Alkali Suite Research Articles

Lunar Evolution in Light of the Chang'e-5 Returned Samples

The Chinese spacecraft Chang'e-5 (CE-5) landed on the northern Ocean Procellarum and returned 1,731 grams of regolith. The CE-5 regolith is composed mostly of fragments of basalt, impact glass, agglutinates, and mineral fragments. The basalts could be classified as of a low-Ti and highly fractionated type based on their TiO2 content of ∼5.3 wt% and Mg# of ∼28. Independent of petrographic texture, the CE-5 basalts have a uniform eruption age of 2,030 ± 4 Ma, demonstrating that the Moon remained volcanically active until at least ∼2.0 Ga. Although the CE-5 landing site lies within the so-called Procellarum KREEP [potassium (K), rare earth elements (REE), and phosphorus (P)] Terrane, neither the CE-5 basalts nor the mantle source regions of those basalts were enriched in KREEP components, such as incompatible elements, water, sulfur, or chlorine. Therefore, it would be a new and stimulating task in the future to look for the triggering mechanism of the young volcanism on the Moon. ▪The CE-5 spacecraft returned 1,731 grams of lunar regolith in December 2020. It was the first new lunar sample since the last collection in August 1976.▪CE-5 regolith is basaltic in chemical composition, with only ∼1% highland materials of anorthosite, Mg suite, alkali suite, and KREEP.▪The CE-5 basalt is low Ti and highly differentiated. It was extruded at ∼2.0 Ga, being the youngest lunar basalt identified so far from the Moon.▪The triggering mechanism of the ∼2.0 Ga lunar volcanism is not clearly understood because its mantle source was dry and contained low abundances of KREEP elements.

Lunar meteorite Northwest Africa 11962: A regolith breccia containing records of titanium‐rich lunar volcanism and the high alkali suite

AbstractNorthwest Africa (NWA) 11962 is a lunar regolith breccia composed of a wide range of different clasts. The lunar origin of this meteorite is confirmed by oxygen isotope analysis and the Fe/Mn ratio in pyroxene and olivine. In the present study, the clasts and the matrix of NWA 11962 are characterized by optical and electron microscopy along with electron microprobe analyses and micro‐Raman spectroscopy. The meteorite has a glassy impact melt matrix, which accounts for 35% of the surface area in the two thin sections examined, and which contains a very large variety of different lithic clasts, monomineralic clasts, and glass fragments. The presence of volcanic and impact‐related glass spherules led to the classification of this meteorite as a regolith breccia. Lithic clasts include numerous fragments of quartz monzogabbro and lunar felsite, which are quite rare lithologies in the lunar alkali suite. However, the most abundant components in the breccia are gabbroic clasts. The mineral chemistry of the pyroxenes in the gabbroic clasts and the chemistry of various types of glass fragments in the breccia indicate an origin of the regolith from an area with low‐Ti to high‐Ti mare basalt volcanism. In addition to the peculiar petrographic characteristics of NWA 11962, the possible pairing relationships with other lunar meteorites are discussed.

Timing of geological events in the lunar highlands recorded in shocked zircon-bearing clasts from Apollo 16.

Apollo 16 soil-like regolith breccia 65745,7 contains two zircon-bearing clasts. One of these clasts is a thermally annealed silica-rich rock, which mineralogically has affinities with the High Alkali Suite (Clast 1), and yields zircon dates ranging from 4.08 to 3.38 Ga. The other clast is a KREEP-rich impact melt breccia (Clast 2) and yields zircon dates ranging from 3.97 to 3.91 Ga. The crystalline cores of both grains, which yield dates of ca 3.9 Ga, have undergone shock pressure modification at less than 20 GPa. We interpret that the U-Pb chronometer in these zircon grains has been partially reset by the Imbrium basin-forming event when the clasts were incorporated into the Cayley Plains ejecta blanket deposit. The zircon grains in Clast 1 have been partially decomposed, resulting in a breakdown polymineralic texture, with elevated U, Pb and Th abundances compared with those in the crystalline zircon. These decomposed areas exhibit younger dates around 3.4 Ga, suggesting a secondary high-pressure, high-temperature event, probably caused by an impact in the local Apollo 16 highlands area.

