SCANNING TRANSMISSION ELECTRON MICROSCOPY – ENERGY DISPERSIVE SPECTROSCOPY (STEM-EDS) REVEALS COMPOSITIONAL HETEROGENEITY AND ZONATION OF SMECTITES FROM OLDUVAI GORGE LOCALITY 80

A set of clay samples from Olduvai Gorge, Tanzania (Locality 80) were previously investigated by TEM-EDS and determined to be dominated by smectite minerals with compositional differences and accessory authigenic minerals potentially providing constraints on the geochemical environment during the time of formation. Compositional differences in smectites have been used previously as a paleoclimate indicator using the Clay Index of Aridity (Deocampo, 2009) and may also have implications for their economic value. Textural differences were observed both among and between smectites at every depth investigated. Compositionally, the majority existed as either an Al-rich or Mg-rich smectite. Dominantly, the texture of the Al-rich phase is lamellar while the Mg-rich phase crystalizes in a compact, platy texture. Tentatively, the lamellar texture is interpreted as detrital in origin whereas the platy texture is interpreted as having crystallized via water column precipitation. To further assess the differences between these two textures, STEM-EDS was utilized and has shown more heterogeneity than expected with potential zoning of K and Na, differential distribution of Fe, Al, and Mg in some instances, and the observation of halite nanocrystals. The unequal distribution of K and Na may reflect differential distribution in the cation exchange site of the smectites. Mg does not correlate with Mn but more closely follows Fe and Al, though still showing divergence from each other. Ti and V are extensively paired, a pattern that has been observed repeatedly and is expected for Fe-Ti oxides. Additionally, nanoscale exsolution was identified on a volcanic or detrital ilmenite crystal. Zonation is commonly observed in other silicate minerals and this is possibly an extension of this well-known macroscale phenomenon. The only other observation of natural zonation or heterogeneity of components in natural clay-sized phyllosilicates is that observed in illite by Arkai et al. (2004). Compositional heterogeneity and zonation of smectite nanoparticles could represent a phase capturing a changing geochemical environment, broadly analogous to plagioclase in igneous systems.