1Mineral chemistry of Turkana basalts and implications for basin development Karla Knudson, Louise Miltich, Nick Swanson-Hysell Department of Geology, Carleton College, Northfield, MN 55057 Abstract Geochemistry of plagioclase, pyroxene, and ilmenite phenocrysts from Turkana basin basalts were analyzed in order to determine if they resulted from a common magma chamber, as suggested by previous studies by Haileab et al. (2004). Electron microprobe results indicated chemically-similar phenocrysts throughout all samples. SiO2 content for the Gombe basalts was between 48 and 51 wt%, and each of the oxides Al2O3, Fe2O3, TiO2, MgO, MnO, CaO, Na2O, K2O, P2O5 showed a range of about 1 wt%. Normalized composition of most plagioclase phenocrysts was 60 to 70 wt% anorthite, with some samples exhibiting weak zoning, becoming more anorthite rich toward the edge. Most ilmenite grains had normalized percentage values for Fe2O3, TiO2, and FeO each around 30-35%, regardless of their location at the core or the rim of the grain. Pyroxene phenocryst chemistry showed compositional zoning and are mostly diopside-hedenbergite with little to no jadite component. Geothermometer equations based on pyroxene chemistry were used to calculate crystallization temperatures between 1093°C and 1182°C. Thermobarometer equations yielded an estimated upper limit of pressure at 5.37 kBar, and suggest that the parent magma chamber was located less than 18 km below the earth’s surface. Introduction Today, the Turkana basin is filled with basalt from Pleiocene tectonic activity. The period of volcanic activity has been constrained to a short time interval, 3.9-4.18 Ma (Haileab et al., 2004). The basalts that were produced throughout the basin have been correlated petrographically and chemically and grouped together as the Gombe basalts (Haileab et al., 2004). In order to further correlate these basalts and to determine if they2resulted from a common magma chamber we studied the geochemistry of individual phenocrysts. Using an electron microprobe, we examined the composition of pyroxene, plagioclase, and ilmenite crystals in basalt samples from throughout the basin. We found the phenocrysts from each sample to be chemically similar, further supporting the findings from whole rock geochemistry work, which categorize these basalts as stemming from a similar magma source (Haileab et al., 2004). Geothermobarometry techniques were used to characterize the parent magma chamber and cooling conditions of these basalts. We were able to determine that these basalts formed at low pressures (<5 kBar); however actual pressures could not be resolved using this technique. Using pyroxene core chemistry we determined initial crystallization temperatures between 1127° C and 1182° C. In this study, using the phenocryst geochemistry and the derived temperatures and pressures of the crystallizing magma, we have generated a model for basin development in the Turkana depression. We propose that thermal erosion and extension resulted in pooling of magma at the base of the crust. This magma erupted, creating a single flood basalt that covered the floor of the basin, resulting in the nearly homogenous basalts found in the Turkana region today. Geological Setting The Turkana basin is the locus of a period of regional tectonic rifting activity that began shortly before 4 Ma in northeastern Africa (Haileab et al., 2004). Regional extension, sediment deposition, and basaltic volcanism occupied a relatively narrow band3between 6°45’ N and 2°45’N and extended from southern Ethiopia through northern Kenya (Fig. 1). Today, the northern portion of the basin accommodates the Omo river while the southern part is occupied by Lake Turkana. The depression is bounded on either side by normal faulting that has created the plateaus east of Lake Turkana and a series of half grabens in the rifting region (Fig. 1). The region is composed of Precambrian basement rocks that are overlain by Paleogene sediments and volcanics followed by sediments interlayered with thin flood basalt flow resulting from Pliocene and Pleistocene rifting. The Pliocene and Pleistocene strata that outcrop in the Omo region are referred to as the Mursi Basalts (including basalts in the Mursi, Nkalabong, Usno, Shungura, Nachukui and Koobi Fora Formations) (Haileab et al., 2004) while those that outcrop on or near the Sergei and Gombe plateaus east of Lake Turkana are grouped as the Gombe Basalts (Haileab et al., 2004). Seismic reflection data indicates that the basalt layer extends across the entire area now covered by Lake Turkana and in some areas in the North where it is covered by Omo River sediments (Haileab et al., 2004). The basalts throughout the region were emplaced within a very short time span (3.99-4.18 Ma) and through whole rock geochemistry have been reconciled to a single group allowing them to be referred to as the Gombe Group basalts (Haileab et al., 2004). Petrography Whole rock geochemical analysis has shown the Turkana basalts to be remarkably homogeneous (Haileab et al., 2004). Samples from across the region are medium grained, compositionally borderline between alkaline and tholeiitic and chemically4distinct from surrounding volcanics (Haileab et al., 2004). They are composed mostly of plagioclase (50%), clinopyroxene (20%), glass (20%) and titanium-rich ilmenite and rarely olivine. The texture is aphyric with glass and small (1mm) subhedral phenocrysts of interlocking plagioclase forming the matrix into which smaller (.5mm) euhedral crystals of clinopyroxene have grown. Ilmenite occurs as thin, branching, dendritic crystals. Alteration is uncommon except where some of the glass has been altered to chlorite. Chemical composition is similar across samples with SiO2 content for the Gombe basalts lying between 48 and 51wt%, and each of the oxides Al2O3, Fe2O3, TiO2, MgO, MnO, CaO, Na2O, K2O, P2O5 showing a range of only about 1 wt%. MgO, K2O, P2O5, Na2O, TiO2 and MnO each compose a small weight percent of total composition (<5 wt%), while CaO, Al2O3, and Fe2O3 each compose between 8 and 15 wt% of the total. These basalts are higher in TiO2 and SiO2 than nearby volcanics and comparatively depleted in MgO, CaO and Fe (Haileab et al., 2004). Methods Thin sections used in this study were provided by Professor Bereket Haileab and contained basalts which were collected from the Turkana depression. All chemical analyses were