An unusual clast in lunar meteorite MacAlpine Hills 88105: A unique lunar sample or projectile debris?

Lunar meteorite MacAlpine Hills (MAC) 88105 is a well‐studied feldspathic regolith breccia dominated by rock and mineral fragments from the lunar highlands. Thin section MAC 88105,159 contains a small rock fragment, 400 × 350 μm in size, which is compositionally anomalous compared with other MAC 88105 lithic components. The clast is composed of olivine and plagioclase with minor pyroxene and interstitial devitrified glass component. It is magnesian, akin to samples in the lunar High Mg‐Suite, and also alkali‐rich, akin to samples in the lunar High Alkali Suite. It could represent a small fragment of late‐stage interstitial melt from an Mg‐Suite parent lithology. However, olivine and pyroxene in the clast have Fe/Mn ratios and minor element concentrations that are different from known types of lunar lithologies. As Fe/Mn ratios are notably indicative of planetary origin, the clast could either (1) have a unique lunar magmatic source, or (2) have a nonlunar origin (i.e., consist of achondritic meteorite debris that survived delivery to the lunar surface). Both hypotheses are considered and discussed.     

An analysis of Apollo lunar soil samples 12070,889, 12030,187, and 12070,891: Basaltic diversity at the Apollo 12 landing site and implications for classification of small‐sized lunar samples

Lunar mare basalts provide insights into the compositional diversity of the Moon's interior. Basalt fragments from the lunar regolith can potentially sample lava flows from regions of the Moon not previously visited, thus, increasing our understanding of lunar geological evolution. As part of a study of basaltic diversity at the Apollo 12 landing site, detailed petrological and geochemical data are provided here for 13 basaltic chips. In addition to bulk chemistry, we have analyzed the major, minor, and trace element chemistry of mineral phases which highlight differences between basalt groups. Where samples contain olivine, the equilibrium parent melt magnesium number (Mg#; atomic Mg/[Mg + Fe]) can be calculated to estimate parent melt composition. Ilmenite and plagioclase chemistry can also determine differences between basalt groups. We conclude that samples of approximately 1–2 mm in size can be categorized provided that appropriate mineral phases (olivine, plagioclase, and ilmenite) are present. Where samples are fine‐grained (grain size <0.3 mm), a “paired samples t‐test” can provide a statistical comparison between a particular sample and known lunar basalts. Of the fragments analyzed here, three are found to belong to each of the previously identified olivine and ilmenite basalt suites, four to the pigeonite basalt suite, one is an olivine cumulate, and two could not be categorized because of their coarse grain sizes and lack of appropriate mineral phases. Our approach introduces methods that can be used to investigate small sample sizes (i.e., fines) from future sample return missions to investigate lava flow diversity and petrological significance.     

Lunar crater depths from orbiter IV long-focus photographs

The paper presents method and results for the determination of the depths of more than 1900 small lunar craters from measures of shadows on the long-focus pictures obtained by Lunar Orbiter IV. The method for converting the measured shadow length into the true length in nature of the shadow hypotenuse is new and is applicable to other planetary bodies provided comparable spacecraft ephemerides are available. The measures were made with a simple surveyor's plotting scale on the standard Orbiter IV photographic enlargements. The results indicate that the smaller lunar (D < 30 km) craters are appreciably deeper than is indicated by earlier work using imagery obtained at terrestrial observatories.     

The effects of space weathering on Apollo 17 mare soils: Petrographie and chemical characterization

Abstract— The lunar soil characterization consortium, a group of lunar‐sample and remote‐sensing scientists, has undertaken the extensive task of characterization of the finest fractions of lunar soils, with respect to their mineralogical and chemical makeup. These compositional data form the basis for integration and modeling with the reflectance spectra of these same soil fractions. This endeavor is aimed at deciphering the effects of space weathering of soils on airless bodies with quantification of the links between remotely sensed reflectance spectra and composition. A beneficial byproduct is an understanding of the complexities involved in the formation of lunar soil. Several significant findings have been documented in the study of the <45 μm size fractions of selected Apollo 17 mare soils. As grain size decreases, the abundance of agglutinitic glass increases, as does the plagioclase, whereas the other minerals decrease. The composition of the agglutinitic glass is relatively constant for all size fractions, being more feldspathic than any of the bulk compositions; notably, TiO₂ is substantially depleted in the agglutinitic glass. However, as grain size decreases, the bulk composition of each size fraction continuously changes, becoming more Al‐rich and Fe‐poor, and approaches the composition of the agglutinitic glasses. Between the smallest grain sizes (10–20 and < 10 μm), the I_S/FeO values (amount of total iron present as nanophase Fe⁰) increase by greater than 100% (>2x), whereas the abundance of agglutinitic glass increases by only 10–15%. This is evidence for a large contribution from surface‐correlated nanophase Fe⁰ to the I_S/FeO values, particularly in the <10 μm size fraction. The surface nanophase Fe⁰ is present largely as vapor‐deposited patinas on the surfaces of almost every particle of the mature soils, and to a lesser degree for the immature soils (Keller et al., 1999a). It is reasoned that the vapor‐deposited patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe⁰.     

