The volatile history of past volcanic eruptions

Thesis

Volatile elements play an important role in almost every aspect of sub-volcanic systems, from the generation and storage of magma, to the timing and style of volcanic activity. Currently, the most common method for assessing pre-eruptive magmatic volatile contents is through analysis of trapped melt inclusions. However, the reliability of this record is uncertain, necessitating development of new, independent petrologic methods for determining the pre-eruptive volatile contents of past eruptions. This thesis combines physical and chemical models with empirical analyses to develop the use of apatite as a magmatic volatile 'probe'.

The first research chapter investigates well-documented difficulties in electron microprobe analysis of apatite volatile concentrations. These are found to be caused by electron-beam induced heating and electric field generation. In determining these effects, it is possible to identify optimal operating procedures for apatite analysis. The next chapter explores the theoretical evolution of apatite volatile compositions as a function of magmatic evolution, building on previous work to develop thermodynamic models that relate crystal compositions to fluid systematics during fractional crystallisation. These provide a qualitative framework for interpreting apatite compositions in natural volcanic systems.

The remainder of the thesis is dedicated to identifying new insights that can be gained from the use of apatite as a magmatic volatile 'probe'; this method is applied to constrain pre-eruptive processes at Campi Flegrei, Italy. Texturally-constrained apatite analyses are used to create a time-series of magmatic volatile evolution in the build-up to eruption. This reveals that volatile saturation occurred late in magmatic evolution, and represents a potential eruption trigger. Apatites from different eruptions show a long-term temporal variability in the H2O contents of primitive melts feeding Campi Flegrei, which correlates with different epochs of activity. Melt inclusions from all eruptions have reequilibrated post-entrapment. This study demonstrates the potential utility of apatite for investigating pre-eruptive volatile behaviour in apatite-saturated magmas.

Authors: Stock, M

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Petrology; Mineralogy; Geochemistry; Volcanology
• Subjects: Earth sciences; Geology