Oldest Immiscible Silica‐rich Melt on the Moon Recorded in a ~4.38 Ga Zircon

AbstractThe temporal duration of lunar‐evolved magmatism is still poorly constrained. In lunar meteorite Northwest Africa (NWA) 10049, a melt inclusion‐bearing zircon fragment provides a new tool to understand the composition and age of the melts from which zircon directly crystallized. The studied zircon‐hosted melt inclusions are silica rich and iron poor (e.g., ~80–90 wt% SiO2; <0.5 wt% FeO), compositionally similar with immiscible silica‐rich melts found in Apollo rocks. Nano‐SIMS U–Pb analyses of the zircon yielded a minimum crystallization age of 4,382 ± 40 Ma, older than the ages for Apollo highly evolved alkali suite lithologies (~3.8–4.33 Ga). Our study shows that the melt inclusion‐bearing zircon in NWA 10049 is the oldest microscale evidence for documenting immiscible silica‐rich melts in lunar samples, suggesting that lunar‐evolved silica‐rich melts were prevalent as early as ~4.38 Ga. This work implies that there would be a prolonged silicic magmatism occurred on the Moon.

Multiple lithic clasts in lunar breccia Northwest Africa 7948 and implication for the lithologic components of lunar crust

AbstractThis study presents the petrography, mineralogy, and bulk composition of lunar regolith breccia meteorite Northwest Africa (NWA) 7948. We identify a range of lunar lithologies including basaltic clasts (very low‐titanium and low‐titanium basalts), feldspathic lithologies (ferroan anorthosite, magnesian‐suite rock, and alkali suite), granulites, impact melt breccias (including crystalline impact melt breccias, clast‐bearing impact melt breccias, and glassy melt breccias), as well as regolith components (volcanic glass and impact glass). A compositionally unusual metal‐rich clast was also identified, which may represent an impact melt lithology sourced from a unique Mg‐suite parent rock. NWA 7948 has a mingled bulk rock composition (Al2O3 = 21.6 wt% and FeO = 9.4 wt%) and relatively low concentrations of incompatible trace elements (e.g., Th = 1.07 ppm and Sm = 2.99 ppm) compared with Apollo regolith breccias. Comparing the bulk composition of the meteorite with remotely sensed geochemical data sets suggests that the sample was derived from a region of the lunar surface distal from the nearside Th‐rich Procellarum KREEP Terrane. Our investigations suggest that it may have been ejected from a nearside highlands‐mare boundary (e.g., around Mare Crisium or Orientale) or a cryptomare region (e.g., Schickard‐Schiller or Mare smythii) or a farside highlands‐mare boundary (e.g., Mare Australe, Apollo basin in the South Pole–Aitken basin). The distinctive mineralogical and geochemical features of NWA 7948 suggest that the meteorite may represent lunar material that has not been reported before, and indicate that the lunar highlands exhibit wide geological diversity.

Rubidium isotopic composition of the Earth, meteorites, and the Moon: Evidence for the origin of volatile loss during planetary accretion

Understanding the origin of volatile element variations in the inner Solar System has long been a goal of cosmochemistry, but many early studies searching for the fingerprint of volatile loss using stable isotope systems failed to find any resolvable variations.An improved method for the chemical purification of Rb for high-precision isotope ratio measurements by multi-collector inductively-coupled-plasma mass-spectrometry. This method has been used to measure the Rb isotopic composition for a suite of planetary materials, including carbonaceous, ordinary, and enstatite chondrites, as well as achondrites (eucrite, angrite), terrestrial igneous rocks (basalt, andesite, granite), and Apollo lunar samples (mare basalts, alkali suite). Volatile depleted bodies (e.g. HED parent body, thermally metamorphosed meteorites) are enriched in the heavy isotope of Rb by up to several per mil compared to chondrites, suggesting volatile loss by evaporation at the surface of planetesimals. In addition, the Moon is isotopically distinct from the Moon in Rb. The variations in Rb isotope compositions in the volatile-poor samples are attributed to volatile loss from planetesimals during accretion. This suggests that either the Rb (and other volatile elements) were lost during or following the giant impact or by evaporation earlier during the accretion history of Theia.

