OLDOINYO LENGAI: CLASTS & CHARACTERISTICS OF THE WORLD'S STRANGEST ACTIVE VOLCANO

Oldoinyo Lengai, an active volcano located in northern Tanzania along the East African Rift, is the world’s only active natrocarbonatite volcano. However, its composition has fluctuated through time between phonolite and nephelinite magmas. This location is also vital in our understanding of human evolution and group behavior due to the exceptional preservation of over 400 early Homo sapiens footprints preserved in the volcanic ash layers at the base of the volcano. In order to map the flow direction and transportation history of the area and to better understand the flow mechanics of the volcano, large volcanic clast samples entrained within the ash and their weathering products were classified using a combination of petrography, powder x-ray diffraction, scanning electron microscopy and cathodoluminescence (both cold-cathode CL and SEM-CL). Nepheline and augite have been identified as the primary minerals from most of the samples, but some contain accessory minerals such as wollastonite, biotite, phlogopite, sanidine, a variety of oxides and sulfides, and melilite garnet. Calcite, apatite, ijolite and phillipsite were found to be the alteration minerals for most of the samples, and are most likely due to weathering processes. These secondary minerals glow brightly under cold-cathode CL and can be distinguished by their vivid colors. Chemical zonation is particularly present in the nepheline, augite, and apatite, which reflect original fractional crystallization processes. This zoning is most apparent under cold-cathode CL, where zones can be easily recognized by euhedral color change throughout the sample. The zonation of nepheline was classified through SEM-EDS as Na-K. The combination of these techniques has shown a complex history within the lava flow, evident by mineral growth, zonation, and weathering.