Lunar highlands

Example 5.7. The oldest lunar rock known to date is a lunar highland anorthosite 60025. The data for different minerals in a single rock are shown in the table below (Carlson and Lugmair, 1988). Find the age. 

We also have over 120 lunar meteorites in our collections. Because the Moon has no atmosphere, the irradiation history of these meteorites can include an extended period in the lunar regolith. The transit times from the Moon to the Earth range from a few x 104 years to nearly 10 Myr. Detailed analysis of exposure ages and terrestrial ages indicate that at least three impact events in the lunar highlands and five events in the lunar mare ejected the meteorites that have been recovered to date. 

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). 

Distribution of iron on the lunar surface, as measured by the Clementine spacecraft, compared with the average iron contents (arrows) of Apollo highlands crustal rocks and lunar highlands meteorites. The meteorites, which presumably come from the nearside and farside, more closely match the global iron peak. The compositional ranges for various lunar rock types are shown as horizontal bars. 

Pieters, C. M. (1986) Composition of the lunar highland crust from near-infrared spectroscopy. Rev. Geophys., 24, 557-78. 

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... 

LSPET (Lunar Sample Preliminary Examination Team) The Apollo 16 Lunar Samples A petrographic and chemical description of samples from the lunar highlands. Preprint 1972. 

B., Thacker R., and Vilcsek E. (1977) On the chemistry of lunar samples and achondrites. Primary matter in the lunar highlands a re-evaluation. Proc. 8th Lunar Sci. Conf 2191-2213. 

Lunar highlands Eerroan anorthosite 60025 U-Pb Hanan and Tilton (1987) 4.50 0.01... 

Lunar highlands Eerroan anorthosite 60025 Sm-Nd Carlson and Lugmair (1988) 4.44 0.02... 

Lunar highlands Norite from breccia 15445 Sm-Nd Shih et al. (1993) 4.46 0.07... 

Lunar highlands Eerroan noritic anorthosite in breccia 67016 Sm-Nd Alibert et al. (1994) 4.56 0.07... 

Figure 8 Initial strontium isotope composition of early lunar highland rocks relative to other early solar system objects. APB Angrite Parent Body CEPB Cumulate Eucrite Parent Body BSSI Bulk Solar System Initial (source Halliday and Porcelli, 2001).

It may also be that a hotter Earth had a surface that was inherently unstable. Some argued that the earliest crust was like the lunar highlands—made from a welded mush of crystals that had previously floated on the magma ocean. Others have suggested that it was made of denser rocks more like those of the Earth s present oceanfloor (Galer and Goldstein, 1991). But firm evidence has so far been sparse. 

Delano J. W. and Ringwood A. E. (1978) Siderophile elements in the lunar highlands nature of the indigenous component and implications for the origin of the Moon. Proc. 9th Lunar Planet. Sci. Conf, 111-159. 

Neal C. R. and Taylor L. A. (1991) Evidence for metasomatism of the lunar highlands and the origin of whitlockite. Geochim. Cosmochim. Acta 55, 2965-2980. 

Ryder G. and Spudis P. D. (1980) Volcanic rocks in the lunar highlands. In Proceedings of the Conference on the Lunar Highlands Crust (eds. J. J. Papike and R. B. Merrill). Pergamon, New York, pp. 353-375. 

Schaeffer O. A. and Husain L. (1973) Early lunar history ages of 2 to 4 mm soil fragments from the lunar highlands. Proc. 4th Lunar Sci. Conf, 1847—1864. 

Shervais J. W. and McGee J. J. (1999) KREEP cumulates in the western lunar highlands ion and electron microprobe study of alkali-suite anorthosites and norites from Apollo 12 and 14. Am. Mineral. 84, 806-820. 

Breccias and KREEP are more likely to he found in the higher elevations of the Moon s surface known as the lunar highlands. Breccias are metamorphic rocks formed when the heat and pressure caused by the impact of a meteorite, comet, or other body on the Moon s surface fuses a section of regolith, converting it into a rock. KREEP is similar to the basalts found in lunar maria except for its higher concentrations of potassium, phosphorus, and rare earth elements. 

Planetary crusts may be divided into three types. Primary cmsts form as a result of the initial melting of the body. The feldspathic crust of the lunar highlands forms this type of example (Fig. 11). Secondary crusts arise through later partial melting of solid planetary mantles and in the rocky inner planets of the solar system, produce basaltic melts. The lunar maria and the surfaces of Mars and Venus as well as our oceanic crust are examples (Figs. 11, 12). Remelting and reprocessing of the basaltic crust as it is returned to the mantle produces our familiar continental cmst. This is an example of a tertiary cmst, and it appears to be the sole example of this type in the solar system. 

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