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

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

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

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

U‐Pb SIMS ages of Apollo 14 zircon: Identifying distinct magmatic episodes

U‐Pb ages of zircon in four different Apollo 14 breccias (14305, 14306, 14314, and 14321) were obtained by secondary ion mass spectrometry. Some of the analyzed grains occur as cogenetic, poikilitic zircon grains in lithic clasts, revealing magmatic events at ~4286 Ma, ~4200–4220 Ma, and ~4150 Ma. The age distribution of the crystal clasts in the breccias exhibits a minor peak at ~4210 Ma, which can be attributed to a magmatic event, as recorded in zircon grains located in noritic clasts. An age peak at ~4335 Ma is present in all four breccias, as well as zircon grains from different Apollo landing sites, enhancing the confidence that these grains recorded a global zircon‐forming event. The overall age distribution among the four breccias exhibits minor differences between the breccias collected farther away from the Cone Crater and the ones collected within the continuous ejecta blanket of the Cone Crater. A granular zircon grain yielded a ²⁰⁷Pb/²⁰⁶Pb age of 3936 ± 8 Ma, which is interpreted as an impact event. A similar age of 3941 ± 5 Ma (n = 17, MSWD = 0.89, P = 0.58) was obtained for a large zircon grain (~430 × 340 μm in size). This grain might have crystallized in the same impact melt sheet which formed the granular zircon or the age is representative of the final extrusion of KREEP magma. The majority of zircon grains, however, occur as isolated crystal clasts within the matrix and their ages cannot be correlated with any real events (impact or magmatic) nor can the possibility be excluded that these ages represent partial resetting of the U‐Pb system.     

Origin and transportation history of lunar breccia 14311

In this paper, we compare the U‐Pb zircon age distribution pattern of sample 14311 from the Apollo 14 landing site with those from other breccias collected at the same landing site. Zircons in breccia 14311 show major age peaks at 4340 and 4240 Ma and small peaks at 4110, 4030, and 3960 Ma. The zircon age patterns of breccia 14311 and other Apollo 14 breccias are statistically different suggesting a separate provenance and transportation history for these breccias. This interpretation is supported by different U‐Pb Ca‐phosphate and exposure ages for breccia 14311 (Ca‐phosphate age: 3938 ± 4 Ma, exposure age: ~550–660 Ma) from the other Apollo 14 breccias (Ca‐phosphate age: 3927 ± 2 Ma, compatible with the Imbrium impact, exposure age: ~25–30 Ma). Based on these observations, we consider two hypotheses for the origin and transportation history of sample 14311. (1) Breccia 14311 was formed in the Procellarum KREEP terrane by a 3938 Ma‐old impact and deposited near the future site of the Imbrium basin. The breccia was integrated into the Fra Mauro Formation during the deposition of the Imbrium impact ejecta at 3927 Ma. The zircons were annealed by mare basalt flooding at 3400 Ma at Apollo 14 landing site. Eventually, at approximately 660 Ma, a small and local impact event excavated this sample and it has been at the surface of the Moon since this time. (2) Breccia 14311 was formed by a 3938 Ma‐old impact. The location of the sample is not known at that time but at 3400 Ma, it was located nearby or buried by hot basaltic flows. It was transported from where it was deposited to the Apollo 14 landing site by an impact at approximately 660 Ma, possibly related to the formation of the Copernicus crater and has remained at the surface of the Moon since this event. This latter hypothesis is the simplest scenario for the formation and transportation history of the 14311 breccia.     

Magma source transition of lunar mare volcanism at 2.3 Ga

Mare basalts provide insights into the composition and thermal history of the lunar mantle. The ages of mare basalts suggest a first peak of magma activity at 3.2–3.8 Ga and a second peak at ~2 Ga. In this study, we reassess the correlation between the titanium contents and the eruption ages of mare basalt units using the compositional and chronological data updated by SELENE (Kaguya). Using morphological and geological criteria, we calculated the titanium content of 261 mare units across a representative area of each mare unit. In the Procellarum KREEP Terrane, where the latest eruptions are located, an increase in the mean titanium content is observed during the Eratosthenian period, as reported by previous studies. We found that the increase in the mean titanium content occurred within a relatively short period near approximately 2.3 Ga, suggesting that the magma source of the mare basalts changed at this particular age. Moreover, the high‐titanium basaltic eruptions are correlated with a second peak in volcanic activity near ~2 Ga. The high‐titanium basaltic eruptions occurring during the last volcanic activity period can be explained by the three possible scenarios (1) the ilmenite‐bearing cumulate rich layer in the core‐mantle boundary formed after the mantle overturn, (2) the basaltic material layers beneath the lunar crust formed through upwelling magmas, and (3) ilmenite‐bearing cumulate blocks remained in the upper mantle after the mantle overturn.     

