Chemical differentiation of subduction zone magmas

Thesis

Arc volcanoes are the product of magmatism at subduction zones. Identifying the origin and evolution of magmas erupted at arc volcanoes is key to understanding where melts are generated, and accumulate, in the underlying crust, and key processes at play in the crustal region. Many volcanoes erupt a limited compositional range of magmas over their lifetime, proposed to reflect the mineralogy of the crystal-rich mush in the source region. La Soufrière volcano, St. Vincent, in the Lesser Antilles is an example of such volcano, erupting predominantly basaltic andesite magmas for hundreds of thousands of years.

Major and trace element geochemistry for lavas and dykes collected from the La Soufrière crater, and western flanks, find magmas to be compositionally homogenous basaltic andesites with rare basalts and andesites. New 40Ar/39Ar dates reveal an unconformity in the crater and find magmatism on the western flanks to be very young (≤ 4.4 ± 4.2 ka). Erupted basaltic andesite magmas contain a wide range of crystal cargo, and primitive relict mineral cores support differentiation from a high-magnesium basaltic parent magma. High A plagioclase (≥ An85) is abundant in all samples and indicates a hydrous and oxidising source region.

Near-liquidus multiple saturation experiments find erupted basaltic andesite magmas to originate in the lower crust at 0.6 ± 0.1 GPa, 1020 ± 10ºC, at NNO+2 and with ~ 8 wt. % H2O in the melt, last equilibrating with an amphibole-gabbro source assemblage. Mineral compositions match erupted crustal xenolith data, and source pressure and temperature estimates agree with geophysical observations and calculated magmatic temperatures for the 2020-2021 eruption at La Soufrière. Oxidation-exsolution textures observed in the 2020-2021 lava dome, but absent from the explosive scoria, are reproduced at ≤ 900ºC and in the presence of air, indicating formation post- or syn-emplacement.

Authors: Morrison-Evans, BE

• DOI: 10.5287/ora-amopoeko8
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English