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Thick continental lithosphere (tectosphere) beneath African cratons has been stable for ~2.5 b.y. despite its mechanical interaction with sublithospheric mantle. Water is known to have significant influence on mechanical stiffness, and the depletion of water is often considered to be a key to preserving the thick lithosphere. Although water-rich environments indicated by the present water content of cratonic xenoliths appear to contradict this hypothesis, these water contents might have been modified at later stages due to the high diffusivity of hydrogen in minerals. Deformation microstructures such as lattice-preferred orientation indicate water-poor conditions (<200 ppm H/Si) during long-term plastic deformation in the continental lithosphere. Analysis of convective instability further constrains the water content to be less than 100 ppm H/Si. We suggest that the continental tectosphere beneath southern Africa must have a low water content, at least one order of magnitude less than oceanic upper mantle, and that the present-day water content of cratonic xenoliths most likely reflects localized metasomatism before eruption.

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