Chemical Fractionation after the Moon-forming Giant

Introduction: The Moon is generally thought to have formed from a circumterrestrial disk generated by the impact of a Mars-sized body onto a nearly-formed Earth [1]. The gravitational energy released in such an event is sufficent to completely melt and partially vaporize the Earth and lunar-forming material. This fluid phase of the evolution is not well understood, but may have delayed lunar accretion by ~10 3 years [2]. Despite widespread isotopic heterogeneity in the inner Solar System, the Earth and Moon are identical with respect to oxygen [3] and tungsten isotopes [4]. Because impact simulations derive most of the lunar material from the impactor [5,6], the " terrestrial " character of lunar isotopes has been used to argue for an episode of isotopic equilibration of the proto-lunar disk and post-impact Earth through turbulent exchange while the system is in a fluid state [7]. However, such a scenario prompts the question of why chemical differences exist between the silicate Earth and Moon. Here, we ask whether liquid-vapor fractionation during the post-impact high-temperature era could have evolved chemical differences between the Earth and the proto-lunar disk, even as it eliminated isotopic heterogeneity. One consequence of the equilibration hypothesis is that liquid-vapor equilibrium played a central role in lunar formation. At sufficiently high temperatures, the isotopic fractionation between co-existing phases approaches zero [8]. However, because silicate mantles are multi-component systems, the chemical composition of a silicate liquid and its co-existing vapor will, in general, be different. This behavior of high temperature equilibrium – of similar isotopic but different chemical composition of co-existing phases – makes this process a prime candidate for explaining isotopic similarities and chemical differences between the sili-cate Earth and Moon. Here, we are testing the hypothesis that the Moon acquired its major-element composition by being composed of a sample that is vapor-rich. There are several differences between the composition of the silicate Earth and Moon that may serve to constrain the role of liquid-vapor fractionation during lunar formation. Here, we focus on the elevated Fe/Mg ratio of the lunar mantle relative to Earth's, a feature inferred on the basis of petrologic and geo-physical constraints [9]. We choose this feature for several reasons. First, both iron and magnesium are major elements in silicate mantles, and their lunar abundances are therefore constrained – even in unsam-pled regions – by geophysical measurements. Second,