Ar/Ar dating of Apollo 12 regolith: Implications for the age of Copernicus and the source of nonmare materials

Twenty-one 2–4 mm rock samples from the Apollo 12 regolith were analyzed by the ⁴⁰Ar/³⁹Ar geochronological technique in order to further constrain the age and source of nonmare materials at the Apollo 12 site. Among the samples analyzed are: 2 felsites, 11 KREEP breccias, 4 mare-basalt-bearing KREEP breccias, 2 alkali anorthosites, 1 olivine-bearing impact-melt breccia, and 1 high-Th mare basalt. Most samples show some degree of degassing at 700–800 Ma, with minimum formation ages that range from 1.0 to 3.1 Ga. We estimate that this degassing event occurred at 782 ± 21 Ma and may have been caused by the Copernicus impact event, either by providing degassed material or by causing heating at the Apollo 12 site. ⁴⁰Ar/³⁹Ar dating of two alkali anorthosite clasts yielded ages of 3.256 ± 0.022 Ga and 3.107 ± 0.058 Ga. We interpret these ages as the crystallization age of the rock and they represent the youngest age so far determined for a lunar anorthosite. The origin of these alkali anorthosite fragments is probably related to differentiation of shallow intrusives. Later impacts could have dispersed this material by lateral mixing or vertical mixing.     

Lunar geochemistry as told by lunar meteorites

About 36 lunar meteorites have been found in cold and hot deserts since the first one was found in 1979 in Antarctica. All are random samples ejected from unknown locations on the Moon by meteoroid impacts. Lithologically and compositionally there are three extreme types: (1) brecciated anorthosites with high Al₂O₃ (26–31%), low FeO (3–6%), and low incompatible elements (e.g., <1 μg/g Th), (2) basalts and brecciated basalts with high FeO (18–22%), moderately low Al₂O₃ (8–10%) and incompatible elements (0.4–2.1 μg/g Th), and (3) an impact-melt breccia of noritic composition (16% Al₂O₃, 11% FeO) with very high concentrations of incompatible elements (33 μg/g Th), a lithology that is identified as KREEP on the basis of its similarity to Apollo samples of that designation. Several meteorites are polymict breccias of intermediate composition because they contain both anorthosite and basalt. Despite the large range in compositions, a variety of compositional parameters together distinguish lunar meteorites from terrestrial materials. Compositional and petrographic data for lunar meteorites, when combined with mineralogical and compositional data obtained from orbiting spacecraft in the 1990s, suggest that Apollo samples identified with the magnesian (Mg-rich) suite of nonmare rocks (norite, troctolite, dunite, alkali anorthosite, and KREEP) are all products of a small, geochemically anomalous (noritic, high Th) region of crust known as the Procellarum KREEP Terrane and are not, as generally assumed, indigenous to the vast expanse of typical feldspathic crust known as the Feldspathic Highlands Terrane. Magnesian-suite rocks such as those of the Apollo collection do not occur as clasts in the feldspathic lunar meteorites. The misconception is a consequence of four historical factors: (1) the Moon has long been viewed as simply bimodal in geology, mare or highlands, (2) one of the last, large basin-forming bolides impacted in the Procellarum KREEP Terrane, dispersing Th-rich material, (3) although it was not known at the time, the Apollo missions all landed in or near the anomalous Procellarum KREEP Terrane and collected many Th-rich samples formed therein, and (4) the Apollo samples were interpreted and models for lunar crust formation developed without recognition of the anomaly because global data provided by orbiting missions and lunar meteorites were obtained only years later.     

Lunar materials: Their mineralogy,petrology and chemistry

The manned Apollo 11, 12, 14 and 15 and the automated Luna 16 lunar missions have provided us with lunar rock and regolith (soil) samples from a number of geologically distinct sites. The mare regions were sampled by Apollo 11, 12 and Luna 16, whereas Apollo 14 landed on a terrain with more relief, the Fra Mauro Formation which represents an ejecta blanket from the Imbrian Basin, and Apollo 15 touched down near the lunar highlands. The samples collected consist of a mixture, mainly of basalt, breccia and regolith (soil-particulate matter, generally < 1 cm in size). The basalts show considerable variation in texture, mineralogy and chemistry and probably represent fragments from various parts of relatively thin and extensive lava flows in the maria. The breccias represent regolith material which was indurated to varying degrees by impact events. The regolith is a product of the breakdown, again by impact, of coherent rock masses of basalt and breccia.     

