Lamellar pyroxene-spinel symplectites in lunar olivine from the Luna 24 regolith

Cr-Ca lamellae in a magnesian olivine grain (section 1611) from the Luna 24 regolith were investigated in detail by electron microprobe analysis (EMPA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that the lamellae are parallel to the (100) plane of oxygen closest packing in olivine and consist of regular vermicular intergrowths of two phases, diopside (Di) and chromite (Chr), in the volume proportion Di: Chr ≈ 3: 1. The bulk chemical composition of the lamellae is approximated as Ca₂Mg₂Fe²⁺(Cr³⁺)₂Si₄O₁₆. They are identical in phase composition to type A, F, and E symplectites from Apollo lunar samples [9]. Based on morphology and phase composition, the lamellar aggregates in the olivine grain from the Luna 24 regolith were classified as pyroxene (Px)-spinel (Spl) symplectites of a lamellar type, the formation of which was related to olivine oxidation at IW ≤ logfO₂ ≤ QFM. The obtained data indicate a solid-phase mechanism of lamella formation and the existence of a lamellar precursor phase, which transformed subsequently into the Px-Spl symplectite. It was supposed that uvarovite-knorringite garnet produced by the oxidation of olivine at high pressures and t > 800°C could be the transitional phase during symplectite formation. The subsequent conversion of the garnet into the low-pressure assemblage of Px-Spl symplectites could occur via cellular decomposition in accordance with the reaction Ca₂MgCr₂Si₃O₁₂ + (Mg,Fe)₂SiO₄ = 2CaMgSi₂O₆ + FeCr₂O₄. The reported results are the first data of a detailed nanomineralogical investigation of lamellar Px-Spl symplectites in lunar olivine.     

Petrology of some lithic fragments from Luna 20

Microscopic and electron microprobe studies were made of polished thin sections of part of a 30-mg sample of 250–500 μm lunar soil returned by Luna 20 from a point between Mare Fecunditatis and Mare Crisium. Very fine-grained lithic (crystalline) rock fragments, composing about one fifth of the total sample, have mineralogical compositions equivalent to various types of gabbro, anorthositic gabbro, gabbroic anorthosite and troctolite, with minor basalt. The textures now observed in these fragments are in large part metamorphic. Twentyseven electron microprobe analyses of minerals from these fragments are presented, including olivine, plagioclase, pyroxene, spinel, nickel-iron and a Zr-Ti-REE mineral possibly similar to ‘phase B’ of Lovering and Wark (1971). Analyses of seven melt inclusions and twenty-eight defocused beam analyses of lithic fragments are also given. Some of the fragments contain ‘gas’ inclusions which, along with the fine grain size, are believed to indicate final crystallization under low pressure near surface conditions. The almost complete absence of granophyric material in this sample raises the question of whether or not there are at least two distinct magmas for the plagioclase-rich terrae rocks from which this soil sample was derived in part.     

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.     

The lunar regolith: Chemistry and petrology of Luna 24 grain size fractions

Chemical data are reported for the first time for lunar soil size fractions smaller then 2 μm. We report chemical data for 30 elements by INAA in eight size fractions (370−200, 200−94, 94−74, 74−40, 40−10, 10−5, 5−2 and <2 μm) and petrology of five size fractions (down to 40−10 μm) in two Luna 24 soils, 24176 and 24214. Consistent with our previous results for lunar soils, the compositions of coarser fractions (>10 μm) are quite similar to each other but quite different from the fine fractions (<10 μm). The finer fractions (10–5, 5–2, <2 μm) become increasingly feldspathic and enriched in large-ion lithophile elements (LILE) with decreasing grain size. Chemical data for the finer fractions provide direct evidence in favor of efficient comminution of rock mesostasis and feldspar leading to their preferential incorporation into the finer fractions. High concentrations of meteoritic indicator elements (Ni, Au, Ir) in the finer fractions are consistent with the comminution process by micrometeorite impacts. The chemical data strongly support the F³ (fusion of the finest fraction) model for agglutinate formation.Based on grain size distribution, petrology, and LILE patterns of size fractions, the Luna 24 soils are less reworked than most lunar soils. The Luna 24 regolith appears to have formed as a result of mixing more mature and fine grained material with less mature coarse material in different proportions at different depth intervals.     

