Although the giant impact hypothesis is the most accepted model for the formation of the Moon the origin of its volatile depletion is still matter of debate. The heavy isotopic composition of moderately volatile elements like K, Rb or Zn is often interpreted as supporting evidence for a large-scale volatile depletion event. On the other hand, elevated abundances of water and other volatile elements in some lunar rocks are interpreted in favour for a less volatile depleted interior and more local volatile-loss processes like magmatic degassing. In the latter model, the heavy stable isotope composition of lunar rocks would be a result of late stage magmatic degassing into vacuum. Here we explore the processes affecting volatile elements in mare basalts through the scope of copper and zinc isotopes.
We report new data for mass-dependent stable isotopes of copper and zinc determined from the same rock aliquot of low- and high-Ti mare basalts. Thanks to the combine data set and the high quality of double spike Zn isotopic data, we resolve the effects of fractional crystallization and late magmatic degassing. Based on these results, fractional crystallization and late-stage magmatic degassing cannot explain volatile depletion and the heavy isotopic composition of most mare basalts and their mantle sources. The homogeneous Zn isotopic composition of low and high-Ti basalt mantle sources suggest that volatile loss and mass-dependent isotope fractionation occurred before the formation of these lunar mantle reservoirs, likely during or briefly after the giant impact.