Melting Behaviour of Carbonated MORB: the transition zone carbon filter

The convecting mantle represents Earth’s largest reservoir for volatile storage on geological timescales. Continuous outgassing in volcanic settings worldwide indicates that volatile recycling during subduction of oceanic crust is critical for the maintenance of mantle volatile contents. Subducting basaltic crust initially contains a cocktail of volatiles, but it loses effectively its entire water cargo as hydrous fluids between 70 and 300 km depending on slab temperature [1], which may leave a residual anhydrous carbonated MORB assemblage. The fate of this carbon during subduction to greater depths is important geologically due to its potential role in causing mantle melting and metasomatism. Large discrepancies exist among studies of carbonated eclogite, and results are scarce beyond 10 GPa. Here we present results of experiments on MORB containing 2.5 wt.% CO₂between 3 and 21 GPa.

We observe a subsolidus phase assemblage dominated by garnet, clinopyroxene and SiO₂ at all pressures. At pressures lower than 7 GPa CO₂ is the stable carbon phase in all runs due to the reaction dol + 2coes = cpx + CO₂ [2]. Solid dolomite, magnesite and/or Na₂(Ca,Mg,Fe)₄(CO₃)₅ are observed in subsolidus experiments at higher pressure. Near-solidus melts above 7 GPa are carbonatites, with Ca# > 0.5 and alkali contents that increase with pressure. The solidus temperature of 1200 °C at 3 GPa rises to 1375 °C at 13 GPa. At higher pressure the melting temperature drops sharply by > 200 °C to ~ 1150 °C. This creates a ledge in the solidus at 13 – 15 GPa, just above or within the uppermost transition zone, which coincides with the appearance of Na₂(Ca,Mg,Fe)₄(CO₃)₅. Temperature paths for the majority of worldwide slabs [3] intersect this ledge and produce carbonatite melt that will metasomatise the overlying mantle, potentially causing a region of increased diamond formation. Only material in the coldest slabs will pass beneath the ledge and carry carbon deeper into the Earth. Thus, this ledge may act as a carbon filter for downgoing slabs, and might be responsible for some low velocity anomalies seen above the 410 km seismic discontinuity.

[1] Maruyama & Okamoto (2007) Gond Res 11, 148-165. [2] Martin and Hammouda (2011) Eur J Min 23, 5-16. [3] Syracuse et al. (2010) Phys Earth Plan Int 183, 73-90.