Stability of dolomite in a hydrous mantle, with implications for the mantle solidus

The stability of dolomite, limited by the reaction enstatite + dolomite = diopside + forsterite + CO₂, has been studied in the system CaO-MgO-SiO₂-CO₂-H₂O at 26 and 31 kbar. The decarbonation curves (contoured for vapor composition) can be extended to a wider range of pressures with thermodynamic functions.

At 31 kbar a portion of the decarbonation curve is intersected by the peridotite solidus. This solidus curve is univariant (independent of volatile composition) because the composition of vapor is buffered by the reaction shown above, within a range of CO₂/(CO₂ + H₂O). This range, called the zone of invariant vapor composition (ZIVC), widens considerably with increasing pressure.

An analogous ZIVC-type solidus can be calculated that is applicable, at pressures greater than about 22 kbar, to natural peridotite compositions containing small amounts of CO₂ and H₂O (dolomite peridotite). The solidus is equally applicable to dolomite-phlogopite peridotite, because the phlogopite content of the mantle is insufficient to buffer the vapor composition.

At pressures below about 22 kbar, dolomite is not stable in the melting range of peridotite. The vapor composition is then buffered by an amphibole reaction at a relatively high (but unknown) value of CO₂ /(CO₂ + H₂O). The change in reaction is accompanied by a relatively sudden increase in the solidus temperature.