Petrology of fine-grained rock fragments and petrologic implications of single crystals from the Luna 20 soil

Only fine-grained rocks are present in the Luna 20 samples, and coarser grained rocks are represented by fragments of single crystals. A petrologic study has been made of 47 fine-grained crystalline rocks, microbreccias, and glassy aggregates. In addition, a total of 33 single crystals of pyroxene, plagioclase, olivine and spinel, in the size range 125 to 500 μ, have been examined using electron microprobe and single crystal X-ray diffraction techniques.The most abundant fine-grained crystalline rocks in the samples we have examined are recrystallized anorthositic norite and anorthositic troctolite. Gabbroic rocks, anorthosite, and KREEP basalt are present but not common. Most of the single crystals of pyroxene and plagioclase could have been derived from coarser grained noritic, troctolitic and anorthositic rocks. However, three of the 14 pyroxene crystals, and 2 of the 5 olivine crystals have $\text{Fe}\text{(Fe + Mg)}$ contents greater than 0.45 and are believed to have been derived from mare basalts or related rocks. Two relatively sodic crystals of plagioclase were found. One is a crystal zoned at least over the range An₈₅ to An₆₃, and the second is a homogeneous crystal of albite (~An₃).     

Valence state of iron in a condensate from the Luna 16 regolith

This paper reports the results of an X-ray photoelectron spectroscopic study of the condensate phase of regolith sample L1639 returned by the Luna 16 mission. The reduced Si⁰, Si²⁺, Al⁰, Ti²⁺, and Ti³⁺ forms were detected in the sample. Iron occurs in all valence states, and Fe³⁺ species were detected for the first time in the condensate. Minor Fe³⁺ concentrations were observed in the upper layers of the sample containing the maximum amounts of condensate products. The fraction of ferric Fe is 22%, and the Fe⁰: Fe²⁺: Fe³⁺ proportion is 33: 45: 22. The appearance of ferric Fe in the lunar condensate is explained by the reaction of FeO disproportionation occurring either at the stage of the expansion and cooling of impact-related vapor or directly in the condensed phase on the surface of regolith particles. This interpretation is supported by the results of a model experiment on augite vaporization and condensation. The experiment simulating impact vaporization was carried out on a laser set-up at a temperature of ∼3000–4000 K and a pulse duration of ∼10⁻³ s in a He atmosphere (P = 1 atm). The results of analyses provided compelling evidence that the condensate produced after augite vaporization contains Fe in all oxidation states, and the proportions of different valence forms approach the stoichiometry of the disproportionation reaction.     

Noble Gases in the Lunar Regolith

The most fundamental character of lunar soil is its high concentrations of solar-wind-implanted elements, and the concentrations and behavior of the noble gases He, Ne, Ar, and Xe, which provide unique and extensive information about a broad range of fundamental problems. In this paper, the authors studied the forming mechanism of lunar regolith, and proposed that most of the noble gases in lunar regolith come from the solar wind. Meteoroid bombardment controls the maturity of lunar soil, with the degree of maturation decreasing with grain size; the concentrations of the noble gases would be of slight variation with the depth of lunar soil but tend to decrease with grain size. In addition, the concentrations of noble gases in lunar soil also show a close relationship with its mineral and chemical compositions. The utilization prospects of the noble gas ^3He in lunar regolith will be further discussed.     

