Terrestrial Pb Evolution Defines A Young Moon

Modern terrestrial Pb has an average composition that lies to the right of the geochron implying that the U-Pb system experienced a significant global fractionation event to increase the 238U/204Pb ratio (? value) from its post- accretionary level. Using a new ? value of 1.7 for ordinary chondrites as a lower limit and Mars (?=3) as an upper limit for Earth’s post-accretionary ? value, Earth apparently accreted moderately depleted in volatiles, including Pb. Accepting a present-day ?2 value of between 9.25-9.65 for Earth’s mantle, Pb evolution models require that the U-Pb fractionation event must have occurred after 4.10 Ga. This Pb loss event is linked to the giant impact that formed the Moon. The efficiency of Pb loss by devolatization during high temperatures following the impact is demonstrated by the lack of initial Pb in angrite and eucrite meteorites that formed up to ca. 10 Myr after their parent body formation. Accepting a 4.36 Ga age for lunar ferroan anorthosites requires at least 30 Myr of cooling from a magma ocean and, thus, brackets the age of the giant impact between 4.41 and 4.39 Ga. Such a young age for the Moon’s formation implies that reported pre-4.41 Ga crystallization ages for lunar rocks, including that for a single zircon, cannot date primary crystallization. The collateral loss of other volatiles (including water) to space and highly siderophile elements (HSE) to the core immediately after the impact implies that water and HSE in the mantle and on the surface of Earth today arrived as a late, post-impact veneer.