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Magmatic volatile contents and storage time-scales for rift-related volcanism in Ethiopia

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

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

This project will combine field-work at several volcanic centres in Ethiopia with state-of-the-art geochemical analysis to better understand the generation, ascent and eruption of basaltic magmas in the East African rift. Continental rifting in Ethiopia is accompanied by extensive magmatic and volcanic activity, fuelled by melts generated in the sub-rift mantle. In addition to feeding numerous volcanoes along the rift system, these magmas also play an important role in facilitating tectonic extension of the lithosphere (e.g. Buck, 2006). In the Main Ethiopian Rift (MER), where rifting is relatively advanced, there is a clear spatial link between the ascent and eruption of melts and lithospheric tectonics, with volcanism and magma intrusion focussed along discrete zones of extensional faulting (Hayward and Ebinger, 1996). Magma production here reflects the interplay between asthenospheric dynamics (temperature, composition etc.) and decompression due to lithospheric thinning, with the relative importance of these factors remaining a matter of debate (e.g. Armitage et al., 2013). Following partial melting, these magmas travel upwards into the lithosphere where the conditions of pre-eruptive storage and spatial patterns of volcanism at the rift surface reflect the interaction of the ascending melts with lithospheric structures and stress fields (Rooney et al., 2007, 2011).

A major gap in the current understanding of magmatic activity in Ethiopia is the lack of constraints on the volatile contents of the magmas and their mantle source. Since the presence of volatile components, such as H2O and CO2, strongly affects the melting behaviour of the mantle, constraints on magmatic volatiles is essential to refining models of magma generation (e.g. Ferguson et al., 2013) and ultimately for modelling the chemical and physical structure of the Ethiopian mantle (e.g. Rooney et al., 2012) and the cycling of volatiles from the mantle to the crust/atmosphere. Variations in the composition of magmas between different intra-rift zones of magmatic-tectonic extension have been previously linked to changes in the local tectonics, influencing the maturity of magmatic pathways and depths of crustal reservoirs (Rooney et al., 2007). However, thus far no detailed studies have been undertaken to quantify the time-scales and depths of magma storage beneath basaltic volcanoes in the MER and how this vary with local tectonic setting.

This project, through the development of exciting novel geochemical datasets for Ethiopian basalts, will provide important new insights into the production and ascent of magmas erupted at monogenetic cones in the MER. This will be done using the record of magmatic processes available from melt inclusions trapped within olivine crystals and also from analysis of the olivine phenocrysts themselves. The melt inclusion data will provide important information on the pre-eruptive volatile contents and compositional diversity of basaltic magmas, illuminating the range of primary melt compositions and allowing for an estimation of the concentrations of volatile elements in the mantle source and the volcanic flux of volatiles from the mantle to the atmosphere. This will be combined with studies of intra-crystal Fe-Mg diffusion to quantify residence timescales within crustal reservoirs (e.g., Hartley et al., 2016), constraining the pre-eruptive histories of magmas and testing whether these vary significantly in different part of the rift. Using these datasets, you will develop an integrated ‘source-to-surface’ model of basaltic magmatism in the MER. You 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 (e.g. Rooney et al., 2007). 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 fissure eruption in Ethiopia (Afar) in 2009. Scoria deposits formed around the vent during explosive lava-fountaining activity contain loose olivine crystals, some of which contain melt inclusions. This project will sample similar deposits from monogenetic cones in the MER (see Rooney et al., 2011).    

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 an often 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 international co-supervisors from the USA (Prof. Tyrone Rooney, Michigan State University) and 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

Armitage, J. J., Ferguson, D. J., Goes, S., Hammond, J. O., Calais, E., Rychert, C. A., & Harmon, N. (2015). Upper mantle temperature and the onset of extension and break-up in Afar, Africa. Earth and Planetary Science Letters, 418, 78-90.

Buck, W.R., 2006. The role of magma in the development of the Afro-Arabian Rift System. Geological Society, London, Special Publications, 259(1), pp.43-54.

Ferguson, D. J., Maclennan, J., Bastow, I. D., Pyle, D. M., Jones, S. M., Keir, D., ... & Yirgu, G. (2013). Melting during late-stage rifting in Afar is hot and deep. Nature, 499(7456), 70-73.

Hartley, M. E., Morgan, D. J., Maclennan, J., Edmonds, M., & Thordarson, T. (2016). Tracking timescales of short-term precursors to large basaltic fissure eruptions through Fe–Mg diffusion in olivine. Earth and Planetary Science Letters, 439, 58-70.

Hayward, N. J., & Ebinger, C. J. (1996). Variations in the along‐axis segmentation of the Afar Rift system. Tectonics, 15(2), 244-257. 

Moore, L. R., Gazel, E., Tuohy, R., Lloyd, A., Esposito, R., Steele-MacInnis, M., ... & Bodnar, R. J. (2015). Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets. American Mineralogist, 100(4), 806-823.

Rooney, T., Furman, T., Bastow, I., Ayalew, D., & Yirgu, G. (2007). Lithospheric modification during crustal extension in the Main Ethiopian Rift.Journal of Geophysical Research: Solid Earth, 112(B10).

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.

Rooney, T. O., Herzberg, C., & Bastow, I. D. (2012). Elevated mantle temperature beneath East Africa. Geology, 40(1), 27-30. 

Related undergraduate subjects:

  • Chemistry
  • Earth science
  • Environmental science
  • Physics