The Extent, Nature, and Origin of K and Rb Depletions and Isotopic Fractionations in Earth, the Moon, and Other Planetary Bodies

Moderately volatile elements (MVEs) are depleted and isotopically fractionated in the Moon relative to Earth. To understand how the composition of the Moon was established, we calculate the equilibrium and kinetic isotopic fractionation factors associated with evaporation and condensation processes. We also reassess the levels of depletions of K and Rb in planetary bodies. Highly incompatible element ratios are often assumed to be minimally affected by magmatic processes, but we show that this view is not fully warranted, and we develop approaches to mitigate this issue. The K/U weight ratios of Earth and the Moon are estimated to be 9704 and 2448, respectively. The Rb-87/Sr-86 atomic ratios of Earth and the Moon are estimated to be 0.072 5 and 0.015 4, respectively. We show that the depletions and heavy isotopic compositions of most MVEs in the Moon are best explained by evaporation in 99%-saturated vapor. At 99% saturation in the protolunar disk, Na and K would have been depleted to levels like those encountered in the Moon on timescales of similar to 40-400 days at 3500-4500 K, which agrees with model expectations. In contrast, at the same saturation but a temperature of 1600-1800 K relevant to hydrodynamic escape from the lunar magma ocean, Na and K depletions would have taken 0.1-103 Myr, which far exceeds the 1000 yr time span until plagioclase flotation hinders evaporation from the magma ocean. We conclude that the protolunar disk is a much more likely setting for the depletion of MVEs than the lunar magma ocean.

Automated summary

This article investigates the depletion and isotopic fractionation of moderately volatile elements in the Moon relative to Earth, and calculates the equilibrium and kinetic fractionation factors associated with evaporation and condensation processes. The study finds that the protolunar disk is more likely than the lunar magma ocean to be the place where moderately volatile elements are depleted.