Element mobility during zeolitic and argillic alteration of volcanic ash in a closed-basin lacustrine environment: Case study Olduvai Gorge, Tanzania

Abstract

Pliocene–Pleistocene Bed I of Olduvai Gorge, Tanzania provides an ideal test for models of element mobility during closed-basin saline–alkaline tephra alteration. Samples of a single phonolitic lapilli-rich layer of marker Tuff IF were collected across a transect of paleo-lake Olduvai, including authigenic K-feldspar and phillipsite bearing samples from the central saline–alkaline lake basin, less altered phillipsite and chabazite-bearing samples from the intermittently dry lacustrine and lake margin deposits, clay-dominated samples from the distal lake margin, and less altered or fresh samples from a freshwater wetlands deposit. In addition to authigenic mineral assemblage, bulk composition (as measured for lapilli separates) also varies across the basin. Of the major elements, Si and Al varied the least across the environments sampled. Ca, Na, and K were variable but did not display significant leaching as would occur under more dilute and neutral conditions. K in particular varied in concentration depending on environment, showing depletion in the lake margin argillic samples and concentration in the phillipsite and K-feldspar dominated zeolitic samples of the central basin. Mg followed the opposite pattern, with higher concentrations in the argillic samples. The overall pattern of element enrichment and depletion helps reconstruct the Olduvai Basin diagenetic history. Ordinarily immobile elements Ti and Zr showed significant variation, especially in the most altered samples. Zr concentrations were up to 50% lower in the zeolitized samples, while they were conserved or slightly enriched in the least altered and argillic samples. Of the trace elements analyzed, only Ta and Nb were consistently immobile, showing variations of under 10%. These observations suggest that Zr and Ti should not be treated as immobile elements if exposure to saline–alkaline fluids is suspected, as they can become more mobile under specific high-pH conditions.