The age and petrography of two Luna 20 fragments and inferences for widespread lunar metamorphism

Ages were determined by the ⁴⁰Ar-³⁹Ar method on two metaclastic rocks returned from the lunar highlands north of Mare Fecunditatis by the Luna 20 probe. Both samples gave very well-defined argon retention ages of 3.90 ± 0.04 AE which are indistinguishable from each other within a resolution of 0.02 AE. Both fragments, 22006 and 22007, are highly recrystallized polymict breccias; there is no evidence for loss of radiogenic ⁴⁰Ar, and the age almost surely dates the time of recrystallization. The cosmic ray exposure ages of these fragments are similar and high: 900 million years for 22006, 1300 million years for 22007. 22007 also contains substantial trapped argon with a high ${}^{40}\text{Ar}{}^{36}\text{Ar}$ ratio.The Luna 20 results greatly extend the area of the Moon's surface exhibiting a well-defined record of metamorphism at 3.9 AE. So far, lunar history in the interval 4.6?3.9 AE is not preserved in the ages of surface rocks. This obliteration suggests lunar-wide metamorphic conditions occurring or terminating at this time as a result of major impacts.     

Carbon chemistry of Luna 16 and Luna 20 samples

Luna 16 and Luna 20 samples were analyzed for volatilizable species using vacuum pyrolysis to 1400°C. The major gaseous products evolved (ranging from 10–650 μg/g) were H₂O, CO, CO₂, N₂ and CH₄. Minor components (all < 10 μg/g) included NH₃, HCN, NO, SO₂, H₂S, C₂H₂, C₂H₄, C₂H₆, C₃H₆ and higher hydrocarbons, benzene, toluene, and the polymeric contaminants Teflon^® and silicone oil. The total carbon and nitrogen contents (μg/g) for these sieved samples (< 125 μm) were: Luna 16—C 418, N 134 and Luna 20—C 380, N 80.     

Composition of the Descartes region,lunar highlands

Analytical data for 40 elements are reported for Apollo 16 soils 60601, 61181, 61501, 64801, 67701, 68501, 65701 and breccias 60015, 60017, 60018, 60315, 61016, 61175, 65015 and 66055. The soils are uniform except for the North Ray Crater rim sample which is richer in Al₂O₃.The breccia components show great diversity in composition. Low-K Fra Mauro basalt, Highland basalt (anorthositic gabbro) and plagioclase are important constituents. Medium-K Fra Mauro basalt is an important constituent of breccias 65015 and 60315.The breccias contain many meteorite fragments and high nickel contents, evidence of the early highland bombardment.Most of the refractory elements (REE, Th, U, Zr, Hf, Nb, Ba) show strong positive correlations, interpreted as resulting from mixing. The REE patterns of the breccias show extreme variation relative to chondrites. There is a good inverse correlation between REE and the europium anomaly ($\text{Eu}\text{Eu}{}^{\mathrm{x}}$). The $\text{La}\text{Yb}$ ratio is constant at 3.1 except in plagioclase. Eu depletion or enrichment is interpreted as due to addition or removal of plagioclase.The Cayley and Descartes formations cannot be distinguished chemically and the differences in surface expression are not due to chemical distinctions. They are interpreted as structural differences, related to early highland cratering and mare basin formation.The complex soil and breccia compositions are related to mixing of four components. These are Low-K Fra Mauro basalt, Highland basalt (anorthositic gabbro) and subordinate plagioclase and Medium-K Fra Mauro basalt. These compositions have been used in a computer program (PETMIX III) to provide fits for the analytical data in terms of the end-members.An average highland composition is proposed, based on the Apollo 15 and 16 orbital data for Si, Al, Mg and Th. Abundances for most other elements are derived from the interelement relationships and correlations, and checked by the mixing program.The resulting composition consists of 69 per cent Highland basalt (anorthositic gabbro) and 31 per cent Low-K Fra Mauro basalt. There is no significant Eu anomaly. The abundances are: SiO₂: 45.2 per cent; TiO₂: 0.68 per cent; Al₂O₃: 24.9 per cent; FeO: 6.3 per cent; MgO: 8.5 per cent; CaO: 13.8 per cent; Na₂O: 0.4 per cent; K₂O: 0.11 per cent; Cr₂O₃: 0.11 per cent; Ba: 144 ppm; Th: 1.8 ppm; U: 0.46 ppm; Pb: 1.6 ppm; Zr: 156 ppm; Hf: 3.2 ppm; Nb: 10.8 ppm; Y: 32 ppm; ΣREE: 85 ppm.     

