Reactive Preservation of Carbonate in Earth’s Mantle Transition Zone

Abstract

Calcium‐rich carbonate may be preserved in fast‐descending slabs to reach the mantle transition zone (MTZ), which is known to be at least locally hydrous. At MTZ pressures, the melting curve of CaCO3 crosses the geotherm and is further depressed by water; hence, Ca‐rich carbonate may be mobilized by hydrous melting and escape the MTZ. Here we show that aragonite reacts with wadsleyite to produce magnesite under the pressure and temperature conditions of cold slabs in the MTZ. Water considerably enhances conversion of Ca‐rich carbonate into more refractory magnesite, helping to retain carbonate in the deep mantle.