The kinetics of the reaction of majorite plus ferropericlase to ringwoodite: Implications for mantle upwellings crossing the 660 km discontinuity

We have measured the kinetics of reaction between MgO and majoritic garnet at 20 GPa and 1773-2123 K as a proxy for the reaction between perovskite and ferropericlase during mantle upwelling across the 660 km seismic discontinuity. Ringwoodite forms a layer between MgO and garnet and, in the case of aluminous garnets the interface between ringwoodite and garnet develops a fingering instability resulting in a complex intergrowth at this interface. By contrast, the MgO-ringwoodite interface is always planar for an initial planar MgO-garnet interface. Two thicknesses are therefore defined; (1) a layer thickness, X1, which is the maximum thickness of ringwoodite which forms a plane-parallel bounded layer next to the MgO, and (2) an interface thickness, X2, which is the maximum extent of the intergrowth region away from the ringwoodite layer. The growth of both of these regions can be described by apparent rate constants, ki, which are Arrhenius with ln⁡(k10)=−6.36±0.25 m2/s and E1=456±40 kJ/mol for the ringwoodite layer, and ln⁡(k20)=−9.2±3.3 m2/s and E2=371±53 kJ/mol for the intergrowth region. The fingering instability is caused by the incompatibility of aluminium in ringwoodite and its low chemical diffusivity in garnet which results in an increase of surface area at the ringwoodite-garnet interface to minimise the aluminium concentration at the interface. The intergrowth region contains a fine-grained mixture of ringwoodite and garnet which coarsens very slowly with time. This might result in a transient weakening of upwelling regions of mantle just above the 660 km seismic discontinuity allowing some viscous decoupling between the upper and lower mantle.