Gallium isotopic evidence for extensive volatile loss from the Moon during its formation

TL;DR

This study uses gallium isotopic measurements in lunar samples to demonstrate that the Moon experienced extensive volatile loss through evaporation during its formation, resulting in a dry lunar surface compared to Earth.

Contribution

It provides new isotopic evidence supporting a global-scale volatile depletion event during the Moon's formation, particularly after the giant impact and magma ocean phase.

Findings

Lunar samples show enrichment in heavier Ga isotopes indicating volatile loss.

Ferroan anorthosites are isotopically heterogeneous, suggesting secondary surface processes.

Volatile depletion likely occurred during the Moon's early evaporation event.

Abstract

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.