Lead supervisor: Marian Holness, Earth Sciences
Co-supervisor: Michael Stock, Trinity College Dublin
Brief summary:
This project is aimed at constraining the nature of the plumbing system feeding volcanoes, using entrained crystal enclaves and plutonic xenoliths erupted by the volcanoes of the Galapagos archipelago, and xenoliths associated with the Skaergaard intrusion of East Greenland
Importance of the area of research concerned:
There has been much interest in the nature of the plumbing systems feeding volcanoes, driven by the absence of geophysical evidence for large bodies of essentially crystal-free magma, culminating in the recent proposal that all volcanoes are underlain by a complex, long-lived mush zone that extends through the full thickness of the crust. The physical behaviour of magma, and the explosivity of eruptions, is fundamentally controlled by the extent to which crystals and residual liquid can be separated (i.e. fractionation): the dynamics of mush systems, the size and longevity of any liquid-rich magma bodies, and the relative rates of solidification and replenishment are all determined by the structure of the crustal plumbing system. While much of the development of the concept of trans-crustal mush zones has concentrated on arc volcanoes above subduction zones, little has been done in those associated with mantle plumes. This project is focussed on examining two examples: the Galapagos archipelago, formed on oceanic crust; and the Palaeogene Skaergaard intrusion of East Greenland intruded into continental crust.
Project summary :
The Skaergaard intrusion is associated with the Palaeogene arrival of the Iceland plume at the base of the lithosphere during North Atlantic opening. Rapid emplacement was followed by solidification as a closed system. Both the walls and base of the intrusion, and a contemporaneous cross-cutting dyke, contain numerous plutonic xenoliths. Lavas from several of the islands of the currently active Galapagos Archipelago (e.g. Rabida) contain well-documented suites of crystalline enclaves which preserve a similar window into the deep magma plumbing system as those from Skaergaard. However, unlike Skaergaard, these plume-derived magmas are intruded directly into oceanic crust: comparing these two xenolith records provides an opportunity to assess the controls of crustal thickness and composition on magmatic architecture.
What will the student do?:
Geophysical observation of currently active volcanic terranes have insufficient resolution to determine much about the lower crust, but abundant information about deeper levels of magmatic plumbing systems is preserved in crystalline enclaves and xenoliths entrained by later magma. The student will undertake a detailed petrographic and geochemical examination of plutonic xenoliths both from the margins of the Skaergaard intrusion, from the contemporaneous Campsite dyke, and from Galapagos volcanoes. The extent to which crystallisation occurred in a static environment, as opposed to a dynamic convecting body of magma, will be determined using recent advances in microstructural interpretation. The timescales of solidification will be constrained using microstructural observations (e.g. dihedral angles, grain shape, grain boundary morphology) and diffusion chronometry of compositionally zoned grains. The depth of solidification will be constrained using standard geobarometric methods. The range of different magmatic histories recorded in the xenolith suites will be interrogated to place constraints on the depth, extent, longevity and dynamics of the magmatic plumbing system.
References - references should provide further reading about the project:
Holness, M.B., Stock, M.J. & Geist, D. 2019. Magma chambers versus mush zones: constraining the architecture of sub-volcanic plumbing systems from microstructural analysis of crystalline enclaves. Phil. Trans. R. Soc. vol. A377: 20180006. doi.org/10.1098/rsta.2018.0006
Holness, M.B., Richardson, C. & Andersen, J.C.O. 2013. The campsite dykes: a window into the early post-solidification history of the Skaergaard intrusion, East Greenland. Lithos, vol. 182-183, pp. 134-149
Applying
You can find out about applying for this project on the Department of Earth Sciences page.