Petrology, chemistry and origin of Apollo 15 regolith breccias

Variations in modal petrology, mineral compositions and bulk compositions were determined for ten Apollo 15 regolith breccias for comparison with local soils and assessment of the intrasite petrologic variability of the Apollo 15 regolith. Based on the above criteria the breccias are of local origin and mimic the soils from the corresponding sampling stations, with the exception of station 2 breccia 15205. This sample formed from an anomalous regolith and although not considered exotic to the site is not representative of the soil at the site. KREEP basalt and green glass components vary from trace amounts to dominant in the breccias, evidence that these materials entered the regolith prior to formation of the breccias. Breccias from the edge of Hadley Rille are modally richer in highland fragments than the soils, whereas at the base of Hadley Delta the reverse is true. This is explained by the loss of material into the Rille to be replaced by basalt-derived material, making the soils more basalt-rich. At the base of Hadley Delta highland material is accumulating and the soils are becoming more highland-rich. Over billions of years these processes have developed differences between the present day, evolving soils and “fossil” non-evolving soils represented by the regolith breccias. This shows that there has been little change in the geology and the morphology of the Apollo 15 site, probably since the eruption of mare basalts at the site (˜3.3 b.y.).     

Antarctic lunar meteorites Yamato-793169, Asuka-881757, MIL 05035, and MET 01210 (YAMM): Launch pairing and possible cryptomare origin

The Antarctic lunar meteorite Meteorite Hills (MET) 01210 is a polymict regolith breccia, dominantly composed of mare basalt components. One relatively large (2.7 × 4.7 mm) basalt clast in MET 01210 (MET basalt) shows remarkable mineralogical similarities to the lunar-meteorite crystalline mare basalts Yamato (Y)-793169, Asuka (A)-881757, and Miller Range (MIL) 05035. All four basalts have similar rock texture, mineral assemblage, mineral composition, pyroxene crystallization trend, and pyroxene exsolution lamellae. The estimated TiO₂ contents (∼2.0 wt%) of the MET basalt and MIL 05035 are close to the bulk-rock TiO₂ contents of Y-793169 and A-881757. These similarities suggest that Y-793169, A-881757, MIL 05035, and the MET basalt came from the same basalt flow, which we designate the YAMM basalt. The source-basalt pairing of the YAMM is also supported by their similar REE abundances, crystallization ages (approx. 3.8-3.9 Ga), and isotopic compositions (low U/Pb, low Rb/Sr, and high Sm/Nd). The pyroxene exsolution lamellae, which are unusually coarse (up to a few microns) by mare standards, imply a relatively slow cooling in an unusually thick lava and/or subsequent annealing within a cryptomare. Reported noble gas and CRE data with close launch ages (∼1 Ma) and ejection depths (deeper than several meters) among the four meteorites further indicate their simultaneous ejection from the moon. Despite the marginally close terrestrial ages, pairing in the conventional Earth-entry sense seems unlikely because of the remote recovery sites among the YAMM meteorites.The high abundance (68%) of mare components in MET 01210 estimated from a two-component mixing model calculation could have resulted from either lateral mixing at a mare-highland boundary or vertical mixing in a cryptomare. The proportion of mare materials in MET 01210 is greater than in Apollo core samples at the mare-highland boundary. The burial depth (>several meters deep) inferred from the lack of surface irradiation of MET 01210 exceeds the typical mare regolith thickness (a few meters). Thus, the source of the YAMM meteorites is likely a terrain of locally high mare-highland mixing within a cryptomare. We searched for a possible source crater of the YAMM meteorites within the well-defined cryptomare, based on the multiple constraints obtained from this study and published data. An unnamed 1.4 km-diameter crater (53°W, 44.5°S) on the floor of the Schickard crater is the most suitable source for the YAMM meteorites.The ²³⁸U/²⁰⁴Pb (μ) value of the YAMM basalts is extremely low, relative to those of the Apollo mare basalts, but comparable to those of the Luna 24 very low-Ti basalts. The low-μ source indicates a derivation from a less differentiated mantle with a lack of KREEP components. Although the chemical sources of materials and heat source of melting might be independent, the heat source that generated the source magma of the YAMM and Luna 24 basalts may not be related to KREEP, unlike the case of the Apollo basalts. The distinct chemical and isotopic compositions of mantle sources between the Apollo basalts and the YAMM/Lunar 24 basalts imply differences in mantle composition and thermal evolution between the Procellarum KREEP Terrane (PKT) and non-PKT regions of the nearside.     

