Timescales of magma residence and transport underneath Iceland

• Main Author: Mutch, Euan James Forsyth
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Robinson College 2019
• Subjects: 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; Skuggafjöll; Bárðarbunga; Grímsvötn; 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
• Online Access: https://doi.org/10.17863/CAM.41326; https://www.repository.cam.ac.uk/handle/1810/294228

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 ...