Katherine A. Adelsberger

Geochemical and Mineralogic Characterization of Middle Stone Age Tools of Laetoli, Tanzania, and Comparisons with Possible Source Materials

Laetoli preserves a rich Middle Stone Age (MSA) artifact assemblage. We examine basaltic MSA materials collected through surface survey at a number of Laetoli localities, as well as representative samples of nearby volcanic rock exposures (i.e., Ogol Lavas, Lemagurut), using petrographic (mineralogic, textural) and geochemical (major and trace element composition) techniques. Petrographic analysis allows for the informal discrimination of three groups of artifacts based on lithologic criteria (Types A, B, and C). The petrographic basis for these groupings is supported by whole-rock geochemical characteristics. Type C artifacts are readily distinguished from Type A and B artifacts, as well as from local volcanic sources, using both major and trace element data. Type A and B artifacts are more similar in character, and geochemical data suggest that Type A artifacts were likely manufactured from the Ogol Lavas. Type B artifacts share some petrographic and geochemical characteristics with volcanic lavas from Ogol, Lemagurut, and Olmoti, but are not a straightforward match for any single source location. The compositions of Type C artifacts do not match those of any nearby volcanic sources used for comparison in this study, nor do they match published data from other volcanic centers in the region. These data demonstrate that multiple raw material sources were utilized in the manufacture of basaltic MSA artifacts at Laetoli. In addition, the dissimilar petrographic and geochemical features of the MSA artifacts and nearby bedrock exposures indicate more distal sources for the artifacts, despite the local availability of basalt. Raw materials for tool-making might have been transported to the site by streams or by Middle Stone Age hominins.