Reflectance Spectral Characteristics of Lunar Surface Materials

Based on a comprehensive analysis of the mineral composition of major lunar rocks (highland anorthosite, lunar mare basalt and KREEP rock), we investigate the reflectance spectral characteristics of the lunar rock-forming minerals, including feldspar, pyroxene and olivine. The affecting factors, the variation of the intensity of solar radiation with wavelength and the reflectance spectra of the lunar rocks are studied. We also calculate the reflectivity of lunar mare basalt and highland anorthosite at 300 nm, 415 nm, 750 nm, 900 nm, 950 nm and 1000 nm. It is considered that the difference in composition between lunar mare basalt and highland anorthosite is so large that separate analyses are needed in the study of the reflectivity of lunar surface materials in the two regions covered by mare basalt and highland anorthosite, and especially in the region with high Th contents, which may be the KREEP-distributed region.     

Spectral properties of plagioclase and pyroxene mixtures and the interpretation of lunar soil spectra

The effects of particle size and mineral proportions on the spectral characteristics of plagioclase and pyroxene mixtures are investigated. Size separates (<25 μm, 25–45 μm, 45–75 μm, 75–125 μm, 125–250 μm, and 250–500 μm) have been prepared for the following labradorite/enstatite compositional mixtures: 100/0%, 95/5%, 85/15%, 50/50%, and 0/100%. Spectrally, the labradorite and enstatite samples are representative of the plagioclase feldspars and the orthopyroxenes: the labradorite exhibits a weak, broadband centered near 1.25 μm and the enstatite exhibits two well-defined bands centered near 0.9 and 1.9 μm. From analysis of the plagioclase bands of the mixtures, it is found that (1) the amount of plagioclase necessary for the plagioclase band to be observed as a discrete absorption band is dependent on particle size and (2) plagioclase can be detected by flattening of the pyroxene reflectance “peak” between the 0.9- and 1.9-μm absorption bands if significant amounts of plagioclase are present. Analogs for immature and mature lunar highland soils have been created to examine the combined effects of particle size and mineral proportions on spectra of plagioclase and pyroxene mixtures. bidirectional reflectance spectra of these soil analogs are used to examine the detectability of plagioclase in soil-like particle size distributions. Plagioclase in significant amounts is detected by the flattening of the pyroxene reflectance “peak” between the 0.9- and 1.9-μm absorption bands, and the plagioclase absorption band itself is observed with 85% plagioclase present. The soil analogs reveal that particle size accounts for only a minor spectral difference between immature and mature lunar highland soils. From comparisons with spectra of returned lunar samples, agglutinates are found to dominate the spectral variations associated with soil maturity. Spectra of the immature soil analogs can be used to estimate the minimum pyroxene abundance for immature regions observed remotely.     

Reflectance spectra of analog basalts; implications for remote sensing of lunar geology

Lunar mare basalts, highland anorthosites and KREEP are the three major lunar rock types reported from the lunar surface. In the present study, we interpret the reflectance spectral behavior of lunar analog basalts including massive basalt, vesicular basalt and amygdaloidal basalt collected from the Deccan basaltic region, which are considered as equivalent of lunar mare basalts. Reflectance spectra of analog basalts were measured at three different environments: in the field, under controlled field conditions and in the lab. In field conditions the reflectance spectra were measured under 350-1050 nm spectral range. During controlled field and lab condition, reflectance spectra were measured under 350-2500 nm range covering the UV, visible, NIR, and SWIR regions. The spectral characteristics of basalts measured under different environments and their merits and demerits were discussed. However, lab spectra have given clear, reliable diagnostic spectral information for our present objective. The major oxides and minerals of analog basalts were compared with lunar mare basalts. The presence of Ca-pyroxene, ferrous and ferric iron and their diagnostic spectral features in basalts are discussed for study of lunar mare region.     

