Planet size controls the redistribution of heat producing elements and volatiles from mantle to crust

Upon melting inside planetary upper mantles, trace elements which are incompatible in the solid rock – such as heat producing elements or volatiles - are redistributed into the melt. If the melt is less dense than the surrounding material, the melt transports the elements towards the surface, where it enriches the crust and leaves a depleted upper mantle behind. 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 the help of mineral/melt partition coefficients, we are able to quantify the amount of the redistributed elements and can therefore infer the impact on the aforementioned planetary processes. Mineral/melt partition coefficients depend highly on pressure, temperature, and composition. However, due to a lack of high-pressure experiments and models, they were typically taken as constant in mantle evolution models.

In this study, we developed a 1D interior evolution model and included a pressure, temperature, and melt composition dependent mineral/melt partition coefficient model that is applicable for higher pressures (Schmidt & Noack, 2021). We apply the model to the five planetary bodies Mercury, Venus, Earth, Moon, and Mars and show that the planet size has a significant effect on the partition coefficients and therefore on the redistribution of heat producing elements and volatiles. This makes most partition coefficients based on low-pressure experiments with an Earth-based composition quite inaccurate in interior evolution models. We quantify the resulting effects on the thermal evolution, crust production, and outgassing rate. Additionally, we vary other starting parameters and compare how this affects the amount of the elements that were redistributed into the crust or outgassed into the atmosphere. These findings help us to understand the effect of depth-dependent redistribution for different types of rocky planets and might be relevant for a wide range of mantle evolution models which include mantle melting and trace element redistribution.

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.