Lunar meteorites from Oman

Abstract– Sixty named lunar meteorite stones representing about 24 falls have been found in Oman. In an area of 10.7 × 10³ km² in southern Oman, lunar meteorite areal densities average 1 g km^?2. All lunar meteorites from Oman are breccias, although two are dominated by large igneous clasts (a mare basalt and a crystalline impact‐melt breccia). Among the meteorites, the range of compositions is large: 9–32% Al₂O₃, 2.5–21.1% FeO, 0.3–38 μg g^?1 Sm, and <1 to 22.5 ng g^?1 Ir. The proportion of nonmare lunar meteorites is higher among those from Oman than those from Antarctica or Africa. Omani lunar meteorites extend the compositional range of lunar rocks as known from the Apollo collection and from lunar meteorites from other continents. Some of the feldspathic meteorites are highly magnesian (high MgO/[MgO + FeO]) compared with most similarly feldspathic Apollo rocks. Two have greater concentrations of incompatible trace elements than all but a few Apollo samples. A few have moderately high abundances of siderophile elements from impacts of iron meteorites on the Moon. All lunar meteorites from Oman are contaminated, to various degrees, with terrestrial Na, K, P, Zn, As, Se, Br, Sr, Sb, Ba, U, carbonates, or sulfates. The contamination is not so great, however, that it seriously compromises the scientific usefulness of the meteorites as samples from randomly distributed locations on the Moon.     

The Flux of Lunar Meteorites onto the Earth

Numerous new finds of lunar meteorites in Oman allow detailed constraints to be obtained on the intensity of the transfer of lunar matter to the Earth. Our estimates show that the annual flux of lunar meteorites in the mass interval from 10 to 1000 g to the entire Earth's surface should not be less than several tenths of a kilogram and is more likely equal to tens or even a few hundred kilograms, i.e., a few percent of the total meteorite flux. This corresponds to several hundred or few thousand falls of lunar meteorites on all of Earth per year. Even small impact events, which produce smaller than craters on the Moon smaller than 10 km in diameter, are capable of transferring lunar matter to the Earth. In this case, the Earth may capture between 10 to 100% of the mass of high-velocity crater ejecta leaving the Moon. Our estimates for the lunar flux imply rather optimistic prospects for the discovery of new lunar meteorites and, consequently, for the analyses of the lunar crust composition. However, the meteorite-driven flux of lunar matter did not play any significant role in the formation of the material composition of the Earth's crust, even during the stage of intense meteorite bombardment.     

Petrography and geochemistry of the LaPaz Icefield basaltic lunar meteorite and source crater pairing with Northwest Africa 032

Abstract— We report on the bulk composition and petrography of four new basaltic meteorites found in Antarctica—LAP (LaPaz Icefield) 02205, LAP 02224, LAP 02226, and LAP 02436—and compare the LAP meteorites to other lunar mare basalts. The LAP meteorites are coarse‐grained (up to 1.5 mm), subophitic low‐Ti basalts composed predominantly of pyroxene and plagioclase, with minor amounts of olivine, ilmenite, and a groundmass dominated by fayalite and cristobalite. All of our observations and results support the hypothesis that the LAP stones are mutually paired with each other. In detail, the geochemistry of LAP is unlike those of any previously studied lunar basalt except lunar meteorite NWA (Northwest Africa) 032. The similarities between LAP and NWA 032 are so strong that the two meteorites are almost certainly source crater paired and could be two different samples of a single basalt flow. Petrogenetic modeling suggests that the parent melt of LAP (and NWA 032) is generally similar to Apollo 15 low‐Ti, yellow picritic glass beads, and that the source region for LAP comes from a similar region of the lunar mantle as previously analyzed lunar basalts.     

