Light element geochemistry of the Apollo 16 site

Pronounced variations in abundances and isotopic compositions of some light elements in soils from the Apollo 16 site are interpreted in terms of differing degrees of solar wind exposure for an originally, and approximately, homogeneous regolith. Carbon abundances in soils are compatible with a model in which equilibrium is established, after 10⁴-10⁵ yr, between solar wind input and loss by H stripping. However, this model does not explain the observed C isotopic distribution, suggesting that other sources of C or other processes, or both, are also important. Carbon abundances in rocks from Apollo 16 are higher (average 40 ppm) than at other landing sites although their isotopic compositions, ?35 < δ¹³C < ?16%. PDB, are normal. Abundances of N and, to a less extent, He and H in soils correlate with C as does a fraction of metallic Fe attributed to in situ reduction of indigenous Fe²⁺ by solar wind H.Fillet soil 67461 apparently contains solar wind C and N in a relatively unfractionated form, yielding an upper limit to solar wind (δ¹³C of ?16%., PDB and a value of 3.4 for $\text{C}\text{N}$ in the solar wind.Sulfur at the Apollo 16 site represents a paradox in that, although abundances in soils are apparently controlled by local rock S contents, they also correlate, for all but one sample, with δ³⁴S, which itself is apparently controlled by surface exposure age. A complex lunar S cycle is suggested.     

Trapped solar wind noble gases and exposure age of Luna 16 lunar fines

He, Ne, Ar, Kr and Xe concentrations and isotopic abundances were measured in three bulk grain size fractions prepared from sample L-16-19, No. 120 (C level, 20–22 cm depth) returned by the Luna 16 mission. The expected anticorrelation between the concentrations of trapped solar wind noble gases and grain size is observed. Elemental abundances of solar wind trapped noble gases are similar to those previously found in corresponding grain size fractions of the Apollo 11 and 12 fines. The trapped ratio ${}^4\text{He}{}^{20}\text{Ne}$ varies in the soils from different lunar maria due to diffusion losses. A rough correlation of ${}^4\text{He}{}^{20}\text{Ne}$ with the proportion of ilmenite in these samples is apparent. The elemental and isotopic ratios of the surface correlated noble gases in Luna 16 resemble those previously found in Apollo fines. Based on ²¹Ne, ⁷⁸Kr and ¹²⁶Xe a cosmic ray exposure age of 360 my was determined. This age is similar to those obtained for the soils from other lunar maria.     

Light element geochemistry of the Apollo 12 site

Analytical techniques of improved sensitivity have revealed details of the concentrations and isotopic compositions of light elements for a comprehensive suite of samples from the Apollo 12 regolith. These samples show a wide spread in maturity, although maximum contents observed for solar wind elements are less than observed at other sites, possibly reflecting relative recency of craters at the Apollo 12 site. Isotopic composition of nitrogen is consistent with the idea that ¹⁵N/¹⁴N in the solar wind has increased with time, at least a major part of this increase having occurred in the past 3.1 Gyr. Sulfur isotope systematics support a model in which sulfur is both added to the regolith, by meteoritic influx and lost, by an isotopically selective process. Most soils from this site are heavily contaminated with terrestrial carbon.     

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.     

Do lunar and meteoritic archives record temporal variations in the composition of solar wind noble gases and nitrogen? A reassessment in the light of Genesis data