Petrologic Characteristics of the Lunar Surface.

Petrologic analysis of the lunar surface is critical for determining lunar formation and evolution. Here, we report the first global petrologic map that includes the five most important lunar lithological units: the Ferroan Anorthositic (FAN) Unit, the Magnesian Suite (MS) Unit, the Alkali Suite (AS) Unit, the KREEP Basalt (KB) Unit and the Mare Basalt (MB) Unit. Based on the petrologic map and focusing on four long-debated and important issues related to lunar formation and evolution, we draw the following conclusions from the new insights into the global distribution of the five petrologic units: (1) there may be no petrogenetic relationship between MS rocks and KB; (2) there may be no petrogenetic link between MS and AS rocks; (3) the exposure of the KREEP component on the lunar surface is likely not a result of MB volcanism but is instead mainly associated with the combined action of plutonic intrusion, KREEP volcanism and celestial collision; (4) the impact size of the South Pole-Aitken basin is constrained, i.e., the basin has been excavated through the whole crust to exhume a vast majority of lower-crustal material and a very limited mantle components to the lunar surface.

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

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

Fluorine and chlorine abundances in lunar apatite: Implications for heterogeneous distributions of magmatic volatiles in the lunar interior

Apatite has been analyzed from mare basalts, the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks using an electron probe microanalysis routine developed specifically for apatite. We determined that all the lunar apatite grains analyzed are predominantly fluorine rich; however, they also contain varying concentrations of chlorine and a missing structural component that, after ruling out other possibilities, we attribute to OH. Apatite grains from mare basalts are compositionally distinct from the apatite grains in the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks, which all had similar apatite compositions. Apatite grains in mare basalts are depleted in chlorine, and many of the analyzed grains have stoichiometry that suggests a significant OH component (i.e., >0.08 structural formula units), whereas apatite grains in the magnesian suite, alkali suite, and KREEP-rich impact melts are enriched in chlorine and do not typically have a missing structural component that could be attributed to OH − (within the detection limit of 0.08 sfu). From these data, we infer that residual liquids in the mare basalts were enriched in H 2O and fluorine relative to chlorine at the time of apatite crystallization, whereas residual liquids in magnesian-suite, alkali-suite, and KREEP-rich impact melts were enriched in chlorine relative to H 2O and fluorine at the time of apatite crystallization. The relative volatile abundance that we determined for the mare basalts is identical to the previously determined relative volatile abundance for the lunar picritic glasses. This result indicates that the observed relative volatile abundance signature of the picritic glass source is the same as that in the mare basalt source regions. The magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks likely reflect a volatile source with different volatile abundances than the sources of mare volcanics. Moreover, the magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks may reveal the relative volatile abundance of urKREEP, the residual melt of the magma ocean. This difference in relative magmatic volatile abundance among the lithologic groups investigated cannot be explained by degassing of a single source composition (relative to magmatic volatiles). The most reasonable explanation for the compositional disparity is a difference in the relative volatile abundances in the magmatic source regions of the Moon. Therefore, we conclude that the Moon has a heterogeneous distribution of magmatic volatiles within its interior, with a chemical divide (with respect to magmatic volatiles) existing between magmas that arise by partial melting of the lunar mantle and magmas that have seen significant contamination by a KREEP component.

