How Phase Transitions Impact Changes in Mantle Convection Style Throughout Earth’s History: From Stalled Plumes to Surface Dynamics

Mineral phase transitions can either hinder or accelerate mantle flow. In the present day Earth, the formation of the bridgmanite + ferropericlase assemblage from ringwoodite at 660 km depth has been found to cause weak and intermittent layering of mantle convection. However, for the higher temperatures in Earth’s past or on other planets, different phase transitions might have governed mantle dynamics and shaped mantle structure. 

Here, we apply a recently developed entropy formulation in mantle convection models with plate-like behavior to investigate the effect of phase transitions on changes in convection style throughout Earth's history. We have extended this method to include chemical heterogeneity, and we have implemented and tested the approach in the geodynamics software ASPECT. Our benchmark results show that this multicomponent entropy averaging method effectively captures the system's thermodynamic effects. Furthermore, we apply the entropy formulation in 2-D and 3-D geodynamic models, incorporating thermodynamic properties computed by HeFESTo. Our models reveal the impact of the endothermic transition from wadsleyite to garnet (majorite) and ferropericlase (occurring between 420–600 km depth and over the 2000–2500 K temperature range) in a mantle with potential temperatures hotter than 1700 K, which impedes rising mantle plumes. 

When encountering this phase transition, the plume conduits tilt significantly, and the plume heads spread out laterally. This change in plume morphology accumulates hot material in the transition zone, spawning secondary plumes.  Partial melt generated within these hot, stalling plumes may lead to chemical differentiation as plume material spreads laterally. On a larger scale, the phase transition can reduce the mass flux of plumes by ~90%. The stalling of plumes creates a long-lasting global hot layer and impedes mass exchange between lower and upper mantle, resulting in global thermal and chemical heterogeneity.

Our models reveal a systematic change in convection style during planetary secular cooling. The wadsleyite to garnet (majorite) + ferropericlase phase transformation only occurs at high temperatures and therefore layering of plumes becomes less frequent and eventually stops as the mantle cools down. This indicates that mantle convection may have been partially layered early in Earth's history, or may be layered today in terrestrial planets with a hotter mantle. As the mantle potential temperature decreases and layering ceases, we observe an increase of surface mobility, suggesting that such a change in convection patterns also affects plate tectonics.