Constraining the source regions of lunar meteorites using orbital geochemical data

Lunar meteorites provide important new samples of the Moon remote from regions visited by the Apollo and Luna sample return missions. Petrologic and geochemical analysis of these meteorites, combined with orbital remote sensing measurements, have enabled additional discoveries about the composition and age of the lunar surface on a global scale. However, the interpretation of these samples is limited by the fact that we do not know the source region of any individual lunar meteorite. Here, we investigate the link between meteorite and source region on the Moon using the Lunar Prospector gamma ray spectrometer remote sensing data set for the elements Fe, Ti, and Th. The approach has been validated using Apollo and Luna bulk regolith samples, and we have applied it to 48 meteorites excluding paired stones. Our approach is able broadly to differentiate the best compositional matches as potential regions of origin for the various classes of lunar meteorites. Basaltic and intermediate Fe regolith breccia meteorites are found to have the best constrained potential launch sites, with some impact breccias and pristine mare basalts also having reasonably well‐defined potential source regions. Launch areas for highland feldspathic meteorites are much less well constrained and the addition of another element, such as Mg, will probably be required to identify potential source regions for these.     

Compositional and lithological diversity among brecciated lunar meteorites of intermediate iron concentration

Abstract— We present new compositional data for 30 lunar stones representing about 19 meteorites. Most have iron concentrations intermediate to those of the numerous feldspathic lunar meteorites (3–7% FeO) and the basaltic lunar meteorites (17–23% FeO). All but one are polymict breccias. Some, as implied by their intermediate composition, are mainly mixtures of brecciated anorthosite and mare basalt, with low concentrations of incompatible elements such as Sm (1–3 μg/g). These breccias likely originate from points on the Moon where mare basalt has mixed with material of the FHT (Feldspathic Highlands Terrane). Others, however, are not anorthosite‐basalt mixtures. Three (17–75 μ/g Sm) consist mainly of nonmare mafic material from the nearside PKT (Procellarum KREEP Terrane) and a few are ternary mixtures of material from the FHT, PKT, and maria. Some contain mafic, nonmare lithologies like anorthositic norites, norites, gabbronorites, and troctolite. These breccias are largely unlike breccias of the Apollo collection in that they are poor in Sm as well as highly feldspathic anorthosite such as that common at the Apollo 16 site. Several have high Th/Sm compared to Apollo breccias. Dhofar 961, which is olivine gabbronoritic and moderately rich in Sm, has lower Eu/Sm than Apollo samples of similar Sm concentration. This difference indicates that the carrier of rare earth elements is not KREEP, as known from the Apollo missions. On the basis of our present knowledge from remote sensing, among lunar meteorites Dhofar 961 is the one most likely to have originated from South Pole‐Aitken basin on the lunar far side.     

Lunar meteorite Queen Alexandra Range 93069 and the iron concentration of the lunar highlands surface

Abstract— Lunar meteorite Queen Alexandra Range 93069 is a clast-rich, glassy-matrix regolith breccia of ferroan, highly aluminous bulk composition. It is similar in composition to other feldspathic lunar meteorites but differs in having higher concentrations of siderophile elements and incompatible trace elements. Based on electron microprobe analyses of the fusion crust, glassy matrix, and clasts, and instrumental neutron activation analysis of breccia fragments, QUE 93069 is dominated by nonmare components of ferroan, noriticanorthosite bulk composition. Thin section QUE 93069,31 also contains a large, impact-melted, partially devitrified clast of magnesian, anorthositic-norite composition. The enrichment in Fe, Sc, and Cr and lower Mg/Fe ratio of lunar meteorites Yamato 791197 and Yamato 82192/3 compared to other feldspathic lunar meteorites can be attributed to a small proportion (5–10%) of low-Ti mare basalt. It is likely that the nonmare components of Yamato 82192/3 are similar to and occur in similar abundance to those of Yamato 86032, with which it is paired. There is a significant difference between the average FeO concentration of the lunar highlands surface as inferred from the feldspathic lunar meteorites (mean: ～5.0%; range: 4.3–6.1%) and a recent estimate based on data from the Clementine mission (3.6%).     

