The life and times of Big Bertha: lunar breccia 14321

The lithic units of polymict breccia 14321 (Big Bertha) have been grouped according to composition, texture, degree of metamorphism, and additional criteria based on a systematic study of the interrelationships of all clast-matrix pairs. From this information it has been possible to reconstruct the assembly and metamorphic history of this breccia. The earliest formed fragmental component of 14321 (microbreccia-1) is dominated by KREEP-rich norite, extruded and subsequently brecciated and lithified in an ejecta blanket at approximately 1000°C in the general region of Mare Imbrium after the Serenitatis impact but prior to the Imbrium impact. This early microbreccia component and lesser amounts of mare-type basalt, microgranite, rhyolite glass, anorthosite and olivine microbreccia were assembled at the Apollo 14 site as part of the Fra Mauro ejecta blanket from the Imbrium impact. The resulting microbreccia-3 incorporates all the lithic types above and accretionary lapilli structures (microbreccia-2) in a dark matrix annealed at approximately 700°C. A later impact on the Fra Mauro excavated and mutually abraded microbreccia-3 and a local, 14321-type, basalt which were assembled into polymict breccia 14321. Final placement of 14321 at its sampling location was accomplished during the minor Cone Crater impact event.     

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.     

Apollo 12 revisited

We present compositional data for 358 lithic fragments (2-4-mm size range) and 15 soils (<1-mm fines) from regolith samples collected at the Apollo 12 site. The regolith is dominated by mare basalt, KREEP impact-melt breccias (crystalline and glassy), and regolith breccias. Minor components include alkali anorthosite, alkali norite, granite, quartz monzogabbro, and anorthositic rocks from the feldspathic highlands. The typical KREEP impact-melt breccia of Apollo 12 (mean Th: 16 μg/g) is similar to that of the Apollo 14 site (16 μg/g), 180 km away. Both contain a minor component (0.3% at Apollo 12, 0.6% at Apollo 14) of FeNi metal that is dissimilar to metal in ordinary chondrites but is similar to metal found in Apollo 16 impact-melt breccias. The Apollo 12 regolith contains another variety of KREEP impact-melt breccia that differs from any type of breccia described from the Apollo sites in being substantially richer in Th (30 μg/g) but with only moderate concentrations of K. It is, however, similar in composition to the melt breccia lithology in lunar meteorite Sayh al Uhaymir 169. The average composition of typical mature soil corresponds to a mixture of 65% mare basalt, 20% typical KREEP impact-melt breccia, 7% high-Th impact-melt breccia, 6% feldspathic material, 2.6% alkali noritic anorthosite, and 0.9% CM chondrite. Thus, although the site was resurfaced by basaltic volcanism 3.1-3.3 Ga ago, a third of the material in the present regolith is of nonmare origin, mainly in the form of KREEP impact-melt breccias and glass. These materials occur in the Apollo 12 regolith mainly as a result of moderate-sized impacts into surrounding Fra Mauro and Alpes Formations that formed craters Copernicus (93 km diameter, 406 km distance), Reinhold (48 km diameter, 196 km distance), and possibly Lansberg (39 km diameter, 108 km distance), aided by excavation of basalt interlayers and mixing of regolith by small, local impacts. Anomalous immature soil samples 12024, 12032, and 12033 contain a lesser proportion of mare basalt and a correspondingly greater proportion of KREEP lithologies. These samples consist mainly of fossil or paleoregolith, likely ejecta from Copernicus, that was buried beneath the mixing zone of micrometeorite gardening, and then brought to the near surface by local craters such as Head, Bench, and Sharp Craters.     

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.     

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.     

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.     

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.     

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.     

A protracted timeline for lunar bombardment from mineral chemistry,Ti thermometry and U–Pb geochronology of Apollo 14 melt breccia zircons

New zircon U–Pb and trace element investigations from Apollo 14 lunar impact breccia sample #14311 reveal at least three distinct (Concordia, 2σ) age populations at 4334 ± 10, 4245 ± 10 and 3953 ± 10 Ma. Titanium-in-zircon thermometry (Ti ^xln ) results correlated with U–Pb ages range from ~800–1200 ºC. Lattice strain models used to infer zircon versus whole-rock rare earth element contents, and partitioning calculations against lunar impact breccia component compositions, quantitatively constrain formation conditions for the different age populations. A compilation of new data with published work shows that Apollo 14 zircons older than ca. 4300 Ma formed by igneous processes associated with lunar crust formation. Compositional variability in the ca. 4240 Ma zircon age population is interpretable, however, via a mixture of inherited and melt-generated components from one or more large impacts perhaps related to a marked increase in bombardment flux. Ages from the youngest zircon group at ca. 3950 Ma coincide with the classical “late heavy bombardment” (LHB) as documented from previous lunar geochronologies. These results lend support to the idea that instead of a simple unimodal LHB scenario, or a monotonic decline in impacts, the Moon was battered by multiple cataclysms since ca. 4240 Ma. Such a “Picket fence”-like bombardment to the Moon best describes the mode and tempo of impacts that accompanied the late stages of solar system formation and giant planet migration.     