Since about half a century samples from the lunar and asteroidal regoliths been used to derive information about elemental and isotopic composition and other properties of the present and past solar wind, predominantly for the noble gases and nitrogen. Secular changes of several important compositional parameters in the solar wind were proposed, as was a likely secular decrease of the solar wind flux. In 2004 NASA’s Genesis mission returned samples which had been exposed to the solar wind for almost 2.5 years. Their analyses resulted in an unprecendented accuracy for the isotopic and elemental composition of several elements in the solar wind, including noble gases, O and N. The Genesis data therefore also allow to re-evaluate the lunar and meteorite data, which is done here. In particular, claims for long-term changes of solar wind composition are reviewed.Outermost grain layers from relatively recently irradiated lunar regolith samples conserve the true isotopic ratios of implanted solar wind species. This conclusion had been made before Genesis based on the agreement of He and Ne isotopic data measured in the aluminum foils exposed to the solar wind on the Moon during the Apollo missions with data obtained in the first gas release fractions of stepwise in-vacuo etch experiments. Genesis data allowed to strengthen this conclusion and to extend it to all five noble gases. Minor variations in the isotopic compositions of implanted solar noble gases between relatively recently irradiated samples (<100 Ma) and samples irradiated billions of years ago are very likely the result of isotopic fractionation processes that happened after trapping of the gases rather than indicative of true secular changes in the solar wind composition. This is particularly important for the ³He/⁴He ratio, whose constancy over billions of years indicates that hardly any ³He produced as transient product of the pp-chains has been mixed from the solar interior into its outer convective zone. The He isotopic composition measured in the present-day solar wind therefore is identical to the (D + ³He)/⁴He ratio at the start of the suns’s main sequence phase and hence can be used to determine the protosolar D/H ratio.Genesis settled the long-standing controversy on the isotopic composition of nitrogen in lunar regolith samples. The ¹⁵N/¹⁴N ratio in the solar wind as measured by Genesis is lower than in any lunar sample. This proves that nitrogen in regolith samples is dominated by non-solar sources. A postulated secular increase of ¹⁵N/¹⁴N by some 30% over the past few Ga is not tenable any longer. Genesis also provided accurate data on the isotopic composition of oxygen in the solar wind, invaluable for cosmochemisty. These data superseded but essentially confirmed one value – and disproved a second one – derived from lunar regolith samples shortly prior to Genesis.Genesis also confirmed prior conclusions that lunar regolith samples essentially conserve the true elemental ratios of the heavy noble gases in the solar wind (Ar/Kr, Kr/Xe). Several secular changes of elemental abundances of noble gases in the solar wind had been proposed based on lunar and meteoritic data. I argue here that lunar data – in concert with Genesis – provide convincing evidence only for a long-term decrease of the Kr/Xe ratio by almost a factor of two over the past several Ga. It appears that the enhancement of abundances of elements with a low first ionisation potential in the solar wind (FIP effect) changed with time.Finally, Genesis allows a somewhat improved comparison of the present-day flux of solar wind Kr and Xe with the total amount of heavy solar wind noble gases in the lunar regolith. It remains unclear whether the past solar wind flux has been several times higher on average than it is today.     

Irradiation records in regolith materials. I: isotopic compositions of solar-wind neon and argon in single lunar mineral grains

We have applied a stepwise pyrolytic extraction technique to eleven individual lunar regolith grains to investigate the compositions of light noble gases embedded in grain surfaces by solar wind irradiation, with emphasis on the rather poorly known isotopic composition of solar-wind argon. Results are intriguing: average ²⁰Ne/²²Ne ratios observed in early pyrolytic releases from ilmenite grains separated from lunar soils 71501, 79035 and 10084 agree very well with both direct measures of the solar wind neon composition in the Apollo foils and with values obtained in first releases from acid-etched ilmenites by the Zürich laboratory, whereas these same pyrolytic and acid-etch fractions carry argon isotopic signatures that significantly disagree—average ³⁶Ar/³⁸Ar ratios near 5.8 for thermal extraction compared to 5.4–5.5 for chemical etching at Zürich. Consideration of the isotopic and elemental data from these grains in the context of first-order diffusive modeling calculations points to gas release at low temperatures, without significant isotopic or elemental fractionation, from isolated grain-surface reservoirs of solar wind composition. The physical nature of these reservoirs is presently unknown. In this interpretation the preferred solar wind ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios deduced from this study are respectively 13.81 ± 0.08 and 0.0333 ± 0.0003, both within error of the Zürich acid-etch values, and ³⁶Ar/³⁸Ar = 5.77 ± 0.08. It may be possible to reconcile the discrepancy between the acid-etch and pyrolytic estimates for the solar wind ³⁶Ar/³⁸Ar ratio in the context of arguments originally advanced by Benkert et al. (1993) to account for their He and Ne isotopic compositions. At the other, high-temperature end of the release profile from one of these grains there are clear isotopic indications of the presence of a Ne constituent with ²⁰Ne/²²Ne close to the 11.2 ratio found at Zürich and attributed by these workers to a deeply-sited component implanted by solar energetic particles.     

