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□ Does any theory of the origin of the Moon account for these differences in crust plus mantle compositions between the Earth and the Moon? As explained in Section 5.2.1, these differences can be accounted for by the collision theory of the origin of the Moon that was outlined in Section 2.2.4.

7.1.7 Radiometric Dating of Lunar Events

Two types of event have been radiometrically dated for the lunar rock samples. For the mare basalts and many highland rocks we have the age of solidification; for the breccias as well as the solidification ages of component fragments, we also have the time at which the rock fragments were impact welded. In addition, isotopes of hafnium (Hf) and tungsten (W) have been used to establish that the lunar core formed by 25-30 Ma after the formation of the Moon: 182Hf decays into 182W with a half-life of only 9 Ma; 182W is removed by iron, so 182W depletion in rocks is the telltale measurement, but there are many complications that will not concern us.

Turning to crust and mantle events, the oldest dates are solidification ages of 4460 Ma for some highland samples of anorthosite. This is not long after the Moon formed, at about 4500 Ma. Most other highland rocks are of comparable antiquity, though towards the lower end of the age range, about 3800 Ma, the radiometric clocks could well have been reset by the impacts that made the samples available on the mare. The earliest ages are in accord with the great antiquity of the highlands inferred from the high impact crater densities there. Mare basalt solidification ages range from about 3950 Ma to 3150 Ma. Breccia welding ages range down to 3100 Ma ago, except for an age of only about 900 Ma for breccia from the impact that formed the crater Copernicus (Figure 6.8(a)). The fresh-looking crater Tycho is even younger, perhaps as little as 100 Ma.

The older breccias help us to date the formation of the mare basin impacts, and the mare basalt solidification ages help us to date the mare infills. The breccia welding ages range from 4000 Ma for Maria Serenitatis, Nectaris, and Humorum to 3900 Ma for Mare Imbrium. Other basins, such as Mare Tranquillitatis, are inferred to be older, because that have been modified by the earliest dated basins. Other ancient basins have presumably been entirely obliterated. Table 7.1 gives the ages of some lunar basins, along with the mare infill basalt solidification ages. As noted earlier, you can see that infill was delayed for several hundred million years, the most recent, that of Mare Procellarum, being completed about 3200 Ma ago. It is not known what caused such delays. There are no radiometric dates for the lunar far side, but Mare Orientale, from ejecta relationships, seems to be a bit younger than Mare Imbrium, and so has an estimated age of 3800 Ma.

Table 7.1 Ages of some lunar basins and mare infill

Basin

Basin age/ Ma Infill age/ Ma

Tranquillitatis

Fecunditatis

Serenitatis

Crisium

Imbrium

Procellarum

Orientale

Before 4000 Before 4000 4000 3900

About 3800

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