Feldspathic Crust Research Articles

The moon before mare

The crust of the Moon experienced a unique geodynamic evolution, beginning with its crystallization from a magma ocean, continuing through a period of heavy impact bombardment, and followed by extensive basaltic mare volcanism. All these events have left crucial records imprinted in the form of topographic features and gravity anomalies. Here, we invert gravity and topography data using a two-layer thin-shell loading model under the premise of pre-mare isostasy to investigate the global structure of the crust and solve for feldspathic crust and mare thickness, together with mare-induced flexure. The tectonic record and partially buried crater population are used to constrain the bulk of mare volcanism to have been emplaced on a 40 km elastic lithosphere, although mare within large impact basins may have formed on a thinner elastic lithosphere. The mare thickness and associated flexure are removed to calculate a map of the surface and crust of the Moon before mare volcanism. The pre-mare surface in the Oceanus Procellarum region is found to be ∼2 km lower than the surrounding nearside, and several possible explanations, including a giant impact, pore space annealing, isostatic adjustment, and crustal erosion induced by a mantle plume or thermal anomaly, are discussed. The pre-mare elevation map further sheds light on the ring structure of Imbrium, which is seen to resemble that of Orientale. Imbrium's outermost ring is observed to be at a larger radial distance to the northeast relative to the south, indicating that some level of lithospheric variability affected ring formation at the time of impact. The western part of Imbrium's ring within Oceanus Procellarum is not found in the pre-mare topography, implying that it either never formed or that some processes erased its signature from gravity and topography. The feldspathic, pre-mare, crust is found to be ∼7 km thinner within large nearside basins than in models not accounting for the high-density mare. The pre-fill floor of these basins was ∼6 km deeper than currently observed, and together with their updated crustal structure, these new insights have implications for impact simulations that try to reproduce the crustal structure of nearside mare basins.

Extensive and ancient feldspathic crust detected across north Hellas rim, Mars: Possible implications for primary crust formation

Abstract Due to its size and observable record of ancient rock, Mars is key to understanding crustal formation on planetary bodies, including Venus and Earth, which may have derived their first stable crust from mantle-overturn melting. Recent evidence that ancient martian crust contains an evolved component supports inferences of a pervasive, buried feldspathic component to the crust. With data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), we searched for feldspathic lithologies in pre-Noachian (older than ca. 4.1 Ga) crustal blocks uplifted by the Hellas basin-forming impact. We present evidence for ancient feldspathic rocks exposed across an ~2200 × 600 km area north of Hellas. Given their pre-Noachian age and stratigraphic position directly above putative mantle material, it is possible that these outcrops represent martian primary crust. Our discovery supports the hypothesis that there exists a pervasive, subsurface feldspathic component to the martian crust—a hypothesis that has, until now, been supported only by inferences from geodynamic data and small-scale observations.

Lunar meteorites from northern Africa

AbstractWe report bulk composition data for 235 stones of ~77 lunar meteorites from northern Africa and 33 lunar meteorites from elsewhere. About 27% of the African meteorites are typical feldspathic lunar rocks, all breccias of similar bulk composition, for example, 3–5% FeO and low concentrations of incompatible elements. Nevertheless, these meteorites have a large range inMg'(whole‐rock mole% Mg/[Mg + Fe]), 57–77 (mean: 65.5), and this parameter does not correlate with either albite content of the plagioclase or concentrations of incompatible elements. From this observation, we conclude that feldspathic lunar meteorites do not support the hypotheses that the anorthositic, precursor plutonic rocks of the meteorite breccias all crystallized from a common magma. Among feldspathic lunar meteorites,Mg'increases with normative olivine abundance but is uncorrelated with normative pyroxene abundance, that is, high‐Mg'feldspathic rocks of the early lunar crust were troctolitic, not noritic. The NWA 5744 (Northwest Arica) clan of lunar meteorites are anorthositic troctolites (59–75% plagioclase, 15–26% olivine, Fo77–80) with very low concentrations of incompatible elements (2.6 × CI) compared to troctolites in the Apollo collection. They may represent the lower feldspathic crust or differentiated impact melt from portions of the highlands distant from the PKT (Procellarum KREEP Terrane). About 31% of the lunar meteorites from northern Africa, all breccias, are, by lunar standards, moderately mafic (5–13% FeO) but more magnesian than mixtures of typical feldspathic meteorite and mare basalt, implying that they represent moderately mafic regions of the highlands or originate from the South Pole–Aitken basin. With ~6 µg g−1Th, two NWA meteorites likely originate from the PKT. Including the unique NWA 773 clan, at least six of the northern Africa meteorites are, or are dominated by, unbrecciated basalts and gabbros of mare affinity. We estimate that the ~341 lunar meteorite stones (Africa and elsewhere) for which there are data represent 131–147 terrestrially unpaired meteorites and 109–134 lunar launch sites.