Relationship between positive cerium anomaly and adsorbed water in Antarctic lunar meteorites

Abstract— Concentrations of adsorbed water in single mineral grains of Antarctic lunar meteorites were determined with micro infrared (IR) spectroscopy. A relationship was found between the mineral's ability to adsorb water and the extent of Ce anomaly in rare earth element (REE) patterns precisely determined by the isotope dilution method using a thermal ionization mass spectrometer. Asuka (A) 881757, a lunar meteorite from the mare basalt without Ce anomaly, showed no trace of IR absorption due to adsorbed water. On the contrary, Yamato (Y) 791197-109, Y-86032-98, Y-86032-95, Y-791197-115 and Y-82192-55A from the lunar highland exhibiting positive Ce anomaly showed IR absorption due to adsorbed water in some of their minerals. The detected water would be of terrestrial origin, because it was not structurally bound and easy to exchange judging from the spectral band shape. The contrast in concentration of adsorbed water between the lunar highland and the mare basalt is derived from a difference in the density of microfractures in mineral grains. Average concentrations of adsorbed water in the lunar highland meteorites were 3.8 mg/cm³ for pyroxene and olivine, and 1.7 mg/cm³ for plagioclase. This contrast between minerals is noteworthy because it has been known that Ce anomaly of pyroxene and olivine is larger than that of plagioclase both for Antarctic lunar meteorites and some lunar rocks. Furthermore, more adsorbed water was detected for minerals in meteorites that exhibit larger Ce anomaly. The present observations demonstrate that the extent of Ce anomaly correlated with the concentration of adsorbed water, which suggests that active mineral surface resulting in adsorption of water could be a trace of interaction forming Ce anomaly. Terrestrial weathering on Antarctica and REE fractionation on the Moon are discussed for possible origins of the Ce anomaly.     

Effects of space weathering on diagnostic spectral features: Results from He+ irradiation experiments

We performed ion irradiation of mineral samples with 50 keV He⁺, aimed to investigate ion irradiation effects on diagnostic spectral features. Reflectance spectra of samples in 0.375–2.5 μm are measured before and after ion irradiation. Silicates, including Luobusha olivine, plagioclase and basaltic glass, have shown reddening and darkening of reflectance spectra at the VIS–NIR range. Olivine is more sensitive to ion irradiation than plagioclase and basaltic glass. Irradiated Panzhihua ilmenite exhibits higher reflectance and stronger absorption features, which is totally different from lunar soil and analog silicate materials in other experiments. Using continuum removal and MGM fit, we extracted and compared absorption features of olivine spectra before and after irradiation. Ion irradiation can induce band strength decrease of olivine but negligible band centers shift. We estimate band centers shift caused by ion irradiation are quite limited, even less than variations due to chemical composition in silicates. It provides one possible explanation for no systematic shift in band positions in lunar soil. Irradiated Luobusha olivine spectrum matches spectra of olivine-dominated asteroids. Our results suggest space weathering should be new clues to explain the subtle difference between A-type asteroid spectra and laboratory spectra of olivine.     

Properties of the hermean regolith: iii. disk-resolved vis-NIR reflectance spectra and implications for the abundance of iron

Disk-resolved reflectance spectra of the surface of Mercury (longitudes 240-300°), obtained in the visual (vis) and near-infrared (NIR) spectral region, are presented and analyzed. The observations were made at the 2.6-m Nordic Optical Telescope with the ALFOSC low-resolution spectrograph on 20 and 22 June 1999 in the wavelength range 520-970 nm with a footprint size of 700 km on the mid-disk of Mercury. A method which enables more accurate correction for telluric line absorptions and atmospheric extinction than that applied on previously published vis-NIR spectra of Mercury is introduced. The resulting reflectance spectra are remarkably linear, lack significant absorption features, and have optical slopes comparable to remotely sensed lunar pure anorthosites. The relation between spectral slope and photometric geometry found by Warell (2002, Icarus 156, 313-317) is confirmed and is explained as caused by strongly backscattering particles with embedded submicroscopic metallic iron in a mature regolith. With the theoretical maturation model of Hapke (2001, J. Geophys. Res. 106 (E5), 10039-10073) an abundance of 0.05-0.3 wt% submicroscopic metallic iron in the regolith for silicate grain sizes in the range 10-80 μm is determined, implying a ferrous iron content in mafic minerals intrinsically lower than that of the lunar highlands. A binary crustal composition model with anorthite linearly mixed with pyroxene provides better spectral fits than a pure anorthitic composition. Comparison with mature lunar pure anorthosite spectra yields a confident upper limit to the FeO content of 3 wt% under the assumption that the surfaces are similarly matured, but this figure probably represents a considerable overestimate. The average mercurian regolith does not seem to be substantially more weathered than the most mature lunar highland soils in terms of abundance of submicroscopic metallic iron, indicating that a steady-state maturation level has been reached. However, the strong relation between optical spectral slope and photometric geometry may imply that the majority of regolith particles are more fine-grained than their lunar counterparts and that the regolith is admixed with complex agglutinate weathering products which are more abundant and more transparent than those of the lunar highlands. This is consistent with more energetic impacts and a higher rate of impact melt production in an iron-poor regolith. An observed relation between the spectral slope and latitude provides evidence that the Ostwald ripening process may be operating at equatorial latitudes on Mercury.     

