Seismic tomography and mantle xenolith studies suggest that Archean cratons are underlain by thick lithospheric keels composed of relatively cold and refractory peridotite. Growth and stabilization of this thick lithosphere is generally coincident with growth of the overlying continental crust, based on whole rock Os model ages (Walker et al., 1989; Pearson et al., 1995) and Nd model ages for garnets included in diamond (Richardson et al., 1984). These observations have led to the perception that, once formed, Archean lithospheric mantle keels are long-lived features resistant to recycling or removal by later tectonic processes such as collision or rifting.
The Tanzanian craton in east Africa presents an excellent opportunity to investigate the effects of younger tectonic processes on Archean lithosphere. Here, the Archean (>2.9 Ga) Tanzanian craton has experienced pan-African collisional tectonics at its eastern margin and Tertiary to Recent rifting, as the east African rift propagates southward from Kenya. Heat flow within the craton is low (40 mW/m2; Nyblade et al., 1990), and the presence of diamond-bearing kimberlites (~50 Ma) are consistent with the presence of a thick mantle lithospheric keel. In contrast, a negative Bouger gravity anomaly of the Tanzanian craton, coupled with its high elevation, have been interpreted to suggest the lithosphere may be relatively thin, and eroded by the rifting (Ebinger et al., 1989).
We are investigating lithospheric mantle history through the study of mantle xenoliths carried in Holocene rift volcanics. The Lashaine ankaramite occurs ~100 km to the east of the Archean craton within rift volcanics that erupt through pan African rocks of the Mozambique fold belt. Peridotites from here have major element characteristics similar to peridotites from the Kaapvaal craton: they show pressures and temperatures of equilibration that lie near a 44 mW/m2 geotherm, they are LREE enriched but refractory in terms of their major element compositions and they are enstatite-rich (Rudnick et al., 1994). A whole rock TRD age (assumes total Re depletion in the rock at the time of melt extraction, therefore these ages represent the minimum age of melt depletion) for one of the deepest, garnet-bearing lherzolites is 1.9 Ga, establishing that old lithospheric mantle is preserved beneath the rift to depths of ~140 km. Peridotites from the Labait cinder cone, which lies on the craton-fold belt boundary, are generally less refractory than those from Lashaine and plot near a higher geotherm of 55 mW/m2 (Dawson et al., 1996). The Labait sample studied here is a harzburgite with large primary chromites (Cr# = 75) and magnesian olivine (Fo92.5). The chromites are zoned with increasing Ti, Fe and Al and decreasing Cr toward the rims (which have Cr#'s between 62 and 68). Where enstatite occurs, it is surrounded by patchy, fluid-inclusion rich, low Al2O3 (0.8-1.0 wt.%) and Cr2O3 (0.3-0.5 wt.%) diopside, which is intergrown with small, euhedral aluminous spinels, sulfides and glass. These rims and patches are interpreted to have formed from a relatively recent metasomatic episode. TRD for the whole rock (~2 ppb Os) is 2.0±0.2 Ga, whereas that for a separate of primary chromite (59 ppb Os) is 2.7±0.2 Ga. The latter is considered a minimum age as the chromites contain presumably younger metasomatic rims. This rock formed 2.7 Ga ago, establishing that Archean-aged lithosphere underlies the craton boundary and was metasomatized recently (based on textures). These data also suggest that mantle metasomatism reduces TRD ages of xenoliths, either through addition of radiogenic Os (in the secondary sulfides?) or through addition of Re. However, bulk rock 187Re/188Os ratios remain low (e.g., ~0.015) and, assuming an age of 2.7 Ga, the measured 187Os/188Os is unsupported. Thus, if the difference between whole rock and chromite is due to Re addition during metasomatism, this Re has been recently lost from the sample.
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