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.     

18O16O ratios in Luna 16 fines

Partial fluorination experiments developed on a 26 mg sample of Luna 16 fines show a big ¹⁸O enrichment in the first oxygen evolved similar to those observed by Epstein and Taylor on Apollo samples and probably related to the high solar wind content of this sample.     

Luna 20 soil: abundance of 17 trace elements

Luna 20 soil is remarkably similar to Apollo 16 soil, in its content of 17 mainly volatile or siderophile elements: Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Rb, Re, Sb, Se, Te, Tl, U, and Zn. Like other highland soils, it seems to contain an ancient meteoritic component of fractionated, volatile-poor composition. The bulk soil has a high $\text{Tl}\text{Cs}$ ratio (9.4 × 10^?2), similar to that in Apollo 16 soils (5.4 × 10^?2), but higher than that in samples from other sites (1.1 × 10^?2). It is severely contaminated with Ag, Cd, Re, and Sb, judging from a comparison with a 1.7 mg soil breccia sample from the coarse fraction of the soil.     

Light element geochemistry and spallogenesis in lunar rocks

The abundances and isotopic compositions of carbon, nitrogen and sulfur were measured in eleven lunar rocks. Samples were combusted sequentially at three temperatures to resolve terrestrial contamination from indigenous volatiles.Sulfur abundances in Apollo 16 highland rocks range from 73 to 1165 μg/g-whereas sulfur contents in Apollo 15 and 17 basalts range from 719 to 1455 μg/g and correlate with TiO₂ content. Lunar rocks as a group have a remarkably uniform sulfur isotopic composition, which may reflect the low oxygen fugacity of the basaltic magmas. Much of the range of reported δ³⁴S_cd values (?2 to + 2.5 permil) is caused by systematic analytical discrepancies between laboratories.Lunar rocks very likely contain less than 0.1 μg/g of nitrogen. The measured spallogenic production rate, 4.1 × 10^?6 μg ¹⁵N/g sample/m.y., agrees remarkably closely with previous estimates. An estimate which includes all available data is 3.7 × 10^?6 μg¹⁵N/g sample/m.y.Lunar basalts may contain no indigenous lunar carbon in excess of procedural blank levels (~0.7 μg/g). Highlands rocks consistently release about 1 to 5 μg/g of carbon in excess of blank levels, but this carbon might either derive from ancient meteoritic debris or be a mineralogie product of terrestrial weathering. The average measured spallogenic ¹³C production rate is 4.1 × 10^?6 μg¹³C/g sample/m.y. The ¹³C spallation exposure ages of rocks 15058 and 15499 are 184 and 135 m.y., respectively.     

The mineralogy and petrology of the Luna 20 soil sample

Fragments of igneous rocks, glasses and minerals comprise 25 per cent of the studied sample of the Luna 20 soil. Basalt fragments in the Luna 20 soil are similar to basalts from the mare regions of the Moon—in that they are characterized by the presence of iron-rich olivines and pyroxenes. On the basis of the FeO contents of plagioclases, it appears possible to distinguish between the plagioclase of the mare and highland regions of the Moon. Other igneous rock fragments are anorthosite, gabbroic anorthosite and anorthositic gabbro. The most abundant rock type (75 per cent of the sample) is microbreceia. One third of the fragments of microbreccia have undergone thermal metamorphism resulting in the homogenization of phases and the development of poikioblastic and hornfelsic textures. Excluding the basalt fragments, the dominant minerals in the Luna 20 soil are anorthite (An_93–98), magnesium-rich orthopyroxenes, intermediate clinopyroxenes and olivine (< Fa₅₀). Chemically, the Luna 20 and Apollo 16 soil samples are similar, but the Luna 20 soil is slightly depleted in aluminum and calcium and enriched in iron and magnesium relative to the Apollo 16 soils. The slight difference in bulk chemistry of the two soils may be a result of the presence of a minor amount of mare material in the Luna 20 soil and its apparent absence in the Apollo 16 soils.     