Minerals mapping of the lunar surface with Clementine UVVIS/NIR data based on spectra unmixing method and Hapke model

The distribution of minerals on the lunar surface is information which could contribute to studying lunar origin and evolution. In this paper, the distribution of clinopyroxene, orthopyroxene, olivine, ilmenite, and plagioclase on the lunar surface has been mapped based on Hapke radiative transfer model and linear unmixing of spectra with Clementine UVVIS/NIR data. The results have been validated on the basis of minerals modal abundance data of the Apollo samples, and problems in the minerals abundance mapping have been analyzed. The validation based on analysis data of Apollo samples indicates that plagioclase mapped in this paper represents the total abundance of plagioclase and agglutinitic glass. The minerals mapping results show that the lunar surface is mainly composed of pyroxene, plagioclase, agglutinitic glass, and ilmenite. Basalt in the lunar mare is mainly composed of clinopyroxene and ilmenite, and lunar highland is mainly composed of plagioclase and agglutinitic glass. Orthopyroxene is mainly distributed on the north of Mare Imbrium, on the south of Maria and Aitken Basin. According to our results, there is probably no large area of olivine distribution on the lunar surface which is different from earlier published results. Therefore, emphasis should be put on the olivine distribution in the minerals mapping using hyperspectral data such as M³ of Chandrayaan-1 and IIM of ChangE-1.     

Polarized absorption spectra of single crystals of lunar pyroxenes and olivines

Measurements have been made of the polarized absorption spectra (360-2200 nm.) of compositionally zoned pyroxene minerals in rocks 10045, 10047 and 10058 and olivines in rocks 10020 and 10022. Specimens in the form of petrographic thin sections were mounted on polarizing microscopes equipped with three-axis universal stage attachments and inserted into a Cary 17 spectrophotometer. The Apollo 11 pyroxenes with relatively high Ti/Fe ratios were chosen initially to investigate the presence of crystal field spectra of Fe²⁺ and Ti³⁺ ions in the minerals.Broad intense bands at about 1000 and 2100 nm. arise from spin-allowed, polarization-dependent transitions in Fe²⁺ ions in pyroxenes. Several weak sharp peaks occur in the visible region. Peaks at 402, 425, 505, 550 and 585 nm. represent spin-forbidden transitions in Fe²⁺ ions, while broader bands at 460–470 nm. and 650–660 nm. are attributed to Ti³⁺ ions. Charge transfer bands, which in terrestrial pyroxenes often extend into the visible region, are displaced to shorter wavelengths in lunar pyroxenes. This feature correlates with the absence of Fe³⁺ ions in these minerals. The magnitudes of the intensity ratios: band 465 nm. (Ti³⁺) to band 1000 nm. (Fe²⁺) are similar to Ti/Fe ratios from lunar pyroxene bulk chemical analyses, suggesting that an appreciable amount of titanium occurs as Ti³⁺ ions in the lunar pyroxenes. The 505 nm. spin-forbidden peak in Fe²⁺, together with absorption at 465 nm. by Ti³⁺, contribute to the pink or pale reddish-brown colors of lunar pyroxenes in transmitted lights.The absorption spectral measurements not only provide information on the redox behavior and crystal chemistry of lunar pyroxenes, but also form a basis for interpreting spectral reflectivity properties of lunar rocks and the Moon's surface.     