Large-Scale Separation of K-frac and REEP-frac in the Source Regions of Apollo Impact-Melt Breccias,and a Revised Estimate of the KREEP Composition

Mafic impact-melt breccias (IMB) from the Apollo landing sites—particularly Apollo 14, Apollo 15, Apollo 16, and Apollo 17—are abundant and form compositionally distinct groups. These groups exhibit a range of major-element compositions and incompatible-element enrichments. Although concentrations of incompatible elements span a significant range, inter-element ratios vary little and have been used in the past to infer a common KREEP component (KREEP = rich in potassium, rare-earth elements, phosphorus, and other alkali and high-field-strength elements). On the basis of an extensive, high-precision data set for melt-breccia groups from different Apollo landing sites, variations in trace-element signatures of the mafic impact-melt breccias reflect significant differences in KREEP components of source regions. These differences are consistent with variable enrichment or depletion of source regions in those trace elements that fractionated during the latest stages of residual-melt evolution and are more or less related to “lunar granite.” Compared to other sites, the source region of Apollo 14 impact melts had an excess of the elements that are concentrated in lunar granite, suggesting either than this source region was enriched in such a component (K-frac) or that it lost a corresponding mafic component (REEP-frac). Because these are impact-melt breccias formed in large (probably basin) impacts, the indicated geochemical separations must have occurred on a broad scale.

Variations in the incompatible-element concentrations of the IMB groups reported in this paper are used to calculate a revised KREEP incompatible-element composition. On the basis of several extremely enriched lunar samples that retain the incompatible elements in KREEP-like ratios, the KREEP composition is extended to a level of 300 ppm La, or about three times the concentration of high-potassium KREEP as estimated by Warren (1989).     

SIMS U-Pb study of zircon from Apollo 14 and 17 breccias: Implications for the evolution of lunar KREEP

We report the results of a SIMS U-Pb study of 112 zircons from breccia samples from the Apollo 14 and 17 landing sites. Zircon occurs in the breccia matrices as rounded, irregular shaped, broken and rarely euhedral grains and as constituent minerals in a variety of lithic clasts ranging in composition from ultra-mafic and mafic rocks to highly evolved granophyres. Crystallisation of zircon in magmatic rocks is governed by the zirconium saturation in the melt. As a consequence, the presence of zircon in mafic rocks on the Moon implies enrichment of their parent melts in the KREEP component. Our SIMS results show that the ages of zircons from mafic to ultramafic clasts range from ca. 4.35 Ga to ca. 4.00 Ga demonstrating multiple generations of KREEPy mafic and ultramafic magmas over this time period. Individual zircon clasts in breccia matrices have a similar age range to zircons in igneous clasts and all represent zircons that have been incorporated into the breccia from older parents. The age distributions of zircons from breccias from both the Apollo 14 and Apollo 17 landing sites are essentially identical in the range 4.35-4.20 Ga. However, whereas Apollo 14 zircons additionally show ages from 4.20 to 3.90 Ga, no zircons from Apollo 17 samples have primary ages less than ca. 4.20 Ga. Also, in contrast to previous suggestions that the magmatism in the lunar crust is continuous our results show that the zircon age distribution is uneven, with distinct peaks of magmatic activity at ca. 4.35 Ga, ca. 4.20 Ga in Apollo 14 and 17 and a possible third peak in zircons from Apollo 14 at ca. 4.00 Ga. To explain the differences in the zircon age distributions between the Apollo 14 and 17 landing sites we propose that episodes of KREEP magmatism were generated from a primary reservoir, and that this reservoir contracted over time towards the centre of Procellarum KREEP terrane. We attribute the peaks in KREEP magmatism to impact induced emplacement of KREEP magma from a primary mantle source or to a progressive thermal build-up in the mantle source until the temperature exceeds the threshold for generation of KREEP magma, which is transported into the crust by an unspecified possibly plume-like process.     

Petrogenesis and chronology of lunar meteorite Northwest Africa 4472: A KREEPy regolith breccia from the Moon

Northwest Africa (NWA) 4472 is a polymict lunar regolith meteorite. The sample is KREEP-rich (high concentrations of potassium, rare earth elements and phosphorus) and comprises a heterogeneous array of lithic and mineral fragments. These clasts and mineral fragments were sourced from a range of lunar rock types including the lunar High Magnesian Suite, the High Alkali Suite, KREEP basalts, mare basalts and a variety of impact crater environments. The KREEP-rich nature of NWA 4472 indicates that the sample was ejected from regolith on the nearside of the Moon in the Procellarum KREEP Terrane and we have used Lunar Prospector gamma-ray remote sensing data to show that the meteorite is most similar to (and most likely sourced from) regoliths adjacent to the Imbrium impact basin.U-Pb and Pb-Pb age dates of NWA 4472 phosphate phases reveal that the breccia has sampled Pre-Nectarian (4.35 Ga) rocks related to early episodes of KREEP driven magmatism. Some younger phosphate U-Pb and Pb-Pb age dates are likely indicative of impact resetting events at 3.9-4 Ga, consistent with the suggested timing of basin formation on the Moon. Our study also shows that NWA 4472 has sampled impact melts and glass with an alkali-depleted, incompatible trace element-rich (high Sc, low Rb/Th ratios, low K) compositional signature not related to typical Apollo high-K KREEP, or that sampled by KREEPy lunar meteorite Sayh al Uhaymir (SaU) 169. This provides evidence that there are numerous sources of KREEP-rich protoliths on the Moon.     

