  The distribution and isotopic composition of volatile elements in planetary
materials holds a key to the characterization of the early solar system and the
Moon's formation. The Moon and Earth are chemically and isotopically very
similar. However, the Moon is highly depleted in volatile elements and the
origin of this depletion is still debated. We present gallium isotopic and
elemental measurements in a large set of lunar samples to constrain the origin
of this volatile depletion. We show that while Ga has a geochemical behavior
different from zinc, both elements show a systematic enrichment in the heavier
isotopes in lunar mare basalts and Mg-suite rocks compared to the silicate
Earth, pointing to a global-scale depletion event. On the other hand, the
ferroan anorthosites are isotopically heterogeneous, suggesting a secondary
distribution of Ga at the surface of the Moon by volatilization and
condensation. The isotopic difference of Ga between Earth and the Moon and the
isotopic heterogeneity of the crustal ferroan anorthosites suggest that the
volatile depletion occurred following the giant impact and during the lunar
magma ocean phase. These results point toward a Moon that has lost its volatile
elements during a whole-scale evaporation event and that is now relatively dry
compared to Earth.
