Chronology and complexity of early lunar crust

Abstract

The least equivocal age and petrographic data on lunar rock samples suggest that: 1. (1) the lunar anorthostic crust was formed by about 120 Ma after the primary accretion of the Moon at 4.56 Ga; it may have been completed earlier. It was formed in a large magma system probably at least several hundred kilometers in depth, with rare earths not appreciably fractionated from chondritic-relative abundances; 2. (2) at least some members of the diverse Mg-suites of rocks (norites, troctoliles, dunites) crystallized very soon after lunar accretion—within a very few 100s of Ma after 4.56 Ga—and cannot presently be distinguished chronologically from anorthositic crust formation. However, their parental magmas had clearly evolved from chondritic incompatible trace-element patterns; 3. (3) a trace-element-rich material (KREEP) was formed by about 4.3 Ga ago (as residues from the previous episodes) with a remarkably constant rare-earth pattern. This residue was subsequently reworked in melting and impact processes such that most samples which contain it have ages around 3.9–4.0 Ga, whether impact or volcanic in immediate origin; 4. (4) the onset of ferrous mare basalt volcanism began much earlier (about 4.33 Ga) than was once assumed, and was in process before the end of the most intense period of bombardment (3.9–4.0 Ga ago). The sources for this mare volcanism are complementary in characteristics to KREEP and were also in place by 4.4 or 4.3 Ga ago at a depth of several hundred kilometers. These characteristics of lunar materials demonstrate that lunar crustal formation happened quickly, in a complex fashion, and affected depths in the mantle to at least several hundreds of kilometers.