Petrography and geochemistry of lunar meteorites Dhofar 1673, 1983, and 1984

The Dhofar 1673, Dhofar 1983, and Dhofar 1984 meteorites are three lunar regolith breccias classified based on their petrography, mineralogy, oxygen isotopes, and bulk chemistry. All three meteorites are dominated by feldspathic lithic clasts; however, impact melt rock clasts and spherules are also found in each meteorite. The bulk chemistry of these samples is similar to other feldspathic highland meteorites with the Al₂O₃ content only slightly lower than average. Within the lithic clasts, the Mg # of mafic phases versus the anorthite content of feldspars is similar to other highland meteorites and is found to plot intermediate of the ferroan‐anorthositic suite and magnesian suite. The samples lack any KREEPy signature and have only minor indications of a mare basalt component, suggesting that the source region of all three meteorites would have been distal from the Procellarum KREEP Terrane and could have possibly been the Feldspathic Highland Terrane. All three meteorites were found within 500 m of each other in the Dhofar region of Oman. This, together with their similar petrography, stable isotope chemistry, and geochemistry indicates the possibility of a pairing.     

Atlas of reflectance spectra of terrestrial,lunar, and meteoritic powders and frosts from 92 to 1800 nm

The reflectance spectra of powdered samples of selected minerals, meteorites, lunar materials, and frosts are presented as an aid in the interpretation of present and future remote sensing data of Solar System objects. Spectra obtained in separate wavelength regions have been combined and normalized, yielding coverage from 92 to 1800 nm. Spectral features include reflectance maxima in the far-ultraviolet region, produced by valence-conduction interband transitions, and reflectance minima in the near-ultraviolet, visible, and near-infrared regions, produced by charge transfer and crystal field transitions. Specific maxima and minima are diagnostic of mineral type and composition; additionally, the minerals present in mixtures such as meteorites and lunar samples can be determined.     

Density,porosity, and magnetic susceptibility of achondritic meteorites

As part of a large‐scale survey of meteorite bulk and grain densities, porosities, and magnetic susceptibilities, we measured these properties for 174 stones from 106 achondritic meteorites. These include four lunar meteorites, 15 stones from 10 shergottites, nakhlites, and chassignites (SNCs), 96 stones from 56 howardites, eucrites, and diogenites (HEDs), 17 stones from nine aubrites, two angrites, and 16 stones from 10 ureilites, four stones of three acapulcoites, as well as four stones of three lodranites, and 15 stones from eight primitive achondrites. Those meteorites derived from basalts and crustal material of differentiated parent bodies have lower densities and magnetic susceptibilities, on an average, than the more primitive achondrites, which have a higher percentage metal. A notable exception is the one chassignite in the study (Chassigny), which has a high grain density of 3.73 ± 0.04 g cm^?3. Ureilites have magnetic susceptibilities consistent with primitive achondrites, but lower grain densities. Porosities do not vary considerably between most of the groups, with most stones 5–14% porous, although on an average, ureilites and brachinites have lower porosities, with most stones less than 7% porous. For primitive achondrites, the higher metal content causes finds to exhibit weathering effects similar to what is observed in ordinary chondrites, with a reduction in grain density, magnetic susceptibility, and porosity as compared with unweathered falls. For lunites, SNCs, and HEDs, no such effect is observed. We also observe that grain density and magnetic susceptibility used in conjunction distinguish shergottites, nakhlites, and chassignites from each other. Shergottites and nakhlites have low grain densities (averaging 3.31 and 3.41 g cm^?3, respectively) whereas Chassigny is 3.7 g cm^?3. In magnetic susceptibility, shergottities and chassignites are similar (averaging 2.85 and 2.98 in log units of 10^?9 m³ kg^?1, respectively) with nakhlites averaging higher at 3.42.     

