Redistribution of trace elements from mantle to crust is controlled by planet size

Inside the upper mantle, incompatible trace elements are redistributed from solid mantle rocks into partial melt. The melt that accumulated the trace elements and that is less dense than the surrounding material rises towards the surface and as a result enriches the crust and depletes the upper mantle. In the case of heat producing elements, this process can affect the thermal evolution and crust production of a planet, whereas in the case of volatiles, the outgassing and atmosphere evolution can be influenced. With mineral/melt partition coefficients, we can quantify the amount of redistributed elements. Due to a lack of high-pressure models and experimental results, partition coefficients were generally taken as constant in mantel evolution models, however, they dependent heavily on pressure, temperature and composition.

In this study, we inserted a P,T,X-dependent clinopyroxene/melt partition coefficient model that is applicable for higher pressure [1] into a mantle evolution code and investigated the effects. Due to their implications for the thermal and atmosphere evolution, we focused both on heat producing elements (Uranium, Thorium, Potassium) and volatiles (H₂O). As a result, we found that the planet size influences partitioning behavior due to differences in depth and temperature inside the melt zones in the upper mantle. With these results, we can infer the impact on various planetary processes, such as the outgassing of water, crust production, and thermal evolution.

[1] Schmidt, J.M. and Noack, L. (2021): Clinopyroxene/Melt Partitioning: Models for Higher Upper Mantle Pressures Applied to Sodium and Potassium, SysMea, 13(3&4), 125-136.