Temperature-Dependent Near-Infrared Spectral Properties of Minerals, Meteorites, and Lunar Soil

The near-IR spectral properties of minerals, meteorites, and lunar soil vary with temperature. The manner in which these materials vary is diagnostic of aspects of their composition. We quantify the spectral dependence on temperature by reporting the change in relative reflectance with temperature as a function of wavelength. We call this quantity, ΔR/ΔT (in units of K⁻¹), as a function of temperature the “thermo-reflectance spectrum.” The thermo-reflectance spectra of olivine and pyroxene are distinct, and most of the observable structure in thermo-reflectance spectra of the ordinary and carbonaceous chondrites can be understood in terms of a mixture of the thermo-reflectance spectra of olivine and pyroxene. The magnitude of thermo-reflectance spectra of meteorites and lunar soils is much less than that of pure minerals. Lunar soils are particularly subdued. While conventional analysis of remotely obtained spectra of the Moon can neglect temperature effects, spatially resolved measurements of the surface of the asteroid Vesta will likely have a strong temperature-dependent component based on measurements of a eucrite and a howardite.     

Spinel assemblages in lunar meteorites Graves Nunataks 06157 and Dhofar 1528: Implications for impact melting and equilibration in the Moon's upper mantle

Magnesium‐rich spinel assemblages occur in the two lunar vitric breccia meteorites—Dhofar (Dho) 1528 and Graves Nunataks (GRA) 06157. Dho 1528 contains up to ~0.7 mm cumulate Mg‐rich spinel crystals associated with Mg‐rich olivine, Mg‐ and Al‐rich pyroxene, plagioclase, and rare cordierite. Using thermodynamic calculations of these mineral assemblages, we constrain equilibration depths and discuss an origin of these lithologies in the upper mantle of the Moon. In contrast, small, 10 to 20 μm spinel phenocryst assemblages in glassy melt rock clasts in Dho 1528 and GRA 06157 formed from the impact melting of Mg‐rich rocks. Some of these spinel phenocrysts match compositional constraints for spinel associated with “pink spinel anorthosites” inferred from remote sensing data. However, such spinel phenocrysts in meteorites and Apollo samples are typically associated with significant amounts of olivine ± pyroxene that exceed the compositional constraints for pink spinel anorthosites. We conclude that the remotely sensed “pink spinel anorthosites” have not been observed in the collections of lunar rocks. Moreover, we discuss impact‐excavation scenarios for the spinel‐bearing assemblages in Dhofar 1528 and compare the bulk rock composition of Dho 1528 to strikingly similar compositions of Luna 20 samples that contain ejecta from the Crisium impact basin.     

