Effect of water on the magnesite–iron interaction, with implications for the fate of carbonates in the deep mantle

The subduction of carbonates beyond 250–300 km, where redox conditions favour the presence of metallic iron (Fe), will result in redox reactions with the Fe dispersed in the silicate rocks. Here, we studied the effect of water on the carbonate–Fe interaction in the hydromagnesite–Fe system at 6, 8 and 16 GPa and the peridotite–CO ₂ –H ₂ O–Fe system at 8 GPa, using a multianvil apparatus. In all of the studied samples, we observed the formation of magnesiowüstite, graphite and carbide. Additionally, in the peridotite–CO ₂ –H ₂ O–Fe system, magnesiowüstite reacted with pyroxenes, resulting in olivine enrichment. Kinetic calculations performed at 8 GPa showed that, at the pressure–temperature (P–T) parameters of the ‘hot’ ‘medium’ and ‘cold’ subduction, about 40, 12 and 4 vol% of carbonates, respectively, would be reduced in the hydrous system within 1 Myr, assuming direct contact with Fe. Based on the present results, it is suggested that carbonates will largely be consumed during the characteristic subduction time to the mantle transition zone by reaction with the reduced mantle in the presence of hydrous fluid.