Lu-Hf zircon evidence for rapid lunar differentiation

摘要

Zircons from the Apollo 14 polymict breccias 14304, 14305, and 14321 crystallized from highly incompatible-trace-element enriched sources over an extended period of early lunar history from 4.4 to 4.0?Ga. These zircons contain the least radiogenic ~(176)Hf/~(177)Hf yet observed in lunar samples, consistent with their source magmas containing a component (KREEP) that evolved during chemical differentiation of a global lunar magma ocean. The initial ~(176)Hf/~(177)Hf isotopic compositions of the zircons as a function of their crystallization ages suggest separation of the KREEP source, and thus crystallization of the lunar magma ocean, by ~ 4.48?Ga to no later than 4.38?Ga.