Geophysical and Geochemical Characterization of Menengai Geothermal System, Kenya

Abstract

Menengai volcano is one of the late Quaternary caldera volcanoes formed on a massive shield in the inner-trough of the Kenya rift valley associated with a high thermal gradient as a result of shallow magmatic intrusions. Menengai geothermal area lies within the central part of this rift section, which is characterized by an intricate geological set up associated with regional uplift referred to as the Kenya dome. In this study, the geothermal characteristics of the field were evaluated by using chemical and isotope data as well as 3-D inversions of both magnetotelluric (MT) and gravity data. The purpose was to image the subsurface geothermal structure with a view of confirming and delineating the extent of the geothermal resource. Integration of gravity data and resistivity data show a good relationship where regions with relatively low densities corresponded well with high resistivity zones. The high-density body of Olrongai (i.e., more than 2500 kg m−3) appears to mantle the resistive part (i.e., more than 90 Ω.m) while the resistive body inside Menengai caldera seems to surround a high-density structure. Anomalously high 3He/4He isotope from geothermal wells lies atop the dense body. The results reveal a possible pocket of a molten body that was emplaced as a dike at a shallow level, connected to a deeper magma chamber.