Reevaluating the fate of subducted magnesite in the Earth's lower mantle

The role that subducted carbonates play in sourcing and storing carbon in the deep Earth's interior is uncertain, primarily due to poor constraints on the stability of carbonate minerals when interacting with mantle phases. Magnesite (${\text{MgCO}}_3$) is the most prominent carbonate phase to be present at all mantle pressure-temperature conditions. In this study, we combined multi-anvil apparatus and laser-heated diamond anvil cell experiments to investigate the stability of magnesite in contact with iron-bearing bridgmanite. We examined the presence of melt, decarbonation, and diamond formation at shallow to mid-lower mantle conditions (25 to 68 GPa; 1350 to 2000 K). Our main observation indicates that magnesite is not stable at shallow lower mantle conditions. At 25 GPa and under oxidizing conditions, melting of magnesite is observed in multi-anvil experiments at temperatures corresponding to all geotherms except the coldest ones. Whereas, at higher pressures and under reducing conditions, in our laser-heated diamond-anvil cell experiments, diamond nucleation is observed as a sub-solidus process even at temperatures relevant to the coldest slab geotherms. Our results indicate that magnesite was reduced and formed diamonds when in contact with the ambient peridotite mantle at depths corresponding to the shallowest lower mantle (33 GPa). Thus, we establish that solid magnesite decomposes at depths of ∼700 km as it contacts the ambient mantle. Consequently, the recycling of carbonates will hinder their transport deeper into the lower mantle.