Change in carbonate budget and composition during subduction below metal saturation boundary

It is generally accepted that carbonates can be subducted to the mantle depths,where they are reduced with iron metal to produce a diamond.In this work,we found that this is not always the case.The mantle carbonates from inclusions in diamonds show a wide range of cation compositions(Mg,Fe,Ca,Na,and K).Here we studied the reaction kinetics of these carbonates with iron metal at 6-6.5 GPa and 1000-1500℃.We found that the reduction of carbonate with Fe produces C-bearing species(Fe,Fe-C melt,Fe3C,Fe7C3,C)and wüistite containing Na2O,CaO,and MgO.The reaction rate constants(k=△x2/2t)are log-linear rel-ative to 1/T and their temperature dependences are determined to be kMgco3(m2/s)=4.37×10-3 exp[-251(kJ/mol)/RT]kcaMg(CO3)2(m2/s)=1.48×10-3 exp[-264(kJ/mol)/RT]kcaco3(m2/s)=3.06×10-5 exp[-245(kJ/mol)/RT]and kNa2co3(m2/s)=1.88×10-10 exp[-155(kJ/mol)/RT].According to obtained results at least,45-70 vol％of carbonates preserve during subduction down to the 660-km discontinuity if no melting occurs.The slab stagnation and warming,subsequent carbonate melting,and infiltration into the mantle saturated with iron metal are accompanied by a reduction of car-bonate melt with Fe.The established sequence of reactivity of carbonates:FeCO3≥MgCO3＞CaMg(CO3)2＞CaCO3＞＞Na2CO3,where K2CO3 does not react at all with iron metal,implies that during reduc-tion carbonate melt with Fe evolves toward alkali-rich.The above conclusions are consistent with the findings of carbonates in inclusions in diamonds from the lower mantle and high concentrations of alka-lis,particularly K,in mantle carbonatite melts entrapped by diamonds from kimberlites and placers worldwide.