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

Abstract

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 and 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 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% fractionational 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 (ΔFMQ +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.