Experimental investigation of silicate-carbonate system at high pressure and high temperature

Melting and subsolidus relations in the (Mg,Fe)SiO3‐(Mg,Fe)CO3, (Mg,Fe) 2SiO4‐(Mg,Fe)CO3, and (Mg,Fe)O‐(Mg,Fe)CO3 systems have been investigated at 14, 15, 16 and 25 GPa, 1973 K and 2173 K, using a 1000 t uniaxial multi anvil split sphere apparatus. The iron‐magnesium partition coefficients between magnesite and silicates or oxides have been measured in subsolidus assemblages. Iron is always partitioned preferentially in the silicate and oxide phases, the order of increasing partitioning being pyroxene, olivine, silicate perovskite, wadsleyite and magnesiowüstite. A thermodynamic model of iron‐magnesium distribution between magnesite and all these phases, based on Gibbs free energy minimization, is established. Melting of pyroxene‐magnesite and olivine‐magnesite pseudo binary systems is eutectic, with eutectic points close to 1973 K and 60 mol % carbonate at 15 GPa in both systems. In the more complex mantle system, it is likely that such melts would form in the transition zone by heating and homogenization of deep subducted carbonates. The melts formed in the olivine‐carbonate system are characterized by high Mg+Fe/Si ratios and thus unlikely to be primary kimberlitic magmas, a conclusion in agreement with previous studies in the peridotite‐CO2 system. On the other hand, the observed pyroxene‐magnesite melts formed at transition zone conditions have Mg+Fe/Si ratios that are comparable to those of natural kimberlites, suggesting that melting of carbonated pyroxenites at high pressures could be a source of kimberlitic magmas.