Donwilhelmsite, [CaAl4Si2O11], a new lunar high-pressure Ca-Al-silicate with relevance for subducted terrestrial sediments

AbstractWe report on the occurrence of a new high-pressure Ca-Al-silicate in localized shock melt pockets found in the feldspatic lunar meteorite Oued Awlitis 001 and discuss the implications of our discovery. The new mineral crystallized as tiny, micrometer-sized, acicular grains in shock melt pockets of roughly anorthitic bulk composition. Transmission electron microscopy based three-dimensional electron diffraction (3D ED) reveals that the CaAl4Si2O11 crystals are identical to the calcium aluminum silicate (CAS) phase first reported from static pressure experiments. The new mineral has a hexagonal structure, with a space group of P63/mmc and lattice parameters of a = 5.42(1) Å; c = 12.70(3) Å; V = 323(4) Å3; Z = 2. This is the first time 3D ED was applied to structure determination of an extraterrestrial mineral. The International Mineralogical Association (IMA) has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the U.S. lunar geologist Don E. Wilhelms. On the Moon, donwilhelmsite can form from the primordial feldspathic crust during impact cratering events. In the feldspatic lunar meteorite Oued Awlitis 001, needles of donwilhelmsite crystallized in ~200 mm sized shock melt pockets of anorthositic-like chemical composition. These melt pockets quenched within milliseconds during declining shock pressures. Shock melt pockets in meteorites serve as natural crucibles mimicking the conditions expected in the Earth's mantle. Donwilhelmsite forms in the Earth's mantle during deep recycling of aluminous crustal materials, and is a key host for Al and Ca of subducted sediments in most of the transition zone and the uppermost lower mantle (460–700 km). Donwilhelmsite bridges the gap between kyanite and the Ca-component of clinopyroxene at low pressures and the Al-rich Ca-ferrite phase and Ca-perovskite at high-pressures. In ascending buoyant mantle plumes, at about 460 km depth, donwilhelmsite is expected to break down into minerals such as garnet, kyanite, and clinopyroxene. This process may trigger minor partial melting, releasing a range of incompatible minor and trace elements and contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.

The Critical Point and the Supercritical State of Alkali Feldspars: Implications for the Behavior of the Crust During Impacts.

The position of the vapor‐liquid dome and of the critical point determine the evolution of the outermost parts of the protolunar disk during cooling and condensation after the Giant Impact. The parts of the disk in supercritical or liquid state evolve as a single thermodynamic phase; when the thermal trajectory of the disk reaches the liquid‐vapor dome, gas and melt separate leading to heterogeneous convection and phase separation due to friction. Different layers of the proto‐Earth behaved differently during the Giant Impact depending on their constituent materials and initial thermodynamic conditions. Here we use first‐principles molecular dynamics to determine the position of the critical point for NaAlSiO and KAlSiO feldspars, major minerals of the Earth and Moon crusts. The variations of the pressure calculated at various volumes along isotherms yield the position of the critical points: 0.5–0.8 g cm and 5500–6000 K range for the Na‐feldspar, 0.5–0.9 g cm and 5000–5500 K range for the K‐feldspar. The simulations suggest that the vaporization is incongruent, with a degassing of O starting at 4000 K and gas component made mostly of free Na and K cations, O, SiO and SiO species for densities below 1.5 g cm. The Hugoniot equations of state imply that low‐velocity impactors (8.3 km s) would at most melt a cold feldspathic crust, whereas large impacts in molten crust would see temperatures raise up to 30000 K.

