Evaporation from the Lunar Magma Ocean Was Not the Mechanism for Fractionation of the Moon’s Moderately Volatile Elements

The cause of chemical depletion and isotopic fractionation of moderately volatile elements (MVEs) in the Moon is a long-standing problem. Here we examine MVE isotopic fractionation during Moon formation using potassium as a primary example. We show that the degree of isotopic fractionation due to evaporation of the lunar magma ocean (LMO) depended critically on the vapor pressure above the LMO. Based on our analysis of evaporation and escape of the resulting rock-vapor atmosphere, LMO evaporation alone could not have caused the observed MVE isotopic compositions. The combination of exposure of the liquid of the LMO to the surface for on the order of 10 2 –10 3 yr, near-equilibrium evaporation at the melt surface, and hydrodynamic escape facilitated by proximity of the Earth leads to no resolvable isotope or elemental fractionation. This study provides a basis for the simplifying assumption that rock vapor immediately above a magma ocean will be in thermodynamic equilibrium with the melt at the surface of the planetary body even where hydrodynamic escape is operative.