Water and carbon in the Earth's mantle

The concentrations of H₂O and C in mid-ocean ridge basalts indicate that the upper, degassed, part of the mantle contains approximately 200 ppm H₂O and 80 ppm C. Estimates for the bulk silicate earth are less precise, but, from geochemical and cosmochemical arguments, values of 550-1900 ppm for H₂O and 900-3700 ppm for C are plausible. The implication is that the (undegassed) lower mantle is enriched relative to the upper mantle in these volatile components, but concentrations there are only of the order of 2000 ppm. No known hydrate is stable in the normal asthenosphere. In the absence of hydrates, water must reside in nominally anhydrous phases such as olivine and β-(Mg,Fe)₂SiO₄. The latter has recently been found to dissolve greater than 1.5wt% H₂O and partitioning of H₂O between β-(Mg,Fe)₂SiO₄ and olivine is greater than 10:1. The strong preference of H₂O for β-(Mg,Fe)₂SiO₄ means that mantle water content influences the width of the seismic discontinuity at 410 km, given that the latter corresponds to the olivine to β-(Mg,Fe)₂SiO₄ transformation. The seismically observed width of the discontinuity (less than 10 km) constrains the water content of upper mantle olivine to be 0-500 ppm. A similar type of constraint can probably be applied to the γ-spinel to perovskite plus magnesiowüstite transformation. Thus, arbitrary amounts of water cannot be assigned to the deeper parts of the mantle without considering the seismological implications. In contrast to hydrates, carbonates in peridotite are extremely refractory; they are likely to survive subduction and are stable over a wide range of mantle P-T conditions. Storage of carbon depends on redox relationships with Fe, however, and recent experimental results indicate that the deeper parts of the upper mantle and transition zone are relatively reduced. Subducted carbonate should therefore be reduced by Fe to diamond and stored in this form. Reoxidation to CO₂ or carbonate occurs in the shallower parts of the asthenosphere or in the lithosphere. The reduction of subducted carbonate to C is a hypothesis consistent with the 'eclogitic suite' of diamond inclusions, minerals included within diamond which could reasonably be the remnants of subducted basaltic crust.