Magmatic evolution of lunar highland rocks estimated from trace elements in plagioclase: A new bulk silicate Moon model with sub-chondritic Ti/Ba, Sr/Ba, and Sr/Al ratios

The compositions of host magmas of ferroan anorthosites (FAN-host magmas) were estimated from secondary ion mass spectrometry analyses of plagioclase in lunar highland rocks. The evolution of the magmas was investigated by considering phase relations based on the MELTS algorithm and by re-examining partition coefficients for trace elements between plagioclase and melts. Data little affected by post-magmatic processes were selected by using plagioclase with relatively primitive Sc and Co contents. The FAN-host magma contained 90–174ppm Sr, 40–119ppm Ba and 0.5–1.3% TiO2, and had sub-chondritic Sr/Ba and Ti/Ba ratios. It is difficult to account for the formation of FAN-host magma on the basis of magma evolution processes of previously proposed bulk silicate Moon models with chondritic ratios for refractory elements at global scale. Therefore, the source of the FAN-host magma must have had primordial sub-chondritic Sr/Ba and Ti/Ba ratios. The FAN-host magmas were consistent in refractory elements with the estimated host mafic magma for feldspathic crust based on lunar meteorites, and some very-low-Ti mare rocks from lunar meteorites. Here, we propose an alternative bulk silicate Moon model (the cBSM model), which is enriched in crustal components of proto-bodies relative to the present whole Earth–Moon system.

Connecting Lunar Meteorites to Source Terranes on the Moon

The number of named stones found on Earth that have proven to be meteorites from the Moon is approx. 180 so far. Since the Moon has been mapped globally in composition and mineralogy from orbit, it has become possible to speculate broadly on the region of origin on the basis of distinctive compositional characteristics of some of the lunar meteorites. In particular, Lunar Prospector in 1998 [1,2] mapped Fe and Th at 0.5 degree/pixel and major elements at 5 degree/pixel using gamma ray spectroscopy. Also, various multispectral datasets have been used to derive FeO and TiO2 concentrations at 100 m/pixel spatial resolution or better using UV-VIS spectral features [e.g., 3]. Using these data, several lunar meteorite bulk compositions can be related to regions of the Moon that share their distinctive compositional characteristics. We then use EPMA to characterize the petrographic characteristics, including lithic clast components of the meteorites, which typically are breccias. In this way, we can extend knowledge of the Moon's crust to regions beyond the Apollo and Luna sample-return sites, including sites on the lunar farside. Feldspathic Regolith Breccias. One of the most distinctive general characteristics of many lunar meteorites is that they have highly feldspathic compositions (Al2O3 approx. 28% wt.%, FeO <5 wt.%, Th <1 ppm). These compositions are significant because they are similar to a vast region of the Moon's farside highlands, the Feldspathic Highlands Terrane, which are characterized by low Fe and Th in remotely sensed data [4]. The meteorites provide a perspective on the lithologic makeup of this part of the Moon, specifically, how anorthositic is the surface and what, if any, are the mafic lithic components? These meteorites are mostly regolith breccias dominated by anorthositic lithic clasts and feldspathic glasses, but they do also contain a variety of more mafic clasts. On the basis of textures, we infer these clasts to have formed by large impacts that excavated and mixed rocks from depth within the lunar crust and possibly the upper mantle. One of the key questions is whether the mafic materials are ferroan or magnesian, which remote sensing does not clearly distinguish, and if mafic, whether they might contain mantlederived components such as olivine (dunite). Many but not all have mainly ferroan mafic components, consistent with a ferroan crustal source that is complementary to the ferroan anorthositic suite and that represents primary magma-ocean-derived feldspathic crust. Meteorites such as ALH 81005 [5] and Shir 161 [6], however, contain coarse-grained magnesian mafic clasts (Fig. 1a) derived from deeply seated and melted material associated with impact basins. Comparison to LP gamma-ray data [2] supports an origin for magnesian feldspathic meteorites such as these (e.g., Shir 161) as shown in Fig. 1b. Sayh al Uhaymir (SaU) 169. Another distinctive but much less common composition is represented by relatively mafic impact-melt breccia that is rich in incompatible elements known as KREEP. These meteorites can be related to the western nearside Procellarum KREEP Terrane, especially through a combination of Fe and Th contents. Among the most enriched is SaU 169, which has been related to high- Th impact-melt breccia found at the Apollo 12 site [7]. Through detailed EPMA and ion microprobe analysis we have shown that these two rock types are related in age and origin.

