PGEs and Re-Os in CE-5 Lunar Soil: Implications for Late Accretion to the Moon

The late accretion of exotic materials is significant in the study of the formation and evolution of the Earth and the Moon. The importance of platinum-group elements (PGEs) in tracking the late accretion stages of planetary formation has long been recognized. In previous studies, estimates of the flux of exotic materials added to the Moon have primarily been based on measurements of siderophile element concentrations in lunar regolith samples returned by the Apollo or Lunar missions. However, due to the analytical limitations at that time, only a few individual siderophile elements, such as Ni, Ir, Ge, Re, and Au, could be quantified. Among these elements, Ni is moderately siderophile, while Ge is moderately volatile, which means neither is the most ideal tracer for identifying the exotic materials in the moon. Advances in analytical techniques have significantly enhanced both the precision and accuracy of measurements for PGEs and Os isotopes. High-precision analytical techniques have established characteristic of PGEs patterns and Os isotope ratios in different meteorite types by ICPMS and TIMS. However, to date, no detailed study has been conducted on PGEs and Os isotopes in mature lunar soil.

The CE-5 lunar soil (CE-5LS) collection site is located in an area far from the Apollo and Luna mission regions, and previous studies have confirmed that the surface basalts in the CE-5 sampling area are more than 1 billion years younger than those in the Apollo and Lunar mission regions[1, 2]. This implies that the exotic material flux and composition within the CE-5LS may differ significantly from those in the Apollo lunar soil.

In this study, 1100 mg of CE-5LS samples were magnetically separated. And PGEs and Os isotopes were analyzed on the magnetic and non-magnetic fractions, respectively. The results indicate that the influx of exotic material at the CE-5 landing site amounted to approximately 0.8%, markedly lower than estimates based on the accumulation of exotic material in Apollo soil samples (1%–5%)[3-7]. Given that the accumulation of extraterrestrial material on the Moon correlates positively with the Moon's age, this conclusion is reasonable. The PGE patterns and Os isotope ratios in CE-5LS are consistent with those analysed in chondrites. Consequently, the exotic material accrated onto the Moon is predominantly chondrites.

 

Acknowledgment

The authors had the great honour of applying for and receiving approval to carry out studies on the CE-5 lunar samples allocated by the CNSA. This work was financially supported by the National Key Research and Development Project of China (2020YFA0714804).

 

Reference

[1] Che X. C., et al. (2021). Science 374:887.

[2] Li Q. L., et al. (2021). Nature 600:54.

[3] Ganapathy R., et al. (1970). Geochimica et Cosmochimica Acta Supplement 1:1117.

[4] Baedecker P. A., et al. (1974). Lunar and Planetary Science Conference Proceedings 2:1625-1643.

[5] Laul J. C., et al. (1974). Lunar and Planetary Science Conference Proceedings 2:1047-1066.

[6] Boynton W. V., et al. (1975). Lunar and Planetary Science Conference Proceedings 2:2241-2259.

[7] Higuchi H. and Morgan J. W. (1975). Lunar and Planetary Science Conference Proceedings 2:1625-1651.