AUTHIGENIC MINERALS IN ALTERED VOLCANIC UNITS FROM EAST AFRICAN PALEOLAKE CORES: A PROXY FOR PALEOENVIRONMENT

Paleolake Olduvai, Tanzania, was a Pleistocene lake associated with the East African Rift System. From 2.4-0.5 Ma, it records changes in climate, tectonics, and volcanism associated with the Ngorongoro Volcanic Highlands, all captured in cores recovered by the 2014 Olduvai Gorge Coring Project (OGCP). The cores provide a record of the Olduvai Beds, along with older lacustrine and volcaniclastic deposits (Naibor Soit and Ngorongoro Formations) not exposed in any outcrop.

Paleolake conditions are often reconstructed using biological paleoclimate indicators. While dominated by volcanic and volcaniclastic units, some intervals of the Ngorongoro Formation (Fm) show evidence for aqueous deposition. In these intervals, mineralogy and geochemistry may provide reliable paleoenvironmental proxies. Mineral proxies are sensitive to shifts in lake water chemistry. Volcanic glass in a semi-arid, closed basin will interact with fluid and under saline-alkaline conditions, eventually forming very early diagenetic near-surface authigenic minerals (zeolites, clays, even feldspars).

The authigenic minerals in the Olduvai Beds register the effects of persistent saline-alkaline fluids. We focus on volcaniclastic core samples from the Ngorongoro Fm, using XRD and SEM to identify authigenic minerals to see how the degree of alteration correlates with paleoenvironmental conditions.

XRD results from the Ngorongoro Fm indicate a range from unaltered (volcanic glass and juvenile volcanic minerals) to slightly altered (zeolites erionite and chabazite, clays, volcanic glass and minerals). Minerals associated with higher levels of alteration (e.g., zeolites analcime and phillipsite, authigenic K-feldspar) were not observed. Erionite was observed in SEM for some samples that did not show zeolites in their XRD patterns (likely below detection). Most samples, including those seemingly unaltered, display some degree of alteration.

The mineral assemblages are consistent with mildly saline-alkaline conditions compared to those observed in the younger Olduvai Beds, indicating an increase in alkalinity over time. These results show that early diagenetic near-surface authigenic minerals in volcaniclastic units may serve as paleoenvironmental indicators when lacustrine proxies are unavailable.