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Microanalytical techniques and experimental studies of the volatile and fO2 history of magmas using melt inclusions

Bristol student theses: Doctoral ThesisDoctor of Philosophy (PhD)

Authors

  • Ery Hughes

Research units

Abstract

Melt inclusions (MIs) are tiny melt pockets trapped inside crystals, which allow us to sample the magma prior to eruption and therefore infer magmatic processes that can help us to understand eruption dynamics and the role of volcanoes in the global volatile cycle. This thesis addresses gaps in our current use of MIs to enable us to measure the magmatic fO2 and volatile content. EPMA can estimate the H2O concentration of small MIs inaccessible to other techniques using ‘volatiles-by-difference’ (VBD), but a literature review reveals that VBD overestimates H2O by ~1 wt% for hydrous glass. Win X-ray modelling shows the effects of sub-surface charging can account for this. An accuracy of ±0.1 wt% on VBD can be achieved by calibrating using standards. The only technique currently available to measure Fe2+/FeT in MIs (an fO2 proxy) requires synchrotron access. Here, a technique using EPMA is presented, which is more accessible. On basaltic and peralkaline hydrous glass composition, the accuracy on Fe2+/FeT is ±0.1 at a spatial resolution of ~20–60 μm. The controls on electron beam induced redox changes during analysis are investigated. CO2 concentrations of MIs alone are not able to constrain the initial carbon concentration of magmas. Groundwork towards using δ13C of hydrous basaltic glasses are carried out, both technically using SIMS and experimentally. Three new standards have been characterised as no hydrous basaltic glass standards were previously available. The first geochemical data from the 23/11/13 Etna paroxysmal eruption was collected on MIs using these new (and other) techniques. An injection of deeper, more primitive magma mixed with a shallower, more evolved magma, which likely triggered the event. The melt composition is intermediate between the 2001–2003 highly explosive, flank eruptions and the 2004–2007 effusive eruptions. The volatile isotope data are consistent with other data available for Etna.

Details

Original languageEnglish
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Award date5 Feb 2019

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