Trace element and volatile redistribution from mantle to crust in rocky planetary bodies

Inside the upper mantle of terrestrial planets and other rocky planetary bodies, melting events influence the further evolution of the mantle- crust system significantly. Upon partial melting, trace elements and volatiles that are incompatible with the solid material partition into the melt. If the melt is buoyant, it rises towards the surface where it enriches the crust while depleting the mantle. The change in element quantity in mantle and crust influences, for example in the case of heat producing elements (K, Th, and U) the thermal conditions whereas in the case of water it can affect the outgassing significantly. The amount of redistributed material is often quantified with partition coefficients, which are dependent on pressure, temperature, and composition. However, since there is a lack of high-pressure experiments and models, most studies in the past have typically taken partition coefficients as constant in mantle evolution models.

Our study combines a partition coefficient model that is adjusted for higher upper mantle pressures (Schmidt and Noack, 2021) with a 1D interior evolution model that starts after the magma ocean phase of a planet. We apply the model to the five planetary solar system bodies Mercury, Venus, stagnant-lid and mobile-lid Earth, Moon, Mars (Schmidt et al., in review), as well as planets of varying Earth-masses (Schmidt and Noack, in prep.). We observe that the partition coefficients of K and H₂O are sensitive to pressure changes. However, while the P-T-X dependent partition coefficient calculation for heat producing elements exhibits only minor impacts on the thermal evolution, the effects on the H₂O-redistribution are significant and imply that the outgassing of water in higher-mass planets might be overestimated if the effects of pressure on the partitioning is not taken into account.

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.

Schmidt, J.M., Vulpius, S., Brachmann, C., Noack, L.: Redistribution of trace elements from mantle to the crust in rocky solar system bodies, in review.

Schmidt, J.M., Noack, L.: Planet mass controls the mineral/melt partitioning of trace elements in the upper mantle of rocky planets, in preparation.