Lead isotope evidence for a young formation age of the Earth–Moon systemOPEN ACCESS
J.N. Connelly, M. Bizzarro
Earth and Planetary Science Letters, Volume 452, 15 October 2016, Pages 36-43
“Highlights
• We use Pb isotopes to constrain the timing of the Moon-forming giant impact.
• A two-stage U–Pb model explains the Pb isotopic composition of Earth’s mantle.
• U/Pb ratio of Earth’s mantle increases by devolatization during the giant impact.
• The Moon-forming impact formed between 4.426 and 4.417 Ga.
• ∼98% of terrestrial volatiles are lost during the giant impact.”
“A model of a giant impact between two planetary bodies is widely accepted to account for the Earth–Moon system. Despite the importance of this event for understanding early Earth evolution and the inventory of Earth’s volatiles critical to life, the timing of the impact is poorly constrained. We explore a data-based, two-stage Pb isotope evolution model in which the timing of the loss of volatile Pb relative to refractory U in the aftermath of the giant impact is faithfully recorded in the Pb isotopes of bulk silicate Earth. Constraining the first stage Pb isotopic evolution permits calculating an age range of 4.426–4.417 Ga for the inflection in the U/Pb ratio related to the giant impact. This model is supported by Pb isotope data for angrite meteorites that we use to demonstrate volatility-driven, planetary-scale Pb loss was an efficient process during the early Solar System. The revised age is ∼100 Myr younger than most current estimates for the age of the Moon but fully consistent with recent ages for lunar ferroan anorthosite and the timing of Earth’s first crust inferred from the terrestrial zircon record. The estimated loss of ∼98% of terrestrial Pb relative to the Solar System bulk composition by the end of the Moon-forming process implies that the current inventory of Earth’s most volatile elements, including water, arrived during post-impact veneering by volatile-rich bodies.”
