The Re,Os isotopic system has been applied to dating melt extraction events in peridotites with great success owing to the fact that Re is moderately incompatible during mantle melting, whereas Os is strongly compatible. Melting thus dramatically lowers the Re/Os ratio of residual peridotites and slows the Os isotopic evolution from the time of melting. The elevated Os concentration of the residual mantle material is resistant to subsequent alteration during mantle metasomatism. Estimates of the time of the original melting event can be determined1) from Re-Os isochrons, 2) from ReOs model ages (TMA) -analogous to Nd model ages, or 3) from Re-depletion model ages (TRD). Ye.D ages assume complete Re extraction during the original melting event therefore the measured 187Os/188Os represents the minimum age of melt depletion when compared to the evolution of primitive mantle. In practice, true Re-Os isochrons are rare, owing to the relative mobility of Re during surface weathering and, in the case of xenoliths, Re addition during hostrock reactions. However, other less mobile elements such as Ca, A1 or the H R E E can be used as proxies for Re contents and linear correlations between these elements and ~87Os/188Os have been used estimate the initial ~870s/lSSOs ratio, hence time of melt extraction (i.e. Reisberg and Lorand, 1995). As melt extraction is generally linked to the timing of lithospheric growth, these ages are used to date lithosphere formation, which for cratonic xenolith suites generally corresponds very well to the age of the overlying crust (Carlson and Irving, 1994; Pearson et al., 1995). We have applied the Re-Os technique to dating peridotite xenoliths from the Labiat tuff cone, a riftrelated Pleistocene olivine melilitite that erupted near the Archaean-Proterozoic boundary of the Tanzanian craton. Peridotites found at Labait span a wide range of compositions and mineralogies (Lee and Rudnick, 1998) and include spinel facies harzburgites and lherzolites, chromite-bearing harzburgites, garnet harzburgites and lherzolites as well as Fe-rich dunites, pyroxenites and glimmerites. P-T estimates for the garnet peridotites fall in a scattered field near a geotherm o f 5 0 mW/m 2. There is an overall correlation between bulk composition and depth, with more fertile peridotites occurring at deeper levels (Lee and Rudnick, 1998). Trace element compositions show LILE enrichments, similar to cratonic mantle xenoliths elsewhere. Metasomatic overprinting by carbonatites and Fe-rich silicate melts is inferred from petrography and trace element compositions. At least one stage of this metasomatism is rift-related, based on U-Pb dating of Pleistocene metasomatic zircons (Rudnick et aL, 1998). Bulk rock Os concentrations range from a love of 0.42 ppb in the glimmerite to a high of 4.8 ppb in a garnet-free harzburgite, showing a general correlation with fertility: Harzburgites, lherzolites, and the metasomatic rocks (glimmerite and pyroxenite) have the highest, intermediate and lowest the lowest Os concentrations, respectively. Os contents do not correlate with the amount of sulphides optically visible in the samples. Re concentrations range from a low of 0.036 ppb in the glimmerite to a high of 0.404 ppb in a garnet lherzolite. Re contents do not correlate well with fertility. A 187Re/188Os vs 187Os/~88Os isochron plot shows only a very poor correlation. Moreover, TMA ages give unreasonable values. Collectively, these observations point to recent Re mobility. It is noteworthy that the garnet lherzolite for which we have petrographic and chemical evidence of overprinting has the highest Re concentration and the highest Re/Os ratio. This suggests that Re addition occurred in some samples immediately before their entrainment in the host melilitite and was associated with Fe-enrichment, but