9600030 Chesley To investigate the influence of tectonic processes on cratonic lithosphere, a detailed examination of xenoliths both on and off the Tanzanian craton is planned. Investigation of the petrography, mineral chemistry, thermobarometry, major and trace element geochemistry and, most importantly, Os isotopic compositions on selected xenoliths will serve to characterize the composition of the Tanzanian lithospheric mantle and the vertical and lateral extent of cratonic mantle. Os isotopic measurements on whole rock and chromite mineral separates will constrain the minimum age of melt extraction in these rocks, hence timing of lithosphere formation. In addition, the effects of silicate and carbonatite metasomatism on the Re-Os systematics in peridotites will be evaluated by comparison of the chromite results (which should yield the initial Os isotopic composition) with whole rock data, which may record enhancement of the Re/Os ratio due to metasomatic re-introduction of Re. These petrologic and geochemical data will help constrain models of lithosphere structure derived from seismic and gravity surveys that are currently underway in Tanzania. Furthermore, integration of these new geophysical data with the petrological and geochemical studies outline here will allow delineation of the important processes involved in lithospheric mantle growth and modification.
