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E436: Crystal-melt segregation and the generation of large crystal-poor silicic eruptions (Lead Supervisor: Marian Holness, Earth Sciences)

Supervisors: Marian Holness (Earth Sciences), Mike Stock (Earth Sciences) and Rex Taylor (Ocean and Earth Science, University of Southampton)

Importance of the area of research:

Silicic magma systems produce the largest and most hazardous volcanic eruptions on Earth. These involve the expulsion of hundreds of cubic km of magma from crustal sub-volcanic storage regions in which magma has undergone extensive cooling and crystallisation. But here lies a great paradox of igneous petrology: the extent of crystallisation required to produce such silica-rich melts creates low permeability crystal-rich mushes containing highly viscous evolved liquid, yet large volumes of essentially crystal-free magma can be extracted from these storage regions. Understanding the processes that drive crystal-melt segregation and the mechanisms of melt migration in these mush systems is therefore critical to understanding the formation of large crystal-poor silicic magma bodies. This would represent a major contribution to our knowledge of sub-volcanic processes, which would ultimately improve our ability to monitor active volcanoes.

Project summary:

One of the major challenges in understanding how magmas migrate in sub-volcanic systems is our inability to make direct observations. Most of our understanding to date is derived from indirect geochemical evidence and numerical models. However, in rare cases, magmatic mush can be exhumed as cumulate nodules during eruptions. These preserve a unique record of the processes operating in the sub-volcanic systems. One such suite of cumulate nodules has been identified in the pyroclastic deposits of Tenerife (Canary Islands). This project will undertake a detailed petrographic and geochemical study of these nodules, to investigate the mechanisms of crystal-melt segregation and melt migration through crustal mush systems.

What the student will do:

The student will undertake a petrologic investigation of exhumed cumulate mush nodules from Tenerife. This will involve making detailed microstructural observations, to characterise melt distribution and migration pathways. State-of-the-art scanning electron microscopy techniques (CL, EBSD) will be used to assess grain-scale deformation and to quantify petrographic observations. This work will be supplemented by high-resolution major and trace element geochemical microanalysis of minerals and glasses (SEM, EPMA, LA-ICP-MS), to investigate spatial and temporal compositional variability in the mush system. Fieldwork in the Canary Islands will be undertaken in year 2, to supplement the existing sample set. The data will be used to make the first direct geological observations of melt migration in sub-volcanic cumulate mushes, which will be compared with existing models of crystal-melt segregation.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Sliwinski, J.T., Bachmann, O., Ellis, B.S., Davila-Harris, P., Nelson, B.K. & Dufek, J. 2015 Eruption of shallow crystal cumulates during explosive phonolitic eruptions on Tenerife, Canary Islands. Journal of Petrology, vol. 56, pp. 2173-2194.

Bachmann, O. & Bergantz, G. 2008 The magma reservoirs that feed supereruptions. Elements, vol. 4, pp.17-21.

Holness, M.B., Anderson, A.T., Martin, V.M., Maclennan, J., Passmore, E. & Schwindinger, K. 2007 Textures in partially solidified crystalline nodules: a window into the pore structure of slowly cooled mafic intrusion. Journal of Petrology, vol. 48, pp. 12

Follow this link to find out about applying for this project.

Other projects available from the Lead Supervisor can be viewed here.

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