Identification and characterization of science-rich landing sites for lunar lander missions using integrated remote sensing observations

Despite more than 52 years of lunar exploration, a wide range of first-order scientific questions remain about the Moon’s formation, temporal evolution, and current surface and interior properties. Addressing many of these questions requires obtaining new in situ analyses or return of lunar surface or shallow subsurface samples, and hence rely on the selection of optimal landing sites. Here, we present an approach to optimize science-rich lunar landing site selection studies based on the integration of remote sensing observations. Currently available remote sensing data, as well as features of interest published in the recent literature, were integrated in a Geographic Information System. This numerical database contains geographic information about all these findings, which can be consulted and used to simultaneously display multiple features and parameters of interest. To illustrate our approach, we identified the optimal landing sites to address the two top priorities (or goals) relative to Concept 3 of the National Research Council of the National Academies (2007), namely to ‘Determine the extent and composition of the primary feldspathic crust, (ur)KREEP layer, and other products of differentiation’ and to ‘Inventory the variety, age, distribution and origin of lunar rock types’. We review site requirements and propose possible landing sites for both these goals. We identified 29 sites that best fulfill both these goals and compare them with the landing sites of planned future lunar lander missions. Finally, we detail two of these science-rich sites (Aristarchus and Theophilus craters) which are particularly accessible through their location on the nearside.

Compositional diversity and geologic insights of the Aristarchus crater from Moon Mineralogy Mapper data

[1] The Moon Mineralogy Mapper (M3) acquired high spatial and spectral resolution data of the Aristarchus Plateau with 140 m/pixel in 85 spectral bands from 0.43 to 3.0 μm. The data were collected as radiance and converted to reflectance using the observational constraints and a solar spectrum scaled to the Moon-Sun distance. Summary spectral parameters for the area of mafic silicate 1 and 2 μm bands were calculated from the M3 data and used to map the distribution of key units that were then analyzed in detail with the spectral data. This analysis focuses on five key compositional units in the region. (1) The central peaks are shown to be strongly enriched in feldspar and are likely from the upper plagioclase-rich crust of the Moon. (2) The impact melt is compositionally diverse with clear signatures of feldspathic crust, olivine, and glass. (3) The crater walls and ejecta show a high degree of spatial heterogeneity and evidence for massive breccia blocks. (4) Olivine, strongly concentrated on the rim, wall, and exterior of the southeastern quadrant of the crater, is commonly associated the impact melt. (5) There are at least two types of glass deposits observed: pyroclastic glass and impact glass.

Scientific Exploration of the Moon

The Moon is a geochemically differentiated object. It has a feldspathic crust (highlands regions) composed of three petrological suites. Underlying that crust is a compositionally heterogeneous upper mantle from which ferrobasalts and picrites (mare regions) were generated. Lunar samples retain a memory of the time-dependent flux of meteorites and comets, which has implications for the origin of sustainable life on Earth and the orbital evolution of the outer planets. Permanently shadowed regions at the lunar poles may contain reservoirs of volatile ices, which would have important resource potential for scientific bases. Geophysical data show that the Moon has a thick, seismically active lithosphere, a partially molten zone beneath that lithosphere, and a small metallic core. The pace of scientific exploration has quickened since 2003 with the successes of spacecraft from Europe, Japan, the People's Republic of China, and India. Upcoming launches of spacecraft from these same nations and the United States herald a new era of lunar discoveries.

