Chemistry and thermal history of metal particles in Luna 20 soils

Individual metal particles from Luna 20 thin sections 521, 513 and 514 as well as several small metallic inclusions in silicate particles from Luna 20 thin sections 501 and 502 were examined using optical microscopy and the electron microprobe. All the metallic particles and inclusions analyzed are of meteoritic Co-Ni content as are most of the metallic particles from the Fra Mauro and the Apollo 16 highlands sites. It is proposed that most of the metal at these 3 sites had its origin in the meteoritic projectiles that bombarded and accumulated in the early lunar crust. It is apparent that the metallic particles and some of the metallic inclusions in the Luna 20 soil have been subjected to reheating on the Moon and this process has removed any evidence of the original meteoritic microstructure of the metal.     

Reduction of mare basalts by sulfur loss

Metallic Fe content and S abundance are inversely correlated in mare basalts. Either S volatilization from the melt results in reduction of Fe²⁺ to Fe⁰ or else high S content decreases Fe⁰ activity in the melt, thus explaining the correlation. All considerations favor the model that metallic iron in mare basalts is due to sulfur loss. The Apollo 11 and 17 mare basalt melts were probably saturated with S at the time of eruption; the Apollo 12 and 15 basalts were probably not saturated.Non-mare rocks show a positive correlation of S abundance with metallic Fe content; it is proposed that this is due to the addition of meteoritic material having a fairly constant Fe⁰/S ratio. If true, metallic Fe content or S abundance in non-mare rocks provides a measure of degree of meteoritic contamination.     

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.     

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.     

Chemical composition of Luna 20 rocks and soil and Apollo 16 soils

The abundances of 24 major, minor and trace elements have been measured by INAA in Luna 20 metaigneous rocks 22006,1 and 22007,1, breccia 22004 and soil 22001,9 and in Apollo 16 soils 62281, 66041 and 66081. An additional 12 trace meteoritic and non-meteoritic elements have also been determined in 22001 and 62281 soils by RNAA. The bulk compositions of L 20 and Ap 16 rocks and soils show close similarity between the two highland sites. There are appreciable differences in bulk compositions between the L 20 highland and the L 16 mare site (120 km apart), suggesting little intermixing of rocks and soils from either site. Luna 20 rocks 22006 and 22007 are nearly identical in chemical composition to Ap 16 metaigneous rocks 61156 and 66095. Luna 20 rocks are feldspathic and are similar to low K-type Fra Mauro basalts. Such rocks and anorthositic gabbros appear to be the major components in highland soils. Luna 20 soil can be distinguished from Ap 16 soils by lower abundances of Al₂O₃, CaO and large ion lithophilic elements. Luna 20 breccia 22004 probably is compacted soil. All L 20 samples show negative Eu anomalies with $\text{Sm}\text{Eu}$ ratios of 5.8, 7.2, 3.9 and 3.3 for rocks 22006, 22007, breccia 22004 and soil 22001, respectively. Norite-KREEP is insignificant, ≤1 per cent, at the L 20 highland site. The derivation of the L 20 soil may be explained by ≈33 per cent of L 20 metaigneous rocks and ≈ 65 per cent anorthositic gabbroic breccia rocks like 15418 (with a positive Eu anomaly) and ≈ 2 per cent meteoritic contributions. Interelement correlations observed previously for maria are also found in highland samples. Luna 20 and Ap 16 soils are low in alkalis. Both soils show an apparent Cd-Zn rich component similar to that observed at the mare sites and high 11 abundances relative to mare sites. The Ap 16 (62281) soil contains a fractionated meteoritic component (probably ancient) of ≈ 1.5 per cent in addition to ≈ 1.9 per cent Cl like material. Luna 20 soil may simply contain 1.9 per cent Cl equivalent.     

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.     

Osmium isotope and highly siderophile element systematics of lunar impact melt breccias: Implications for the late accretion history of the Moon and Earth