The geochemistry and provenance of Apollo 16 mafic glasses

The regolith of the Apollo 16 lunar landing site is composed mainly of feldspathic lithologies but mafic lithologies are also present. A large proportion of the mafic material occurs as glass. We determined the major element composition of 280 mafic glasses (>10 wt% FeO) from six different Apollo 16 soil samples. A small proportion (5%) of the glasses are of volcanic origin with picritic compositions. Most, however, are of impact origin. Approximately half of the mafic impact glasses are of basaltic composition and half are of noritic composition with high concentrations of incompatible elements. A small fraction have compositions consistent with impact mixtures of mare material and material of the feldspathic highlands. On the basis of major-element chemistry, we identified six mafic glass groups: VLT picritic glass, low-Ti basaltic glass, high-Ti basaltic glass, high-Al basaltic glass, KREEPy glass, and basaltic-andesite glass. These glass groups encompass 60% of the total mafic glasses studied. Trace-element analyses by secondary ion mass spectroscopy for representative examples of each glass group (31 total analyses) support the major-element classifications and groupings. The lack of basaltic glass in Apollo 16 ancient regolith breccias, which provide snapshots of the Apollo 16 soil just after the infall of Imbrium ejecta, leads us to infer that most (if not all) of the basaltic glass was emplaced as ejecta from small- or moderate-sized impacts into the maria surrounding the Apollo 16 site after the Imbrium impact. The high-Ti basaltic glasses likely represent a new type of basalt from Mare Tranquillitatis, whereas the low-Ti and high-Al basaltic glasses possibly represent the composition of the basalts in Mare Nectaris. Both the low-Ti and high-Al basaltic glasses are enriched in light-REEs, which hints at the presence of a KREEP-bearing source region beneath Mare Nectaris. The basaltic andesite glasses have compositions that are siliceous, ferroan, alkali-rich, and moderately titaniferous; they are unlike any previously recognized lunar lithology or glass group. Their likely provenance is within the Procellarum KREEP Terrane, but they are not found within the Apollo 16 ancient regolith breccias and therefore were likely deposited at the Apollo 16 site post-Imbrium. The basaltic-andesite glasses are the most ferroan variety of KREEP yet discovered.     

Re-examination of the formation ages of the Apollo 16 regolith breccias

The lunar regolith is exposed to irradiation from the solar wind and to bombardment by asteroids, comets and inter-planetary dust. Fragments of projectiles in the lunar regolith can potentially provide a direct measure of the sources of exogenous material being delivered to the Moon. Constraining the temporal flux of their delivery helps to address key questions about the bombardment history of the inner Solar System.Here, we use a revised antiquity calibration (after Eugster et al., 2001) that utilises the ratio of trapped ⁴⁰Ar/³⁶Ar (‘parentless’ ⁴⁰Ar derived from radioactive decay of ⁴⁰K, against solar wind derived ³⁶Ar) to semi-quantitatively calculate the timing of the assembly of the Apollo 16 regolith breccias. We use the trapped ⁴⁰Ar/³⁶Ar ratios reported by McKay et al. (1986). Our model indicates that the Apollo 16 ancient regolith breccia population was formed between ∼3.8 and 3.4 Ga, consistent with regoliths developed and assembled after the Imbrium basin-forming event at ∼3.85 Ga, and during a time of declining basin-forming impacts. The material contained within the ancient samples potentially provides evidence of impactors delivered to the Moon in the Late-Imbrian epoch. We also find that a young regolith population was assembled, probably by local impacts in the Apollo 16 area, in the Eratosthenian period between ∼2.5 and 2.2 Ga, providing insights to the sources of post-basin bombardment. The ‘soil-like’ regolith breccia population, and the majority of local Apollo 16 soils, were likely closed in the last 2 Ga and, therefore, potentially provide an archive of projectile types in the Eratosthenian and Copernican periods.     

