Comparative chemical history of the Earth, the Moon and parent body of achondrite

Abstract

Chronological studies on the lunar samples suggest that major chemical fractionation occurred at 4.4 Ga. It is inferred from both whole-rock Rb-Sr isochron and Nd-Sm systematics. It is stressed that any models on the lunar petrogenesis and evolution should reconcile with this early fractionation. A model for chemical evolution of the Moon (extensive fractional crystallization of a molten layer, followed by impact melting and mixing of melts) is discussed to account for phase relations and r.e.e. abundances. Similar chronological characteristics are observed for achondrite parent body. Achondrite parent body experienced a similar evolutionary history to the Moon starting with a slightly different initial composition (major elements). In the Earth, on the contrary, chemical differentiation has continued (or is still continuing) as indicated by chronological and isotopic evidence.