Petrogenesis and chronology of lunar meteorite Northwest Africa 4472: A KREEPy regolith breccia from the Moon

Northwest Africa (NWA) 4472 is a polymict lunar regolith meteorite. The sample is KREEP-rich (high concentrations of potassium, rare earth elements and phosphorus) and comprises a heterogeneous array of lithic and mineral fragments. These clasts and mineral fragments were sourced from a range of lunar rock types including the lunar High Magnesian Suite, the High Alkali Suite, KREEP basalts, mare basalts and a variety of impact crater environments. The KREEP-rich nature of NWA 4472 indicates that the sample was ejected from regolith on the nearside of the Moon in the Procellarum KREEP Terrane and we have used Lunar Prospector gamma-ray remote sensing data to show that the meteorite is most similar to (and most likely sourced from) regoliths adjacent to the Imbrium impact basin. U–Pb and Pb–Pb age dates of NWA 4472 phosphate phases reveal that the breccia has sampled Pre-Nectarian (4.35 Ga) rocks related to early episodes of KREEP driven magmatism. Some younger phosphate U–Pb and Pb–Pb age dates are likely indicative of impact resetting events at 3.9–4 Ga, consistent with the suggested timing of basin formation on the Moon. Our study also shows that NWA 4472 has sampled impact melts and glass with an alkali-depleted, incompatible trace element-rich (high Sc, low Rb/Th ratios, low K) compositional signature not related to typical Apollo high-K KREEP, or that sampled by KREEPy lunar meteorite Sayh al Uhaymir (SaU) 169. This provides evidence that there are numerous sources of KREEP-rich protoliths on the Moon.

REE characteristics and Nd isotopic compositions of Lunar pristine rocks: Implications for petrogenesis of pristine rocks and source regions

This paper compile the rare-earth elements and Nd isotope data for lunar pristine rocks from investigations in recent years. Using these data, we compared the REE characteristics of lunar pristine rocks and Nd isotopic compositions of their source regions. Based on the Lunar Magma Ocean model, we then studied their formation and petrogenetic correlations of Mg suite, alkali suite, and KREEP, with especial emphasis on the importance of assimilation during early magmatism. And Nd isotopic compositions of mare basalt samples suggest that mantle sources of mare basalts should be heterogeneous, which has not yet been explained by several current models.

Lunar meteorite regolith breccias: An in situ study of impact melt composition using LA-ICP-MS with implications for the composition of the lunar crust

Dar al Gani (DaG) 400, Meteorite Hills (MET) 01210, Pecora Escarpment (PCA) 02007, and MacAlpine Hills (MAC) 88104/88105 are lunar regolith breccia meteorites that provide sampling of the lunar surface from regions of the Moon that were not visited by the US Apollo or Soviet Luna sample return missions. They contain a heterogeneous clast population from a range of typical lunar lithologies. DaG 400, PCA 02007, and MAC 88104/88105 are primarily feldspathic in nature, and MET 01210 is composed of mare basalt material mixed with a lesser amount of feldspathic material. Here we present a compositional study of the impact melt and impact melt breccia clast population (i.e., clasts that were generated in impact cratering melting processes) within these meteorites using in situ electron microprobe and LA-ICP-MS techniques. Results show that all of the meteorites are dominated by impact lithologies that are relatively ferroan (Mg# 10), and have low incompatible trace element (ITE) concentrations (i.e., typically 10 ppm Sm), High Magnesium Suite (typically >70 Mg#) or High Alkali Suite (high ITEs, Sc/Sm ratios <2) target rocks. Instead the meteorite mafic melts are more ferroan, KREEP-poor and Sc-rich, and represent mixing between feldspathic lithologies and low-Ti or very low-Ti (VLT) basalts. As PCA 02007 and MAC 88104/05 were likely sourced from the Outer-Feldspathic Highlands Terrane our findings suggest that these predominantly feldspathic regions commonly contain a VLT to low-Ti basalt contribution.

