Magmatic evolution of the Moon

Although incomplete because of the imperfect and somewhat random sampling o f rock types by the Apollo and Luna missions (1969-1976), the history of lu nar magmatism has been reconstructed by numerous researchers over the past three decades. These reconstructions have illustrated the continuous nature of lunar magmatism (from 4.6 to similar to 2.0 Ga) and the large influence of early differentiation and catastrophic bombardment on lunar mantle dyna mics, magmatism, and eruptive style. In this review, we group magmatism int o multiple stages of activity based on sampled rock types and evaluate the models for each stage. Stage 1 is early lunar differentiation and associated magmatism. Partial me lting of the Moon soon after accretion was responsible for producing an ano rthositic crust and a differentiated lunar interior. The extent of lunar me lting and mantle processing depends strongly on the mechanisms that induced melting. Estimates for the time over which melting and crystallization occ urred range from tens to hundreds of millions of years. Stage 2 is the disr uption of lunar magma ocean cumulates. Soon after the crystallization of mo st of the lunar magma ocean, the cumulate pile experienced gravitational ov erturn. This resulted in transport of late-forming cumulates into the deep lunar mantle and mixing of magma ocean cumulates on a variety of scales. St age 3 is the post-magma ocean highland magmatism. Whereas the ferroan anort hositic crust was probably produced during the crystallization of a magma o cean, the slightly younger Mg suite and alkali suite plutonic rocks may hav e been generated by decompressional melting of early magma ocean cumulates during cumulate pile overturn. A KREEP and crustal signature was incorporat ed into these primitive basaltic magmas through assimilation near the base of the lunar crust or through melting of a hybridized mantle. The alkali su ite could represent either the differentiation products of Mg suite parenta l magmas or a separate, but contemporaneous episode of basaltic magmatism. Stage 4 is pre-basin volcanism. Sample analysis and remote sensing data ind icate that early lunar volcanism (KREEP basalts and high-alumina basalts) w as contemporaneous with periods of highlands plutonism and catastrophic bom bardment of the lunar surface. The relationship between early stages of lun ar volcanism and the contemporaneous plutonism is not clear. The KREEP basa lts may be volcanic equivalents to both the Mg suite and alkali suite. Stag e 5 is the late remelting of magma ocean cumulates and eruption of mare bas alts. Basin-associated eruption of mare basalts occurred during and followi ng the late stages of catastrophic bombardment. This volcanic activity was possibly an extension of the thermal event that initiated pre-basin volcani sm. Mare basalts exhibit a wide range of composition resulting from nearsur face fractionation of chemically distinct primary basaltic magmas. Most lik ely, mare basalts were produced by small to moderate degrees of partial mel ting of hybrid cumulate sources in the deep lunar mantle. Alternatively, th e mixed chemical signatures observed in many mare basalts may be interprete d as indicating assimilation of late-stage, evolved cumulates by melts prod uced deep in the cumulate pile. The wide range of compositions exhibited by the mare basalts compared with earlier episodes of basaltic magmatism may reflect the thermal regime in the lunar mantle that limited the extent of p artial melting and melt-source homogenization.