Dry separation of respirable lunar dust: Providing samples for the lunar airborne dust toxicity advisory group

In order to study the toxicity of lunar dust, the respirable size fraction of lunar soil needed to be separated with an apparatus that possesses the following capabilities: no use of liquid; fully recoverable sample; and use of only small sample quantities (<1 gm). We report the design of a simple apparatus that meets these requirements and implements an inertial-impaction mechanism established in aerosol science. Lunar soil was agitated at a frequency of 100 Hz using a vibration table with a containment chamber under a constant air flow. The air flow carried the lofted lunar dust particles past four impactors (four T-junction connectors), upon which a fraction of large particles were captured during the impaction. The fine particles in the air flow were then collected by an end-of-the line membrane filter. Detailed examination of particles on the filter showed that the majority (∼80-90 wt%) are <3 μm (geometric diameter), suggesting a high level of effectiveness for the apparatus.     

Petrogenesis of lunar impact melt rock meteorite Oued Awlitis 001

Oued Awlitis 001 is a highly feldspathic, moderately equilibrated, clast‐rich, poikilitic impact melt rock lunar meteorite that was recovered in 2014. Its poikilitic texture formed due to moderately slow cooling, which judging from textures of rocks in melt sheets of terrestrial impact structures, is observed in impact melt volumes at least 100 m thick. Such coherent impact melt volumes occur in lunar craters larger than ~50 km in diameter. The composition of Oued Awlitis 001 points toward a crustal origin distant from incompatible‐element‐rich regions. Comparison of the bulk composition of Oued Awlitis 001 with Lunar Prospector 5° γ‐ray spectrometer data indicates a limited region of matches on the lunar farside. After its initial formation in an impact crater larger than ~50 km in diameter, Oued Awlitis 001 was excavated from a depth greater than ~50 m. The cosmogenic nuclide inventory of Oued Awlitis 001 records ejection from the Moon 0.3 Ma ago from a depth of at least 4 m and little mass loss due to ablation during its passage through Earth's atmosphere. The terrestrial residence time must have been very short, probably less than a few hundred years; its exact determination was precluded by a high concentration of solar cosmic ray‐produced ¹⁴C. If the impact that excavated Oued Awlitis 001 also launched it, this event likely produced an impact crater >10 km in diameter. Using petrologic constraints and Lunar Reconnaissance Orbiter Camera and Diviner data, we test Giordano Bruno and Pierazzo as possible launch craters for Oued Awlitis 001.     

The distribution of sulfur in lunar rocks and its relationship to carbon content

A summary of total sulfur abundances representative of the Apollo Missions is presented. Lunar crystalline rocks range from 0 to 3100μg S g⁻¹. Lunar soils range from 310 to 1300μg S g⁻¹. Rock mixing models evaluate the distribution of sulfur and define indigenous rock components and extralunar contributions of sulfur in lunar soils. Extralunar sulfur shows a positive correlation with a CC-1 like meteoritic component and solar wind derived total carbon content in the Apollo 16 and 17 lunar soils. Presented at the 25th International Geological Congress, Sydney, Australia, Section 15, Planetology. Contribution No. 105 from the Center for Meteorite Studies.     