On the origin of impact glass in the Apollo 16 regolith

This study addresses the issue of what fraction of the impact glass in the regolith of a lunar landing site derives from local impacts (those within a few kilometers of the site) as opposed to distant impacts (10 or more kilometers away). Among 10,323 fragments from the 64-210-μm grain-size fraction of three Apollo 16 regolith samples, 14% are impact glasses, that is, fragments consisting wholly or largely of glass produced in a crater-forming impact. Another 16% are agglutinates formed by impacts of micrometeorites into regolith. We analyzed the glass in 1559 fragments for major- and minor-element concentrations by electron probe microanalysis and a subset of 112 of the fragments that are homogeneous impact glasses for trace elements by secondary ion mass spectrometry. Of the impact glasses, 75% are substantially different in composition from either the Apollo 16 regolith or any mixture of rocks of which the regolith is mainly composed. About 40% of the impact glasses are richer in Fe, Mg, and Ti, as well as K, P, and Sm, than are common rocks of the feldspathic highlands. These glasses must originate from craters in maria or the Procellarum KREEP Terrane. Of the feldspathic impact glasses, some are substantially more magnesian (greater MgO/FeO) or have substantially lower concentrations of incompatible elements than the regolith of the Apollo 16 site. Many of these, however, are in the range of feldspathic lunar meteorites, most of which derive from points in the feldspathic highlands distant from the Procellarum KREEP Terrane. These observations indicate that a significant proportion of the impact glass in the Apollo 16 regolith is from craters occurring 100 km or more from the landing site. In contrast, the composition of glass in agglutinates, on average, is similar to the composition of the Apollo 16 regolith, consistent with local origin.     

Petrology, geochemistry, and age of low-Ti mare-basalt meteorite Northeast Africa 003-A: A possible member of the Apollo 15 mare basaltic suite

Northeast Africa 003 (NEA 003) is a lunar meteorite found as a two paired stones (6 and 118 g) in Libya, 2000 and 2001. The main portion (∼75 vol%) of the 118 g meteorite, used for this study, (NEA 003-A) consists of mare-basalt and a smaller adjacent portion (∼25 vol%) is a basaltic breccia (NEA 003-B). NEA 003-A has a coarse-grained magmatic texture consisting mainly of olivine, pyroxene and plagioclase. The late-stage mineral association is composed mainly of elongated plagioclase, ilmenite, troilite, fayalite, Si-K-rich glass, apatite, and a rare SiO₂ phase. Other accessory minerals include ulvöspinel, chromite, and trace Fe-Ni metal. Olivine and pyroxene contain shock-induced fractures, and plagioclase is completely converted into maskelynite.The Fe/Mn values of the whole rock, olivines and pyroxenes, and the bulk-rock oxygen isotopic composition provide evidence for the lunar origin of NEA 003-A meteorite. This is further supported by the presence of Fe-Ni metal and the anhydrous mineral association.NEA 003-A is geochemically and petrographically distinct from previously described mare-basalt meteorites and is not paired with any of them. The petrography and major element composition of NEA 003-A is similar to the composition of low-Ti olivine mare basalts from Apollo 12 and olivine-normative basalts from Apollo 15. The NEA 003-A meteorite shows obvious geochemical similarities in trace elements contents with Apollo 15 olivine-normative basalts and could represent a yet unknown geochemically primitive member of the olivine-normative basalt series. The meteorite is depleted in rare earth elements (REE) and incompatible trace elements indicating a primitive character of the parental magma. The bulk-rock chemical composition demonstrates that the parent melt of NEA 003-A was not contaminated with KREEP components as a result of magma mixing or assimilation processes. Results of crystallization modelling and low minimum cooling rate estimates (∼0.07 °C/h) suggest that the parent melt of NEA 003-A crystallized in the lower part of a lava flow containing cumulate olivine (∼10%) and was probably derived from more primitive picritic magma by fractional crystallization processes.Sm-Nd dating yields an age of 3.09 ± 0.06 Ga which corresponds to the period of lower Eratosthenian lunar volcanic activity, and the near-chondritic ε_Nd value of −0.4 ± 0.3 indicates that the meteorite could be derived from a slightly enriched mantle source similar to the Apollo 15 green glasses. Ar-Ar step release results are inconsistent with Sm-Nd ages suggesting that NEA 003-A was exposed to one or more impact events. The most extensive event took place at 1.8 Ga and the shock intensity was likely between 28 and 45 GPa. The absence of solar Ar suggests that NEA 003-A has not been directly exposed at the lunar surface but the cosmic ray exposure age of 209 ± 6 Ma suggests that NEA 003-A resided in the upper regolith for part of its history.     

