The age and early evolution of the Moon revealed by the Rb-Sr systematics of lunar ferroan anorthosites

The formation of the Moon by a giant impact of an object called Theia onto proto-Earth marks the end of the main stage of Earth’s accretion. However, the timing of this event is controversial, with estimates ranging between ∼50 and ∼220 million years (Ma) after solar system formation. The ⁸⁷Rb-⁸⁷Sr system has the potential to resolve this debate, as formation of the Moon resulted in strong fractionation of rubidium from strontium. To better determine the initial ⁸⁷Sr/⁸⁶Sr of the Moon, we obtained Rb-Sr isotope data for several lunar ferroan anorthosites, which define an initial ⁸⁷Sr/⁸⁶Sr of 0.6990608±0.0000005 (2 s.e.) at 4.360±0.003 Ga. Modeling the pre-giant impact Rb-Sr isotopic evolution of Theia and the proto-Earth reveals that while in the canonical giant impact model no Rb-Sr model age can be determined, all other current impact models yield a Moon formation age of 4.502±0.020 Ga, or 65±20 Ma after solar system formation. When compared to the chronology of lunar samples, this age implies that solidication of the lunar magma ocean took ∼70 Ma, and that the Moon underwent a global re-melting event ∼150 Ma after its formation.