The Barringer Award Address Presented 1996 July 25, Berlin,Germany: Impact experiments related to the evolution of planetary regoliths

Abstract— Impact-induced comminution of planetary surfaces is pervasive throughout the solar system and occurs on submillimeter to global scales, resulting in comminution products that range from fine-grained surface soils, to massive, polymict ejecta deposits, to collisionally fragmented objects. Within this wide range of comminution products, we define regoliths in a narrow sense as materials that were processed by repetitive impacts to dimensional scales comparable to or smaller than that of component minerals of the progenitor rock(s). In this paper, we summarize a wide variety of impact experiments and other observations that were primarily intended to understand the evolution of regoliths on lunar basalt flows, and we discuss some of their implications for asteroidal surfaces. Cratering experiments in both rock and noncohesive materials, combined with photogeologic observations of the lunar surface, demonstrate that craters <500 m in diameter contribute most to the excavation of local bedrock for subsequent processing by micrometeorites. The overall excavation rate and, thus, growth rate of the debris layer decreases with time, because the increasingly thicker fragmental layer will prevent progressively larger projectiles from reaching bedrock. Typical growth rates for a 5 m thick lunar soil layer are initially (～≥3 Ga ago) a few mm/Ma and slowed to <1 mm/Ma at present. The coarse-grained crater ejecta are efficiently comminuted by collisional fragmentation processes, and the mean residence time of a 1 kg rock is typically 10 Ma. The actual comminution of either lithic or monomineralic detritus is highly mineral specific, with feldspar and mesostasis comminuting preferentially over pyroxene and olivine, thus resulting in mechanically fractionated fines, especially at grain sizes <20 μm. Such fractionated fines also participate preferentially in the shock melting of lunar soils, thus giving rise to “agglutinate” melts. As a consequence, agglutinate melts are systematically enriched in feldspar components relative to the bulk composition of their respective host soil(s). Compositionally homogeneous, impact derived glass beads in lunar soils seem to result from micrometeorite impacts on rock surfaces, reflecting lithic regolith components and associated mineral mixtures. Cumulatively, experimental and observational evidence from lunar mare soils suggests that regoliths derive substantially from the comminution of local bedrock; the addition of foreign, exotic components is not necessary to explain the modal and chemical compositions of diverse grain size fractions from typical lunar soils. Regoliths on asteroids are qualitatively different from those of the Moon. The modest impact velocities in the asteroid belt, some 5 km s^?1, are barely sufficient to produce impact melts. Also, substantially more crater mass is being displaced on low-gravity asteroids compared to the Moon; collisional processing of surface boulders should therefore be more prominent in producing comminuted asteroid surfaces. These processes combine into asteroidal surface deposits that have suffered modest levels of shock metamorphism compared to the Moon. Impact melting does not seem to be a significant process under these conditions. However, the role of cometary particles encountering asteroid surfaces at presumably higher velocities has not been addressed in the past. Unfortunately, the asteroidal surface processes that seemingly modify the spectral properties of ordinary chondrites to match telescopically obtained spectra of S-type asteroids remain poorly understood at present, despite the extensive experimental and theoretical insights summarized in this report and our fairly mature understanding of lunar surface processes and regolith evolution.     

The optical properties of the finest fraction of lunar soil: Implications for space weathering

Abstract— The fine fraction of lunar soils (<45 μm) dominates the optical properties of the bulk soil. Definite trends can be seen in optical properties of size separates with decreasing particle size: diminished spectral contrast and a steeper continuum slope. These trends are related to space weathering processes and their affects on different size fractions. The finest fraction (defined here as the <10 μm fraction) appears to be enriched in weathering products relative to the larger size fractions, as would be expected for surface correlated processes. This <10 μm fraction tends to exhibit very little spectral contrast, often with no distinguishable ferrous iron absorption bands. Additionally, the finest fractions of highland soils are observed to have very different spectral properties than the equivalent fraction of mare soils when compared with larger size fractions. The spectra of the finest fraction of feldspathic soils flatten at longer wavelengths, whereas those of the finest fraction of basaltic soils continue to increase in a steep, almost linear fashion. This compositional distinction is due to differences in the total amount of nanophase iron that accumulates in space weathering products. Such ground‐truth information derived from the <10 μm fraction of lunar soils provides valuable insight into optical properties to be expected in other space weathering environments such as the asteroids and Mercury.     

