their formation, including mixing of primitiveCommunicated by Timothy L. Many eruptions have been proposed to have been triggered by magma replenishment shortly before eruption (Bacon and Barnes 1978; Clynne 1999; Edmonds and Woods 2018; Eichelberger 1980; Oldenburg et al 1989; Scaillet et al 2000; Sparks et al 1977; Trial and Spera 1990), with replenishment times that can vary from days to weeks (e.g., Devine et al 2003; Kahl et al 2013; Kent et al 2010; Scarlato et al 2021) In this scenario, longer pre-eruptive timescales (over years) may occur due to the prolonged hybridisation and/ or mixing processes of intruded magma that consecutively interacted with the pre-existing magma reservoir, as has been proposed for Merapi (Costa et al 2013b) and Rabaul volcanoes (Fabbro et al 2020). Between Phase III and Phases IV and V, the duration of each phase decreased from years to months, while the average extrusion rate increased (Christopher et al 2014; Wadge et al 2014)

Objectives

Results

Discussion

Conclusion