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Explosive basaltic volcanism in the Ethiopian rift

Dr David Ferguson (SEE), Dr Dan Morgan (SEE), Prof Gezahegn Yirgu (Addis Ababa), Dr Marie Edmonds (Cambridge)

Contact email: d.j.ferguson@leeds.ac.uk

The Ethiopian rift valley is one of the most volcanically active regions on Earth and provides an exceptional natural laboratory to study the interplay between magmatism and rift zone tectonics. Explosive mafic volcanism involves magmas that rapidly transit through the lithosphere, preserving geochemical signatures from the sub-rift mantle. The alignment of basaltic volcanic cones along faults at the surface of the Ethiopian rift implies a close relationship between magma ascent and lithospheric tectonics (e.g. Rooney et al., 2011), which may include a feedback between tectonic extension and weakening of lithosphere via magma intrusion. This project will combine field sampling of basaltic volcanoes in Ethiopia with state-of-the-art geochemical analysis to investigate the processes associated with the generation and eruption of these magmas, and to evaluate the interplay between volcanism and rift tectonics. It will take a source-to-surface approach, seeking to understand the initial generation of magma in the mantle, melt ascent through the lithosphere, and finally, eruption at the surface.

Aims of the project

An important aspect of this work will be constraining the magmatic volatile contents through a detailed investigation of olivine hosted melt inclusions. Variations in volatile components, such as H2O and CO2, strongly affects the process of magma generation and knowledge of these is essential for refining models of magma generation in the Ethiopian rift. By examining how the volatile, major and trace element compositions of magmas vary in different parts of the rift the researcher will be able to identify how magmatic processes may be influenced by changes in local tectonic setting. In addition to looking at variations in melt generation, this project will also examine the dynamics of pre-eruptive melt storage. This will be achieved by investigating olivine crystal ages via Fe-Mg diffusion chronometry. This data will provide quantitative constraints on the timescales associated with pre-eruptive magma storage and melt replenishment and be used to evaluate the extent to which this varies between the rift margins and axis. Finally, this results from the project will be combined with other relevant datasets, such as those from on-going geophysical investigations in the Ethiopian rift, to produce a synthesis of basaltic volcanism in this region.

The student will receive experience and training in a variety of field, laboratory and data analysis skills relevant to the study of volcanic rocks and magmatic processes. This work will benefit from engagement with a large community of multi-disciplinary scientists in the UK and elsewhere who are actively conducting research on magmatism and rifting in Ethiopia, including the on-going NERC funded RiftVolc project.

Objectives

You will undertake fieldwork in Ethiopia to sample scoria deposits from several groups of monogenetic volcanoes within the Main Ethiopian Rift. These will primarily target regions where previous studies have indicated that variations in magmatic processes exist due to differences in the local tectonic setting. Olivine crystals separated from the scoria will be assessed to identify those containing melt inclusions suitable for geochemical analysis and those with intra-crystal Fe-Mg zonation suitable for diffusion modelling. The analytical/work programme will involve:

  1. Analysis of melt inclusions by secondary ion mass spectrometry (SIMS) to measure concentrations of trace and volatile elements in the melt inclusion glass (most likely using the NERC Ion Micro-Probe facility in Edinburgh). 
  2. Raman spectroscopy of melt inclusion bubbles to accurately quantify CO2 contents (e.g. Moore et al., 2015). 
  3. Electron microprobe (EMPA) and electron back-scatter diffraction (EBSD) imaging and analysis of olivine crystals to quantify intra-crystal Fe-Mg variations.  
  4. Diffusion modelling of the Fe-Mg data in the olivine crystals to extract quantitative time-scale information for magma residence in the lithosphere. 
  5. Develop models for magma generation, ascent and storage at the study sites. This will include new constraints on the flux of volatiles associated with basaltic magmatism in the MER and can be integrated with other on-going geophysical and geochemical studies.

Figure 1: Basaltic cinder cones in the Ethiopian rift valley.     

Potential for high impact outcome

This project will increase understanding of volcanic processes in a region of extensive volcanic activity where the development of new infrastructure (cities, roads etc.) is occurring at a rapid pace. This includes data on the timescales over which magmas feeding explosive basaltic volcanoes reside in the crust prior to eruption. In scientific terms, it will provide a significant contribution to understanding magmatic processes in the MER, which is a natural laboratory for studies of continental rifting. It will provide robust constraints on the volatile budget of MER magmas, a vital parameter for understanding asthenosphere dynamics and to quantify volatile fluxes from the rift zone. The results will have important implications for various on-going interdisciplinary studies on continental rifting and have the potential to generate high-impact publications.

Training

The student will work under the supervision of Dr. David Ferguson and Dr. Dan Morgan in Leeds and Dr. Marie Edmonds in Cambridge. They will also interact with an international co-supervisor from Ethiopia (Prof. Gezahegn Yirgu, University of Addis Ababa). They will gain high-level experience and expertise in: i) field sampling in volcanic terrains; ii) preparation of samples for geochemical analysis; iii) data collection using a variety of analytical instrumentation; and iv) diffusion modelling to extract quantitative timescale information.

Within SEE they will have the opportunity to engage with researchers in the volcanology, high-T geochemistry and tectonics/geodynamics research groups, many of whom will have overlapping interests with aspects of this work. They will also benefit from membership of NERC’s Centre for the Observation and Modelling of Earthquakes and Tectonics (COMET), a collaboration between several UK universities, and from interaction with the NERC funded RiftVolc project, which is focused on volcanism in the MER and also involves researchers at several UK institutions. In addition, the student will also have access to a broad range of Faculty- and University-led training courses and workshops at Leeds (http://www.emeskillstraining.leeds.ac.uk/).

References

Rooney, T. O., Bastow, I. D., & Keir, D. (2011). Insights into extensional processes during magma assisted rifting: Evidence from aligned scoria cones. Journal of Volcanology and Geothermal Research, 201(1), 83-96.

Related undergraduate subjects:

  • Chemistry
  • Earth science
  • Environmental science
  • Geochemistry
  • Geology
  • Geoscience
  • Natural sciences