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 kimberlites
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.
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