Tungsten isotopes and the early evolution of the Moon

Key events in the early history of the Moon include its formation by a giant impact, the solidification of the lunar magma ocean, and late accretion. The ¹⁸²Hf-¹⁸²W system (t_1/2 ~9 Ma) constitutes a versatile tool to study each of these processes because they can all result in measurable ¹⁸²W variations. Here we review the ¹⁸²W record of lunar rocks and highlight key constraints on the early evolution of the Moon. Tungsten isotope studies on lunar samples demonstrate that there are no resolvable ¹⁸²W variations within the Moon, implying that lunar magma ocean differentiation later than ~70 Ma after Solar System formation. Nevertheless, the Moon is characterized by a uniform ~25 parts-per-million ¹⁸²W excess over the present-day bulk silicate Earth (BSE). One possibility is that this ¹⁸²W difference is radiogenic in origin, in which case the Hf-W system can potentially be used to date the formation of the Moon. However, this interpretation is problematic for two reasons. First, mixing processes during the giant impact very likely modified the ¹⁸²W composition of the Moon and led to distinct initial ¹⁸²W compositions of the Moon and Earth. Second, the pre-late accretion ¹⁸²W compositions of the Moon and BSE overlap within uncertainty, and hence there is no resolved radiogenic ¹⁸²W difference between the BSE and the Moon. Consequently, the Hf-W system does not provide reliable constraints on the age of the Moon. Instead, the Hf–W systematics are fully consistent with 'young' ages of the Moon, well after the effective lifetime of ¹⁸²Hf.