Identifying impact events within the lunar cataclysm from Ar–Ar ages and compositions of Apollo 16 impact melt rocks

Lunar impact melt breccias provide a unique record of the timing and frequency of collisional events during the early history of the inner Solar System prior to the development of a significant rock record on Earth. The predominance of ages clustering between 3.8 and 4.0 Ga was a major, unexpected discovery obtained from geochronological studies of lunar impact melts, and is the basis of the concept that a cataclysmic bombardment of large planetesimals struck the Earth and Moon, and possibly the entire inner Solar System, about 3.85 ± 0.10 billion years ago. As a test of the cataclysm hypothesis, we measured high-resolution (20–50 steps) ⁴⁰Ar–³⁹Ar age spectra on 25 samples of Apollo 16 impact melt breccias using a continuous laser heating system on sub-milligram fragments. Twenty-one of these 25 breccias produced multi-step plateaus that we interpret as crystallization ages, with 20 of these ages falling in the range 3.75–3.96 Ga. We propose that at least four different melt-producing impact events can be distinguished based on the ages, bulk compositions, and petrographic characteristics of Apollo 16 melt breccias. The recognition of multiple impact events within the Apollo 16 melt breccia suite shows that numerous impact events occurred on the lunar surface within a relatively narrow time interval, providing additional evidence of a heavy bombardment of the Moon during the early Archean.     

A laser probe Ar/Ar and INAA investigation of four Apollo granulitic breccias

Infrared laser probe ⁴⁰Ar/³⁹Ar geochronology, instrumental neutron activation analysis (INAA) and analytical electron microscopy have been performed on four 0.5 × 1.0 × 0.3 cm polished rock tiles of Apollo 16 and 17 granulitic breccias (60035, 77017, 78155, and 79215). Pyroxene thermometry indicates that these samples were re-equilibrated and underwent peak metamorphic sub-solidus recrystallization at 1000-1100 °C, which resulted in homogeneous mineral compositions and granoblastic textures.⁴⁰Ar/³⁹Ar data from this study reveal that three samples (60035, 77017, and 78155) have peak metamorphic ages of ∼4.1 Ga. Sample 79215 has a peak metamorphic age of 3.9 Ga, which may be related to Serenitatis basin formation. All four samples contain moderately high concentrations of meteoritic siderophiles. Enhanced siderophile contents in three of the samples provide evidence for projectile contamination of their target lithologies occurring prior to peak metamorphism.Post-peak metamorphism, low-temperature (<300 °C) events caused the partial resetting of argon in the two finer-grained granulites (60035 and 77017). These later events did not alter the mineralogy or texture of the rocks, but caused minor brecciation and the partial release of argon from plagioclase. Interpretation of the low-temperature data indicates partial resetting of the argon systematics to as young as 3.2 Ga for 60035 and 2.3 Ga for 77017. Cosmic ray exposure ages range from 6.4 to ∼339 Ma.Our results increase the amount of high-precision data available for the granulitic breccias and lunar highlands crustal samples. The results demonstrate the survival of pre-Nectarian material on the lunar surface and document the effects of contact metamorphic and impact processes during the pre-Nectarian Epoch, as well as the low-temperature partial resetting of ages by smaller impact events after 3.9 Ga.The mineralogy and chemical composition of these rocks, as well as exhumation constraints, indicate that the source of heat for metamorphism was within kilometers of the surface via burial beneath impact-melt sheets or hot ejecta blankets.     

Mineralogy, geochemistry, and Ar-Ar geochronology of lunar granulitic breccia Northwest Africa 3163 and paired stones: Comparisons with Apollo samples