Ultraviolet spectral reflectance properties of common planetary minerals

Ultraviolet spectral reflectance properties (200-400 nm) of a large number of minerals known or presumed to exist on the surfaces of Mars, the Moon, and asteroids, and in many meteorites, were investigated. Ultraviolet reflectance spectra (200-400 nm) of these minerals range from slightly blue-sloped (reflectance decreasing toward longer wavelengths) to strongly red-sloped (reflectance increasing toward longer wavelengths). Most exhibit one or two absorption features that are attributable to FeO charge transfers involving Fe³⁺ or Fe²⁺. The UV region is a very sensitive indicator of the presence of even trace amounts (<0.01 wt%) of Fe³⁺ and Fe²⁺. The major Fe³⁺O absorption band occurs at shorter wavelengths (∼210-230 nm), and is more intense than the major Fe²⁺O absorption band (∼250-270 nm). Ti-bearing minerals, such as ilmenite, rutile and anatase exhibit UV absorption bands attributable to Ti⁴⁺O charge transfers. While the positions of metal-O charge transfer bands sometimes differ for different minerals, the variation is often not diagnostic enough to permit unique mineral identification. However, iron oxides and oxyhydroxides can generally be distinguished from Fe-bearing silicates in the 200-400 nm region on the basis of absorption band positions. Within a given mineral group (e.g., low-calcium pyroxene, olivine, plagioclase feldspar), changes in Fe²⁺ or Fe³⁺ abundance do not appear to result in a measurable change in absorption band minima positions. Absorption band positions can vary as a function of grain size, however, and this variation is likely due to band saturation effects. The intensity of metal-O charge transfers means that some minerals will exhibit saturated UV absorption bands even for fine-grained (<45 μm) powders. In cases where absorption bands are not saturated (e.g., Fe²⁺O bands in some plagioclase feldspars and pyroxenes), changes in Fe²⁺ content do not appear to cause variations in band position. In other minerals (e.g., olivine), changes in band positions are correlated with compositional and/or grain size variations, but this is likely due to increasing band saturation rather than compositional variations. Overall, we find that the UV spectral region is sensitive to different mineral properties than longer wavelength regions, and thus offers the potential to provide complementary capabilities and unique opportunities for planetary remote sensing.     

New lunar meteorite NWA 10986: A mingled impact melt breccia from the highlands—A complete cross section of the lunar crust

Northwest Africa (NWA) 10986 is a new mingled lunar meteorite found in 2015 in Western Sahara. This impact melt breccia contains abundant impact melt glass and clasts as large as 0.75 mm. Clasts are predominantly plagioclase and pyroxene‐rich and represent both highland and basalt lithologies. Highland lithologies include troctolites, gabbronorites, anorthositic norites, and troctolitic anorthosites. Basalt lithologies include crystalline clasts with large zoned pyroxenes representing very low titanium to low titanium basalts. In situ geochemical analysis of minerals within clasts indicates that they represent ferroan anorthosite, Mg‐suite, and gabbronorite lithologies as defined by the Apollo sample collection. Clasts representing magnesian anorthosite, or “gap” lithologies, are prevalent in this meteorite. Whole rock and in situ impact glass measurements indicate low incompatible trace element concentrations. Basalt clasts also have low incompatible trace element concentrations and lack evolved KREEP mineralogy although pyroxferroite grains are present. The juxtaposition of evolved, basaltic clasts without KREEP signatures and highland lithologies suggests that these basaltic clasts may represent cryptomare. The lithologies found in NWA 10986 offer a unique and possibly a complete cross section view of the Moon sourced outside of the Procellarum KREEP Terrane.     

Mineralogy of the lunar crust: Results from Clementine

Abstract— The central peaks of 109 impact craters across the Moon are examined with Clementine ultraviolet-visible (UVVIS) camera multispectral data. The craters range in diameter from 40 to 180 km and are believed to have exhumed material from 5–30 km beneath the surface to form the peaks, including both upper and lower crustal rocks depending on whether craters have impacted into highlands or basins. Representative five-color spectra from spectrally and spatially distinct areas within the peaks are classified using spectral parameters, including “key ratio” (which is related to mafic mineral abundance) and “spectral curvature” (linked to absorption band shape, which distinguishes between low- and high-Ca pyroxene and olivine). The spectral parameters are correlated to mineralogical abundances, related in turn to highland plutonic rock compositions. The derived rock compositions for the various central peaks are presented in a global map. From these results, it is evident that the lunar crust is compositionally diverse, both globally and at local 100 m scales found within individual sets of central peaks. Although the central peaks compositions imply a crust that is generally consistent with previous models of crustal structure, they also indicate a more anorthositic crust than generally assumed, with a bulk plagioclase content of ～81%, evolving from “pure” anorthosite near the surface towards more mafic, low-Ca pyroxene-rich compositions with depth (comparable to anorthositic norite). Evidence for mafic plutons occurs in both highlands and basins and represent all mafic highland rock types. However, the lower crust is more compositionally diverse than the highlands, with both a greater range of rock types and more diversity within individual sets of central peaks.     

