The distribution and behaviour of rhenium and osmium amongst mantle minerals and the age of the lithospheric mantle beneath Tanzania K.W. Burton a;b; *, P. Schiano a;b , J.-L. Birck a , C.J. Alle ©gre a , M. Rehka «mper c , A.N. Halliday c , J.B. Dawson d a Laboratoire de Ge ¨ochimie et Cosmochimie, CNRS, IPG-Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France b Laboratoire Magmas et Volcans, OPGC-CNRS UMR 6524, Universite Blaise-Pascal, 5 Rue Kessler, 63038 Clermont-Ferrand, France c Institut fu «r Isotopengeologie und Mineralische Rohsto¡e, Department fu «r Erdwissenschaften, ETH- Zentrum NO, CH-9092 Zurich, Switzerland d Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH10 6LE, UK Received 30 August 1999; received in revised form 16 August 2000; accepted 31 August 2000 Abstract 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 abundances 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 two different samples yield 187 Re^ 188 Os ages of 15.4 þ 6.1 and 31.4 þ 6.3 Myr, respectively. Comparison with 232 Th^ 208 Pb (267.1 þ 4.4 Myr) 147 Sm^ 143 Nd (164 þ 18 Myr) and 87 Rb^ 87 Sr (in equilibrium at the present-day) ages for the same silicate minerals suggests differential closure between these isotope systems, and a closure temperature of v 670³C for the Re^Os system. Remarkably, sulphide inclusions were not affected by diffusional equilibration between the silicates, and preserve significantly older age information. Model calculations suggest that sulphide^silicate equilibration ceased some 200^300 Ma, and the Os isotope composition of the sulphide ( 187 Os/ 188 Os = 0.10432 þ 0.00013) suggests a minimum age of 3.4 Gyr. Most xenoliths possess Os isotope compositions that are less radiogenic than the present-day chondritic mantle indicating that they experienced Re-loss some time ago. Samples showing evidence for modal metasomatism have high Re concentrations and Re/Os ratios, but their relatively unradiogenic Os isotope compositions suggests that the metasomatism occurred recently, consistent with data for metasomatic vein minerals. In contrast, some dunites possess both high Re/Os ratios and radiogenic Os isotope compositions. These samples differ from those affected by modal metasomatism in having low Re and exceptionally low Os concentrations. These results provide quantitative constraints on the distribution of Re and Os amongst mantle minerals, highlight the potential of Re^Os isotope dating of sulphide inclusions for establishing the early history of mantle mineral assemblages, and demonstrate that mantle processes 0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII:S0012-821X(00)00259-4 * Corresponding author. Present address: Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK. Fax: +44-1908-655151; E-mail: k.w.burton@open.ac.uk Earth and Planetary Science Letters 183 (2000) 93^106 www.elsevier.com/locate/epsl