Fluorine and chlorine abundances in lunar apatite: Implications for heterogeneous distributions of magmatic volatiles in the lunar interior

Apatite has been analyzed from mare basalts, the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks using an electron probe microanalysis routine developed specifically for apatite. We determined that all the lunar apatite grains analyzed are predominantly fluorine rich; however, they also contain varying concentrations of chlorine and a missing structural component that, after ruling out other possibilities, we attribute to OH. Apatite grains from mare basalts are compositionally distinct from the apatite grains in the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks, which all had similar apatite compositions. Apatite grains in mare basalts are depleted in chlorine, and many of the analyzed grains have stoichiometry that suggests a significant OH component (i.e., >0.08 structural formula units), whereas apatite grains in the magnesian suite, alkali suite, and KREEP-rich impact melts are enriched in chlorine and do not typically have a missing structural component that could be attributed to OH⁻ (within the detection limit of 0.08 sfu). From these data, we infer that residual liquids in the mare basalts were enriched in H₂O and fluorine relative to chlorine at the time of apatite crystallization, whereas residual liquids in magnesian-suite, alkali-suite, and KREEP-rich impact melts were enriched in chlorine relative to H₂O and fluorine at the time of apatite crystallization. The relative volatile abundance that we determined for the mare basalts is identical to the previously determined relative volatile abundance for the lunar picritic glasses. This result indicates that the observed relative volatile abundance signature of the picritic glass source is the same as that in the mare basalt source regions. The magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks likely reflect a volatile source with different volatile abundances than the sources of mare volcanics. Moreover, the magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks may reveal the relative volatile abundance of urKREEP, the residual melt of the magma ocean. This difference in relative magmatic volatile abundance among the lithologic groups investigated cannot be explained by degassing of a single source composition (relative to magmatic volatiles). The most reasonable explanation for the compositional disparity is a difference in the relative volatile abundances in the magmatic source regions of the Moon. Therefore, we conclude that the Moon has a heterogeneous distribution of magmatic volatiles within its interior, with a chemical divide (with respect to magmatic volatiles) existing between magmas that arise by partial melting of the lunar mantle and magmas that have seen significant contamination by a KREEP component.     

The petrology and geochemistry of Miller Range 05035: A new lunar gabbroic meteorite

Miller Range (MIL) 05035 is a lunar gabbroic meteorite. The mineralogy, Fe/Mn ratios in olivine and pyroxene, bulk-rock chemical composition and the bulk oxygen isotope values (δ¹⁷O = 2.86-2.97‰ and δ¹⁸O = 5.47-5.71‰) are similar to those of other mare basalts, and are taken as supporting evidence for a lunar origin for this meteorite. The sample is dominated by pyroxene grains (54-61% by area mode of thin section) along with large plagioclase feldspar (25-36% by mode) and accessory quartz, ilmenite, spinel, apatite and troilite. The bulk-rock major element composition of MIL 05035 indicates that the sample has a very low-Ti (VLT) to low-Ti lunar heritage (we measure bulk TiO₂ to be 0.9 Wt.%) and has low bulk incompatible trace element (ITE) concentrations, akin to samples from the VLT mare basalt suite. To account for these geochemical characteristics we hypothesize that MIL 05035’s parental melt was derived from a mantle region dominated by early cumulates of the magma ocean (comprised principally of olivine and orthopyroxene). MIL 05035 is likely launch paired with the Asuka-881757 and Yamato-793169 basaltic lunar meteorites and the basaltic regolith breccia MET 01210. This group of meteorites (Y/A/M/M) therefore may be a part of a stratigraphic column consisting of an upper regolith environment underlain by a coarsening downwards basalt lava flow.     

Geochemistry, petrology and ages of the lunar meteorites Kalahari 008 and 009: New constraints on early lunar evolution

Kalahari 008 and 009 are two lunar meteorites that were found close to each other in Botswana. Kalahari 008 is a typical lunar anorthositic breccia; Kalahari 009 a monomict breccia with basaltic composition and mineralogy. Based on minor and trace elements Kalahari 009 is classified as VLT (very-low-Ti) mare basalt with extremely low contents of incompatible elements, including the REE. The Lu-Hf data define an age of 4286 ± 95 Ma indicating that Kalahari 009 is one of the oldest known basalt samples from the Moon. It provides evidence for lunar basalt volcanism prior to 4.1 Ga (pre-Nectarian) and may represent the first sample from a cryptomare. The very radiogenic initial ¹⁷⁶Hf/¹⁷⁷Hf (εHf = +12.9 ± 4.6), the low REE, Th and Ti concentrations indicate that Kalahari 009 formed from re-melting of mantle material that had undergone strong incompatible trace element depletion early in lunar history. This unusually depleted composition points toward a hitherto unsampled basalt source region for the lunar interior that may represent a new depleted endmember source for low-Ti mare basalt volcanism. Apparently, the Moon became chemically very heterogeneous at an early stage in its history and different cumulate sources are responsible for the diverse mare basalt types.Evidence that Kalahari 008 and 009 may be paired includes the similar fayalite content of their olivine, the identical initial Hf isotope composition, the exceptionally low exposure ages of both rocks and the fact that they were found close to each other. Since cryptomaria are covered by highland ejecta, it is possible that these rocks are from the boundary area, where basalt deposits are covered by highland ejecta. The concentrations of cosmogenic radionuclides and trapped noble gases are unusually low in both rocks, although Kalahari 008 contains slightly higher concentrations. A likely reason for this difference is that Kalahari 008 is a polymict breccia containing a briefly exposed regolith, while Kalahari 009 is a monomict brecciated rock that may never have been at the surface of the Moon.Altogether, the compositions of Kalahari 008 and 009 permit new insight into early lunar evolution, as both meteorites sample lunar reservoirs hitherto unsampled by spacecraft missions. The very low Th and REE content of Kalahari 009 as well as the depletion in Sm and the lack of a KREEP-like signature in Kalahari 008 point to a possible source far from the influence of the Procellarum-KREEP Terrane, possibly the lunar farside.     

