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Title: Timescales of magma residence and transport underneath Iceland
Author: Mutch, Euan James Forsyth
ISNI:       0000 0004 7968 4918
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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The focus of this thesis is the development and application of a new method of diffusion chronometry known as DFENS (Diffusion chronometry using Finite Elements and Nested Sampling). This method combines a flexible finite element numerical model with a nested sampling Bayesian inversion to provide robust uncertainty estimates and account for observations from multiple elements within a single phase, or multiple phases. In this thesis, the DFENS methodology is applied to three eruptions from Iceland with different compositions and storage depths in order to characterise the timescales of pre-eruptive residence and transport: the Skuggafjöll eruption, Bárðarbunga volcanic system; the Borgahraun eruption, Theistareykir volcanic system; and the Laki eruption, Grímsvötn volcanic sytem. These findings are then synthesised within an Icelandic and global framework. The Skuggafjöll eruption contains zoned macrocrysts of olivine and plagioclase that record a shared magamatic history of growth from trace element depleted melts followed by sequestration in a crystal mush environment and then entrainment into a trace element enriched melt prior to eruption. Equilibrated plagioclase core Mg profiles suggest storage in a mush that was colder than the original primitive liquid, but hotter than the final carrier liquid. Olivine and plagioclase DFENS models indicate that mush disaggregation and mixing took place approximately 1 year before eruption. Petrological observations have shown the monognetic Borgarhraun eruption was fed by mixed primary magmas that ponded near the Moho (approximately 24 km depth). Diffusion chronometry conducted on olivine macrocrysts show that crystal entrainment into the carrier-liquid took place approximately 13 days before eruption with some crystals having residence times of less than 5 days. This is the first estimate of Moho to surface transport times anywhere on the global spreading ridge system and implies a rapid connection between the lower and upper crust with melt transport rates of 0.01 to 0.06 m s⁻¹. Spinel crystals contained in Borgarhraun wehrlite nodules are zoned in Cr and Al. The core composition and degree of disequilibrium observed in the spinel crystals within each nodule correlates with their size suggesting diffusion is responsible for the zoning. Diffusive exchange in these spinels is primarily driven by cooling and exchange with the interstitial mush liquid. 2D diffusion modelling of Cr-Al exchange in the spinels indicates that the crystal mush was stored for approximately 1500 years. This is the first direct estimate of the storage times of primary magmas in the lower crust and for active basaltic mushes in general. The Laki eruption contains several populations of plagioclase crystals within its crystal cargo. Those with oscillatory zoned mantles and high-anorthite cores are equilibrated in terms of their Mg content and indicate storage in a mush environment that was colder than the final carrier liquid. Large equilibrated crystals of 3 mm diameter indicate minimum storage times under these conditions of 2100 years. Crystals with simpler zoned mantles have Mg core disequilibria that record diffusion timescales of 60 to 150 years before eruption. It is likely that the Laki crystal cargo was incrementally assembled over a protracted period of time. Diffusion timescales of final entrainment for olivine and plagioclase macrocrysts from different eruptive fissures are typically less than 20 days, which is shorter than the average interval between eruptive episodes, and supports the concept of pulsed mush disaggregation throughout the eruption. Combined, the observations and modelling results of this thesis suggest there is a dichotomy in the temporal evolution of basaltic systems. Long-term crystal storage in colder mush environments on the order of hundreds to thousands of years throughout the crust are interspersed with periods of rapid magma transport (days to weeks).
Supervisor: John, Maclennan ; Edmonds, Marie Sponsor: Natural Environment Research Council (NERC) ; University of Cambridge
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Iceland ; Magma ; Volcanology ; Petrology ; Geology ; Earth Sciences ; Olivine ; Plagioclase ; Spinel ; DFENS ; Timescales ; Basalt ; FEniCS ; MulitNest ; Crust ; Mantle ; Modelling ; SIMS ; EPMA ; Finite element ; Bayesian inversion ; Laki ; Borgarhraun ; Skuggafjo¨ll ; Ba´rðarbunga ; Gri´msvo¨tn ; Geothermobarometry ; Mineral ; Picrite ; Theistareykir ; Tephra ; Lava ; Mush ; Zoning ; Eruption ; Disequilibrium ; Crystal clocks ; EBSD ; BSE ; EDS ; QEMSCAN ; Tholeiite ; Volcano ; Magma residence ; Magma ascent ; Magma storage ; Geospeedometry ; Igneous ; Diffusion chronometry ; Mafic