The oxygen isotope composition, petrology and geochemistry of mare basalts: Evidence for large-scale compositional variation in the lunar mantle

To investigate the formation and early evolution of the lunar mantle and crust we have analysed the oxygen isotopic composition, titanium content and modal mineralogy of a suite of lunar basalts. Our sample set included eight low-Ti basalts from the Apollo 12 and 15 collections, and 12 high-Ti basalts from Apollo 11 and 17 collections. In addition, we have determined the oxygen isotopic composition of an Apollo 15 KREEP (K - potassium, REE - Rare Earth Element, and P - phosphorus) basalt (sample 15386) and an Apollo 14 feldspathic mare basalt (sample 14053). Our data display a continuum in bulk-rock δ¹⁸O values, from relatively low values in the most Ti-rich samples to higher values in the Ti-poor samples, with the Apollo 11 sample suite partially bridging the gap. Calculation of bulk-rock δ¹⁸O values, using a combination of previously published oxygen isotope data on mineral separates from lunar basalts, and modal mineralogy (determined in this study), match with the measured bulk-rock δ¹⁸O values. This demonstrates that differences in mineral modal assemblage produce differences in mare basalt δ¹⁸O bulk-rock values. Differences between the low- and high-Ti mare basalts appear to be largely a reflection of mantle-source heterogeneities, and in particular, the highly variable distribution of ilmenite within the lunar mantle. Bulk δ¹⁸O variation in mare basalts is also controlled by fractional crystallisation of a few key mineral phases. Thus, ilmenite fractionation is important in the case of high-Ti Apollo 17 samples, whereas olivine plays a more dominant role for the low-Ti Apollo 12 samples.Consistent with the results of previous studies, our data reveal no detectable difference between the Δ¹⁷O of the Earth and Moon. The fact that oxygen three-isotope studies have been unable to detect a measurable difference at such high precisions reinforces doubts about the giant impact hypothesis as presently formulated.     

Two oxygen isotopic components with extra-selenial origins observed among lunar metallic grains - In search for the solar wind component

Oxygen isotopic analyses were performed in the surface layers of lunar metallic grains from lunar regolith samples 71501 and 79035, presumably exposed at the Moon surface at different times. We were able to reproduce the two extreme O components previously found [Hashizume K. and Chaussidon M. (2005) A non-terrestrial ¹⁶O-rich isotopic composition for the protosolar nebula. Nature434, 619-622; Ireland T. R., Holden P., Norman M. D. and Clarke J. (2006) Isotopic enhancements of ¹⁷O and ¹⁸O from solar wind particles in the lunar regolith. Nature440, 776-778], with a range observed of −12 ± 5 < Δ¹⁷O < +33 ± 3‰ (1σ). The relatively minor ¹⁶O-rich component corresponding to an end-member Δ¹⁷O value lower than −20‰ is likely the solar component. This comes from the fact that its concentration roughly agrees with the maximum solar wind abundance expected among the grains from the two samples. At variance the ¹⁶O-poor component is 5-10 times more abundant and thus likely non-solar. The δ¹⁸O range found for the ¹⁶O-poor component may reflect various processes such as isotope exchange reaction during oxidation of metallic iron and/or isotope fractionation by evaporation/condensation at the surface of the Moon or during implantation at depth in the lunar metallic grains. The present study suggests that planetary solid materials in bulk are systematically depleted in ¹⁶O relative to the solar isotopic composition, suggesting the existence of non-mass-dependent isotopic fractionations associated to the formation of solids in the accretion disk.     

Major element compositions of Luna 20 glass particles

Major element analyses of nineteen Luna 20 glass particles indicate that most of the Luna 20 glasses have Al₂O₃ contents greater than 21 wt.% and compositions similar to Apollo 10 and Luna 20 rocks and soils. Three of the glass particles have low Al₂O₃ (< 13 wt.%) and high FeO (> 18 wt.%) contents and were probably derived from one of the adjacent maria. The low glass content of the Luna 20 soil indicates that it is relatively young or less mature than most mare soils that have been studied.     

