The selenium isotope composition of lunar rocks: Implications for the formation of the Moon and its volatile loss

The Moon and Earth share similar relative abundances and isotope compositions of refractory lithophile elements, indicating that the Moon formed from a silicate reservoir that is chemically indistinguishable from the Earth's primitive silicate mantle. In contrast, most volatile elements are depleted in lunar mare basalts compared to Earth's mantle and differ in their isotope composition. However, the depletion of volatile elements is not a simple function of their condensation temperature, indicating multiple mechanisms that established the lunar volatile element budget. Specifically, the chalcophile elements S, Se and Te are not depleted in lunar basalts compared to their terrestrial counterparts. In this study, the abundances and stable isotope compositions of the volatile and chalcophile element Se measured in three lunar mare basalts and seven soils are used to refine the processes that caused volatile element depletion on the Moon. The Se isotope composition of two lunar mare basalts (Se = 1.08 and 0.8‰) is significantly heavier compared to chondrites (−0.20 ± 0.26‰; 2 s.d.) and terrestrial basalts (0.29 ± 0.24‰; 2 s.d.). The offset in the Se isotope composition is attributed to a volatility controlled loss of Se from the Moon. The lack of chalcophile element depletion in lunar mare basalts is then explained by sulphide segregation in the Earth's mantle after the Moon forming impact followed by a late veneer of chondritic material to the Earth. Seven lunar soils were found to have chondritic S/Se ratios, but have Se values that are 6 to 13‰ heavier compared to mare basalts. This fractionation is likely the result of coupled and repeating processes of meteoritic material addition and concomitant partial evaporation. Results from numerical modelling indicate that isotope fractionation in lunar soils is due to partial evaporation of FeSe and FeS with evaporative loss of about 20% for both Se and S.