PlanetBuilder 1.0: An open-source model to analyse the geochemical evolution of rocky planets during accretion and core formation

Abstract. The chemical compositions of the silicate mantle and iron-rich metallic core of rocky planets are primarily determined by core formation during planetary accretion. For large planetary bodies, such as the Earth, accretion and core formation are complex, continuous processes that take place over millions of years, accompanied by increases in interior pressures and temperatures, changes in compositions and sizes of impactors and variations in the extent of core-mantle equilibration. PlanetBuilder is a new open-source Python-based geochemical multi-stage core formation program, designed to model these complex core formation processes. Using experimentally-derived metal-silicate element partitioning and mass-balance equations, the chemical equilibration between multiple series of (differentiated) impactors and the planetary body can be calculated to derive the bulk core and mantle compositions of continuously growing rocky planetary bodies. Our model combines methodologies from several established geochemical tools, and uses a thermodynamic oxygen partitioning model to verify if the metal-silicate equilibration is complete. PlanetBuilder is designed to handle different planetary and impactor compositions and conditions, as well as multiple elements simultaneously. Due to its reliance on oxygen partitioning, the model is not equipped to handle very oxidised impactors or volatile elements well. One of the main features of PlanetBuilder is the option to correct metal-silicate distribution for non-ideal behaviour of elements in metallic iron-rich liquids. Using a canonical Earth formation model as an example, we show that incorporating non-ideality can change predicted mantle element abundances after core formation by tens of percents, well outside the accuracy of terrestrial mantle abundance estimates. Due to our model's sensitivity to small changes in key parameters (e.g. equilibrium constants and exchange coefficients), it can be a valuable tool for reviewing a parameter's influence on predicted core and mantle compositions.