Constraints on the flux of meteoritic and cometary water on the Moon from volatile element (N–Ar) analyses of single lunar soil grains,Luna 24 core

We report new nitrogen and argon isotope and abundance results for single breccia clasts and agglutinates from four different sections of the Luna 24 drill core in order to re-evaluate the provenance of N trapped in lunar regolith, and to place limits on the flux of planetary material to the Moon’s surface. Single Luna 24 grains with ⁴⁰Ar/³⁶Ar ratios <1 show δ¹⁵N values between ?54.5‰ and +123.3‰ relative to terrestrial atmosphere. Thus, low-antiquity lunar soils record both positive and negative δ¹⁵N signatures, and the secular increase of the δ¹⁵N value previously postulated by Kerridge (Kerridge, J.F. [1975]. Science 188(4184), 162–164. doi:10.1126/science.188.4184.162) is no longer apparent when the Luna and Apollo data are combined. Instead, the N isotope signatures, corrected for cosmogenic ¹⁵N, are consistent with binary mixing between isotopically light solar wind (SW) N and a planetary N component with a δ¹⁵N value of +100‰ to +160‰. The lower δ¹⁵N values of Luna 24 grains compared to Apollo samples reflect a higher relative proportion of solar N, resulting from the higher SW fluence in the region of Mare Crisium compared to the central near side of the Moon. Carbonaceous chondrite-like micro-impactors match well the required isotope characteristics of the non-solar N component trapped in low-antiquity lunar regolith. In contrast, a possible cometary contribution to the non-solar N flux is constrained to be ?3–13%. Based on the mixing ratio of SW to planetary N obtained for recently exposed lunar soils, we estimate the flux of micro-impactors to be (2.2–5.7) × 10³ tons yr^?1 at the surface of the Moon. Our estimate for Luna 24 agrees well with that for young Apollo regolith, indicating that the supply of planetary material does not depend on lunar location. Thus, the continuous influx of water-bearing cosmic dust may have represented an important source of water for the lunar surface over the past ～1 Ga, provided that water removal rates (i.e., by meteorite impacts, photodissociation, and sputtering) do not exceed accumulation rates.     

The petrology,geochemistry, and age of lunar regolith breccias Miller Range 090036 and 090070: Insights into the crustal history of the Moon

Meteorites ejected from the surface of the Moon as a result of impact events are an important source of lunar material in addition to Apollo and Luna samples. Here, we report bulk element composition, mineral chemistry, age, and petrography of Miller Range (MIL) 090036 and 090070 lunar meteorites. MIL 090036 and 090070 are both anorthositic regolith breccias consisting of mineral fragments and lithic clasts in a glassy matrix. They are not paired and represent sampling of two distinct regions of the lunar crust that have protoliths similar to ferroan anorthosites. ⁴⁰Ar‐³⁹Ar chronology performed on two subsplits of MIL 090070,33 (a pale clast impact melt and a dark glassy melt component) shows that the sample underwent two main degassing events, one at ~3.88 Ga and another at ~3.65 Ga. The cosmic ray exposure data obtained from MIL 090070 are consistent with a short (~8–9 Ma) exposure close to the lunar surface. Bulk‐rock FeO, TiO₂, and Th concentrations in both samples were compared with 2‐degree Lunar Prospector Gamma Ray Spectrometer (LP‐GRS) data sets to determine areas of the lunar surface where the regolith matches the abundances observed on the sample. We find that MIL 090036 bulk rock is compositionally most similar to regolith surrounding the Procellarum KREEP Terrane, whereas MIL 090070 best matches regolith in the feldspathic highlands terrane on the lunar farside. Our results suggest that some areas of the lunar farside crust are composed of ferroan anorthosite, and that the samples shed light on the evolution and impact bombardment history of the ancient lunar highlands.     

The Distribution of Craters Within Craters

Photometric correction is a necessary step in planetary image pre-processing since the images of planetary surfaces are acquired by orbiting spacecraft at various observational geometries. In this study, visible (748 nm) and near-infrared (948 nm) bands of Hyper Spectral Imager (HySI) onboard Chandrayaan-1 have been used to derive a preliminary photometric correction for lunar data. The purpose of the proposed photometric correction for HySI is to convert observations taken at solar incidence (i), sensor emission (e), and the solar phase angles (α) to a fixed geometry by applying i?=?α?=?30° and e?=?0° to each image. The Lommel–Seeliger function was used to model the lunar limb darkening effect, while topography data from the merged Digital Elevation Model of Lunar Reconnaissance Orbiter—Lunar Orbiter Laser Altimeter (LRO-LOLA) and SELENE Terrain Camera (TC) was used to correct local topographic effects. Data from Moon Mineralogy Mapper (M³), SELENE Multiband Imager (MI) and Clementine Ultraviolet and Visible Camera (UV/VIS) were also used to compare radiance, reflectance and phase functions derived from HySI. Our analysis reveals that HySI is darker than M³ primarily due to low surface radiance conditions observed by HySI. The derived phase functions for the two HySI bands indicate a good correlation between the derived reflectance and phase angle as well as with the phase functions derived for the empirically corrected M³ data. This approach led to the derivation of a photometric correction for maria regions. Finally, it is expected that the proposed correction would be applicable to all HySI images covering the lunar mare region.