Dhofar 081: A new lunar highland meteorite

Abstract— The lunar meteorite Dhofar 081, found as a single fragment of 174 g in the Dhofar region of Oman, is a shocked feldspathic fragmental highland breccia dominated by anorthosite‐rich lithic and mineral clasts embedded into a fine‐grained mostly shock melted clastic matrix. Major mineral phases in the bulk rock are Ca‐rich plagioclase (An_96.5–99.5), pyroxene (FS_21.9–46.2Wo_3.0–41.4), and olivine (Fa_29.3–47.8); accessory phases include Fe‐Ni metal, ilmenite, and Ti‐Cr‐rich spinel. Dhofar 081 contains subordinate crystalline fragments of large anorthosites, intersertal impact‐melt rocks, microporphyritic impact‐melt breccias, dark fine‐grained impact‐melt breccias, large cataclastic feldspars, and irregularly shaped brown glass clasts. Mafic components are rare and no genuine regolith components were found in the sections studied. Minerals in Dhofar 081 show homogeneously distributed shock features: intergranular recrystallization, strong fracturing and mosaicism in feldspar as well as a high density of mostly irregular fractures in pyroxene and olivine. Localized impact melting caused by one or several impacts led to a strong lithification. Based on these effects an equilibration shock pressure of about 15–20 GPa is estimated for the strongest shock event in Dhofar 081. Devitrification of the “glassy” material in the rock indicates thermal annealing after shock melting suggesting that the 15–20 GPa shock event predated the ejection event. According to the concentrations of implanted solar noble gases Dhofar 081 represents a polymict clastic breccia deposit with possibly a minor regolith component. A similar noble gas record of Dhofar 081 and MacAlpine Hills 88104/05 suggests the possibility of a source crater pairing of both meteorites. As indicated by noble gas measurements pairing of Dhofar 081 with the other lunar meteorites found in Oman, Dhofar 025 and Dhofar 026, is unlikely.     

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.     

Valence of Ti,V, and Cr in Apollo 14 aluminous basalts 14053 and 14072

The valences of Ti, V, and Cr in olivine and pyroxene, important indicators of the fO₂ of the source region of their host rocks, can be readily measured nondestructively by XANES (X‐ray absorption near edge structure) spectroscopy, but little such work has been done on lunar rocks, and there is some uncertainty regarding the presence of Ti³⁺ in lunar silicates and the redox state of the lunar mantle. This is the first study involving direct XANES measurement of valences of multivalent cations in lunar rocks. Because high alumina activity facilitates substitution of Ti cations into octahedral rather than tetrahedral sites in pyroxene and Ti³⁺ only enters octahedral sites, two aluminous basalts from Apollo 14, 14053 and 14072, were studied. Most pyroxene contains little or no detectable Ti³⁺, but in both samples relatively early, magnesian pyroxene was found that has Ti valences that are not within error of 4; in 14053, this component has an average Ti valence of 3.81 ± 0.06 (i.e., Ti³⁺/[Ti³⁺ + Ti⁴⁺ = 0.19]). This pyroxene has relatively low atomic Ti/Al ratios (<0.4) due to crystallization before plagioclase, contrary to the long‐held belief that lunar pyroxene with Ti/Al > 0.5 contains Ti³⁺ and pyroxene with lower ratios does not. Later pyroxene, with lower Mg/Fe and higher Ti/Al ratios, has higher proportions of Ti (all Ti⁴⁺) in tetrahedral sites. All pyroxene analyzed contains divalent Cr, ranging from 15 to 30% of the Cr present, and all but one analysis spot contains divalent V, accounting for 0 to 40% (typically 20–30%) of the V present. Three analyses of olivine in 14053 do not show any Ti³⁺, but Ti valences in 14072 olivine range from 4 down to 3.70 ± 0.10. In 14053 olivine, ~50% of the Cr and 60% of the V are divalent. In 14072 olivine, the divalent percentages are ~20% for Cr and 20–60% for V. These results indicate significant proportions of divalent Cr and V and limited amounts of trivalent Ti in the parental melts, especially when crystal/liquid partitioning preferences are taken into account. These features are consistent with an fO₂ closer to IW ? 2 than to IW ? 1. Apollo 15 basalt 15555, analyzed for comparison with A‐14 materials, has olivine with strongly reduced Cr (Cr²⁺/(Cr²⁺ + Cr³⁺) ~0.9). Basalts from different sites may record redox differences between source regions.     