To characterize the compositions of materials accreted to the Earth-Moon system between about 4.5 and 3.8 Ga, we have determined Os isotopic compositions and some highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances in 48 subsamples of six lunar breccias. These are: Apollo 17 poikilitic melt breccias 72395 and 76215; Apollo 17 aphanitic melt breccias 73215 and 73255; Apollo 14 polymict breccia 14321; and lunar meteorite NWA482, a crystallized impact melt. Plots of Ir versus other HSE define excellent linear correlations, indicating that all data sets likely represent dominantly two-component mixtures of a low-HSE target, presumably endogenous component, and a high-HSE, presumably exogenous component. Linear regressions of these trends yield intercepts that are statistically indistinguishable from zero for all HSE, except for Ru and Pd in two samples. The slopes of the linear regressions are insensitive to target rock contributions of Ru and Pd of the magnitude observed; thus, the trendline slopes approximate the elemental ratios present in the impactor components contributed to these rocks. The ¹⁸⁷Os/¹⁸⁸Os and regression-derived elemental ratios for the Apollo 17 aphanitic melt breccias and the lunar meteorite indicate that the impactor components in these samples have close affinities to chondritic meteorites. The HSE in the Apollo 17 aphanitic melt breccias, however, might partially or entirely reflect the HSE characteristics of HSE-rich granulitic breccia clasts that were incorporated in the impact melt at the time of its creation. In this case, the HSE characteristics of these rocks may reflect those of an impactor that predated the impact event that led to the creation of the melt breccias. The impactor components in the Apollo 17 poikilitic melt breccias and in the Apollo 14 breccia have higher ¹⁸⁷Os/¹⁸⁸Os, Pt/Ir, and Ru/Ir and lower Os/Ir than most chondrites. These compositions suggest that the impactors they represent were chemically distinct from known chondrite types, and possibly represent a type of primitive material not currently delivered to Earth as meteorites.     

The composition and origin of metal in howardites

Howardites can be divided into two main groups, Ni-rich (>350ppm Ni) and Ni-poor (<150ppm Ni). In the Ni-rich group Ni occurs principally in metal grains associated with melt rocks and is largely derived from projectiles which caused the melting. The metal in Bununu, Kapoeta and Malvern melt rocks plots in the meteoritic Ni-Co range and in Bununu and Kapoeta is enriched in P. By contrast, most metal grains in primary lithic and crystal clasts in howardites are Ni-poor and plot mainly in the composition field of pristine lunar anorthosite metal. However, there are variations in the abundance and exact composition of primary metal from howardite to howardite and each therefore represents a discrete source region. The matrix metal in Bholgati, Bununu, and Kapoeta shows the diversity of compositions expected in a polymict breccia, with compositions plotting in and between the anorthositic and meteoritic Ni-Co fields. Other howardites show a more limited range of matrix metal compositions, because of limited metal-bearing clasts.Petersburg differs from other howardites in several ways. The metal in primary clasts has a unique $\text{Ni}\text{Co}$ ratio of about 40, which indicates derivation from a different reservoir from other howardite primary clasts. The metal in the matrix consists of large grains intergrown with silicates with compositions clustering tightly at 3.3% Ni, 0.2% Co. This is interpreted as equilibration, possibly as the result of deeper burial for Petersburg than for other howardites.     

The Luna 20 lithic fragments,and the composition and origin of the lunar highlands

Luna 20 soil 22003,1 (250–500 μ) is similar to Apollo 16 soil 61501,47 (250–500 μ) in terms of the percentage of different types of particles. However, among the lithic fragments, the Apollo 16 sample contains a greater percentage of fragments with more than 70 wt. % modal plagioclase and a significantly greater proportion of KREEP-rich particles. Modal analyses of non-mare lithic fragments in Luna 20 and Apollo 11, 14, 15 and 16 indicate that the KREEP-poor highland regions (the bulk of the lunar terrae), though relatively feldspathic, are compositionally inhomogeneous, ranging in plagioclase content from approximately 35 to 100 wt. %. The average plagioclase content lies in the range 45–70 wt.%. Luna 20 pyroxene analyses cluster in two groups, one more magnesian than the other. The groups persist when pyroxene analyses from KREEP-poor noritic, troctolitic and anorthositic lithic fragments from Apollo 11, 14, 15 and 16 and Luna 20 are included. Olivine compositions mimic these pyroxene groups.Within each pyroxene group Cr₂O₃ and TiO₂ decrease as $\text{Fe}\text{(Fe + Mg)}$ increases, suggesting a relationship by fractional crystallization. The two groups suggest that at least two magma compositions were involved. To account for these observations we envisage a Moon-wide magma system in which initial accretionary heterogeneities were imperfectly erased by diffusion and convection. During the cooling of this magma system fractional crystallization was effected by the flotation of plagioclase and sinking of pyroxene, olivine and perhaps ilmenite. The endproduct was an upper layer enriched in plagioclase and a lower layer enriched in mafic silicates. KREEP-rich rocks, which are predominantly noritic in major element composition, may be mechanical mixtures of KREEP-poor norite and material residual after fractional crystallization of the surface magma system.     