     

Atom probe tomography of space‐weathered lunar ilmenite grain surfaces

The surfaces of airless bodies, such as the Moon and asteroids, are subject to space weathering, which alters the mineralogy of the upper tens of nanometers of grain surfaces. Atom probe tomography (APT) has the appropriate 3‐D spatial resolution and analytical sensitivity to investigate such features at the nanometer scale. Here, we demonstrate that APT can be successfully used to characterize the composition and texture of space weathering products in ilmenite from Apollo 17 sample 71501 at near‐atomic resolution. Two of the studied nanotips sampled the top surface of the space‐weathered grain, while another nanotip sampled the ilmenite at about 50 nm below the surface. These nanotips contain small nanophase Fe particles (~3 to 10 nm diameter), with these particles becoming less frequent with depth. One of the nanotips contains a sequence of space weathering products, compositional zoning, and a void space (~15 nm in diameter) which we interpret as a vesicle generated by solar wind irradiation. No noble gases were detected in this vesicle, although there is evidence for ⁴He elsewhere in the nanotip. This lunar soil grain exhibits the same space weathering features that have been well documented in transmission electron microscope studies of lunar and Itokawa asteroidal regolith grains.     

Lunar physical librations and laser ranging

The analysis of lunar laser ranging data requires very accurate calculations of the lunar physical librations. Libration terms are given which arise from the additive and planetary terms in the lunar theory. The large size of the recently discovered terms due to third degree gravitational harmonics will allow some of these harmonics to be measured, in addition to and, by laser ranging to the Moon. Combining the laser ranging determinations of = 630.6 ± 0.5 × 10^–6 and = 226.4 ± 3.0 × 10^–6 with lunar orbiter measurements ofC ₂₀ andC ₂₂ givesC/MR ²=0.395 _-0.010 ^+0.006 . Numerical integration promises to be an effective method of calculating librations. Comparison of numerical integrations with analytic series indicates that the calculation of the series due to third and fourth degree harmonics is not yet as accurate as the more extensively developed second degree terms.Communication presented at the Conference on Lunar Dynamics and Observational Coordinate Systems, held January 15–17, 1973, at the Lunar Science Institute, Houston, Tex., U.S.A.     

Heterogeneous agglutinitic glass and the fusion of the finest fraction (F3) model

Abstract— Evidence in favor of the model fusion of the finest fraction (F³) for the origin of lunar agglutinitic glass has been accruing. They include (1) theoretical expectations that shock pulses should engulf and melt smaller grains more efficiently than larger grains, (2) experimental results of impact shock, albeit at lower than presumed hypervelocity impacts of micrometeorites on the lunar regolith, and (3) new analyses confirming previous results that average compositions of agglutinitic glass are biased towards that of the finest fraction of lunar soils from which they had formed. We add another reason in support of the F³ model. Finer grains of lunar soils are also much more abundant. Hence, electrostatic forces associated with the rotating terminator region bring the finest grains that are obviously much lighter than courser grains to the surface of the Moon. This further contributes to the preferential melting of the finest fraction upon micrometeoritic impacts. New backscattered electron imaging shows that agglutinitic glass is inhomogeneous at submicron scale. Composition ranges of agglutinitic glass are extreme and deviate from that of the finest fraction, even by more than an order of magnitude for some components. Additionally, we show how an ilmenite grain upon impact would produce TiO₂‐rich agglutinitic glass in complete disregard to the requirements of fusion of the finest fraction. We propose an addition to the F³ model to accommodate these observations (i.e., that micrometeorite impacts indiscriminately melt the immediate target regardless of grain size or grain composition). We, therefore, suggest that (1) agglutinitic glass is the sum of (a) the melt produced by the fusion of the finest fraction of lunar soils and (b) the microvolume of the indiscriminate target, which melts at high‐shock pressures from micrometeoritic impacts, and that (2) because of the small volume of the melt and incorporating cold soil grains, the melt quenched so rapidly that it did not mix and homogenize to represent any preferential composition, for example, that of the finest fraction.     