Oxygen isotope constraints on the origin and differentiation of the Moon

We report new high-precision laser fluorination three-isotope oxygen data for lunar materials. Terrestrial silicates with a range of δ 18O values (− 0.5 to 22.9‰) were analyzed to independently determine the slope of the terrestrial fractionation line (TFL; λ = 0.5259 ± 0.0008; 95% confidence level). This new TFL determination allows direct comparison of lunar oxygen isotope systematics with those of Earth. Values of Δ 17O for Apollo 12, 15, and 17 basalts and Luna 24 soil samples average 0.01‰ and are indistinguishable from the TFL. The δ 18O values of high- and low-Ti lunar basalts are distinct. Average whole-rock δ 18O values for low-Ti lunar basalts from the Apollo 12 (5.72 ± 0.06‰) and Apollo 15 landing sites (5.65 ± 0.12‰) are identical within error and are markedly higher than Apollo 17 high-Ti basalts (5.46 ± 0.11‰). Evolved low-Ti LaPaz mare-basalt meteorite δ 18O values (5.67 ± 0.05‰) are in close agreement with more primitive low-Ti Apollo 12 and 15 mare basalts. Modeling of lunar mare-basalt source composition indicates that the high- and low-Ti mare-basalt mantle reservoirs were in oxygen isotope equilibrium and that variations in δ 18O do not result from fractional crystallization. Instead, these differences are consistent with mineralogically heterogeneous mantle sources for mare basalts, and with lunar magma ocean differentiation models that result in a thick feldspathic crust, an olivine–pyroxene-rich mantle, and late-stage ilmenite-rich zones that were convectively mixed into deeper portions of the lunar mantle. Higher average δ 18O (WR) values of low-Ti basalts compared to terrestrial mid ocean ridge basalts (Δ=0.18‰) suggest a possible oxygen isotopic difference between the terrestrial and lunar mantles. However, calculations of the δ 18O of lunar mantle olivine in this study are only 0.05‰ higher than terrestrial mantle olivine. These observations may have important implications for understanding the formation of the Earth–Moon system.

Earth-Moon System, Planetary Science, and Lessons Learned

The origin and evolution of the Moon is now well understood. Many bizarre theories were proposed to account for our satellite before these were swept away by the data from the Apollo missions. We now understand with the benefit of hindsight that we are looking at a very ancient object whose surface has changed little for the past 3.5 b.y. Nevertheless the Moon remains a unique body in the solar system. The Moon formed as the result of a glancing collision with the Earth of a Mars-sized impactor late in the accretional history of the Earth when both bodies had differentiated into metallic cores and silicate mantles. The impactor is derived from the neighborhood of the Earth, as shown by similarities in O and Cr isotopes. Our satellite is mostly derived from the mantle of the impactor rather than from the mantle of the Earth. The Moon melted, perhaps entirely and a thick (50 km?) feldspathic crust, dated at 4460 ± 40 Ma floated on the ocean of magma. The closest analogue may be the crust of Mercury. The interior crystallized into cumulate zones of differing mineralogy from which the mare basalts were later derived by partial melting. The impact history of the lunar surface has provided insights into planetary accretion, the planetesimal hypothesis and the bombardment history in the inner solar system. Debate continues whether the concentration of impact-produced basins constitutes a “cataclysm” or the tail end of accretion. A notable example and a target for future missions is the 2500 km diameter South Pole-Aitken basin that may provide insights into the nature of the deep lunar crust. Remaining significant problems include whether the Moon has a small metallic core, while the deep interior structure is not well understood. New seismic and heat flow data are an urgent priority. …