Lunar pure anorthosite as a spectral analog for Mercury

Abstract— Plans are underway for spacecraft missions to the planet Mercury beginning in the latter part of this decade (NASA's MESSENGER (MErcury, Surface, Space ENvironment, GEochemistry, Ranging) and ESA's BepiColombo). Mercury is an airless body whose surface is apparently very low in ferrous iron. Much of the mercurian surface material is expected to be optically mature, a state produced by the “space weathering” process from direct exposure to the space environment. If appropriate analog terrains can be identified on the Moon, then study of their reflectance spectra and composition will improve our understanding of space weathering of low‐Fe surfaces and aid in the interpretation of data returned from Mercury by the spacecraft. We have conducted a search for areas of the lunar surface that are optically mature and have very low ferrous iron content using Clementine ultraviolet‐visible (UV‐vis) image products. Several regions with these properties have been identified on the farside. These areas, representing mature pure anorthosites (>90% plagioclase feldspar), are of interest because only relatively immature pure anorthosites have previously been studied with Earth‐based spectrometry. A comparison of Mercury with the lunar analogs reveals similarities in spectral characteristics, and there are hints that the mercurian surface may be even lower in FeO content than the lunar pure anorthosites. We also investigate the potential for use of spectral features other than the commonly studied “1 μm” mafic mineral absorption band as tools for compositional assessment when spacecraft spectral measurements of Mercury become available. Most low‐Fe minerals plausibly present on Mercury lack absorption bands, but plagioclase possesses an iron impurity absorption at 1.25 μm. Detection of this diagnostic band may be possible in fresh crater deposits.     

Chronology,geochemistry, and petrology of a ferroan noritic anorthosite clast from Descartes breccia 67215: Clues to the age,origin, structure,and impact history of the lunar crust

Abstract— The petrology, major and trace element geochemistry, and Nd‐Ar‐Sr isotopic compositions of a ferroan noritic anorthosite clast from lunar breccia 67215 have been studied in order to improve our understanding of the composition, age, structure, and impact history of the lunar crust. The clast (designated 67215c) has an unusually well preserved igneous texture. Mineral compositions are consistent with classification of 67215c as a member of the ferroan anorthositic suite of lunar highlands rocks, but the texture and mineralogy show that it cooled more rapidly and at shallower depths than did more typical ferroan anorthosites (FANs). Incompatible trace element concentrations are enriched in 67215c relative to typical FANs, but diagnostic signatures such as Ti/Sm, Sc/Sm, plagiophile element ratios, and the lack of Zr/Hf and Nb/Ta fractionation show that this cannot be due to the addition of KREEP. Alternatively, 67215c may contain a greater fraction of trapped liquid than is commonly present in lunar FANs. ¹⁴⁷Sm‐¹⁴³Nd isotopic compositions of mineral separates from 67215c define an isochron age of 4.40 ± 0.11 Gyr with a near‐chondritic initial ε¹⁴³_Nd of +0.85 ± 0.53. The ⁴⁰Ar‐³⁹Ar composition of plagioclase from this clast records a post‐crystallization thermal event at 3.93 ± 0.08 Gyr, with the greatest contribution to the uncertainty in this age deriving from a poorly constrained correction for lunar atmosphere ⁴⁰Ar. Rb‐Sr isotopic compositions are disturbed, probably by the same event recorded by the Ar isotopic compositions. Trace element compositions of FANs are consistent with crystallization from a moderately evolved magma ocean and do not support a highly depleted source composition such as that implied by the positive initial ε¹⁴³_Nd of the ferroan noritic anorthosite 62236. Alternatively, the Nd isotopic systematics of lunar FANs may have been subject to variable degrees of modification by impact metamorphism, with the plagioclase fraction being more strongly affected than the mafic phases. ¹⁴⁷Sm‐¹⁴³Nd isotopic compositions of mafic fractions from the 4 ferroan noritic anorthosites for which isotopic data exist (60025, 62236, 67016c, 67215c) define an age of 4.46 ± 0.04 Gyr, which may provide a robust estimate for the crystallization age of lunar ferroan anorthosites.     