Properties of the Hermean regolith: V. New optical reflectance spectra, comparison with lunar anorthosites, and mineralogical modelling

We present new optical (0.4-0.65 μm) spectra of Mercury and lunar pure anorthosite locations, obtained quasi-simultaneously with the Nordic Optical Telescope (NOT) in 2002. A comparative study is performed with the model of Lucey et al. (2000, J. Geophys. Res. 105, 20297-20305, and references therein) between iron-poor, mature, pure anorthosite (>90% plagioclase feldspar) Clementine spectra from the lunar farside and a combined 0.4-1.0 μm mercurian spectrum, obtained with the NOT, calculated for standard photometric geometry. Mercury is located at more extreme locations in the Lucey ratio-reflectance diagrams than any known lunar soil, specifically with respect to the extremely iron-poor mature anorthosites. Though quantitative prediction of FeO and TiO₂ abundances cannot be made without a more generally applicable model, we find qualitatively that the abundances of both these oxides must be near zero for Mercury. We utilize the theory of Hapke (2002, Icarus 157, 523-534, and references therein), with realistic photometric parameters, to model laboratory spectra of matured mineral powders and lunar soils, and remotely sensed spectra of lunar anorthosites and Mercury. An important difference between fabricated and natural powders is the high value for the internal scattering parameter necessary to interpret the spectra for the former, and the requirement of rough and non-isotropically scattering surfaces in the modelling of the latter. The mature lunar anorthosite spectra were well modelled with binary mixtures of calcic feldspars and olivines, grain sizes of 25-30 μm and a concentration of submicroscopic metallic iron (SMFe) of 0.12-0.15% in grain coatings. The mercurian spectrum is not possible to interpret from terrestrial mineral powder spectra without introducing an average particle scattering function for the bulk soil that increases in backscattering efficiency with wavelength. The observed spectrum is somewhat better predicted with binary mixture models of feldspars and pyroxenes, than with single-component regoliths consisting of either albite or diopside. Correct spectral reflectance values were predicted with a concentration of 0.1 wt% SMFe in coatings of 15-30 μm sized grains. Since reasonable cosmogonical formation scenarios for Mercury, or meteoritic infall, predict iron concentrations at least this high, we draw the conclusion that the average grain size of Mercury is about a factor of two smaller than for average returned lunar soil samples. The 0.6-2.5 μm spectrum of McCord and Clark (1979, J. Geophys. Res. 178, 745-747) is used to further limit the possible range of mineralogical composition of Mercury. It is found that an intimately mixed and matured 3:1 labradorite-to-enstatite regolith composition best matches both the optical and near-infrared spectra, yielding an abundance of ∼1.2 wt% FeO and ∼0 wt% TiO₂.     

Prognosis of TiO2 abundance in lunar soil using a non-linear analysis of Clementine and LSCC data

We suggest a technique to determine the chemical and mineral composition of the lunar surface using artificial neural networks (ANNs). We demonstrate this powerful non-linear approach for prognosis of TiO₂ abundance using Clementine UV-VIS mosaics and Lunar Soil Characterization Consortium data. The ANN technique allows one to study correlations between spectral characteristics of lunar soils and composition parameters without any restrictions on the character of these correlations. The advantage of this method in comparison with the traditional linear regression method and the Lucey et al. approaches is shown. The results obtained could be useful for the strategy of analyzing lunar data that will be acquired in incoming lunar missions especially in case of the Chandrayaan-1 and Lunar Reconnaissance Orbiter missions.     

X-ray digital imaging petrography of lunar mare soils: modal analyses of minerals and glasses

It is essential that accurate modal (i.e., volume) percentages of the various mineral and glass phases in lunar soils be used for addressing and resolving the effects of space weathering upon reflectance spectra, as well as for their calibration such data are also required for evaluating the resource potential of lunar minerals for use at a lunar base. However, these data are largely lacking. Particle-counting information for lunar soils, originally obtained to study formational processes, does not provide these necessary data, including the percentages of minerals locked in multi-phase lithic fragments and fused-soil particles, such as agglutinates. We have developed a technique for modal analyses, sensu stricto, of lunar soils, using digital imaging of X-ray maps obtained with an energy-dispersive spectrometer mounted on an electron microprobe. A suite of nine soils (90 to 150 micrometers size fraction) from the Apollo 11, 12, 15, and 17 mare sites was used for this study. This is the first collection of such modal data on soils from all Apollo mare sites. The abundances of free-mineral fragments in the mare soils are greater for immature and submature soils than for mature soils, largely because of the formation of agglutinitic glass as maturity progresses. In considerations of resource utilization at a lunar base, the best lunar soils to use for mineral beneficiation (i.e., most free-mineral fragments) have maturities near the immature/submature boundary (Is/FeO approximately or = 30), not the mature soils with their complications due to extensive agglutination. The particle data obtained from the nine mare soils confirm the generalizations for lunar soils predicted by L.A. Taylor and D.S. McKay (1992, Lunar Planet Sci. Conf. 23rd, pp. 1411-1412 [Abstract]).     