Lunar geochemistry as told by lunar meteorites

About 36 lunar meteorites have been found in cold and hot deserts since the first one was found in 1979 in Antarctica. All are random samples ejected from unknown locations on the Moon by meteoroid impacts. Lithologically and compositionally there are three extreme types: (1) brecciated anorthosites with high Al 2O 3 (26–31%), low FeO (3–6%), and low incompatible elements (e.g., <1 μg/g Th), (2) basalts and brecciated basalts with high FeO (18–22%), moderately low Al 2O 3 (8–10%) and incompatible elements (0.4–2.1 μg/g Th), and (3) an impact-melt breccia of noritic composition (16% Al 2O 3, 11% FeO) with very high concentrations of incompatible elements (33 μg/g Th), a lithology that is identified as KREEP on the basis of its similarity to Apollo samples of that designation. Several meteorites are polymict breccias of intermediate composition because they contain both anorthosite and basalt. Despite the large range in compositions, a variety of compositional parameters together distinguish lunar meteorites from terrestrial materials. Compositional and petrographic data for lunar meteorites, when combined with mineralogical and compositional data obtained from orbiting spacecraft in the 1990s, suggest that Apollo samples identified with the magnesian (Mg-rich) suite of nonmare rocks (norite, troctolite, dunite, alkali anorthosite, and KREEP) are all products of a small, geochemically anomalous (noritic, high Th) region of crust known as the Procellarum KREEP Terrane and are not, as generally assumed, indigenous to the vast expanse of typical feldspathic crust known as the Feldspathic Highlands Terrane. Magnesian-suite rocks such as those of the Apollo collection do not occur as clasts in the feldspathic lunar meteorites. The misconception is a consequence of four historical factors: (1) the Moon has long been viewed as simply bimodal in geology, mare or highlands, (2) one of the last, large basin-forming bolides impacted in the Procellarum KREEP Terrane, dispersing Th-rich material, (3) although it was not known at the time, the Apollo missions all landed in or near the anomalous Procellarum KREEP Terrane and collected many Th-rich samples formed therein, and (4) the Apollo samples were interpreted and models for lunar crust formation developed without recognition of the anomaly because global data provided by orbiting missions and lunar meteorites were obtained only years later.

Contrasting Si-saturation in troctolite-anorthosite intrusions along the Manicouagan corridor of the Abitibi-Grenville transect

A survey of troctolite-anorthosite complexes along the Manicouagan corridor of the Abitibi-Grenville Lithoprobe transect reveals the existence of two distinct crystallization sequences in their cumulate rocks. In two intrusions (Brien and Raudot), olivine crystallized first, followed by plagioclase, Fe-oxides, and then clinopyroxene; and magmatic orthopyroxene is absent. In five other intrusions (Lucie, Tétépisca, Blache, Baie-Comeau, and Pentecôte), plagioclase and olivine are followed by orthopyroxene, and then clinopyroxene. These two distinct crystallization sequences correlate with compositional differences in gabbroic marginal rocks and dykes associated with the intrusions. The Raudot intrusion exhibits a fine-grained gabbroic margin that is Fe- and Al-rich, but Si-poor and nepheline normative. In contrast, the marginal rocks of the Blache, Baie-Comeau, and Pentecôte intrusions are typically gabbronorites that are poorer in Fe and richer in Si. Dyke rocks cutting this latter group of intrusions share these compositional characteristics and are typically orthopyroxene normative. The differences between the two types of intrusions appear to reflect the degree of Si-saturation of their parental magmas with respect to the troctolite divide. Together, the array of estimated parental magma compositions of the troctolite-anorthosite intrusions mimics that of Tertiary picritic magmas. The low forsterite (Fo) contents in olivine and the late appearance of clinopyroxene in the cumulates of the intrusions may reflect the interaction of mantle-derived parental magmas with relatively juvenile feldspathic crust.

The great lunar hot spot and the composition and origin of the Apollo mafic (“LKFM”) impact‐melt breccias