Ancient Lunar Crust: Origin, Composition, and Implications

Samples from the Apollo (USA) and Luna (Soviet) missions and from lunar meteorites, coupled with remote sensing data, reveal that the ancient highlands of the Moon are compositionally diverse. The average surface material contains 80 vol% plagioclase. A major suite of rocks, the ferroan anorthosites, averages 96 vol% plagioclase. The feldspathic composition reflects plagioclase flotation in a magma ocean. Late-stage REE-rich magma pooled in the Procellarum region of the lunar nearside. The concentration of heat-producing elements in this region triggered mantle melting and overturn of the cumulate pile, forming two more suites of chemically distinct highland rocks, the magnesian and alkali suites.

The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Jurassic age volcanic rocks of the Stonyford volcanic complex (SFVC) comprise three distinct petrological groups based on their whole-rock geochemistry: (1) oceanic tholeiites; (2) transitional alkali basalts and glasses; (3) high-Al, low-Ti tholeiites. Major and trace element, and Sr–Nd–Pb isotopic data indicate that the oceanic tholeiites formed as low-degree partial melts of normal mid-ocean ridge basalt (N-MORB)-source asthenosphere similar in isotope composition to the East Pacific Rise today; the alkalic lavas were derived from an enriched source similar to that of E-MORB. The high-Al, low-Ti lavas resemble second-stage melts of a depleted MORB-source asthenosphere that formed by melting spinel lherzolite at low pressures. Trace element systematics of the high-Al, low-Ti basalts show the influence of an enriched component, which overprints generally depleted trace element characteristics. Tectonic discrimination diagrams show that the oceanic tholeiite and alkali suites are similar to present-day basalts generated at mid-oceanic ridges. The high-Al, low-Ti suite resembles primitive arc basalts with an enriched, alkali basalt-like overprint. Isotopic data show the influence of recycled components in all three suites. The SFVC was constructed on a substrate of normal Coast Range ophiolite in an extensional forearc setting. The close juxtaposition of the MORB-like olivine tholeiites with alkali and high-Al, low-Ti basalts suggests derivation from a hybrid mantle source region that included MORB-source asthenosphere, enriched oceanic asthenosphere, and the depleted supra-subduction zone mantle wedge. We propose that the SFVC formed in response to collision of a mid-ocean ridge spreading center with the Coast Range ophiolite subduction zone. Formation of a slab window beneath the forearc during collision allowed the influx of ridge-derived magmas or the mantle source of these magmas. Continued melting of the previously depleted mantle wedge above the now defunct subduction zone produced strongly depleted high-Al, low-Ti basalts that were partially fertilized with enriched, alkali basalt-type melts and slab-derived fluids.

