http://www.sciencedirect.com/science/article/pii/S1674987114000590

This paper reviews the origin and evolution of fluid inclusions in ultramafic xenoliths, providing a framework for interpreting the chemistry of mantle fluids in the different geodynamic settings. Fluid inclusion data show that in the shallow mantle, at depths below about 100 km, the dominant fluid phase is CO₂ ± brines, changing to alkali-, carbonate-rich (silicate) melts at higher pressures. Major solutes in aqueous fluids are chlorides, silica and alkalis (saline brines; 5–50 wt.% NaCl eq.). Fluid inclusions in peridotites record CO₂ fluxing from reacting metasomatic carbonate-rich melts at high pressures, and suggest significant upper-mantle carbon outgassing over time. Mantle-derived CO₂ (±brines) may eventually reach upper-crustal levels, including the atmosphere, independently from, and additionally to magma degassing in active volcanoes.