Petrography,mineralogy, and geochemistry of lunar meteorite Sayh al Uhaymir 300

Abstract— We report here the petrography, mineralogy, and geochemistry of lunar meteorite Sayh al Uhaymir 300 (SaU 300). SaU 300 is dominated by a fine‐grained crystalline matrix surrounding mineral fragments (plagioclase, pyroxene, olivine, and ilmenite) and lithic clasts (mainly feldspathic to noritic). Mare basalt and KREEPy rocks are absent. Glass melt veins and impact melts are present, indicating that the rock has been subjected to a second impact event. FeNi metal and troilite grains were observed in the matrix. Major element concentrations of SaU 300 (Al₂O₃ 21.6 wt% and FeO 8.16 wt%) are very similar to those of two basalt‐bearing feldspathic regolith breccias: Calcalong Creek and Yamato (Y‐) 983885. However, the rare earth element (REE) abundances and pattern of SaU 300 resemble the patterns of feldspathic highlands meteorites (e.g., Queen Alexandra Range (QUE) 93069 and Dar al Gani (DaG) 400), and the average lunar highlands crust. It has a relatively LREE‐enriched (7 to 10 x CI) pattern with a positive Eu anomaly (?11 x CI). Values of Fe/Mn ratios of olivine, pyroxene, and the bulk sample are essentially consistent with a lunar origin. SaU 300 also contains high siderophile abundances with a chondritic Ni/Ir ratio. SaU 300 has experienced moderate terrestrial weathering as its bulk Sr concentration is elevated compared to other lunar meteorites and Apollo and Luna samples. Mineral chemistry and trace element abundances of SaU 300 fall within the ranges of lunar feldspathic meteorites and FAN rocks. SaU 300 is a feldspathic impact‐melt breccia predominantly composed of feldspathic highlands rocks with a small amount of mafic component. With a bulk Mg# of 0.67, it is the most mafic of the feldspathic meteorites and represents a lunar surface composition distinct from any other known lunar meteorites. On the basis of its low Th concentration (0.46 ppm) and its lack of KREEPy and mare basaltic components, the source region of SaU 300 could have been within a highland terrain, a great distance from the Imbrium impact basin, probably on the far side of the Moon.     

Meteorites from the United Arab Emirates: Description,weathering, and terrestrial ages

We report on the first meteorite search campaign in the United Arab Emirates (UAE). The geology and proximity of our search region suggest that it is the north‐western extension of the Oman meteorite fields. We found 26 ordinary chondrites, bringing the total number of official meteorites from the UAE to 28. The campaign was organized and conducted in close cooperation with the UAE government and the main masses of the meteorites remained in the country where they will become part of an exhibition. The bulk composition of five meteorite and three soil samples indicates an uptake of U, Mo, Sr, Ba, Li, and Pb from the soil into the meteorites during terrestrial weathering. Terrestrial ages determined from ¹⁴C decay of 21 meteorites range from recent falls to 24.4 ka, with two meteorites having >37 ka and approximately 39 ka, respectively. Weak correlations between weathering degree, meteorite bulk chemical composition, and terrestrial age suggest highly localized weathering conditions, possibly related to abundant occurrences of sabkhas in the search region.     

Occurrence and implications of secondary olivine veinlets in lunar highland breccia Northwest Africa 11273

Lunar breccias preserve the records of geologic processes on the Moon. In this study, we report the occurrence, petrography, mineralogy, and geologic significance of the observed secondary olivine veinlets in lunar feldspathic breccia meteorite Northwest Africa (NWA) 11273. Bulk‐rock composition measurements show that this meteorite is geochemically similar to other lunar highland meteorites. In NWA 11273, five clasts are observed to host veinlets that are dominated by interconnecting olivine mineral grains. The host clasts are mainly composed of mafic minerals (i.e., pyroxene and olivine) and probably sourced from a basaltic lithology. The studied olivine veinlets (~5 to 30 μm in width) are distributed within the mafic mineral host, but do not extend into the adjacent plagioclase. Chemically, these olivine veinlets are Fe‐richer (Fo_41.4–51.9), compared with other olivine grains (Fo_54.3–83.1) in lithic clasts and matrix of NWA 11273. By analogy with the secondary olivine veinlets observed in meteorites from asteroid Vesta (howardite–eucrite–diogenite group samples) and lunar mare samples, our study suggests that the newly observed olivine veinlets in NWA 11273 are likely formed by secondary deposition from a lunar fluid, rather than by crystallization from a high‐temperature silicate melt. Such fluid could be sulfur‐ and phosphorous‐poor and likely had an endogenic origin on the Moon. The new occurrence of secondary olivine veinlets in breccia NWA 11273 reveals that the fluid mobility and deposition could be a previously underappreciated geological process on the Moon.     

