How phase transitions change convection patterns through the Earths history: A modeling study

Mineral phase transitions have an important influence on mantle flow through their effect on buoyancy and latent heat. For example, endothermic transitions can result in denser mineral assemblages at higher temperatures, inhibiting upwelling mantle flow. In the present day, the formation of the bridgmanite + ferropericlase assemblage from ringwoodite has been found to cause weak and intermittent layering of mantle convection. However, for the higher temperatures in Earths past, different phase transitions, such as the transformation from wadsleyite to garnet + ferropericlase, might have controlled mantle dynamics, implying a change in convection patterns during Earths secular cooling. Therefore, it is important to incorporate the thermodynamic effects of realistic phase transitions in mantle dynamics to properly explore their influence on mantle convection throughout Earths history. Our research applies a new numerical technique, which reformulates the energy conservation equation in terms of specific entropy instead of temperature, which better captures the effects of phase transitions. We use the community software ASPECT, with thermodynamic properties computed by HeFESTo, to model global convection under a broad range of core-mantle boundary temperatures and starting mantle adiabats. We focus on the rise of plumes in the early Earth and explore whether endothermic phase transitions between 420-600 km depth and over the 2000-2500 K temperature range may have induced layering. We show that for temperature profiles hotter than the present-day geotherm, the wadsleyite to garnet + ferropericlase transition inhibits the upward rise of plumes while allowing subducted slabs to pass through unimpeded, leading to elevated mantle temperatures in a depth range of 500 to 650 km (see figure below). Our results imply that convection may have been layered early in Earths history and illustrate the crucial role of mantle thermodynamics on Earths chemical and dynamical evolution.