Meteorites lunar

Among the rarest of all meteorites are the lunar meteorites. Isotopic, mineralogical, and compositional properties of these samples provide positive identification as lunar samples because of the unique properties of lunar materials that have been discovered by extensive analyses of lunar materials returned by the manned ApoUo and unstaffed Luna missions. AH but one of the lunar meteorites that have been found to date have been recovered from Antarctica. 

Origin. Typical meteorites have formation ages of 4.55 Gyr and exposure ages of only 10 years, duting which time they existed as meter-sized bodies unshielded to the effects of cosmic rays. With the exception of the SNC (Martian) and lunar meteorites it is widely befleved that most conventional... 

Lunar Meteorites Highlands breccias Mare basalts... 

The Apollo astronauts returned 382 kg of lunar sample to Earth, and this collection was supplemented by 326 g of soil samples collected by the Soviet Luna landers. The first lunar meteorite was found in 1982 in Antarctica. Since that time, over 120 lunar meteorites representing about 60 different fall events have been collected. The total mass of these meteorites is -48 kg. About one-third of these meteorites were recovered in Antarctica by American and Japanese teams, and most of the rest were recovered in the deserts of North Africa and Oman. The lunar meteorites have significantly expanded the areas of the Moon from which we have samples. 

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. 

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. 

Scientific literature on the geochemistry of the Moon and Mars is voluminous, and we can only provide overviews of some of the more recent data and current understanding. Lunar samples, lunar meteorites, and Martian (SNC) meteorites were briefly described in... 

Laboratory analysis of returned lunar samples and lunar meteorites... 

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. 

Lunar rocks Ferroan anorthosite Lunar meteorite... 

Analyses of thorium and FeO in lunar rocks and lunar meteorites can be described by three compositional end members - ferroan anorthosite, KREEP, and mare basalt. The various lunar terranes defined by orbital measurements of Th and FeO, illustrated by shaded and hatched fields, can also be explained by mixtures of these components. Terrane abbreviations are PKT (Procellarum KREEP Terrane) FHT (Feldspathic Highlands Terrane) SPA (South Pole-Aiken Terrane). Modified from Jolliff etal. (2000). 

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). 

Korotev, R. L., Jolliff, B. L., Zeigler, R. A., Gillis, J. J. and Haskin, L. A. (2003) Feldspathic lunar meteorites and their imphcations for compositional remote sensing of the lunar surface and the composition of the lunar crust. Geochimica et Cosmochimica Acta, 67, 4895—4923. 

Feldspar Unlike enstatite and forsterite, feldspar is present in most meteorites. The meteoritic feldspars are dominated by plagioclase which are those feldspar compositions in the continuous solid solution series ranging from albite (NaAlSi308) to anorthite (CaAl2Si2C>8) compositions near the ends of this series are most common. The earliest CL studies of feldspar were prompted by the lunar program about 1970 and a series of papers compared the CL of lunar, meteoritic and doped synthetic plagioclase. Later observations were made for anorthite in the carbonaceous chondrites. 

Beryllium in lunar, meteorite and terrestrial samples was determined successfully by Eisentraut et al. [632]. The sample was pulverized and fused with sodium carbonate, dissolved in dilute hydrochloric acid and transferred into a polyethylene bottle. After adjusting the pH to about 4, it was further adjusted to 5.0 with acetate buffer, both EDTA and trifluoroacetylacetone in benzene were added and the mixture was heated briefly at 95°C. A PTFE column packed with 10% of SE-30 on Gas-Chrom Z was used and a sensitivity of 4 10"14 g of beryllium was reported for the measurement of peak heights with the use of a tritium-foil ECD. 

Figure 28 Bulk oxygen isotopic compositions of SNC meteorites, lunar meteorites, and HED meteorites (source Clayton and Mayeda, 1996).

Figure 31 Lunar meteorite North West Africa (NWA) 773 consists of two distinct lithologies cumulate olivine norite and regolith breccia. The cumulate portion is composed of olivine, pigeonite, augite, feldspar, and opaques (troilite, chromite, Fe-metal). The breccia portion contains fragments of cumulate portion as well as silica glass, hedenbergitic pyroxene, volcanic rocks, and unusual lithic clasts with fayalite + Ba-rich K-feldspar + silica + plagioclase (photograph courtesy of M. Killgore).

Korotev R. L. (2002) Lunar meteorites. Website, http //epsc. wustl.edu/admin/resources/moon meteorites.html. 

Bischoff A. (2001) Fantastic new chondrites, achondrites, and lunar meteorites as the result of recent meteorite search expeditions in hot and cold deserts. Earth Moon Planet. 85, 87-97. 

We summarize results pertaining to the CRE histories of lunar meteorites in Table 4. The meanings of the column headers are discussed in the table notes. Eugster (1989) and Warren (1994, 2001) have written specialized reviews dealing with questions related to the exposure histories of lunar meteorites. Some general observations and conclusions from this work follow. 

Most lunar meteorites travel to Earth in less than 1 Myr and some in less than 0.1 Myr other kinds of meteorites generally take longer. As the Moon is close by, the short transit times are intuitively appealing. They also fall naturally out of theoretical calculations, as does the expectation that a small fraction of the lunar meteorites will have longer exposure ages (cf. Gladman et aL, 1995). 

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