Water content of the mantle xenoliths from Kimberley and implications for explaining textural variations in cratonic roots

Water contents estimates based on infrared spectroscopy and deformation microstructures are used to constrain the origin of textural variations observed in mantle xenoliths from Kimberley, South Africa. Infrared spectra indicate water contents of 10–370 ppm (H2O by weight) in olivine, 20–370 ppm in orthopyroxene, up to 340 ppm in garnet and 30–550 ppm in clinopyroxene, although no systematic differences are detected among texturally distinct xenoliths (i.e. granular and sheared peridotites). In contrast, active slip systems in olivine, as inferred from tilt boundaries, are different between the granular and sheared peridotites; the former deformed via the [100](010) slip system, which is commonly active under hot, dry conditions, whereas the latter deformed via the [001](100) slip system, which is commonly activated in water-rich environments. The discrepancy in water contents estimates based on infrared spectroscopy and deformation microstructures may reflect different stages of the evolution of water content: the present water contents, as analysed by infrared spectroscopy, may have been modified by the most recent events (e.g. eruption by kimberlite magma), whereas the water contents inferred from tilt boundaries may have resulted from the various deformation processes in the deep cratonic mantle. Accordingly, we suggest that the observed heterogeneity in water content within cratonic roots facilitates local deformation, and gives rise to the range of textures seen in cratonic mantle xenoliths. Copyright © 2010 John Wiley & Sons, Ltd.