Evidence from Ar/Ar ages of lunar impact glasses for an increase in the impact rate ∼800 Ma ago

Geochemical and ⁴⁰Ar/³⁹Ar data on nine impact glasses from the Apollo 14, 16, and 17 landing sites indicate at least seven distinct impact events with ages ∼800 Ma. Rock fragments analyzed by Barra et al. [Barra F., Swindle T. D., Korotev R. L., Jolliff B. L., Zeigler R. A., and Olsen E. (2006) ⁴⁰Ar-³⁹Ar dating of Apollo 12 regolith: implications for the age of Copernicus and the source of nonmare materials, Geochim. Cosmochim. Acta,70, 6016-6031] from the Apollo 12 landing site and some Apollo 12 spherules reported by Levine et al. [Levine J., Becker T. A., Muller R. A., Renne P. R. (2005) ⁴⁰Ar/³⁹Ar dating of Apollo 12 impact spherules, Geophys. Res. Let., 32, L15201, doi: 10.1029/2005GL022874.] show ∼800 Ma ages, close to the accepted age of the Copernicus event, 800 ± 15 Ma [Bogard D. D., Garrison D. H., Shih C. Y., and Nyquist L. E. (1994) ³⁹Ar-⁴⁰Ar dating of two lunar granites: The age of Copernicus, Geochim. Cosmochim. Acta, 58, 3093-3100]. These Apollo 12 samples are thought to have been affected by material from the Copernicus event since there is a Copernicus ray going through the Apollo 12 landing site. When all of these data are viewed collectively, including an Apollo 16 glass bomb [Borchardt R., Stöffler D., Spettel B., Palme H. and Wänke H. (1986) Composition, structure, and age of the Apollo 16 subregolith basement as deduced from the chemistry of post-Imbrium melt bombs. In Proceedings, 17th Lunar and Planetary Science Conference, pp. E43-E54], and in the context of diverse compositional range and sample location, there is a suggestion that there may have been a transient increase in the global lunar impact flux at ∼800 Ma. Therefore, the Copernicus impact event could have been one of many. If correct, there should be evidence for this increased impact flux around 800 Ma ago in the age statistics of terrestrial impact samples.     

Imbrium provenance for the Apollo 16 Descartes terrain: Argon ages and geochemistry of lunar breccias 67016 and 67455

In order to improve our understanding of impact history and surface geology on the Moon, we obtained ⁴⁰Ar-³⁹Ar incremental heating age data and major + trace element compositions of anorthositic and melt breccia clasts from Apollo 16 feldspathic fragmental breccias 67016 and 67455. These breccias represent the Descartes terrain, a regional unit often proposed to be ejecta from the nearby Nectaris basin. The goal of this work is to better constrain the emplacement age and provenance of the Descartes breccias.Four anorthositic clasts from 67016 yielded well-defined ⁴⁰Ar-³⁹Ar plateau ages ranging from 3842 ± 19 to 3875 ± 20 Ma. Replicate analyses of these clasts all agree within measurement error, with only slight evidence for either inheritance or younger disturbance. In contrast, fragment-laden melt breccia clasts from 67016 yielded apparent plateau ages of 4.0-4.2 Ga with indications of even older material (to 4.5 Ga) in the high-T fractions. Argon release spectra of the 67455 clasts are more variable with evidence for reheating at 2.0-2.5 Ga. We obtained plateau ages of 3801 ± 29 to 4012 ± 21 Ma for three anorthositic clasts, and 3987 ± 21 Ma for one melt breccia clast. The anorthositic clasts from these breccias and fragments extracted from North Ray crater regolith (Maurer et al., 1978) define a combined age of 3866 ± 9 Ma, which we interpret as the assembly age of the feldspathic fragmental breccia unit sampled at North Ray crater. Systematic variations in diagnostic trace element ratios (Sr/Ba, Ti/Sm, Sc/Sm) with incompatible element abundances show that ferroan anorthositic rocks and KREEP-bearing lithologies contributed to the clast population.The Descartes breccias likely were deposited as a coherent lithologic unit in a single event. Their regional distribution suggests emplacement as basin ejecta. An assembly age of 3866 ± 9 Ma would be identical with the accepted age of the Imbrium basin, and trace element compositions are consistent with a provenance in the Procellarum-KREEP Terrane. The combination of age and provenance constraints points toward deposition of the Descartes breccias as ejecta from the Imbrium basin rather than Nectaris. Diffusion modeling shows that the older apparent plateau ages of the melt brecia clasts plausibly result from incomplete degassing of ancient crust during emplacement of the Descartes breccias. Heating steps in the melt breccia clasts that approach the primary crystallization ages of lunar anorthosites show that earlier impact events did not completely outgas the upper crust.     

