Lunar samples highlands rocks

The nature of the lunar crust is imperfectly known. It is on a firm basis inasmuch as it was derived mainly from studies of samples. The samples, however, were collected from eight locations in a region on the lunar frontside that is not characteristic of the lunar highlands. An important characteristic of the lunar crust is its enrichment, relative to the remainder of the moon, in the constituents of Ca-rich feldspar (CaAl2Si20s). Presumably, feldspar-rich materials crystallized from the molten lunar exterior along with more mafic (Fe, Mg rich) minerals and collected selectively at the lunar surface. Samples of nearly pure feldspar rock, called anorthosite, were included with the lunar samples. Lanthanide distributions (fig. 21.14) in these rocks reflect the strong affinity of feldspar for Eu(II) and the relative preference for lighter members of the lanthanide series. 

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 relatively high abundance in the lunar mare rocks of elements which on Earth are concentrated in the mantle19 — Fe, Mg, and especially Cr (Fig. 3b) -could indicate that the differentiation between crust and mantle was less complete on the Moon than on Earth. Alternatively, if such a differentiation did take place on the Moon, the mare rocks must represent mantle material, i.e. they would have come from a considerable depth. The latter seems more likely. According to the results of the analyses of the Apollo 16 samples (see soil 60601 in Table 2), the elements Fe, Mg, and Cr are indeed much rarer in the lunar highlands, which are thought to represent the ancient lunar crust. 

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... 

Fig. 18. 23 This small rock sample was collected on January 18, 1982 by Ian WhiUans and John Schutt on the Middle-Western ice field on the East Antarctic ice sheet west of the Allan HiUs. It was subsequently identified as a lunar rock by scientists at the Johnson Space Center in Houston, Texas, and was assigned the identifying number ALHA 81005. The rock contains fragments of coarse-grained plagioclase feldspar in a fine-grained black matrix and has been described as an anorthosite breccia from the highlands of the Moon. (Courtesy ofNASA/LPI))...

Fig. 10. The wide variety of lanthanide abundance patterns observed in lunar highlands samples ranging from extreme enrichment in Eu in feldspathic rocks (anorthosites, 61016, 15455) to massive depletion of Eu in KREEP (65015), which represents the final residual liquid from the crystallisation of the magma ocean. Sample 68415 is a granulitic breccia close in composition to that of the average highland crust. (Data are from table 12).

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