The lunar meteorites Northwest Africa (NWA) 3163, 4881, and 4483 are paired stones classified as granulitic breccias. At 2.4 kg, these three stones constitute one of the largest known lunar meteorite masses. Here we describe the petrography, mineralogy, and chemistry of NWA 3163, 4881, and 4483, and present ⁴⁰Ar-³⁹Ar data for two of the meteorites. Two-pyroxene thermometry indicates that the rocks equilibrated at 1050 ± 50 °C, which represents the high-temperature, low-pressure event that generated their characteristic recrystallization textures and reset their Ar systematics. Stepped-heating, in situ infrared laser microprobe ⁴⁰Ar-³⁹Ar geochronology yields a mean age of 3327 ± 29 Ma for NWA 3163, and a more disturbed release spectrum for NWA 4881. NWA 4881 shows an upward-trending pattern, suggesting that it may have had a ⁴⁰Ar-³⁹Ar age of >3.0 Ga, but that it was partially reset at ∼2.6 Ga. NWA 3163 et al. exhibit shock effects, including maskelynitized plagioclase, shock veins, and melt pockets, which are absent in the Apollo granulitic breccias. Although the Apollo and meteorite samples are texturally similar and have comparable bulk compositions and equilibration temperatures, their trace and siderophile element contents point to distinct parental lithologies derived from different regions of the Moon. Based on mineralogical and geochemical differences between the Apollo and meteorite samples, we conclude that the parent rock(s) of the paired NWA meteorites came from an area outside the Imbrium region and that they underwent high-temperature (granulite event) metamorphism long after the Late Heavy Bombardment.     

Solar and cosmogenic argon in dated lunar impact spherules

We have studied lunar impact spherules from the Apollo 12 and Apollo 14 landing sites, examining the isotopic composition of argon released by stepwise heating. Elsewhere, we reported the formation ages of these spherules, determined by the ⁴⁰Ar/³⁹Ar isochron method. Here, we discuss solar and cosmogenic argon from the same spherules, separating these two components by correlating their partial releases with the releases of calcium-derived ³⁷Ar on a “cosmochron” diagram. We use the abundances of cosmogenic argon to derive a cosmic ray exposure age for each spherule, and demonstrate that single scoops of lunar soil contain spherules which have experienced very different histories of exposure and burial. The solar argon is seen to be separated into isotopically lighter and heavier fractions, which presumably were implanted to different depths in the spherules. The abundance of the isotopically heavy solar argon is too great to explain as a minor constituent of the solar particle flux, such as the suprathermal tail of the solar wind. The fact that the spherules have been individually dated allows us to look for possible variations in the solar wind as a function of time, over the history of the Solar System. However, the isotopic composition and fluence of solar argon preserved in the lunar spherules appear to be independent of formation age. We believe that most of the spherules are saturated with solar argon, having reached a condition in which implantation by the solar wind is offset by losses from solar-wind sputtering and diffusion.     

The contribution of the sensitive high-resolution ion microprobe (SHRIMP) to lunar geochronology

The sensitive high-resolution ion microprobe (SHRIMP) developed at the Australian National University (ANU) was the first of the high-resolution ion microprobes. The impact of this instrument on geochronological research over the last twenty years has been immense. This is particularly so for lunar geochronology where it has opened up avenues of research that were not possible using conventional TIMS techniques. The great advantage of SHRIMP is that it provides a means for determining precise U–Pb isotopic ratios on selected micron-size areas on polished grains of zircon and other U-bearing minerals. One of the first projects undertaken on the newly invented SHRIMP I was an investigation of U–Pb ages of lunar zircon. Using SHRIMP, multiple analyses could be made on areas of individual zircons to test the stability of U–Pb systems in shocked grains. Also, by analysing grains “in situ”, textural relationships between the analysed zircon and the components of the sample breccia could be used in the interpretation of the SHRIMP data. As a result of this research it was realised that most lunar zircons have ages up to 500 Ma older than the Imbrium and Serenitatis impacts at ca. 3.9 Ga, demonstrating that the zircons have not been affected by the these impact events although heating and shock effects have profoundly disturbed other dating systems. This has opened the way for research into the early lunar magmatic and bombardment record. For example, recent SHRIMP results have revealed profound differences in the ages of zircons from breccias from the Apollo 14 and Apollo 17 sample sites, raising new questions about the evolution of lunar magmatism. Also, multiple SHRIMP analyses on complex lunar zircons have shown that these grains can record U–Pb disturbance by later impact events. SHRIMP U–Pb age determinations on phosphates in lunar meteorites has identified lunar events not recognised in samples from the Apollo program. SHRIMP-based research on lunar materials is ongoing and, in combination with other chemical and structural evidence, continues to stimulate new ideas on the early evolution of the Moon.     