Origin and significance of cosmogenic signatures in vesicles of lunar basalt 15016

Lunar basalt 15016 (~3.3 Ga) is among the most vesicular (50% by volume) basalts recovered by the Apollo missions. We investigated the possible occurrence of indigenous lunar nitrogen and noble gases trapped in vesicles within basalt 15016, by crushing several cm-sized chips. Matrix/mineral gases were also extracted from crush residues by fusion with a CO₂ laser. No magmatic/primordial component could be identified; all isotope compositions, including those of vesicles, pointed to a cosmogenic origin. We found that vesicles contained ~0.2%, ~0.02%, ~0.002%, and ~0.02% of the total amount of cosmogenic ²¹Ne, ³⁸Ar, ⁸³Kr, and ¹²⁶Xe, respectively, produced over the basalt's 300 Myr of exposure. Diffusion/recoil of cosmogenic isotopes from the basaltic matrix/minerals to intergrain joints and vesicles is discussed. The enhanced proportion of cosmogenic Xe isotopes relative to Kr detected in vesicles could be the result of kinetic fractionation, through which preferential retention of Xe isotopes over Kr within vesicles might have occurred during diffusion from the vesicle volume to the outer space through microleaks. This study suggests that cosmogenic loss, known to be significant for ³He and ²¹Ne, and to a lesser extent for ³⁶Ar (Signer et al. 1977 ), also occurs to a negligible extent for the heaviest noble gases Kr and Xe.     

The early geological history of the Moon inferred from ancient lunar meteorite Miller Range 13317

Miller Range (MIL) 13317 is a heterogeneous basalt‐bearing lunar regolith breccia that provides insights into the early magmatic history of the Moon. MIL 13317 is formed from a mixture of material with clasts having an affinity to Apollo ferroan anorthosites and basaltic volcanic rocks. Noble gas data indicate that MIL 13317 was consolidated into a breccia between 2610 ± 780 Ma and 1570 ± 470 Ma where it experienced a complex near‐surface irradiation history for ~835 ± 84 Myr, at an average depth of ~30 cm. The fusion crust has an intermediate composition (Al₂O₃ 15.9 wt%; FeO 12.3 wt%) with an added incompatible trace element (Th 5.4 ppm) chemical component. Taking the fusion crust to be indicative of the bulk sample composition, this implies that MIL 13317 originated from a regolith that is associated with a mare‐highland boundary that is KREEP‐rich (i.e., K, rare earth elements, and P). A comparison of bulk chemical data from MIL 13317 with remote sensing data from the Lunar Prospector orbiter suggests that MIL 13317 likely originated from the northwest region of Oceanus Procellarum, east of Mare Nubium, or at the eastern edge of Mare Frigoris. All these potential source areas are on the near side of the Moon, indicating a close association with the Procellarum KREEP Terrane. Basalt clasts in MIL 13317 are from a very low‐Ti to low‐Ti (between 0.14 and 0.32 wt%) source region. The similar mineral fractionation trends of the different basalt clasts in the sample suggest they are comagmatic in origin. Zircon‐bearing phases and Ca‐phosphate grains in basalt clasts and matrix grains yield ²⁰⁷Pb/²⁰⁶Pb ages between 4344 ± 4 and 4333 ± 5 Ma. These ancient ²⁰⁷Pb/²⁰⁶Pb ages indicate that the meteorite has sampled a range of Pre‐Nectarian volcanic rocks that are poorly represented in the Apollo, Luna, and lunar meteorite collections. As such, MIL 13317 adds to the growing evidence that basaltic volcanic activity on the Moon started as early as ~4340 Ma, before the main period of lunar mare basalt volcanism at ~3850 Ma.