H2O-poor minerals in equilibrium with volatile-bearing fluids: Insights from diamond-hosted mineral inclusions

We investigated the H 2 O content of 21 garnet and clinopyroxene inclusions extracted from Canadian diamonds using Fourier Transform Infrared (FTIR) spectroscopy. Garnets have H 2 O contents below the FTIR detection limit, while clinopyroxene inclusions have H 2 O contents ranging from below the detection limit to 289 wt.ppm. These H 2 O contents are lower than in the same minerals found in xenoliths from the same localities, supporting the idea that the activity of H 2 O may be low during diamond formation, as previously suggested for samples from other localities (e.g., the Kalahari and Siberian cratons). Considering these findings, both the speciation of COH fluid in the lithosphere and the incorporation of H in nominally anhydrous minerals were thermodynamically modelled along cratonic mantle geotherms to investigate the composition of diamond-forming fluids. The COH speciation model shows that, contrary to what has been previously suggested, diamonds are unlikely to form from a fluid at, or close to, the water maximum, as such conditions require oxygen fugacities higher than those expected in the lithospheric diamond stability field (log f O 2 (ΔIW) from +2 to 0 at ∼8 GPa). The H incorporation modelling shows that to produce inclusions with the H 2 O content detected here and elsewhere (dry to hundreds of ppm wt), fluids with low H 2 O activity are required. The different H incorporation mechanisms relevant for each mineral will lead to variable H 2 O contents, explaining the range observed here. These results indicate that lithospheric monocrystalline diamonds may form mainly from carbonatitic or hydrous-carbonated silicate melts rather than aqueous fluids.