The halogens in Luna 16 and Luna 20 soils

Cl and P₂U₅ do not appear to exhibit the same correlation in soils from the Luna 20 and possibly the Luna 16 sites as they do in samples from the Apollo 11–15 sites. Nevertheless, the coherence between labile Cl and other KREEP-related elements is maintained.     

Structural state of native molybdenum in the lunar regolith

The structural state was determined for zero-valence molybdenum in the lunar regolith. The body- and face-centered molybdenum forms (BCC and FCC, respectively) were identified. Disruption of the structure down to complete amorphization was noted. This might be caused by the long-term influence of the solar wind.     

A lunar differentiation model in light of new chemical data on Luna 20 and Apollo 16 soils

Fines from a Luna 20 soil sample and from three Apollo 16 deep drill core samples have been analyzed for major-minor element abundances by a combined, semi-micro atomic absorption spectrophotometric and colorimetric method. Both the major element and large ion lithophile trace element abundances in these soils, the first from interior highland sites, are greatly influenced by the very high normative plagioclase content, being distinctly richer in Al and Ca, and poorer in K, P, Cr, Mn, Fe, and Ti, than most bulk soil samples from previous lunar missions. The relatively large compositional variations in the Apollo 16 core can be ascribed almost entirely to decreasing plagioclase with increasing depth. The chemical composition of the Luna 20 soil indicates less plagioclase and less KREEP than in the Apollo 16 soils. A lunar differentiation model is presented in which is made the suggestion that KREEP is the result of a second fusion event in a lunar crust consisting of early feldspathic cumulates and primary aluminous ‘liquid’.     

Oxide minerals in lithic fragments from Luna 20 fines

One hundred and seventy-six oxide mineral grains in the Luna 20 samples were analyzed by electron microprobe. Spinel is the most abundant oxide, occurring in troctolite fragments. Next most abundant is ilmenite, which occurs in all rock types except those containing spinel. Chromite also occurs in all rock types except those containing spinel. Minor amounts of ulvöspinel, armalcolite, zirkelite, baddeleyite and an unidentified TiO₂-rich phase were also found.Spinel grains are predominantly spinel-hercynite solid solutions, commonly with very minor chromite. The $\text{Fe}\text{(Fe + Mg)}$ ratio is generally lower than in spinel from Apollo 14 rocks. Chromites in non-mare rocks are similar to those from mare rocks. Ilmenite of mare origin is Mg-poor and Zr-rich compared to non-mare ilmenite; these elements may therefore be useful in determining the origin of ilmenite grains.Phase equilibria considerations suggest that spinel troctolite crystallized from a melt high in alumina; a likely candidate is the high-alumina basalt of Prinzet al. (1973a).Sub-micron wide rods of metallic Fe occur in plagioclase grains and may have formed by sub-solidus reduction processes.     

U-Th-Pb measurements of Luna 20 soil

The concentrations of uranium, thorium and lead and the lead isotopic composition of Luna 20 soil were determined. The data indicate that the Luna 20 soil is mainly a mixture of highland anorthosites and low-K basalt, but little KREEP basalt. The U-Th-Pb systematics are discussed in comparison with other lunar soils, especially with Apollo 16 soils which were collected from a ‘typical’ highland region. The data fit well in the Apollo 16 soil array on a U-Pb evolution diagram, and they exhibit excess lead relative to uranium. This relationship appears to be a characteristic of highland localities. Considering the previous observations of lunar samples, we infer that lead enrichment in the soil relative to uranium occurred between 3.2 and 3.9 b.y. ago and that the soil was disturbed by ‘third events’ about 2.0 b.y. ago. A lunar evolution model is discussed.     

Radiation damage in Luna 20 soil

Micron-sized soil grains from the Luna 20 mission are the most lightly irradiated we have examined, in contrast to micron-sized grains from the Luna 16 soil, which are the most heavily irradiated. Radiation damage in micron-sized grains is inversely correlated with albedo of the soil. The absence of angular, amorphous grains in Luna 20 supports our previous contention that such grains in Luna 16 were produced by intense radiation damage.