Geochemical evolution of the Moon

There is now consensus that the Moon formed following a colhsion of the early Earth with a Mars-sized impactor (as summarized in the 1986 book Origin oftheMoon). The timing of the impact and subsequent assembly of the Moon are not tightly constrained, but models of radiogenic isotopes suggest an age of 40 to 50 Myr after solar system formation (Halliday, 

The pronounced asymmetry of the lunar cmst, with PKT dominating the nearside and FHT dominating the farside, is apparently a result of magma ocean crystallization. Some 

Two scenarios for the crystallization of the lunar magma ocean, involving different amounts of melt. In both models, olivine and orthopyroxene cumulates sink and plagiodase floats, but the thickness of the plagiodase-rich crust is greater for a completely melted Moon. Ilmenite crystallizes late in both models, but mixing of dense, ilmenite-rich rocks with early magnesium-rich cumulates could result from convection. After Ryder (1991). 

The so-called late heavy bombardment, discussed more fully in Chapter 14, was a period of intense impacts by large planetesimals, concentrated in the Nectarian and Early hnbrian periods. The Procellarum and Imbrium impacts exposed subsurface rocks in the PKT. The SPA terrane formed during this time and represents not only the largest impact basin on the Moon, but also the biggest basin in the solar system. It exposes lower crust and perhaps mantle materials. 

In most respects, asteroid 4 Vesta is geochemically similar to the Moon. As judged from howardite-eucrite-diogenite (HED) meteorites (see Chapter 6), Vesta is an ancient, basalt-covered world (Keil, 2002). Its rocks are highly reduced, and its depletions in volatile and siderophile element abundances resemble those of lunar basalts. And like the Moon, Vesta is hypothesized to have had an early magma ocean. The exploration of Vesta is now in progress, and within a few years we may have enough data to discuss it in a similar way that we have considered the Moon. 

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Taylor S. R. and Jakes P. (1974) The geochemical evolution of the Moon. Proc. 5th Lunar Planet. Sci. Conf, 1287-1305. 

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