Oxygen Fugacity in the Lunar Magma Ocean

Models and experimental data investigating the lunar magma ocean (LMO) tend to fix oxygen fugacity (fO2) at a constant buffer during the entirety of crystallization. However, actual evolution of the LMO should not be subjected to buffering but instead evolve as an open system on a global scale. Key results of LMO simulations and experiments include redox-sensitive mechanisms such as the partitioning of Europium, which may not be accurately modeled by enforcing an fO2 buffer. However, these results are crucial to matching the physical evidence for the LMO hypothesis such as the thickness of the lunar anorthosite crust and the Europium anomaly in KREEP basalts that are sourced from enriched late LMO magmas (urKREEP). Our work treats the LMO as an unbuffered system to determine the magnitude of redox change during crystallization, and subsequently determine if this causes observable effects in cumulate and melt major oxide and trace element composition. We modeled equilibrium and fractional crystallization of the LMO and accounted for redox chemistry by including Fe2O3 as an oxide component, allowing us to track fO2 in P,T,X space as the system evolves based on the proportion of Fe2O3 to FeO. We also implemented oxygen fugacity-dependent partitioning of the trace element Europium in order to determine the ratio of Eu to other, non-fugacity-dependent partitioned trace elements e.g. Samarium which allows us to determine the Europium anomaly in late melts for comparison to KREEP basalt composition. We will present results on the magnitude of redox evolution and Eu anomaly variations for different LMO evolution scenarios including early, middle, and late switches between equilibrium and fractional crystallization as well as different initial lunar bulk oxide compositions and fugacity depth profiles.