PhD defence by Geoffrey Kiptoo Mibei

20 October 2021
Geoffrey Kiptoo Mibei is a geologist at the Geothermal Development Company (GDC) since 2010. He was …
Geoffrey Kiptoo Mibei is a geologist at the Geothermal Development Company (GDC) since 2010. He was born within Nakuru County, Kenya. He obtained a BSc in Geology in 2008 and MSc in Geochemistry in 2012 from the University of Nairobi. In 2012, he received a Six Months Fellowship at the United Nations University Geothermal Training Programme (UNU-GTP) in Iceland. In September 2017, he received a UNU-GTP Fellowship to do his PhD in Geology at the University of Iceland.

On Thursday the 28th of October from 15:00-18:00, Geoffrey Kiptoo Mibei will defend his PhD thesis in Geology from the University of Iceland in the Main building, room The Aula and on live stream at

The thesis is titled:
The magmatic evolution, eruptive history and geothermal reservoir assessment of the Paka volcanic complex, Northern Kenya rift

Opponents are:
Karen Fontjin, Associate Professor at Ghent University, Belgium
Agnes Reyes, Petrologist/fluid geochemist at GNS-Science, New Zealand (and first GTP Fellow in 1979)

Administrative Supervisor:
Halldór Geirsson, Associate Professor, Faculty of Earth Sciences at the University of Iceland

Academic supervisors are:
Eniko Bali, Associate Professor at the Faculty of Earth Sciences, at the University of Iceland.
Björn S. Harðarson, Geologist, Iceland Geosurvey
Hjalti Franzson, Geologist, Iceland Geosurvey

Other members of the Doctoral committee:
Guðmundur H. Guðfinnsson, Research Scientist at Institute of Earth Sciences, University of Iceland
The ceremony will be chaired by Freysteinn Sigmundsson, Head of Faculty of Earth Sciences.

Everyone is welcome to attend


In this study, the magmatic evolution, and the eruptive history of the Paka volcanic complex are investigated and its geothermal reservoir is assessed. The study comprises two parts. The first part deals with the eruptive history, magma evolution, and magma reservoir storage conditions. The second part focuses on an assessment of the geothermal reservoir. I propose a revised stratigraphic framework for Paka volcanism where five eruptive sequences have been identified, spanning ~582 to 8 ka. The Paka edifice growth comprises four main eruptive sequences commencing at 390 ka with the main caldera collapse occurring at ~36 ka. The volcanic products in the area include alkaline basalt-trachyte lava and tephra deposits. A minimum bulk volume for the erupted products between 390 to 8 ka is estimated to be ~50 km3. This translates to a magma supply rate of 1.2 x10-4 km3/yr, this is associated with a heat flux between 110-138 mW/m2. Trace element numerical modelling using La and Y indicates that Paka primary magma was generated by partial melting of garnet peridotite mantle (60-80 km depth) by 5-10% partial melting. The magma differentiation processes invoked encompass 70% fractionation crystallization and 10-20% assimilation of syenite in the mid to shallow crustal depths to generate the Paka trachyte. Geothermobarometric models indicate magma reservoirs of basaltic and intermediate composition located diversely at depths of 5-20 km and 15-20 km respectively. A shallow trachyte magma reservoir is estimated to occur at 3.7-5 km depth. Based on an evaluation of the temporarily diverse magma series, the established basalt and trachyte magma reservoir depths within the deep to mid and shallow crustal zones are long-lived and have not changed significantly since ~580 ka. The estimated storage temperature for the basalt and intermediate(s) magma is between 1032-1206 °C, whereas for the trachyte it ranges between 900-938°C under relatively oxidized conditions (∆FMQ +1). The underlying geothermal reservoir within the Paka complex is manifested on the surface by fumaroles, altered surface rocks, and fossil hot springs. Detailed analysis of PK-01 borehole data indicates that the geothermal system is characterized by three hydrothermal alteration zones, namely smectite -mordenite, chlorite-illite, and epidote-actinolite. Evidence suggests notable temperature fluctuations within the deeper levels of the hydrothermal system over time, perhaps associated with faults channelling cold water deep into the system. The reservoir system is liquid-dominated with a boiling zone at ~900 m depth. It is characterized by Na-bicarbonate fluids and has been such over the lifetime of the system.