REDUCTION OF MAGNESITE AT MANTLE CONDITIONS AND INFLUENCE OF WATER-CONTAINING FLUID ON THE REACTION RATE

Stability of carbonates during subduction is an important issue for the global geodynamic processes, diamond formation and for global carbon cycle. The deeper part of the mantle can be too reduced to favor stable carbonates. Extremely reduced conditions (fO₂ IW) result in significant chemical heterogeneity between mantle and oxidized subducted plate, which induces redox reaction, involving silicates, carbonates, metallic Fe,Ni–alloy, and fluids. The complexity of the system impedes particular reconstruction of redox reaction, so composite systems should be studied.

Chemical reaction between magnesite, dolomite and reduced agent, specifically iron, have been investigated at high pressure of 6 GPa. To determine water influence on the reaction, additional experiments were conducted in the MgCO₃–Fe–H₂O and peridotite-MgCO₃–Fe–H₂O at pressure from 4 to 16 GPa and temperature ranging from 650 to 1200^o C. In the “dry” and H₂O-containing systems formation of solid phases, containing magnesiowüstite, graphite, and Fe₇C₃ was detected.

Calculations of kinetic parameters clearly detected, that the rate of product formation in the magnesite – iron – water system increases by two orders of magnitude in comparison with the magnesite – iron system. In the other case, in water containing peridotite, reaction rate constant of magnesite reduction was computed to be 10^-13 m²/sec at 1100^oC and 6 GPa, that is two order of magnitude lower than in the simplified magnesite-iron system. Despite the fact that water acts as a transporting agent for the reactants, the reduction of magnesite will increasingly depend on the rate of diffusion in the silicates that extremely small. The sluggish kinetics of MgCO₃-metallic iron interaction in the water-containing peridotites established in our study suggests that more than 60 % of carbonates could survive during subduction from metal saturation boundary near 250 km depth down to the transition zone and presumably to the lower mantle.