The formation of Earth’s Moon by the impact of a Mars-sized body with the Earth is widely accepted. While the high temperatures predicted by models of this process might be expected to have driven off volatile elements, such elements are preserved in lunar glasses. Scenarios for the origin of such elements include original accretion during the formation of Earth and subsequent accretion from exogenous sources. Each leaves distinct geochemical fingerprints in the form of the isotopic compositions of noble gases.

Patrizia Will and colleagues from the Institute of Geochemistry and Petrology in Switzerland analyzed interior fragments from glasses in six lunar basalts, which are approximately three billion years old, recovered from the La Paz Icefield, Antarctica. The authors report on the isotopic composition of noble gases preserved in the glasses, which solidified from magmas that originated by partial melting of the deep mantle. For the first time, these results show indigenous isotopic signatures in volatile elements, including He and Ne. Signatures of mechanisms that could have contributed exogenous volatiles, such as from solar wind or primordial He or Ne isotopic signatures, were lacking in the analyzed samples. These results led the authors to propose that during the Moon’s formation by a giant impact with Earth, original isotopic signatures from Earth’s mantle were transferred to and preserved in the resulting lunar material and were stored in reservoirs containing incompatible elements that were concentrated in the late stages of lunar magma differentiation. These findings support the observations that volatile elements derived from Earth’s mantle during the formation of the Moon were preserved and not lost to processes that involve high-temperature reactions. READ MORE