Thorium‐rich, mafic impact‐melt breccias from the Apollo 14–17 missions, that is, those breccias identified with the composition known as “LKFM,” are regarded largely as products of basin‐forming impacts that penetrated the feldspathic crust and sampled underlying mafic material and magma‐ocean residuum carrying the compositional signature of KREEP (potassium, rare earth elements, phosphorous). Despite considerable compositional variation among such breccias, compositions of all of them correspond to mixtures of only four components: (1) a norite with composition generally similar to that of Apollo 15 basalt (mean abundance: 58%; range: ∼30–95%), (2) Fo∼90 dunite (mean: 13%, range: 1–27%), (3) feldspathic upper crust (mean: 29%, range: 4–50%), and FeNi metal (0.1–1.7%). Petrographic evidence has shown that much of the feldspathic component, but none of the KREEP component, is clastic. This observation and the high proportion of KREEP norite component in the breccias suggest that the melt zone of the impact or impacts forming the breccias contained little feldspathic material but consisted predominantly of material with the average composition of KREEP norite. The dunite component probably derives ultimately from the upper mantle. These conclusions support the hypothesis that the breccias were not formed in typical feldspathic crust but instead by one or more impacts into what is designated here “the great lunar hot spot,” that is, the anomalous Th‐rich terrane in the Imbrium‐Procellarum area identified by the Apollo and Lunar Prospector gamma‐ray spectrometers. The LKFM composition is a special product of the great lunar hot spot and is not the average composition of the lower crust in typical feldspathic highlands. Similarly, Mg‐suite and alkali‐suite plutonic rocks of the Apollo collection are likely all differentiation products of the hot spot, not of plutons that might occur in typical feldspathic crust.

Crystal‐bearing lunar spherules: Impact melting of the Moon's crust and implications for the origin of meteoritic chondrules

Abstract— Crystal‐bearing lunar spherules (CLSs) in lunar breccia (14313, 14315, 14318), soil (68001, 24105), and impact‐melt rock (62295) samples can be classified into two types: feldspathic and olivine‐rich. Feldspathic CLSs contain equant, tabular, or acicular plagioclase grains set in glass or a pyroxene‐olivine mesostasis; the less common olivine‐rich CLSs contain euhedral or skeletal olivine set in glass, or possess a barred‐olivine texture. Bulk‐chemical and mineral‐chemical data strongly suggest that feldspathic CLSs formed by impact melting of mixtures of ferroan anorthosite and Mg‐suite rocks that compose the feldspathic crust of the Moon. It is probable that olivine‐rich CLSs also formed by impact melting, but some appear to have been derived from distinctively magnesian lunar materials, atypical of the Moon's crust.Some CLSs contain reversely‐zoned “relic” plagioclase grains that were not entirely melted during CLS formation, thin (≤5 μm thick) rims of troilite or phosphate, and chemical gradients in glassy mesostases attributed to metasomatism in a volatile‐rich (Na‐K‐P‐rich) environment. Crystal‐bearing lunar spherules were rimmed and metasomatized prior to brecciation. Compound CLS objects are also present; these formed by low‐velocity collisions in an environment, probably an ejecta plume, that contained numerous melt droplets.Factors other than composition were responsible for producing the crystallinity of the CLSs. We agree with previous workers that relatively slow cooling rates and long ballistic travel times were critical features that enabled these impact‐melt droplets to partially or completely crystallize in free‐flight. Moreover, incomplete melting of precursor materials formed nucleation sites that aided subsequent crystallization.Clearly, CLSs do not resemble meteoritic chondrules in all ways. The two types of objects had different precursors and did not experience identical rimming processes, and vapor fractionation appears to have played a less important role in establishing the compositions of CLSs than of chondrules. However, the many detailed similarities between CLSs and chondrules indicate that it is more difficult to rule out an origin for some chondrules by impact melting than some have previously argued. Differences between CLSs, chondrules, and their host rocks possibly can be reconciled with an impact‐melt origin for some chondrules when different precursors, the higher gravity of the Moon compared to chondrite parent bodies, and the likely presence of nebular gas during chondrule formation are taken into account.

Lunar and terrestrial crusts: a constrast in origin and evolution

Planetary crusts, differing in composition from that of the bulk planet, and containing large concentrations of incompatible elements such as K, U and Th, may arise in two distinctly different ways. The first, exemplified by the Moon, involves flotation of a feldspathic crust during an initial melting episode in which much of the planetary body was molten. The second, probably more common type of crust is formed as a consequence of partial melting, in response to radioactive heating, in the planetary interior and the eruption of basaltic lavas. Examples include the lunar maria and the oceanic crust of the Earth. Subsequent partial melting episodes, involving recycling into the mantle, or lower crustal melting episodes produce more acidic crusts, of which the terrestrial continental crust forms the type example. The upper crust requires at least three successive partial melting events.