Graphite oxidation in the Apollo 17 orange glass magma: Implications for the generation of a lunar volcanic gas phase

An Apollo 17 picritic orange glass composition has been used to experimentally investigate the conditions at which graphite would oxidize to form a CO-rich gas, and ultimately produce lunar fire-fountain eruptions. Isothermal decompression experiments run above the A17 orange glass liquidus temperature (>1350 °C) suggest that the initial CO-rich gas phase produced by graphite oxidation would be generated during magma ascent at a pressure of 40 MPa, 8.5 km beneath the lunar surface. Additional experiments with 2000 ppm S and 1000 ppm Cl showed that the presence of these dissolved gas species would not affect the depth of graphite oxidation, verifying that the first volcanic gas phase would be generated by the oxidation of graphite.A simple ideal chemical mixing model for calculating melt FeO activity in a Fe-metal/silicate melt system was tested with a series of 0.1 MPa controlled oxygen fugacity experiments. Agreement between the model and experiments allows the model to be used to calculate oxygen fugacity in picritic lunar glass compositions such as the A17 orange glass. Using this model in a reanalysis of chemical equilibria between the natural A17 orange glass melt and the metal spherules (Fe₈₅Ni₁₄Co₁) trapped within the glass beads indicates a log oxygen fugacity of −11.2, 0.7 log units, more oxidized than previous estimates. At the A17 orange glass liquidus temperature (1350 ± 5 °C), this f_O2 corresponds to a minimum pressure of 41 MPa on the graphite–C–O surface. The fact that the same critical graphite oxidation pressure was determined in decompression experiments and from the Fe–FeO activity model for the natural A17 orange glass–metal assemblage strongly supports this pressure (8.5 km depth) for volcanic gas formation in lunar basalts. Generation of a gas by oxidation of C in ascending magma is likely to have been important in getting dense lunar magmas to the surface as well as in generating fire-fountain eruptions. The vesicles common in many lunar basalts and the ubiquitous Fe-metal in these rocks are also likely generated by the oxidation of carbon. The presence of carbon in the lunar basalts and the recent discovery of ppm levels of water in lunar basalts indicate that at least parts of the lunar interior still contained volatiles at 3.9 bybp.     

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.     

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.     

Tektites: A post-Apollo view

Prior to the receipt of the lunar samples, it was the scientific consensus that tektites were melted and splashed material formed during large cometary or meteorite impact events. Whether the impact took place on the Earth or the Moon was the topic of a long-standing scientific debate, which raged with particular intensity during the decade previous to the lunar landings.Four definite and separate tektite-strewn fields are known: bediasites (North America, 34 m.y.); moldavites (Czechoslovakia, 14 m.y.); Ivory Coast (1.3 m.y.); and Southeast Asian and Australian fields (0.7 m.y.). A fifth possible occurrence, of high-Na australites, possibly 3–4 m.y. old, remains to be substantiated. The age of infall of the australites is not agreed upon. Radiometric and fission track dates agree with the magnetic stratigraphy for deep-sea core microtektite occurrences at about 0.7 m.y. Terrestrial stratigraphic evidence favours a recent (30,000 years) date.The chemistry of tektites appears to reflect that of the parent material, and losses during fusion appear to be restricted to elements and compounds more volatile than cesium. Terrestrial impact glasses provide small-scale analogues of tektite-forming events, and indicate that only the most volatile components are lost during fusion.The Apollo lunar missions provide critical evidence which refutes the hypothesis of lunar origin of tektites. Tektite chemistry is totally distinct from that observed in lunar maria basalts. These possess Cr contents which are two orders of magnitude higher than tektites, distinctive REE patterns with large Eu depletions, high Fe and low SiO₂ contents, low K/U ratios and many other diagnostic features, none of which are observed in the chemistry of tektites. The lunar uplands compositions, as shown by Apollo 14, 15 and 16 samples and the μ-ray and XRF orbiter data, are high-Al, low-SiO₂ compositions totally dissimilar to those of tektites. The composition of lunar rock 12013 shows typical lunar features and is distinct from that of tektites. The small amounts of lunar K-rich granitic material found in the soils have K/Mg and K/Na ratios 10–50 times those of tektites.The ages of the lunar maria (3.2–3.8 aeons) and uplands (> 4.0 aeons) are an order of magnitude older than the parent material of the Southeast Asian and Australian tektites, which yield Rb-Sr isochrons indicating ages of the order of 100–300 m.y. The lunar lead isotopic compositions are highly radiogenic whereas tektites have terrestrial Pb isotopic ratios. Lunar δ¹⁸ O values are low (< 7 per mil) compared with values of +9.6 to +11.5 per mil for tektites. In summary, a lunar impact origin for tektites is not compatible with the chemistry, age or isotopic composition of the lunar samples. A lunar volcanic origin, recently revived by O'Keefe (1970) encounters most of the same problems. Recent lunar volcanism (< 50 m.y.), if the source of tektites, should contribute tektite glass to the upper layers of the regolith. None has been found. The presence of meteoritic components in tektites, and the high pressure phase coesite, are more readily interpreted as evidence of impact.The element abundances and inter-element variations in tektites do not resemble those in terrestrial igneous rocks, but show a close similarity to terrestrial sandstones. The composition of the Southeast Asian tektites, australites and moldavites resembles that of micaceous sandstones or subgreywackes, the Ivory Coast tektite composition is similar to that of greywacke, and the bediasite chemistry is analogous to that of arkose.No suitable terrestrial impact site has been identified for the bediasites, Southeast Asian tektites and australites. It is suggested that a search for the source of these latter strewnfields be made using satellite photographs to look for wide shallow craters produced by super-Tunguska type events on areas of Mesozoic sandstones. The moldavites were possibly formed during the Ries Crater event but, if so, the precise source of the material remains to be identified. The Ivory Coast tektites are linked by chemistry, isotope and age evidence to the Bosumtwi Crater, Ghana. The overall evidence now supports the origin of tektites by cometary (or meteorite) impact on terrestrial sedimentary rocks.     

