International Geology Review, Vol. 50, 2008, p. 84–88. Copyright © 2008 by V. H. Winston & Son, Inc. All rights reserved. 0020-6814/08/977/84-5 $25.00 84 Annealing Time-Scale of the Cratonic Lithosphere of Southern Africa Inferred from the Shape of Inclusion Minerals IKUO KATAYAMA, 1 Department of Complexity Sciences and Engineering, Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan T SUYOSHI KOMIYA, Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan AND MITSUHIRO T ORIUMI Department of Complexity Sciences and Engineering, Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan Abstract The change in shape of inclusion minerals is a time-dependent process that is controlled by grain-boundary diffusion to minimize the interfacial free energy between inclusion and host minerals. We analyzed olivine inclusions in the orthopyroxene matrix in mantle xenoliths erupted by kimber- lites in the Kalahari craton, southern Africa. Most fine-grained inclusions are rounded, whereas coarse-grained ones show a polyhedral outline with slightly rounded edges. The critical radius of the largest rounded inclusion is in the size range 240–280 µm in several kimberlite suites in this craton; this corresponds to a minimum annealing time-scale of the order of billions of years, which agrees with the isotopic model ages of the mantle xenoliths. The coincidence of the annealing time-scale of the mantle xenoliths with the major age of the crust formation in this region suggests a coupled link between the formation of the Archean craton and that of the underlying lithospheric mantle. Introduction THICK LITHOSPHERIC mantle beneath Archean cra- tons (e.g., Jordan, 1975; Grand, 1987) is character- ized by a distinctive chemical composition that is deficient in incompatible elements compared with that of fertile mantle (Jordan, 1978; Boyd and Nixon, 1978; Boyd, 1989). This led to the idea that the cratonic lithosphere formed as a residue by the removal of a basaltic or komatiitic component (Jordan, 1975, 1978; Boyd, 1989), because incom- patible elements are preferentially partitioned into the magma during partial melting in the mantle. If the formation of cratonic lithosphere coincides with the creation of an Archean craton, such a region is considered to be stable over a couple of billion years. This can be tested by age-dating of mantle xenoliths trapped by kimberlite magma. However, direct dating of mantle xenoliths is challenging because of the low abundance of radiogenic ele- ments in depleted mantle samples and the limitation of the blocking temperature. Enriched Nd and Sr isotope compositions of cratonic xenoliths and dia- mond inclusions suggest isolation of the cratonic lithosphere from the convecting mantle for billions of years (e.g., Menzies and Murthy, 1980; Richard- son et al., 1984), although it might provide an age dominated by incompatible element enrichment such as metasomatism. The Re-Os isotope system is less sensitive to such enrichment events due to the compatible nature of Os and low concentration in a secondary fluid or melt. The analyses of Re-Os isotopic compositions suggest that large portions of cratonic lithosphere were stabilized by the late to middle Archean (Walker et al., 1989; Pearson, 1999; Irvine et al., 2001). In this study, we use an alternative approach to determine the resident time-scale of cratonic litho- sphere. Peridotite xenoliths from the lithospheric mantle contain abundant inclusions within host crystals, and the textural development of the inclu- sions is mainly controlled by diffusion kinetics that minimize the total interfacial free energy between inclusion and host minerals (Toriumi, 1979, 1981). This process rounds inclusions during annealing, and the radius of curvature of rounded edges 1 Corresponding author; current address: Department of Earth and Planetary Science Systems, Hiroshima University, Hiroshima 739-8526, Japan; email: katayama@hiroshima- u.ac.jp