The distribution and behaviour of rhenium and osmium amongst mantle minerals and the age of the lithospheric mantle beneath Tanzania

Rhenium-osmium (Re-Os) isotope and elemental abundances have been obtained for primary mantle minerals, metasomatic phases, and a range of mantle rock types from xenoliths in recent Volcanics in northern Tanzania. Re and Os a bundances for sulphide and coexisting silicates in garnet lherzolites from Lashaine confirm that sulphide dominates the Os budget, but also show that Re is almost exclusively sited in silicate phases. Silicate minerals from t wo different samples yield Re-187-Os-188 ages of 15.4 +/- 6.1 and 31.4 +/- 6.3 Myr, respectively. Comparison with Th-232-Pb-208 (267.1 +/- 4.4 Myr) Sm -147-Nd-143 (164 +/- Is Myr) and Rb-87-Sr-87 (in equilibrium at the present -day) ages for the same silicate minerals suggests differential closure bet ween these isotope systems, and a closure temperature of greater than or eq ual to 670 degreesC for the Re-Os system. Remarkably, sulphide inclusions w ere not affected by diffusional equilibration between the silicates, and pr eserve significantly older age information. Model calculations suggest that sulphide-silicate equilibration ceased some 200-300 Ma, and the Os isotope composition of the sulphide (Os-187/Os-188 = 0.10432 +/- 0.00013) suggests a minimum age of 3.4 Gyr. Most xenoliths possess Os isotope compositions t hat are less radiogenic than the present-day chondritic mantle indicating t hat they experienced Re-loss some time ago. Samples showing evidence for mo dal metasomatism have high Re concentrations and Re/Os ratios, but their re latively unradiogenic Os isotope compositions suggests that the metasomatis m occurred recently, consistent with data for metasomatic Vein minerals. In contrast, some dunites possess both high Re/Os ratios and radiogenic Os is otope compositions. These samples differ from those affected by modal metas omatism in having low Re and exceptionally low Os concentrations. These res ults provide quantitative constraints on the distribution of Re and Os amon gst mantle minerals, highlight the potential of Re-Os isotope dating of sul phide inclusions for establishing the early history of mantle mineral assem blages, and demonstrate that mantle processes themselves (metasomatism and dunite formation) can significantly modify the Os isotope chemistry of mant le rocks. (C) 2000 Elsevier Science B.V. AU rights reserved.