Omenda et al 2019 - Sulfur Geochemisty of Igneous rocks from Central Kenya

Proceedings, 7th African Rift Geothermal Conference Kigali, Rwanda 31st October – 2nd November 2018 SULFUR GEOCHEMISTRY OF IGNEOUS ROCKS FROM THE CENTRAL KENYA PERALKALINE PROVINCE, EAST AFRICA RIFT Peter A Omenda1, Nicholas Mariita2, Joel Muhanga2, Anthony Marrufo3, Benjamin Brunner3, Elizabeth Y Anthony3 Scientific and Engineering Power Consultants, P.0 Box 38991, Nairobi, Kenya Institute of Geothermal Technology, Dedan Kimathi University of Technology, Nyeri, Kenya University of Texas at El Paso, Geological Sciences pomenda@sepco.co.ke Keywords Sulfur content, sulfur isotopes, obsidian, pyroclastic tuffs, ArcGIS data base, Central Kenya Peralkaline Province ABSTRACT We report here a study on sulfur geochemistry for volcanoes associated with geothermal plants in the Central Kenya Peralkaline Province northeast of Nairobi The present study is two pronged: first, to create an ArcGIS database from published and unpublished reports of sulfur geochemistry for magmatic and hydrothermal sources from this portion of Kenya (Clarke et al., 1990) The data base includes well water, fumaroles, and surface waters The second prong is to determine sulfur concentration and isotope composition for samples from Suswa, Eburru, Longonot, Menengai, and Olkaria The samples constitute the magmatic end-member in the magmatic-hydrothermal system Sulfur concentrations are typical for highly evolved magmas throughout the world They are as follows: Suswa phonolite obsidian (282 - 337 ppm), Eburru rhyolite obsidian (86-124 ppm), Longonot rhyolite fine-grained rock (164-186 ppm), Olkaria rhyolite obsidian (82 - 110 ppm), and Menengai trachyte (31 - 39 ppm) The sulfur isotope values are: Suswa phonolite obsidian (0.6 to 2.2 per mil), Eburru rhyolite obsidian (-0.6 to 2.6 per mil), Longonot rhyolite fine-grained rock (1.1 to 2.5 per mil), Olkaria rhyolite obsidian (7.7 to 9.5 per mil), and Menengai trachyte (4.0 to 17.3 per mil) The sulfur content is mostly explained by magmatic differentiation from a mafic, mantle-derived magma during which sulfur, as an incompatible element, increases in concentration The sulfur isotopic composition of Suswa, Longonot, and Eburru overlaps values from other rhyolites with similar processes (Liotta et al., 2012) The shift to higher ⸹34S for Olkaria and Menengai, may be the result of magmatic outgassing, as interpreted for a similar trend for samples from Krakatau (Mandeville et al., 1998) This explanation would require that the magma-fluid system experienced a high oxidation state Because oxidation of the magma chamber during closed-system crystallization exists at low oxidation (Ren et al., 2006), the outgassing hypothesis suggests that we may be seeing the beginning of the complex interactions between strictly magmatic fluids and hydrothermal/meteoric fluids This hypothesis, although intriguing, requires additional data for interpretation A principal focus of our ongoing research is to evaluate these complex magma-fluid processes and pathways Omenda et al Introduction We report here sulfur geochemical data for the geothermal areas of central Kenya (Fig 1) This area is called the Central Kenya Peralkaline Province (CKPP) because the felsic magmatism is characterized by phonolites and pantelleritic and comenditic rhyolites The volcanism is focused on five centers These volcanic centers include: (a) two dome fields: the Eburru Volcanic Complex composed of trachyte and pantellerite (Ren et al., 2006), and the Greater Olkaria Volcanic Complex (GOVC), dominated by comendites with minor outcrops of trachyte (Omenda, 1998; Macdonald et al., 2008); and (b) three trachytic caldera volcanoes: Menengai with a comenditic to pantelleritic trachyte composition (Macdonald et al., 2011), Longonot composed of pantelleritic trachyte and mixed (trachyte/ hawaiite) lavas (Rogers, 2004), and Suswa, with trachyte and phonolite lavas and tuffs (White et al., 2012) The mafic lava fields Elmenteita, Ndabibi, and Tandamara, lie in the rift floor adjacent to Eburru, Olkaria and Suswa, respectively, and are composed of alkali and transitional basalt, basaltic trachyandesite and trachyandesite Figure 1: Map of the Central Kenya Peralkaline Province From (Macdonald et al., 2011) The rationale for this study is to trace sulfur pathways from the magmatic source, through outgassing, and hydrothermal circulation We present here a limited, pilot data set that represents initial stages of our study A major goal of the larger study will be to see which faults establish a through-going path directly to the magma chamber and