Magmatic evolution of the Moon

Although incomplete because of the imperfect and somewhat random sampling of rock types by the Apollo and Luna missions (1969-1976), the history of lunar magmatism has been reconstructed by numerous researchers over the past three decades. These reconstructions have illustrated the continous nature of lunar magmatism (from 4.6 to ~2.0 Ga) and the large influence of early differentia­tion and catastrophic bombardment on lunar mantle dynamics, magmatism, and eruptive style. In this review, we group magmatism into multiple stages of activity based on sampled rock types and evaluate the models for each stage. Stage I is early lunar differentiation and associated magmatism. Partial melting of the Moon soon after accretion was responsible for producing an anorthositic crust and a differentiated lunar interior. The extent of lunar melting and mantle processing depends strongly on the mechanisms that induced melting. Estimates for the tune over which melting and crystallization occurred range from tens to hundreds of millions of years. Stage 2 is the disruption of lunar magma ocean cumulates. Soon after the crystallization of most of the lunar magma ocean, the cumulate pile experienced gravitational overturn. This resulted in transport of late-forming cumulates into the deep lunar mantle and mixing of magma ocean cumulates on a variety of scales. Stage 3 is the post-magma ocean highland magmatism. Whereas the ferroan anorthositic crust was probably produced during the crystallization of a magma ocean, the slightly younger Mg suite and alkali suite plutonie rocks may have been generated by decompressional melting of early magma ocean cumulates during cumulate pile over­turn. A KREEP and crustal signature was incorporated into these primitive basaltic magmas through assimilation near the base of the lunar crust or through melting of a hybridized mantle. The alkali suite could represent either the differentiation products of Mg suite parental magmas or a separate, but contemporaneous episode of basaltic magmatism. Stage 4 is pre-basin voleanism. Sample analysis and remote sensing data indicate that early lunar voleanism (KREEP basalts and high-alumina basalts) was contemporaneous with periods of liiglilands plutonism and catastrophic bombardment of the hmar surface. The relationship between early stages of lunar voleanism and the contemporane­ous plutonism is not clear. The KREEP basalts may be volcanic equivalents to both the Mg suite and alkali suite. Stage 5 is the late remelting of magma ocean cumulates and eruption of mare basalts. Basin-associated eruption of mare basalts occurred during and following the late stages of catastrophic bombardment. This volcanic activity was possibly an extension of the thermal event that initiated pre-basin voleanism. Mare basalts exhibit a wide range of composition resulting from near­surface fractionation of chemically distinct primary basaltic magmas. Most likely, mare basalts were produced by small to moderate degrees of partial melting of hybrid cumulate sources in the deep lunar mantle. Alternatively, the mixed chemical signatures observed in many mare basalts may be interpreted as indicating assimilation of late-stage, evolved cumulates by melts produced deep in the cumulate pile. The wide range of compositions exhibited by the mare basalts compared with earlier episodes of basaltic magmatism may reflect the thermal regime in the lunar mantle that limited the extent of partial melting and melt-source homogenization.

Petrology of the Western Highland Province: Ancient crust formation at the Apollo 14 site

Plutonic rocks found at the Apollo 14 site comprise four lithologic suites: the magnesian suite, the alkali suite, evolved lithologies, and the ferroan anorthosite suite (FAN). Rocks of the magnesian suite include troctolite, anorthosite, norite, dunite, and harzburgite; they are charaterized by plagioclase ≈An95 and mafic minerals with mg#s 82–92. Alkali suite rocks and evolved rocks generally have plagioclase ≈An90 to ≈An40 and mafic minerals with mg#s 82–40. Lithologies include anorthosite, norite, quartz monzodiorite, granite, and felsite. Ferroan anorthosites have plagioclase ≈An96 and mafic minerals with mg#s 45–70. Whole rock geochemical data show that most magnesian suite samples and all alkali anorthosites are cumulates with little or no trapped liquid component. Norites may contain significant trapped liquid component, and some alkali norites may represent cumulate‐enriched, near‐liquid compositions, similar to KREEP basalt 15386. Evolved lithologies include evolved partial cumulates related to alkali suite fractionation (quartz monzodiorite), immiscible melts derived from these evolved magmas (granites), and impact melts of preexisting granite (felsite). Plots of whole rock mg# versus whole rock Ca/(Ca + Na + K) show a distinct gap between rocks of the magnesian suite and rocks of the alkali suite, suggesting either distinct parent magmas or distinct physical processes of formation. Chondrite‐normalized rare earth element (REE) patterns show that rocks of both the magnesian suite and alkali suite have similar ranges, despite the large difference in major element chemistry. Current models for the origin of the magnesian suite call for a komatiitic parent magma derived from early magma ocean cumulates; these melts must assimilate plagiophile elements to form troctolites at low pressures and must assimilate a highly enriched KREEP component so that the resulting mixture has REE concentrations similar to high‐K KREEP. There are as yet no plausible scenarios that can explain these unusual requirements. We propose that partial melting of a primitive lunar interior and buffering of these melts by ultramagnesian early magma ocean cumulates provides a more reasonable pathway to form magnesian troctolites. Alkali anorthosites and norites formed by crystallization of a parent magma with major element compositions similar to KREEP basalt 15386. If the parent magma of the alkali suite and evolved rocks is related to the magnesian suite, then that magma must have evolved through combined assimilation‐fractional crystallization processes to form the alkali suite cumulates.

