2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 5
Presentation Time: 9:20 AM


MCHENRY, Lindsay J., Department of Geosciences, University of Wisconsin-Milwaukee, 3209 N. Maryland Ave, Milwaukee, WI 53211, lmchenry@uwm.edu

Olduvai Gorge is famous for its fossil hominins, stone tools, paleoecological indicators, and (in certain scientific communities), abundant zeolites. A saline-alkaline lake persisted during Bed I and early Bed II, changing in extent and salinity with changes in climate. These saline-alkaline conditions altered trachytic tephra.

Tuff IF marks the boundary between Beds I and II. It is characterized, in the central part of the basin, by surge deposits surrounding a laterally-extensive lapilli-rich layer. This layer contains vitric lapilli ~0.5-2.5 cm across that exhibit varying degrees of alteration between sites. It was deposited at a low lake level, thus alteration likely occurred during the next transgression or in groundwater.

The lapilli layer was sampled at 15 localities in the lake margin, 1 at the center of the paleo-lake, and 3 in outlying areas. Hand-picked lapilli separates of all samples were analyzed by XRD for mineral assemblage and degree of devitrification. For selected samples, SEM imaging helped determine the order of formation. XRF and EMPA of representative samples explored changes in bulk composition and element distribution between phases.

Samples varied from fresh to argillic (mainly smectite) to zeolitic (phillipsite, chabazite, and minor analcite), with authigenic K-spar and jarosite in the lake. Degree and type of alteration depend on distance from the center of the basin, with more zeolite near the center and more clay further out. This is consistent with the “bulls-eye” pattern of zeolite formation in closed-basin lakes. However, samples collected at freshwater wetland sites (Loc 40) were fresher than more distal samples. Freshwater sources thus locally counteracted the basin-wide trends.

Tuff IF bulk composition shows little change between fresh and altered samples in most elements. Si, Ti, Al, Fe, Mn, and Na were largely preserved, Mg was enriched by 25%, and only Ca and K were leached (50% and 30%, respectively). This trend holds whether the samples were altered mainly to zeolite or to clay. For zeolitic samples, the minor associated clay minerals are rich in Fe, Mg, and Ti, accommodating components of the original tephra that do not fit well in zeolite structures. In argillic samples, the clays contain less Fe, Mg, and Ti while the minor associated zeolites are K-poor, as K is present in the clay.