The current study demonstrates how co-existing zeolite and clay minerals formed by the alteration of tephra in a closed-basin lacustrine and lake-margin environment can retain the overall composition of the original bulk tephra for many elements, even when diagenetic conditions and resulting authigenic mineral assemblages change. Zeolite and clay minerals co-exist in the closed-basin, saline-alkaline lacustrine altered tephra of Pleistocene Olduvai Gorge, Tanzania, and their diagenetic histories can be reconstructed using variations in their textures and compositions. The authigenic minerals in the altered tephra of the Olduvai paleolake form a classic ‘bull’s-eye’ pattern, with clay-dominated tephra in the distal lake margin, chabazite and phillipsite in the proximal margin, and phillipsite 3 K-feldspar in the intermittently dry lake and lake center. Fifteen representative samples of altered volcanic ash lapilli (designated Tuff IF) were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), electron probe microanalysis (EPMA), and scanning electron microscopy (SEM) to determine their authigenic mineral assemblages and bulk compositions, and to texturally and compositionally compare their clay mineral and zeolite components.
Textural observations indicate that clay minerals formed first, followed by zeolites and finally feldspars. Clay minerals, however, persist even in the most altered samples. The overall composition of Tuff IF shows only limited change in Fe, Si, Al, and Na between fresh, clay-altered, and zeolite-dominated diagenetic environments, despite significant differences in authigenic assemblage. Where zeolites dominate the assemblage, the remaining clay minerals are rich in Mg, Fe, and Ti, elements that are not readily incorporated in zeolite structures. Where clay minerals dominate, they are more Al-rich. A ‘mixing model’ combining clay-mineral and zeolite compositions yields a close approximation of the original volcanic glass for most elements (exceptions including Mg, Ca, and K). This initial composition was preserved in part by the redistribution of elements between co-existing clay minerals and zeolites.