Surfing the Fra Mauro Shoreline: Highlands Crust at the Apollo 14 Site

Plutonic rocks at the Apollo 14 site may be grouped into four lithologic suites. In order of relative abundance these are—magnesian suite, alkali suite, evolved lithologies, and ferroan anorthosites (FAN). Most of the samples described to date occur as clasts in lunar polymict breccias or in regolith breccias; these clasts rarely are more than a few cm in diameter. The magnesian suite (63 clasts) includes troctolite, anorthosite, norite, dunite, and harzburgite, all characterized by plagioclase ≈An95 and mafic minerals with Mg#s from 82 to 92. Alkali-suite rocks (21 clasts) and evolved rocks (10 clasts) generally have plagioclase An90 to An40, and mafic minerals with Mg#s from 82 to 40. Lithologies include anorthosite, norite, quartz monzodiorite, granite, and felsite. Ferroan anorthosites (5 clasts) have plagioclase ≈An96 and mafic minerals with Mg#s from 45 to 70. Plots of whole-rock [Mg/(Mg+Fe)] versus whole-rock [Ca/(Ca+Na+K)] show a distinct gap between rocks of the magnesian suite and rocks of the alkali suite, suggesting distinct parent magmas or distinct physical processes of formation. Chondrite-normalized REE patterns show that rocks of both the magnesian suite and alkali suite have similar ranges, despite the large difference in major-element chemistry. Most magnesian-suite samples and all alkali anorthosites are cumulates with little or no trapped liquid component. Norites may contain significant trapped liquids, and some alkali norites may represent cumulate-enriched, near-liquid compositions, similar to KREEP basalt 15386. Evolved lithologies include evolved partial cumulates related to alkali-suite fractionation (quartz monzodiorite), immiscible melts derived from these evolved magmas (granites), and impact melts of pre-existing granite (felsite). Recent models for the origin of the magnesian suite envision a komatiitic parent magma derived from early magma-ocean cumulates; these melts must assimilate plagiophile elements to form troctolites at low pressures, and must assimilate a highly enriched KREEP component so that the resulting mixture has REE concentrations similar to high-K KREEP. As yet there are no plausible scenarios that can explain these unusual requirements. In contrast, alkali anorthosites and norites can be shown to form from crystallization of a KREEP-basalt parent magma similar to 15386, along with some magnesian norites and the evolved quartz monzodiorites. If the parent magma of the alkali suite and evolved rocks is related to the magnesian suite, then that magma must have evolved through combined assimilation-fractional crystallization processes to form the alkali-suite cumulates.

Mineral-Chemical and Isotopic Variations in Apollo 16 Impact-Melt Breccias

Lunar impact-melt breccias were formed by intense bombardment of the Moon's surface by meteorites. The compositions of these rocks may record the composition of the underlying lunar crust, but also may contain components that were transported from much larger basin-forming impacts several hundred kilometers away. In this study, a suite of nine impact-melt breccia samples were analyzed for their texture and mineral compositions and a subset of these samples were analyzed for Sr and Nd-isotopic compositions. These new data were integrated into accumulated databases on the mineral-chemical and geochemical compositions of Apollo 16 impact-melt breccias to develop a better understanding of the formation of these rocks and the nature of the materials from which they were derived. Although not pronounced, there are some distinct mineralogical features characteristic of the various breccia groups and subgroups defined in McKinley et al. (1984) and Korotev (1994). Rocks in the various subgroups have isotopic characteristics that are correlated with the geochemical features used to classify them. Mineralogical and chemical features of Group 3 and Group 4 breccias (feldspathic) are clearly inherited from their anorthositic source. The large compositional variation in Group 3 and Group 4 breccias (mafic) is more complicated, requiring a significantly more mafic and incompatible trace element–rich source than anorthosite. Although rocks of the magnesian and alkali suites have these compositional characteristics, the amount of added material required to shift the compositions of feldspathic breccias to mafic breccias is prohibitive. The mafic impact-melt breccias are interpreted to have been emplaced as a result of the Imbrium impact, and thus contain a record of the composition of the middle to lower crust in the Imbrium region. Compositional similarities between Group 1 breccias and Apollo 15 KREEP basalts suggest that these two rocks may ultimately have the same source region.