Labile trace elements in basaltic achondrites: Can they distinguish between meteorites from the Moon,Mars, and V‐type asteroids?

Abstract— We report data for 14 mainly labile trace elements (Ag, Au, Bi, Cd, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U, and Zn) in eight whole‐rock lunar meteorites (Asuka [A‐] 881757, Dar al Gani [DaG] 262, Elephant Moraine [EET] 87521, Queen Alexandra Range [QUE] 93069, QUE 94269, QUE 94281, Yamato [Y‐] 793169, and Y‐981031), and Martian meteorite (DaG 476) and incorporate these into a comparative study of basaltic meteorites from the Moon, Mars, and V‐type asteroids. Multivariate cluster analysis of data for these elements in 14 lunar, 13 Martian, and 34 howardite, eucrite, and diogenite (HED) meteorites demonstrate that materials from these three parents are distinguishable using these markers of late, low‐temperature episodes. This distinguishability is essentially as complete as that based on markers of high‐temperature igneous processes. Concentrations of these elements in 14 lunar meteorites are essentially lognormally distributed and generally more homogeneous than in Martian and HED meteorites. Mean siderophile and labile element concentrations in the 14 lunar meteorites indicate the presence of a CI‐equivalent micrometeorite admixture of 2.6% When only feldspathic samples are considered, our data show a slightly higher value of 3.4% consistent with an increasing micrometeorite content in regolith samples of higher maturity. Concentrations of labile elements in the 8 feldspathic samples hint at the presence of a fractionated highly labile element component, possibly volcanic in origin, at a level comparable to the micrometeorite component. Apparently, the process(es) that contributed to establishing lunar meteorite siderophile and labile trace element contents occurred in a system open to highly labile element transport.     

Dhofar 081: A new lunar highland meteorite

Abstract— The lunar meteorite Dhofar 081, found as a single fragment of 174 g in the Dhofar region of Oman, is a shocked feldspathic fragmental highland breccia dominated by anorthosite‐rich lithic and mineral clasts embedded into a fine‐grained mostly shock melted clastic matrix. Major mineral phases in the bulk rock are Ca‐rich plagioclase (An_96.5–99.5), pyroxene (FS_21.9–46.2Wo_3.0–41.4), and olivine (Fa_29.3–47.8); accessory phases include Fe‐Ni metal, ilmenite, and Ti‐Cr‐rich spinel. Dhofar 081 contains subordinate crystalline fragments of large anorthosites, intersertal impact‐melt rocks, microporphyritic impact‐melt breccias, dark fine‐grained impact‐melt breccias, large cataclastic feldspars, and irregularly shaped brown glass clasts. Mafic components are rare and no genuine regolith components were found in the sections studied. Minerals in Dhofar 081 show homogeneously distributed shock features: intergranular recrystallization, strong fracturing and mosaicism in feldspar as well as a high density of mostly irregular fractures in pyroxene and olivine. Localized impact melting caused by one or several impacts led to a strong lithification. Based on these effects an equilibration shock pressure of about 15–20 GPa is estimated for the strongest shock event in Dhofar 081. Devitrification of the “glassy” material in the rock indicates thermal annealing after shock melting suggesting that the 15–20 GPa shock event predated the ejection event. According to the concentrations of implanted solar noble gases Dhofar 081 represents a polymict clastic breccia deposit with possibly a minor regolith component. A similar noble gas record of Dhofar 081 and MacAlpine Hills 88104/05 suggests the possibility of a source crater pairing of both meteorites. As indicated by noble gas measurements pairing of Dhofar 081 with the other lunar meteorites found in Oman, Dhofar 025 and Dhofar 026, is unlikely.     