The 506 nm absorption feature in pyroxene spectra: Nature and implications for spectroscopy-based studies of pyroxene-bearing targets

High-resolution (0.34 nm) reflectance spectra of a suite of terrestrial ortho- and clinopyroxenes were characterized in the 506-nm region. This region exhibits absorption bands attributed to spin-forbidden transitions in Fe²⁺ located in the M2, and possibly M1, crystallographic site(s). The most intense absorption bands (up to 3.8% deep in <45 μm fractions) are present in low Ca-content orthopyroxene spectra. This region exhibits two (spectral Group I) or more (spectral Group II) absorption bands in the 500-515 nm interval. Group I spectra are associated with the lowest Ca-content samples. For orthopyroxenes, the number of constituent absorption bands and band depths vary as a function of Ca content; increasing Ca content results the appearance of more than two absorption bands and a general reduction in band depths, offsetting an expected increase in band depth with increasing Fe²⁺ content; band depths may also be reduced due to the long wavelength wing of ultraviolet region Fe-O charge transfer absorptions. Band depths and shapes in this region are also a function of grain size, with the strongest bands appearing for larger grain sizes - in the 90-250 μm range. The number and position of constituent absorption bands can be used to constrain factors such as cooling rates, as expressed in the formation of Guinier-Preston zones versus coarser-grained augite exsolution lamellae. Band depths in the spectra of fine-grained (<45 μm) clinopyroxenes do not exceed 1% and are generally lowest for spectral type A clinopyroxenes, where most of the Fe²⁺ is present in the M1 crystallographic site. The appearance of the 506 nm band in the spectra of pyroxene-bearing asteroids can be used to constrain pyroxene composition and structure. The results of this study suggest that detailed analysis of absorption features in the 506 nm region is a powerful tool for determining the composition and structure of pyroxenes. The spectral resolution of the VIR-MS spectrometer aboard the Dawn spacecraft - which will examine Asteroid 4 Vesta, a body possessing surficial pyroxenes - will be sufficient to provide some constraints on pyroxene composition.     

Complex origins of silicate veinlets in HED meteorites: A case study of Northwest Africa 1109

We report on the petrography and mineralogy of three types of silicate veinlets in the brecciated eucrite Northwest Africa (NWA) 1109. These include Fe‐rich olivine, Mg‐rich olivine, and pyroxene veinlets. The Fe‐rich olivine veinlets mainly infill fractures in pyroxene and also occur along grain boundaries between pyroxene and plagioclase crystals, in both nonequilibrated and equilibrated lithic clasts. The host pyroxene of Fe‐rich olivine veinlets shows large chemical variations between and within grains. The Fe‐rich olivine veinlets also contain fine‐grained Fe³⁺‐bearing chromite, highly calcic plagioclase, merrillite, apatite, and troilite. Based on texture and mineral chemistry, we argue that the formation of Fe‐rich olivine was related to fluid deposition at relatively high temperatures. However, the source of Fe‐rich olivine in the veinlets remains unclear. Magnesium‐rich olivine veinlets were found in three diogenitic lithic clasts. In one of these, the Mg‐rich olivine veinlets only occur in one of the fine‐grained interstitial regions and extend into fractures within surrounding coarse‐grained orthopyroxene. Based on the texture of the interstitial materials, we suggest that the Mg‐rich olivine veinlets formed by shock‐induced localized melting and recrystallization. Pyroxene veinlets were only observed in one clast where they infill fractures within large plagioclase grains and are associated with fine‐grained pyroxene surrounding coarse‐grained pyroxene. The large chemical variations in pyroxene and the fracture‐filling texture indicate that the pyroxene veinlets might also have formed by shock‐induced localized melting and rapid crystallization. Our study demonstrates that silicate veinlets formed by a range of different surface processes on the surface of Vesta.