Lunar polymict breccia 14321: a petrographic study

Seven petrographic thin sections of lunar rock sample 14321, ‘Big Bertha’, have been examined. It is a complex rock incorporating diverse lithic and single crystal fragments and represents a sampling of the heterogeneous Fra Mauro formation, considered by the writers to be lithified debris from the Imbrium impact event. Electron probe microanalysis and microscopic study of textures reveal the assembly history of this breccia which in turn allows some interpretation of the nature of the pre-Imbrium crust and the effect of the Imbrium impact and the subsequent transportation to the Apollo 14 site. The present-day polymict breccia 14321 is composed of basaltic clasts originating from the fragmentation of a single or closely related set of lava cooling units, a set of fragmental clasts designated as microbreccia 3 (themselves polymict microbreccias), and a light colored matrix which formed rock 14321 by cementing the two major groups of clasts. The light colored matrix material is derived from the fragmentation and mutual abrasion of the basalt and microbreccia 3. On the basis of consistent textural relations two older sets of microbreccias have been identified within microbreccia 3. Microbreccia 1 clasts are well-rounded, relatively light colored, and noritic. They are always completely enclosed within microbreccia 3, most often forming the central cores of rounded accretionary lapilli structures which we have designated as microbreccia 2. Microbreccias 1, 2, 3, and macrobreccia 14321 represent a chronological series of fragmentation and lithification events. Each of these events involved some thermal and/or shock metamorphism as evidenced by mineralogical and textural criteria, and the chronological order of formation of the breccias also corresponds to a decreasing intensity of associated thermal effects. The petrology and mineralogy of 14321 are described in detail in this paper. A more general interpretation of the combined petrographic and chemical data is given in Duncanet al. (1975a).     

Major element chemistry of glasses in Apollo 14 soil 14156

Glasses in a soil sample (14156) from the middle layer of the trench at the Fra Mauro landing site show a wide range of compositions clustered around certain preferred compositions. Ninety per cent of the glasses are of two major types—Fra Mauro basalt (63 per cent) with high K and 17 weight per cent Al₂O₃ and Highland basalt or anorthositic gabbro (27 per cent) with low K and 25–26 weight per cent Al₂O₃. The glass population is almost identical with that of the comprehensive soil 14259 (Apollo Soil Survey, 1971).     

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).     

Trace element chemistry of Apollo 14 lunar soil from Fra Mauro

Analytical data are presented for Apollo 14 fines ( < 1 mm) sample 14163,136 for 31 trace elements. The heavy REE are enriched monotonically by factors of 105 ± 10 over chondrites. Eu shows a large depletion (30 × chondrites) and the light REE show a smooth progressive enrichment with a slight fall at La. Ba, Cs, Th, U, Nb, Zr and Hf are strongly enriched, relative to chondritic abundances. Thus the outer portions of the moon sampled by the Imbrium event, and now represented by the Fra Mauro Formation, possessed high concentrations (100–200 × chondrites) for many elements, prior to the excavation of the mare basins. A correlation may exist between Gd/Eu and Zr/Hf ratios in lunar materials.     

Volcanism in Mare Fecunditatis and Mare Crisium: Ar-Ar age studies

The laser ⁴⁰Ar/³⁹Ar dating technique has been applied to five Luna 16 basalt fragments and one impact glass, and nine Luna 24 basalt fragments and one breccia. The textures of these basalts are fine-grained ophitic and coarse-grained basalts. The samples contain high levels of solar and lunar atmospheric argon acquired during their residence on the lunar surface. These trapped argon components are predominantly released at low temperature steps and can be distinguished from radiogenic and cosmogenic released at intermediate and high temperature steps. The apparent ages obtained for Luna 16 samples span a narrow range of 3.29 to 3.38 Ga. A young age of 0.988 Ga was obtained for a basaltic impact glass indicating the age of an impact event in the vicinity of Luna 16 landing site. The ages obtained for Luna 24 samples suggest the existence of at least three periods of volcanism occurring over a protracted interval of between 3.45 and 2.52 Ga. The long period of volcanism suggested for the Mare Crisium was likely due to a combination of geophysical and geochemical features in the surrounding and underlying areas of the Crisium Basin. Attempts at dating three Luna 20 samples were inconclusive due to their high trapped argon contents.     

The regolith at the Apollo 15 site and its stratigraphic implications

Regolith samples from the Apollo 15 landing site are described in terms of two major fractions, a homogeneous glass fraction and a non-homogeneous glass fraction. The proportions of different components in the homogeneous glass fraction were determined directly by chemical analyses of individual particles. They are mainly green glass, a mare-like glass, and different types of Fra Mauro and Highland type glasses. The proportions of various components in the remainder of each of the soils were determined indirectly by finding the mix of components that best fits their bulk compositions. The mixing model suggests that the Apennine Front consists mainly of rocks of low-K Fra Mauro basalt composition. These may overlie rocks with the composition of anorthositic gabbro. Green glass, which occurs widely throughout the site is believed to be derived from a green glass layer which darkens upland surfaces and lies beneath the local mare surface.