which are more closely related to shallow circulation and dilution of the magmatic fluids with meteoric water Current status of geothermal development in the Kenya Geothermal resources in Kenya have been under development since 1950’s with deep drilling commencing in 1973 All the high temperature prospects are closely associated with Quaternary volcanoes located in the axis of the rift and a feasibility study in 1976 indicated that development of the geothermal resource was feasible Olkaria geothermal field is so far the largest producing site with current installed capacity of 687.4 MWe from five power plants and wellheads against total potential of about 10,000 MWe estimated to exist within the Kenya’s part of the East African Rift Out of the total, the first power plant was commissioned in three phases totaling 45 MWe between 1981 and 1983 (Ouma, 2010) This was Omenda et al followed by phased development of Olkaria II (with current 105 MWe capacity) in 2003 and 2009, Olkaria III (with current 150 MWe capacity) in 2000-2018, Olkaria IV (with current 150 MWe capacity) in 2014 and a 150 MWe expansion of Olkaria I (units and 5) which came on line in 2014-2015 Olkaria I, II, IV and units and of Olkaria I are operated by KenGen while Olkaria III is operated by OrPower4, Inc KenGen, additionally about 83.3 MWe of well head power plants located in various sites at Olkaria I and Olkaria IV Construction is currently ongoing for 166 MWe Olkaria V plant and financial close has been reached for 140 MWe Olkaria VI plant while production drilling is proceeding for 140 MWe Olkaria VII Also under development is 100 MWe modular power plants at Olkaria Oserian Development Company has an installed capacity of 4.3 MWe for own use in the flower farm KenGen also operates a 2.4 MWe pilot power plant at Eburru field and has plans to expand the plant to 40MWe in the first phase of development Further north, about 100 km from Olkaria, Geothermal Development Company (GDC) started development of Menengai geothermal project located within Menengai caldera in 2011 and so far more than forty wells have been drilled with steam equivalent of 140 MWe on the wellhead Plans are underway to develop 105MWe through PPP arrangement with IPPs The coming years are going to see expansion of geothermal projects in Kenya with deep exploratory drilling planned at Longonot, Homa Hills, Suswa, Barrier, Paka and Korosi volcanoes Direct use of geothermal resources is gaining momentum in Kenya and currently 12 MWt is being utilized at Oserian greenhouses and at a geothermal spa using waste brine from Olkaria II field developed by KenGen Analytical Methods Approximately 30 mg of fresh, whole rock powders with grain size less than 120 mesh were encapsulated in tin capsules Approximately 15 mg of vanadium pentoxide, which serves as a catalyst during combustion, was included Standards included silver sulfide (IAEA S1 and S2) and internal standards Samples were combusted in a elemental analyzer (Pyrocube, Elementar) and analyzed on a isotope ration mass spectrometer (IRMS, VeoVisION, Elementar) Based on the error propagation and repeated measurements of standards, we estimate the error for sulfur content measurements to be ±20 ppm (2σ) For the sulfur isotope composition, we estimate an error of ±0.4‰ (2 σ) The reproducibility of the sulfur isotope values of the standards is better (±0.2‰, σ) but was also carried out on larger samples Therefore, we report the more conservative error estimate in Table The low sulfur concentrations for the Menengai sample results in a large error estimate of the sulfur isotope value for this volcano We are currently working on refining these measurements using the analytical technique of (Arnold et al., 2014) Table Sulfur analytical data Omenda et al Results 4.1 ArcGIS data base The GIS project consists of compiling on a georeferenced map geochemical data from wells, springs, and fumaroles from Clarke et al., 1990 We are targeting as the first priority sulfur data, pH, and temperatures from this report The second stage of the project will be to incorporate data from KenGen and GDC reports Insight from the complete data base will guide collecting additional samples for sulfur isotopic analysis We have completed georeferencing the map of the area and initial input of sulfate concentrations Contouring of the preliminary data set reinforce the results of the Clarke et al study that sulfur geochemistry is quite variable within the study region Our goal is to elucidate whether the variations are more closely correlated with the circular caldera structures or the north-south trending faults