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 degrees C. We found that the reduction of carbonate with Fe produces C-bearing species (Fe, Fe-C melt, Fe3C, Fe7C3, C) and wustite containing Na2O, CaO, and MgO. The reaction rate constants (k = Delta x(2)/2t) are log-linear relative to 1/T and their temperature dependences are determined to be k(MgCO3) (m(2)/s) = 4.37 x 10(-3) exp [-251 (kJ/mol)/RT] k(CaMg(CO3)2) (m(2)/s) = 1.48 x 10(-3) exp [-264 (kJ/mol)/RT] k(CaCO3) (m(2)/s) = 3.06 x 10(-5) exp [-245 (kJ/mol)/RT] and k(Na2CO3) (m(2)/s) = 1.88 x 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 carbonate 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 reduction 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 alkalis, particularly K, in mantle carbonatite melts entrapped by diamonds from kimberlites and placers worldwide. (C) 2022 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.

Automated summary

The study investigates the reaction kinetics and properties of mantle carbonates with iron metal, revealing the reduction, evolution, and preservation of carbonates in the mantle. This has significant implications for understanding diamond formation and mantle material cycling.