Chemical and oxygen isotopic compositions of accretionary rim and matrix olivine in CV chondrites: Constraints on the evolution of nebular dust

A correlation of petrography, mineral chemistry and in situ oxygen isotopic compositions of fine-grained olivine from the matrix and of fine- and coarse-grained olivine from accretionary rims around Ca-Al-rich inclusions (CAIs) and chondrules in CV chondrites is used here to constrain the processes that occurred in the solar nebula and on the CV parent asteroid. The accretionary rims around Leoville, Vigarano, and Allende CAIs exhibit a layered structure: the inner layer consists of coarse-grained, forsteritic and ¹⁶O-rich olivine (Fa_1-40 and Δ¹⁷O = −24‰ to −5‰; the higher values are always found in the outer part of the layer and only in the most porous meteorites), whereas the middle and the outer layers contain finer-grained olivines that are more fayalitic and ¹⁶O-depleted (Fa_15-50 and Δ¹⁷O = −18‰ to +1‰). The CV matrices and accretionary rims around chondrules have olivine grains of textures, chemical and isotopic compositions similar to those in the outer layers of accretionary rims around CAIs. There is a correlation between local sample porosity and olivine chemical and isotopic compositions: the more compact regions (the inner accretionary rim layer) have the most MgO- and ¹⁶O-rich compositions, whereas the more porous regions (outer rim layers around CAIs, accretionary rims around chondrules, and matrices) have the most MgO- and ¹⁶O-poor compositions. In addition, there is a negative correlation of olivine grain size with fayalite contents and Δ¹⁷O values. However, not all fine-grained olivines are FeO-rich and ¹⁶O-poor; some small (<1 μm in Leoville and 5-10 μm in Vigarano and Allende) ferrous (Fa_>20) olivine grains in the outer layers of the CAI accretionary rims and in the matrix show significant enrichments in ¹⁶O (Δ¹⁷O = −20‰ to −10‰). We infer that the inner layer of the accretionary rims around CAIs and, at least, some olivine grains in the finer portions of accretionary rims and CV matrices formed in an ¹⁶O-rich gaseous reservoir, probably in the CAI-forming region. Grains in the outer layers of the CAI accretionary rims and in the rims around chondrules as well as matrix may have also originated as ¹⁶O-rich olivine. However, these olivines must have exchanged O isotopes to variable extents in the presence of an ¹⁶O-poor reservoir, possibly the nebular gas in the chondrule-forming region(s) and/or fluids in the parent body. The observed trend in isotopic compositions may arise from mixtures of ¹⁶O-rich forsterites with grain overgrowths or newly formed grains of ¹⁶O-poor fayalitic olivines formed during parent body metamorphism. However, the observed correlations of chemical and isotopic compositions of olivine with grain size and local porosity of the host meteorite suggest that olivine accreted as a single population of ¹⁶O-rich forsterite and subsequently exchanged Fe-Mg and O isotopes in situ in the presence of aqueous solutions (i.e., fluid-assisted thermal metamorphism).     

Where do the meteorites come from? A re-evaluation of the earth-crossing apollo objects as sources of chondritic meteorites

The total number of Earth-crossing Apollo objects larger than 500 m in radius is estimated to be 600, based on failure of chance rediscovery, lunar crater frequency and completeness-of-search results of Shoemaker, Helin and Gillett. The number of Amor objects (perihelion between 1.0 and 1.3 A.U.) is estimated to be about 500. These estimates are about an order of magnitude higher than those given by previous workers, and these objects appear sufficiently numerous to dominate post-mare lunar and terrestrial cratering (d ≥ 10 km).The terrestrial meteorite and meteorite yield of 100-10⁶ g bodies derived from fragmentation of Apollo objects is re-evaluated using this estimate, together with more recent data on asteroid albedos and on hypervelocity impact. Terrestrial rate of impacts of these fragments at sufficiently low velocities to penetrate the atmosphere is estimated to be ~2 × 10⁸g/yr. This is in the middle of the range of the actual extraterrestrial impact rate based on photographic fireball surveys (Prairie Network), lunar seismometry, and recovery of meteorites. It is likely that most ordinary chondrites are fragments of Apollo objects, provided that these fragments are sufficiently strong to survive atmospheric entry.Possible asteroidal and cometary sources of Apollo objects are reviewed. Several mechanisms for the removal of asteroids into Earth-crossing orbit are qualitatively acceptable, but appear inadequate by at least an order of magnitude to supply the required number. Most Apollo objects are probably the cores of comets which have lost their volatile material by repeated solar evaporation, as proposed by Öpik.The distribution of the component of the Apollo objects' angular momentum perpendicular to the plane of the solar system is tabulated. It is found that considerable non-random clustering of these values exists, for which no adequate explanation is known.     

