A window into magmatic systems: mineralogical constraints on the nature and timing of magma interactions and ascent

Investigations into the time evolution of sub-volcanic magmatic systems are invaluable in order to improve our understanding of volcanic behaviours and associated hazards. The magmatic pathway from source to surface is usually complex and may involve many different processes, such as fractional crystallisation, magma mingling/mixing, and assimilation of wall rock in contact with the magma. These processes are recorded in the erupted deposits and can be found both at macroscopic (e.g., inclusions in the host rock, banded pumice, and xenoliths) and microscopic scales (e.g., rock-forming mineral textures and compositional zoning patterns). 

This thesis focused on mineral-scale studies of volcanic rocks in order to decipher the magmatic histories of volcanoes located in different tectonic settings: Soufriere Hills Volcano at Montserrat Island (Lesser Antilles; subduction zone), Gede Volcano in West Java (Indonesia; subduction zone), and Waitomokia Volcano (New Zealand; intraplate zone).

Soufrière Hills Volcano has been studied through mineralogical textures and chemical compositions of plagioclase in its volcanic rocks. Diffusion modelling of magnesium in these plagioclase reveals timescales for magmatic interactions (basaltic andesite intrusion into the andesitic host) ranging from a few hours up to two months prior the eruption, inferred to be related to syn-eruptive processes.

Gede Volcano produced rocks with plagioclase phenocrysts originally crystallised from the host andesite. These plagioclase show resorption areas from 20 to 400 microns in thickness, which imply timescales ranging from 1 to 10 days between the intrusion of new magma in the host and the eruption.

Waitomokia Volcano has been characterised through olivine core compositions and zoning patterns within juvenile material produced during progressive eruption stages (from initial phreatomagmatic to final magmatic phase). It has been found that magma ascent from the mantle source to the surface is rapid, and yet that magma mixing has an important role in this small-volume plumbing system.

In this work, the recognition of magmatic interactions via mixing and mingling processes within all the volcanic systems investigated shows complexity either in small- and large-scale volcanic systems. The findings from this work about the timescales of storage,  transport and evolution of magma have important implications in the understanding of eruption triggers. The results for these three volcanoes are particularly relevant due to their location nearby populous cities, and therefore this work provides powerful results to support the interpretation of the monitoring signals, fundamental in case of future unrest situation.