TRANSMISSION ELECTRON MICROSCOPY (TEM) INVESTIGATION OF OLDUVAI’S LOCALITY 80 SHOWS UNEXPECTED MINERALOGICAL DIVERSITY: PROGRESS IN THE ACACIA PROJECT

A set of samples from Olduvai’s Locality 80 are being investigated using TEM techniques to better understand the mineralogical diversity of the clay mineral assemblage which in turn will help refine paleoclimate reconstructions, specifically in the context of aridity. Samples investigated thus far with TEM show a great deal of mineralogical diversity. A variety of Mg-smectite minerals, montmorillonite, probable illite, jarosite, iron-oxides, gypsum, titanium oxides and kozulite have been observed in several samples from the lower half of the core and a few samples from the upper half of the core. Thus far samples investigated are dominated by Mg-rich smectites. Multiple morphologies of Mg-rich smectite occur. There is a tendency for the lamellar aggregates to have appreciable Al₂O₃ content (approximately 4 to 6 wt %) where mossy and globular aggregates tend to have compositions defined by MgO and SiO₂. K is the primary interlayer cation with minor amounts of Na for all smectites observed thus far. No evidence has been observed for mineral transformations to Mg-smectite minerals and the mineralogy reflects depositional and not diagenetic processes. This is a critical finding for interpreting mineralogical and chemical results in the context of paleoclimate. Several other minerals were observed in samples. Jarosite is common in samples and the crystals observed are dominantly euhedral and suggest that the minerals formed in place, although the timing of which is not certain. Gypsum is also present in one of the same samples in which jarosite occurs providing some constraints on geochemical conditions. Fe-oxyhydroxides are compact aggregates of nanoscale crystals and occur in samples with and without jarosite and are tentatively interpreted as being detrital in origin. The Fe-oxyhydroxides always have some appreciable Si content (1-3 wt%) and have amounts of As up to 1.3 wt% elemental As. TEM results thus far provide constraints for the interpretations of paleoclimate reconstructions by showing chemically precipitated phyllosilicates appear to be not extensively modified by diagenetic processes and detrital phases can be identified. This ultimately will strengthen interpretations of aridity based on clay mineral assemblages.