Chemical Composition of Fumarolic Gases at the Menengai Geothermal Field- Central Rift Kenya: Variations Monitored Over a Period of Sampling from the Year 2009 to June 2013

Abstract

Physico-chemical characteristics of the fluids circulating in a volcanic system can display significant temporal variations, in response to changes caused by different inputs of; condensation attributed to atmospheric contamination, boiling, re-equilibration in response to cooling and reactions between, juvenile (magmatic) and more surficial (hydrothermal and meteoric) components during the fluid ascend to the surface. This paper seeks to outline the temporal variation of the gas concentrations (CO2, H2S, H2, CH4, O2 and N2) detected in Fumaroles MF-2, MF-3, MF-7 and MF-9 for a period of sampling from the year 2009 to June, 2013. The sampling temperature of the different fumaroles range from 56ºC to 90 ºC depending on the fumarole discharge strength. The Menengai fumarolic gas flow is greatly propelled to the surface along well-formed conduits of faults and fissures that characterize the superposed lava. This invariably gives rise to the atmospheric contamination of the gases over the different phases of sampling and ultimately affecting the chemistry of the gas species through condensation and/or boiling at the time of sampling. The varying relative concentration of CO2, H2S and N2 in the subsequent sampling phases could be primarily due to condensation caused by atmospheric contamination. The predominant surficial atmospheric contamination makes it practically difficult to constrain the temporal variation in response to any physico-chemical processes happening in the reservoir that can be discernible in the surface thermal expressions. The average computed equilibrium reservoir temperatures are of the order of magnitude of 270ºC and above 300ºC with respect to H2S and CO2 geothermometer respectively. The temperatures calculated from the same functions in the subsequent phases of sampling give markedly lower temperatures with an exception of CO2 geothermometer which was in the magnitude of more than 350ºC.