H₂O in olivine and garnet inclusions still trapped in diamonds from the Siberian craton

We investigated the H₂O content of olivine and garnet inclusions still trapped in seven uncut and unpolished kimberlitic diamonds from Udachnaya (Siberain craton). Fourier Transform Infrared (FTIR) spectroscopy analyses of the inclusions show spectra similar to those observed for the same minerals in mantle xenoliths. The olivine inclusions are found to contain 0.5 to 5ppm wt. H₂O while the garnets do not show any clear absorption bands and therefore are considered to be dry. These data contrast with H₂O contents reported for olivines and garnets in xenoliths, 300 and 20ppm wt. H₂O respectively, as well as olivine xenocrysts (up to ~400ppm wt. H₂O), from the same kimberlite. We interpret the high H₂O concentrations in xenolithic minerals or xenocrysts to be the result of interactions with either the kimberlitic magmas or metasomatic fluids/melts, and believe that the inclusions in diamonds investigated here preserve their mantle signature which is not altered during ascent to the surface. Based on the data collected in this study we conclude that the deep roots of the Siberian craton were essentially dry when the diamond formed and that this was possibly the reason for making this portion of the lithosphere stable with respect to the convective mantle. Finally, using H₂O partition coefficient data, we estimate that the diamond-forming melts in this cratonic lithosphere contain below 0.1wt.% H₂O.