Ablation spherules in the Sikhote Alin meteorite and their genesis

Practically all of the examined spherules extracted in 1948?C1949 from soil at the crater field of the 1947 Sikhote Alin meteorite shower are ablation spherules produced during this meteorite fall. The spherules can classified into two textural types: (i) fine-grained, which consist of Ni-bearing magnetite (3?C6 wt % NiO) dendrites, sometimes with minor amounts of interstitial P- and Fe-rich material, and (ii) coarse-grained, which also consist of Ni-bearing magnetite dendrites or grains, sometimes with wuestite, an interstitial material, which resembles that in type (i) or has a silicate composition. The texture, mineralogy, and chemistry (presence of P and Si) of these spherules differ from those of iron cosmic spherules (type I) that occur in the background flux of micrometeorites. The spherules are thought to were produced by the ablation of meteoritic material at elevations of about 12 km (in the region where disintegration starts) and below, at maximum temperatures of 1600?C2180°C and oxygen fugacity of 10^?14 to 10^?1 atm. Conceivably, the ablated material was enriched in silicates compared to the fallen material.     

Trace elements in natural metallic iron from Disko Island,Greenland

The largest occurrence of natural metallic iron on Earth is on the island of Disko, Greenland. Metallic iron is found there in a variety of different types, from small metal particles in basalts to large meter-sized blocks. We have studied three different types of metallic iron: small metal spherules (< 300 m) in basaltic magma; larger metal grains (300 m-3 mm), often composed of aggregates of smaller particles, in similar host rocks; and massive iron lumps (up to several tons). Analytical data for 13 siderophile elements in samples from these three types are presented. All metals analysed have a distinctly crustal pattern of siderophile elements. High Co/Ni, Re/Ir or W/Ir ratios clearly demonstrate that a meteoritic origin for the metallic iron must be excluded. Since the Co/Ni and Re/Ir ratios are approximately chondritic in the upper mantle of the Earth, a mantle origin for the Disko metals can also be ruled out. This supports earlier petrological and geological evidence that the metallic iron was formed through reduction of basaltic magma by carbon derived from Tertiary shales and coals. Significant differences in absolute and relative abundances of siderophile elements occur among the three kinds of metals. The strongly siderophile elements (e.g. Ir, Re, Ni) increase in concentration from the small metal spherules through the larger grains to the massive iron lumps. The contents of less strongly siderophile elements (P, W, Ga) decrease in the same sequence. Evidence is presented that the small metal spherules are formed by in situ reduction. Larger iron metal grains and massive iron lumps are composed of small spherules, accumulated by gravitational settling in a magma reservoir. These metal cumulates have extracted highly siderophile elements from a larger volume of basaltic melt.Part of a Ph.D. thesis by W. Klöck     