A noble gas data collection of lunar meteorites

We collected the published noble gas data of altogether 35 lunar meteorites. This compilation includes the stable isotopes of He, Ne, Ar, Kr, and Xe. We also give a summary of cosmogenic, trapped, and radiogenic noble gas components of lunar meteorites for which data are available in the literature.     

METEORITIC METAL IN APOLLO 16 SAMPLES

One hundred metallic particles from Apollo 16 soils (61181, 65701) and rocks (60018, 60315, 66055) have been investigated microscopically and by electron microprobe analysis. Their cobalt content indicates a meteoritic origin for all but one particle. However, most contain more phosphorus than typical meteoritic metal, possibly due to the reduction of phosphates in the lunar rocks. Compositions of coexisting kamacite and schreibersite indicate temperatures of about 550–650°C which are thought to have occurred during metamorphism. The bulk nickel content of the lunar metal is somewhat low by comparison with most iron meteorites or the metallic component of common stony meteorites. However, this may be due to compositional changes that occurred after emplacement in the lunar surface layer.     

Recognizing mercurian meteorites

Abstract— With the recent realization that some meteorites may come from Mars and the Moon, it is worthwhile to consider whether meteorites from Mercury could exist in our collections and, if so, whether they could be recognized. The current state of ignorance about Mercury both increases the potential scientific value of mercurian meteorites and aggravates the problem of identifying them. Here, we review evidence supporting the possibility of impact launch and subsequent orbital evolution that could deliver rocks from Mercury to Earth and suggest criteria that could help identify a mercurian meteorite. Mercurian rocks are probably differentiated igneous rocks or breccias or melt rocks derived therefrom. Solar nebula models suggest that they are probably low in volatiles and moderately enriched in Al, Ti, and Ca oxides. Mercurian surface rocks contain no more than 5% FeO and may contain plagioclase. A significant fraction may be volcanic. They may possess an unusual isotopic composition. Most pristine mercurian rocks should have solidification ages of ～3.7 to ～4.4 Ga, but younger impact-remelted materials are possible. Because we know more about the space environment of Mercury than we do about the planet itself, surface-exposed rocks would be easiest to identify as mercurian. The unique solar-to-galactic cosmic-ray damage track ratio expected in materials exposed near the Sun may be useful in identifying a rock from Mercury. Mercury's magnetic field stands off the solar wind, so that solar-wind implants in mercurian regolith breccias may be scarce or fractionated compared to lunar ones. Mercurian regolith breccias should contain more agglutinates (or their recrystallized derivatives) and impact vapor deposits than any other and should show a higher fraction of exogenic chondritic materials than analogous lunar breccias. No known meteorite group matches these criteria. A misclassified mercurian meteorite would most likely be found among the aubrites or the anorthositic lunar meteorites.     

The spatial and temporal distribution of lunar mare basalts as deduced from analysis of data for lunar meteorites

In this work we analyze data for lunar meteorites with emphasis on the spatial and temporal distribution of lunar mare basalts. The data are mostly from the Lunar Meteorite Compendium (http://www-curator.jsc.nasa.gov/antmet/lmc/contents.cfm cited hereafter as Compendium) compiled by Kevin Righter, NASA Johnson Space Center, and from the associated literature. Analysis of the data showed that (i) a significant part of the lunar meteorite source craters are not larger than hundreds of meters in diameter; (ii) cryptomaria seem to be rather abundant in lunar highlands; (iii) the ratios of lunar meteorites belonging to three broad petrologic groups (mare basalt/gabbro, feldspatic highland breccias, and mingled breccias which are a mixture of mare and highland components) seem to be roughly proportional to the areal distribution of these rocks on the lunar surface; and (iv) the meteorite mare basalt ages show a range from ～2.5 to 4.3 Ga and fill the gaps in the Apollo/Luna basalt age distribution. The ages of mare basalt clasts from mingled breccias seem to be systematically higher than those of “normal” mare basalts, which supports the suggestion that mingled breccias originated mostly from cryptomaria.     

