Geocheminar winter-2019

Moderately volatile elements (MVE) are significant tracers of volatile depletion in planetary bodies. Investigating the mechanisms of MVE isotopic fractionation in the bulk silicate Moon is the key to our understanding of the thorough scenarios of Moon formation. Here we take the example of potassium to examine MVE isotopic fractionation during rapid accretion, lunar magma ocean evaporation, and vapor-melt exchange on the Moon. Accordingly, neither condensation inside of magma-atmosphere disk or evaporation in lunar magma ocean can reproduce the present distinguishing potassium isotopic composition in the lunar melt relative to terrestrial composition. Instead, both scenarios would generate the lunar melt with potassium isotopic composition either identical to or lighter than terrestrial composition. On the other hand, a silicate steady-state atmosphere can be rapidly formed above the lunar magma ocean. The isotopic fractionated vapor interacts with the lunar surface melt, which can efficiently shift K isotope composition toward positive value relative to bulk silicate Earth. To yield the present K isotopic composition in the bulk silicate Moon only takes 100 ~1000 years. This timescale coincides with previous estimate of duration of lunar magma ocean crystallization. In addition, given the evaporation properties of Mg and Si, the lunar isotopic composition of Mg and Si cannot be varied in such a short timescale. Our model interprets both isotopic difference of MVE and identical isotopic composition of lithospheric elements (e.g., Mg and Si) on the Moon relative to Earth and provides a new perspective on chemical and isotopic evolution of the Moon.