Lunar samples mare basalts

Lunar surface materials (Apollo and Luna returned samples and lunar meteorites) are classified into three geochemical end members - anorthosite, mare basalt, and KREEP. These components are clearly associated with the various geochemically mapped terrains of different age on the lunar surface. The composition of the lunar interior is inferred from the geochemical characteristics of basalts that formed by mantle melting, and geochemistry provides constraints on the Moon s impact origin and differentiation via a magma ocean. 

Mare basalts include lavas that erupted from fissures and pyroclastic deposits that produced glass beads. Six of the nine missions to the Moon that returned samples included basalts. The mare basalts from different sites have distinctive compositions and are classified based on their Ti02 contents, and to a lesser extent on their potassium contents (Fig. 13.3). A further subdivision is sometimes made, based on A1203 contents. Mare basalts are compositionally more diverse than their terrestrial counterparts. Volcanic glass beads, formed by fire fountains of hot lava erupting into the lunar vacuum, were found at several Apollo sites and eventually were shown to be a constituent of virtually every lunar soil. The glasses are ultramafic in composition and formed at very high temperatures. 

Lunar meteorites (see review by Korotev et al., 2003) are mostly brecciated samples of highlands crust (FAN) and regolith, although a few mare basalts are included in this collection. It is likely that the source craters for the meteorites are randomly distributed and thus include materials from the lunar farside. As we will see, these meteorites provide a better estimate of the crustal composition than do the geographically biased samples returned by spacecraft. 

Samples returned by the Apollo and Luna missions can be readily distinguished based on their contents of FeO and thorium. This may seem like an unlikely choice of chemical components for classification, but they nicely discriminate rock types and are easily measured by remote sensing. The FeO and thorium contents of ferroan anorthosites, mare basalts, impact melt breccias, and lunar meteorites are shown by various symbols in Figure 13.4. 

As noted earlier, lunar meteorites are mostly breccias of ferroan anorthosite and related early crustal rocks, although a few mare basalt meteorites are known. The lunar meteorites likely sample the whole Moon. The absence of KREEP-rich breccias so common among Apollo samples collected from the nearside in the lunar meteorite collection implies that KREEP-rich rocks cover only a small area on the Moon. In fact, the lunar highlands meteorites appear to provide a closer match to the average lunar crust than do the Apollo highlands samples (Fig. 13.5), as measured by geochemical mapping (see below). 

The values for the Al/Si ratio of the regolith in mare areas, as determined from moon samples as well as via the x-ray fluorescence technique, also differ from the values found in the rock samples of mare basalts (Tables 2 and 3), the latter being somewhat lower. These differences can be explained by the admixture of anorthositic material from the lunar highlands, as found at the... 

The evidence from seismic experiments is fully in agreement with the model outlined above. In the Fra Mauro region of Oceanus Procellarum the crust is about 65 km thick and overlies a solid mantle 126 The crust, in turn, is divided into two parts covered with a thin regolith. For comparison with the returned lunar samples, the upper layer has been inferred to be mare basalt and the lower one anorthositic gabbros. 

Figure 3 Ages of mare basalts and pyroclastic glasses show no correlation with Ti02. Age data are from previous compilations by BVSP (1981), Ryder and Spudis (1980), Fernandes et al. (2002a), and (for pyroclastic glasses) Shih et al. (2001), plus a lower limit cited for Apollo 17 VLT basalts by Taylor et al. (1991). The Ti02 data are averaged from the compilation of Haskin and Warren (1991). The five major Apollo basalt types are shown with small symbols because each point represents one of many available samples from the given locale, whereas each of the lunar meteorites represents (probably) our only sample from its locale.

There cannot be any doubt that the lunar maria were formed by large lava flows2The basalts from the various landing sites differ in composition (Table 3), but samples from a single mare site are not uniform either. Fig. 5 plots the concentrations of Al vs. Mg in lunar basalts from Apollo 12. The observed trend can be accounted for by magmatic differentiation processes, with the variations in chemical composition reflecting origin at different depths. 

A comprehensive review of the chemical composition of the lunar surface, as ascertained by analysis of samples obtained on the Surveyor and subsequent Apollo and Luna missions, has been collated by Turkevich. The lunar surface is made up of silicate rocks, the principal minerals being calcium-rich feldspars and pyroxenes. In many respects the chemical composition of the maria analysed are similar to those of the terrestrial basalts. The terra regions analysed are distinctly different from the maria in having considerably smaller amounts of iron and titanium and larger amounts of calcium and aluminium. Apollo missions have shown that the lunar mare material is very dry and was produced under relatively reducing conditions. 

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