Thermochemical plumes and mantle phase transitions

Abstract

Integral relations based on boundary layer theory are derived to study the motion of an isolated, two‐dimensional thermal plume through a viscous mantle containing polymorphic phase changes. Analytical results are obtained which show that phase transitions alter average mantle convective velocities by less than 50%. In particular we find that the olivine‐spinel transition, approximated as univariant, can enhance the circulation velocity of upper mantle convection by 30–40%, while it can enhance the overall amplitude of whole mantle convection by a few percent only. Our calculations demonstrate that a possible endothermic phase change located at 650 km will not prevent deep mantle convection unless the Clapeyron slope defining the transition exceeds −0.3 kbar/°K. This large value is more than one order of magnitude greater than what has been proposed for the 650‐km discontinuity. We then extend the method to include compositional buoyancy and effects of the divariant nature of the olivine‐spinel transition. Analysis of the motion of a compositionally buoyant plume (one having an anomalous Mg/Fe ratio relative to the ambient mantle) reveals that the chemical plume locally distorts the transition in a way which contributes buoyancy and enhances convective amplitudes by 10% or less. Finally, we combine thermal and compositional buoyancy to investigate the interaction between a thermochemical plume and a compositionally induced density interface. The results are used to determine the stability criterion for two‐layered mantle convection. We find that a compositionally produced density increase of 4% at 650 km is needed to prevent mixing between the upper and lower mantle. Seismological observations indicate that density jump at the 650‐km discontinuity is between 5% and 9%. If all the density increase observed there is due to change in composition, then two‐layered mantle convection is stable. However, if phase transitions account for part of the density increase, so that the chemically produced component is less than 4%, then two‐layered mantle convection may be unstable.