that cut through the area 4.2 Sulfur geochemistry The two end-members for waters in volcano-centered geothermal reservoirs are the magmatic chamber and meteoric groundwater (Fig 2) This report provides the first sulfur data for the magmatic end-member (Table 1, Figure 3) We discuss interpretation of measured sulfur content followed by isotopic signatures Finally, we place our data within a context of studies of Etna (Liotta et al., 2012) and Krakatau (Mandeville et al., 1998) Omenda et al Figure 2: Sources and pathways of geothermal fluids Modified from (Hutchison et al., 2016) The evolved peralkaline rocks from Suswa have been modeled as residues up to 90 to 96 per cent crystal fractionation of a Ndabibi-like hawaiite (White et al., 2012) The geochemical composition of Eburru and Longonot argue for a similar interpretation for these volcanic centers Sulfur is an incompatible element during prolonged crystal fractionation, which results in increasing sulfur content in the magma through time and thus accounts for its' high measured concentrations This magmatic behavior is also permissive evidence for the lower S content in the Menengai trachyte, which is less differentiated than the rhyolites and phonolites from the other volcanoes The isotopic composition of Eburru, Longonot, and Suswa samples is similar to measured values from evolved volcanic rocks at the Etna volcano, Italy (Liotta et al., 2012) and also accepted values for the mantle reservoir (Faure, 1986) This observation is consistent with the primitive basaltic parental magmas and the lack of crustal assimilation for the CKPP magma systems The Olkaria and Menengai values are distinct from the other volcanic centers and fall on a trend characterized by outgassing of a fluid from the magma accompanied by isotopic fractionation between the fluid and remaining magma (Faure, 1986) Outgassing was given as the preferred interpretation of a similar trend for samples from the famous 1883 Krakatau explosion (Mandeville et al., 1998) Our data set, at this point, is very limited, but it will be intriguing to see whether additional samples add to the trend observed in Figure We note also that the increase in delta 34S requires that the magma-fluid system is buffered at high oxidation states It is a well-established fact that, because of the mantle-derived parental magma, the magma is buffered at low oxidation states (Ren et al., 2006) Thus, a preliminary interpretation for the Menengai and Olkaria samples is that they have seen fluid interaction buffered by a hydrothermal/meteoric fluids Omenda et al Figure 3: Sulfur data for the CKPP samples Superposed on our data set are data from Etna and Krakatau Envelopes enclose the majority of these data in order to emphasize the trend predicted by outgassing in an oxidizing environment See text for further discussion Conclusions We show here that sulfur geochemistry, both concentrations and isotopic values, vary within the Kenya Dome geothermal field As this study progresses, we hope to elucidate the 3-D distribution of magmatic and meteoric fluids and the pathways for their mixing and upwelling Acknowledgments We would like to thank Levi Shako of Geothermal Development Company Ltd (Nakuru) for help  with the GIS project REFERENCES Arnold G L., Brunner B., Müller I A and Røy H (2014) Modern applications for a total sulfur reduction distillation method - what’s old is new again Geochem Trans.15, Clarke M C., Woodhall D G., Allen D and Darling G (1990) Geological, volcanological and hydrogeological controls on the occurrence of geothermal activity in the area surrounding Lake Naivasha, Kenya., Kenya, Ministry of Energy Faure G (1986) Principles of isotope geology Second edition 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238U-230Th-226Ra Disequilibria in Trachyte Lavas from Longonot Volcano, Kenya J Petrol.45, 1747–1776 White J C., Espejel-García V V., Anthony E Y and Omenda P (2012) Open System evolution of peralkaline trachyte and phonolite from the Suswa volcano, Kenya rift Lithos152, 84–104  .. .Omenda et al Introduction We report here sulfur geochemical data for the geothermal areas of central Kenya (Fig 1) This area is called the Central Kenya Peralkaline Province (CKPP)... transitional basalt, basaltic trachyandesite and trachyandesite Figure 1: Map of the Central Kenya Peralkaline Province From (Macdonald et al. , 2011) The rationale for this study is to trace sulfur pathways... analytical technique of (Arnold et al. , 2014) Table Sulfur analytical data Omenda et al Results 4.1 ArcGIS data base The GIS project consists of compiling on a georeferenced map geochemical