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Most of our knowledge of the composition and water content of the cratonic mantle comes from xenoliths found in alkaline magmatic rocks (e.g., kimberlites). However, it is debatable whether such mantle-xenolith samples are representative of the mantle over larger lateral extents or only represent more local metasomatized conduits. To investigate this, we made 3D deterministic and 1D probabilistic models of magnetotelluric (MT) data from the southwestern Kaapvaal Craton and carried out quantitative interpretations. The results indicate that water content decreases with depth between 100 and 160 km, broadly matching the distribution of metasomatism inferred from garnet-xenocrysts. Spatially pervasive phlogopite could exist for a colder geotherm (37 mW/m2) whereas it is not required for the more likely geotherm of 40.2 mW/m2. Most water contents measured from xenolith samples agree with those calculated from the MT models within error, but the discrepancies between them suggest a local metasomatic control over xenolith water contents.
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