On the relationship between the Apollo 16 ancient regolith breccias and feldspathic fragmental breccias,and the composition of the prebasin crust in the Central Highlands of the Moon

Abstract Two types of texturally and compositionally similar breccias that consist largely of fragmental debris from meteorite impacts occur at the Apollo 16 lunar site: Feldspathic fragmental breccias (FFBs) and ancient regolith breccias (ARBs). Both types of breccia are composed of a suite of mostly feldspathic components derived from the early crust of the Moon and mafic impact-melt breccias produced during the time of basin formation. The ARBs also contain components, such as agglutinates and glass spherules, indicating that the material of which they are composed occurred at the surface of the Moon as fine-grained regolith prior to lithification of the breccias. These components are absent from the FFBs, suggesting that the FFBs might be the protolith of the ARBs. However, several compositional differences exist between the two types of breccia, making any simple genetic relationship implausible. First, clasts of mafic impact-melt breccia occurring in the FFBs are of a different composition than those in the ARBs. Also the feldspathic “prebasin” components of the FFBs have a lower average Mg/Fe ratio than the corresponding components of the ARBs; the average composition of the plagioclase in the FFBs is more sodic than that of the ARBs; and there are differences in relative abundances of rare earth elements. The two breccia types also have different provenances: the FFBs occur primarily in ejecta from North Ray crater and presumably derive from the Descartes Formation, while the ARBs are restricted to the Cayley plains. Together these observations suggest that although some type of fragmental breccia may have been a precursor to the ARBs, the FFBs of North Ray crater are not a significant component of the ARBs and, by inference, the Cayley plains. The average compositions of the prebasin components of the two types of fragmental breccia are generally similar to the composition of the feldspathic lunar meteorites. With 30–31% Al₂O₃, however, they are slightly richer in plagioclase than the most feldspathic lunar meteorites (～29% Al₂O₃), implying that the crust of the early central nearside of the Moon contained a higher abundance of highly feldspathic anorthosite than typical lunar highlands, as inferred from the lunar meteorites. The ancient regolith breccias, as well as the current surface regolith of the Cayley plains, are more mafic than (1) prebasin regoliths in the Central Highlands and (2) regions of highlands presently distant from nearside basins because they contain a high abundance (～30%) of mafic impact-melt breccias produced during the time of basin formation that is absent from other regoliths.     

Relationship between positive cerium anomaly and adsorbed water in Antarctic lunar meteorites

Abstract— Concentrations of adsorbed water in single mineral grains of Antarctic lunar meteorites were determined with micro infrared (IR) spectroscopy. A relationship was found between the mineral's ability to adsorb water and the extent of Ce anomaly in rare earth element (REE) patterns precisely determined by the isotope dilution method using a thermal ionization mass spectrometer. Asuka (A) 881757, a lunar meteorite from the mare basalt without Ce anomaly, showed no trace of IR absorption due to adsorbed water. On the contrary, Yamato (Y) 791197-109, Y-86032-98, Y-86032-95, Y-791197-115 and Y-82192-55A from the lunar highland exhibiting positive Ce anomaly showed IR absorption due to adsorbed water in some of their minerals. The detected water would be of terrestrial origin, because it was not structurally bound and easy to exchange judging from the spectral band shape. The contrast in concentration of adsorbed water between the lunar highland and the mare basalt is derived from a difference in the density of microfractures in mineral grains. Average concentrations of adsorbed water in the lunar highland meteorites were 3.8 mg/cm³ for pyroxene and olivine, and 1.7 mg/cm³ for plagioclase. This contrast between minerals is noteworthy because it has been known that Ce anomaly of pyroxene and olivine is larger than that of plagioclase both for Antarctic lunar meteorites and some lunar rocks. Furthermore, more adsorbed water was detected for minerals in meteorites that exhibit larger Ce anomaly. The present observations demonstrate that the extent of Ce anomaly correlated with the concentration of adsorbed water, which suggests that active mineral surface resulting in adsorption of water could be a trace of interaction forming Ce anomaly. Terrestrial weathering on Antarctica and REE fractionation on the Moon are discussed for possible origins of the Ce anomaly.