Stratigraphy of the lunar highland crust; depths of burial of lunar samples from cooling-rate studies

Current models of lunar crustal stratigraphy are based primarily on models of impact cratering coupled with what is known about the distribution of rock types in the ejecta blankets associated with the large multiring basins. These models postulate an upper crust rich in the ferroan anorthosite component underlain by a more mafic (noritic) lower crust (Spudis 1993). We attempted to test these models by determining the depth at which various lunar plutonic rocks formed. Despite intense brecciation and shock, many lunar highland samples retain vestiges of a prior magmatic history. For example, pyroxene samples preserve a record of their thermal history in such phenomena as cation ordering and exsolution textures. By computer simulation of the growth of exsolution lamellae in augite and pigeonite, we deduced cooling rates and from these estimated a depth of burial on the basis of a simple model for the thermal evolution of the lunar crust. Despite the fairly large uncertainties in the calculations, it is clear that the ferroan anorthosite samples we studied formed at a depth between 10 and 20 km, whereas some members of highland magnesian and alkali suites formed as cumulates in magma chambers intruded into the uppermost layers of the crust. As yet there is no evidence to indicate that crystalline rocks from the lower half of the lunar crust are present in the samples returned by the Apollo missions. Most samples derived from the lower crust appear to be impact melts oflow-K Fra Mauro (noritic) composition.

Petrology and geochemistry of alkali gabbronorites from Lunar Breccia 67975

Clasts of an unusual type of lunar highlands igneous rock, alkali gabbronorite, have been found in Apollo 16 breccia 67975. The alkali gabbronorites form two distinct subgroups, magnesian and ferroan. Modes and bulk compositions are highly varied. The magnesian alkali gabbronorites are composed of bytownitic plagioclase (Or2–5An82–89), hypersthene (Wo3–5En49–62), augite (Wo39–42En36–44), a silica mineral, and trace Ba‐rich K‐feldspar. The ferroan alkali gabbronorites are composed of ternary plagioclase (Or11–22An65–74), pigeonite (Wo6–9En35–47), augite (Wo38–40En29–35), Ba‐rich K‐feldspar, and a silica mineral. Trace minerals in both subgroups are apatite, REE‐rich whitlockite, and zircon. The magnesian and ferroan alkali gabbronorites appear to have formed by progressive differentiation of the same, or closely related, parent magmas; the compositional data indicate that these magmas were REE‐rich. The ternary plagioclase is probably a high‐temperature metastable phase formed during crystallization. In composition and mineralogy, the 67975 alkali gabbronorites show many similarities to Apollo 12 and 14 alkali norites, alkali gabbronorites, and alkali anorthosites, and all these rocks together constitute a distinctive alkali suite. In addition, the alkali gabbronorites show some similarities to KREEP basalts, Mg‐norites, and some felsites. These data suggest genetic links between some or all of these types of pristine rocks. Two types of relationships are possible. The first is that alkali‐suite rocks crystallized in plutons of KREEP basalt magma, and KREEP basalts are their extrusive equivalents. The second is that the alkali‐suite rocks and some felsites all crystallized in plutons of Mg‐norite parent magmas, and KREEP basalt magmas formed by remelting of these plutons. Additional studies are needed to resolve which of these hypotheses is correct.