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Highly fractured rocks

Anorthositic gabbro

Olivine-rich gabbro

Figure 7.4 Seismic wave speeds versus depth for the outer part of the Moon.

in particular the calcium-rich variety plagioclase feldspar. Anorthosite in this quantity could be produced by strong differentiation in a widespread magma ocean created by impact melting in the later stages of lunar accretion. To account for the anorthosite the ocean would have needed to be a few hundred kilometres deep, with a peridotite composition similar to that of the Earth's mantle, though depleted in the more volatile elements and modified by the formation of a small iron-rich lunar core. Fractional crystallisation would have established a lunar crust rich in anorthosite, underlain by a mantle rich in olivine. It is this change in composition that is held to be responsible for the increase in seismic speed at about 40 km in Figure 7.4. The seismic speeds are consistent with anorthosite above this level and olivine-rich gabbro below it. It is possible that the heat-producing radioactive isotopes of uranium, thorium, and potassium are concentrated into the crust, and this would help explain why the Moon's interior seems to have cooled early in its history (Section 4.5.1).

The mare rocks differ from the highland rocks through being dominated by the extrusive igneous rock basalt - just the sort of rock that would result from partial differentiation of the mantle after crust formation. Though the maria cover about 17% of the lunar surface, the mare infill is at most only a kilometre or so deep, and so the basalts comprise less than 1% of a lunar crust of mean thickness of roughly 40-50 km.

Taking the crust plus mantle as a whole, analyses show that, compared with the Earth, the lunar crust plus mantle is enriched in refractory compounds but heavily depleted in iron and iron-rich compounds and in siderophile elements such as magnesium. The lunar crust plus mantle is also heavily depleted in volatiles such as water, carbon dioxide, and hydrocarbons (compounds of carbon and hydrogen), and in the more volatile silicates, such as those rich in potassium. Metallic iron particles in the lunar rocks indicate that the lunar surface has never been exposed to oxygen-rich volatiles to much extent - otherwise they would have been dissociated and oxidised all the metallic iron.

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