Chemistry and surface morphology of soil particles from Luna 20 LRL sample 22003

Six siliceous glass spheres, five siliceous glass-bonded agglutinates and one breccia fragment from Luna 20 LRL sample number 22003 were analyzed by optical microscope, scanning electron microscope, scanning electron microprobe and energy-dispersive techniques. The data suggest that most of the glass spheres were probably derived locally by meteoritic impact processes and that most craters on their surfaces may have occurred from impacts of relatively high velocity particles in the impact-produced debris cloud while the glass sphere was at elevated temperatures. This is suggested by the nature of the craters, the partially buried fragments of plagioclase surrounded by radiating fractures and by the apparent absence of craters on the glass surfaces of the glass-bonded agglutinates. One glass sphere has a surface suggestive of a complex multiple impact origin involving liquid siliceous material and numerous siliceous spherules from 0.1 μm to 1 μm in diameter that may have formed from vaporization and condensation processes possibly in a relatively large scale meteoritic impact event.The surfaces of the siliceous glass spheres have several different types of materials. Concentration of metallic iron spherules on the surfaces of the glass spheres is generally lower than for similar Apollo 11 and 12 glass spheres. This is consistent with reduction processes being of primary importance in the formation of this metallic iron. Surface material composed only of Ca, C and O₂, possibly CaCO₃, is probably derived from carbonaceous chondrites. Splashes of material rich in Ca, Al, Fe, K and Cl occur. The origin of the relatively low temperature chlorine-bearing melt is unknown but it may be related to vaporization and condensation processes, possibly volcanic in nature, or possibly to partial fusion of components of carbonaceous chondrites. Siliceous surface material rich in potassium may represent either fused splash material of granitic composition or material enriched by vaporization and condensation processes.     

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.     

Lunar surface geochemistry: Global concentrations of Th, K, and FeO as derived from lunar prospector and Clementine data

Accurate estimates of global concentrations of Th, K, and FeO have an important bearing on understanding the bulk chemistry and geologic evolution of the Moon. We present empirical ground-truth calibrations (transformations) for Lunar Prospector gamma-ray spectrometer data (K and Th) and a modified algorithm for deriving FeO concentrations from Clementine spectral reflectance data that incorporates an adjustment for TiO₂ content. The average composition of soil samples for individual landing sites is used as ground-truth for remotely sensed data. Among the Apollo and Luna sites, Apollo 12 and 14 provide controls for the incompatible-element-rich compositions, Apollo 16 and Luna 20 provide controls for the feldspathic and incompatible-element-poor compositions, and Apollo 11, 15, and 17, and Luna 16 and 24 provide controls for Fe-rich compositions. In addition to these Apollo and Luna sample data we include the composition of the feldspathic lunar meteorites as a proxy for the northern farside highlands to extend the range of the calibration points, thus providing an additional anorthositic end-member composition. These empirical ground-truth calibrations for Lunar Prospector Th and K provide self consistency between the existing derived data and lunar-sample data. Clementine spectral-reflectance data are used to construct a TiO₂-sensitive FeO calibration that yields higher FeO concentrations in areas of high-Fe, low-Ti, mare-basalt-rich surfaces than previous FeO algorithms. The data set so derived is consistent with known sample compositions and regolith mixing relationships.     

基于光谱特征的月球岩性分类方法研究——以Apollo 16登月区域为例

利用NASA行星数据系统提供Apollo计划登月点采样线路影像数据,通过与嫦娥二号CCD数据、印度M~3数据空间校正获得采样路线坐标。开展嫦娥二号CCD数据与印度M~3数据MAP(后验概率)融合并选择Apollo 15、Apollo 16-62231的LSCC测得的标准岩石双向反射率光谱与M~3、嫦娥二号进行交叉定标。本文采用月球岩石光谱谱型全特征分析方法,选取涵盖Apollo计划登月获取的36个基站主要岩性87种、285件岩石样品,利用校正后的M~3数据分析月球典型岩石各阶吸收反射特征,建立月球典型岩石标准遥感影像光谱库,此后应用Apollo 623个岩石样品进行对比得到很好结果,同时完成Apollo 16登月点周围领域岩性分布图,并讨论了研究区的岩石成因,Apollo 16区域形成于高地大撞击,在早期的研究中已经被用于划分月球年代,本文方法对于月球岩石类别研究与理解月球的岩浆演化具有重要的研究价值。     

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.