Lunar anorthosite 60025, the petrogenesis of lunar anorthosites, and the composition of the Moon

Systematic variations of the mineral chemistry of ferroan anorthosite 60025, which is probably a mixture of closely related materials, suggest that lunar anorthosites formed by strong fractional crystallization and near-perfect adcumulate growth, without trapping liquid. The parent liquid for the most primitive samples was saturated with olivine, plagioclase, pigeonite, and chromite, and evolved to one saturated with plagioclase, pigeonite, high-Ca pyroxene, and ilmenite. The parent liquid also had a very low Na 2O content, and combined with strong fractional crystallization this explains the steep trend of anorthosites on an Mg∗ ( atomic 100 × Mg/( Mg + Fe)) v. An diagram. The mineral and chemical data for other anorthosites are consistent with such a model. Near-perfect adcumulation can occur if growth takes place at the crystal-liquid interface without the physical accumulation of crystals grown elsewhere, and is encouraged by the shifts in phase boundaries with pressure. Anorthosites are probably the remnants of a crust floating on, and crystallizing at the surface of, a magma ocean originally of bulk Moon composition. Mineralogical and trace element data suggest that the parental liquid for the most primitive anorthosites had previously crystallized no plagioclase and some but perhaps very little pyroxene. Hence the bulk Moon appears to be similar to that proposed by Ringwood (1976) but to have even lower alkalis, a subchondritic Ca Al ratio, and REE abundances and patterns close to chondritic. The mare basalt sources are not directly complementary to the feldspathic crust, because experimental and trace element data indicate that they are too magnesian and contain too much high-Ca pyroxene. Other crustal rocks, such as the Mg-suite samples, are not closely related to anorthosites; in addition to their chemical differences they have a different crystallization sequence: ol → plag → px, in contrast with the ol → px → plag inferred for anorthosite parental liquid evolution.

Study of the Moon and comparative planetology: Problems and perspectives

On the base of the comparative planetologic study of the Moon and terrestrial planets two fundamental features of their history and structure have been established. Firstly, shell-like structure of the terrestrial planets could be understood only in the terms of the heterogeneous accretion theory. At the final stages of major terrestrial planet formation the leading role belonged to the planetosimals of carbonaceous chondritic composition. Secondly, there are two types of the crust on the planetary surface. Their formation are considered to be independent and differing in the geological time. The primary planetary crust of predominantly feldspathic composition is considered to form during the pregeologic period at the final stage of planetary formation due to the impact-explosive processes. The hydrosphere and atmosphere is thought to appear contemporaneously. The basaltic planetary crust is forming later due to the radioactive decay and superimposed on the primary feldspathic crust.

The earth-moon system. Chemistry and origin

In all lunar highland samples a Mg-rich component is observed. As these samples consist of only three major components and the components vary in their abundances, the composition of the Mg-rich component can be calculated with certain assumptions. Wänke et al., (1977) have recently shown that the Mg-rich component probably consists itself of two portions of different origin, i.e. true-primary matter which was added to the moon when a solid feldspathic crust had already formed and material derived from residual liquids after the separation of olivine and plagioclase. The composition of the primary matter shows a striking similarity to that of the earth's mantle, except that the refractory elements seem to be somewhat enriched and all elements more volatile than Si are considerably depleted. The element ratios of the two strongly correlated elements W and La as well as the P Li ratio indicate that both W and P are highly depleted on the moon. The depletion for W relative to La (both elements belong to the group of refractory elements) is in fact identical to the one observed for terrestrial samples. The only feasible process for the depletion of W and P is removal with metallic iron. From measurements of the partition coefficients of W between silicate phase and metallic NiFe we know that a metal phase of 26% would be required to explain the observed depletion. This by far exceeds the amount tolerable from the lunar density. We conclude that the metal responsible for the removal of W from the lunar silicates is in the core of the earth. Hence, models in which the moon is formed from material evaporated from the earth's mantle are favoured.