The variety of lithologies in the Yamato-86032 lunar meteorite: Implications for formation processes of the lunar crust

We performed a petrologic, mineralogical, geochemical, and isotopic study of several lithologies in the Y-86032 feldspathic breccia. This study leads us to conclude that Y-86032 likely originated on the lunar farside. Y-86032 is composed of several types of feldspathic clasts, granulitic breccias, and minor basaltic clasts set in a clastic matrix. We identify an “An97 anorthosite” that has An contents similar to those of nearside FANs. Mg′ (= molar Mg/(Mg + Fe) × 100) values vary significantly from ∼45 to ∼80 covering the ranges of both nearside FANs and the Mg′ gap between FANs and the Mg-suite. A light-gray feldspathic (LG) breccia making up ∼20% of the investigated slab (5.2 × 3.6 cm²) mainly consists of fragments of anorthosites (“An93 anorthosite”) more sodic than nearside FANs. LG also contains an augite-plagioclase clast which either could be genetically related to the An93 anorthosite or to slowly-cooled basaltic magma intruded into the precursor rock. The Na-rich nature of both An93 anorthosite and this clast indicates that the LG breccia was derived from a relatively Na-rich but incompatible-element-poor source. The Mg′ variation indicates that the “An97 anorthosite” is a genomict breccia of several types of primary anorthosites. Granulitic breccias in Y-86032 have relatively high Mg′ in mafic minerals. The highest Mg′ values in mafic minerals for the “An97 anorthosite” and granulitic breccias are similar to those of Mg-rich lithologies recently described in Dhofar 489. Basaltic clasts in the dark-gray matrix are aluminous, and the zoning trends of pyroxene are similar to those of VLT or LT basalts. The crystallization of these basaltic clasts pre-date the lithification age of the clastic matrix at ∼3.8 Ga. The low K contents of plagioclase in both the anorthositic and basaltic clasts and generally low incompatible element abundances in all the lithologies in Y-86032 indicate that KREEP was not involved during the formation of the precursor lithologies. This observation further suggests that urKREEP did not exist in the source regions of these igneous lithologies. All these facts support the idea that Y-86032 was derived from a region far distant from the PKT and that the lithic clasts and fragments are indigenous to that region. An An97 anorthositic clast studied here has distinct Sm-Nd isotopic systematics from those previously found for another An97 anorthositic clast and “An93 anorthosite”, and suggests either that An97 anorthosites come from isotopically diverse sources, or that the Sm-Nd isotopic systematics of this clast were reset ∼4.3 Ga ago. These lines of geochemical, isotopic, and petrologic evidence suggest that the lunar crust is geochemically more heterogeneous than previously thought.     

Lunar crater Copernicus: search for debris of impacting body at Apollo 12 site

In an attempt to characterize meteoritic material at the Apollo 12 site, 4 KREEP concentrates from soil 12033 have been analyzed by neutron activation analysis. These contain a meteoritic component in which siderophile Ir, Re and Sb are depleted by about a factor of 2, while volatile Se, Zn, Ag and Bi are depleted by a factor of more than 5 relative to Au. This pattern does not closely resemble any major chondrite or iron meteorite group, but is very similar to that observed in high-alkali samples from Apollo 14. The meteoritic component in KREEP at both sites is therefore predominantly derived from Imbrian ejecta. However, a second, small component of primitive composition seems to be present in Apollo 12 KREEP, judging from the slight, uniform enrichments in Ir, Re, Sb, Se and Zn relative to Au. This component does not seem to be due to micrometeorites. If it is attributed to the Copernican projectile, the crater Copernicus may have been formed by a cometary nucleus, 4 km in diameter, with an impact velocity of 30–40 km/sec. These conclusions depend critically on the assumption that the meteoritic component in Apollo 12 KREEP is representative of the entire impact.