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.     

Isotopic composition of surface-correlated chromium in Apollo 16 lunar soils

We have analyzed by thermal ionization mass spectrometry (TIMS) the isotopic composition of Cr in five progressive etches of size-sorted plagioclase grains separated from lunar soils 60601 and 62281. Aliquots of the etch solutions were spiked for isotopic dilution (ID) analysis of Cr and Ca. The Ca ID data indicate that the initial etch steps represent dissolution of an average 0.1 to 0.2 μm depth from the grain surfaces, the approximate depth expected for implanted solar wind. The Cr/Ca ratio in the initial etches is several fold higher than that expected for bulk plagioclase composition, but in subsequent etches decreases to approach the bulk value. This indicates a source of Cr extrinsic to the plagioclase grains, surface-correlated and resident in the outermost fraction of a μm, which we provisionally identify as solar wind Cr. The surface-correlated Cr is isotopically anomalous and by conventional TIMS data reduction has approximately 1 permil excess ⁵⁴Cr and half as great excess ⁵³Cr. In successive etches, as the Cr/Ca ratio decreases and approaches the bulk plagioclase value, the magnitude of the apparent anomalies decreases approaching normal composition. If these results do indeed characterize the solar wind, then either the solar wind is enriched in Cr due to spallation in the solar atmosphere, or the Earth and the various parent bodies of the meteorites are isotopically distinct from the Sun and must have formed from slightly different mixes of presolar materials. Alternative interpretations include the possibility that the anomalous Cr is meteoritic rather than solar or that the observed (solar) Cr is normal except for a small admixture of spallation Cr generated on the Moon. We consider these latter possibilities less likely than the solar wind interpretation. However, they cannot be eliminated and remain working hypotheses.     

Fluorine in lunar samples: implications concerning lunar fluorapatite

The F contents of a number of Apollo 14 and 15 samples range from less than a ppm for anorthosite rock fragments to ~165 ppm for some soils and breccias. Apollo 15 soils tend to have lower F contents (50–70 ppm) than soils from other sites. In most cases samples were run simultaneously with W-1 in which F was determined to be 216 (±11) ppm.The $\text{F}\text{P}{}_2\text{O}{}_5$ ratio is 0·032 ± 0·005 in soils and rocks. A correlation exists in soils between F, P₂O₅, and that fraction of the Cl which is insoluble in hot water. The $\text{F}\text{Cl}{}_{\mathrm{r}}$ ratio in soils and rocks, though different, requires that the phosphate phase involved be fluorapatite; this is consistent with mineralogical observations. F, like Cl, is correlated with KREEP elements at all sites for which data are available.     

Nanophase Fe0 in lunar soils

Back scattered electron and transmission electron imaging of lunar soil grains reveal an abundance of submicrometer-sized pure Fe⁰ globules that occur in the rinds of many soil grains and in the submillimeter sized vesicular glass-cemented grains called agglutinates. Grain rinds are amorphous silicates that were deposited on grains exposed at the lunar surface from transient vapors produced by hypervelocity micrometeorite impacts. Fe⁰ may have dissociated from Fe-compounds in a high temperature (>3000°C) vapor phase and then condensed as globules on grain surfaces. The agglutinitic glass is a quenched product of silicate melts, also produced by micrometeorite impacts on lunar soils. Reduction by solar wind hydrogen in agglutinitic melts may have produced immiscible droplets that solidified as globules. The exact mechanism of formation of such Fe⁰ globules in lunar soils remains unresolved.