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

This 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.     

A review of lunar chronology revealing a preponderance of 4.34–4.37 Ga ages

Data obtained from Sm‐Nd and Rb‐Sr isotopic measurements of lunar highlands’ samples are renormalized to common standard values and then used to define ages with a common isochron regression algorithm. The reliability of these ages is evaluated using five criteria that include whether: (1) the ages are defined by multiple isotopic systems, (2) the data demonstrate limited scatter outside uncertainty, (3) initial isotopic compositions are consistent with the petrogenesis of the samples, (4) the ages are defined by an isotopic system that is resistant to disturbance by impact metamorphism, and (5) the rare‐earth element abundances determined by isotope dilution of bulk of mineral fractions match those measured by in situ analyses. From this analysis, it is apparent that the oldest highlands’ rock ages are some of the least reliable, and that there is little support for crustal ages older than approximately 4.40 Ga. A model age for ur‐KREEP formation calculated using the most reliable Mg‐suite Sm‐Nd isotopic systematics, in conjunction with Sm‐Nd analyses of KREEP basalts, is 4389 ± 45 Ma. This age is a good match to the Lu‐Hf model age of 4353 ± 37 Ma determined using a subset of this sample suite, the average model age of 4353 ± 25 Ma determined on mare basalts with the ¹⁴⁶Sm‐¹⁴²Nd isotopic system, with a peak in Pb‐Pb ages observed in lunar zircons of approximately 4340 ± 20 Ma, and the oldest terrestrial zircon age of 4374 ± 6 Ma. The preponderance of ages between 4.34 and 4.37 Ga reflect either primordial solidification of a lunar magma ocean or a widespread secondary magmatic event on the lunar nearside. The first scenario is not consistent with the oldest ages reported for lunar zircons, whereas the second scenario does not account for concordance between ages of crustal rocks and mantle reservoirs.     

Ancient geologic events on Mars revealed by zircons and apatites from the Martian regolith breccia NWA 7034

Zircons and apatites in clasts and matrix from the Martian breccia NWA 7034 are well documented, timing ancient geologic events on Mars. Furthermore, in this study, zircon trace elemental content, apatite volatile content, and apatite volatile isotopic compositions measured in situ could constrain the evolution of those geologic events. The U‐Pb dates of zircons in basalt, basaltic andesite, trachyandesite igneous clasts, and the matrix are similar (4.4 Ga) suggesting intense volcanism on ancient Mars. However, two metamict zircon grains found in the matrix have an upper intercept date of ~4465 Ma in crystalline, whereas amorphous areas have a lower intercept date of 1634 ± 93 Ma. The younger date is consistent with the date of apatites (1530 ± 65 Ma), suggesting a metamorphic event that completely reset the U‐Pb system in both the amorphous areas of zircon and all apatites. δD values in all apatites negatively correlate with water content in a two‐endmember mixing trend. The D (δD up to 2459‰) and ³⁷Cl heavy core (3.8‰) of a large apatite grain suggest a D‐, ³⁷Cl‐rich fluid during the metamorphic event ~1.6 Ga ago, consistent with the trace elements Y, Hf and Ti and P in zircons. The fluid was also therefore P‐rich. The D‐, ³⁷Cl‐poor H₂O‐rich rim (<313‰) suggests the degassing of water from the Martian Cl‐poor interior at a later time. This D‐, ³⁷Cl‐poor Martian mantle reservoir could have derived from volcanic intrusions postdating the younger metamorphic event recorded in NWA 7034.     

Petrology and geochemistry of feldspathic impact‐melt breccia Abar al' Uj 012, the first lunar meteorite from Saudi Arabia

Abar al' Uj (AaU) 012 is a clast‐rich, vesicular impact‐melt (IM) breccia, composed of lithic and mineral clasts set in a very fine‐grained and well‐crystallized matrix. It is a typical feldspathic lunar meteorite, most likely originating from the lunar farside. Bulk composition (31.0 wt% Al₂O₃, 3.85 wt% FeO) is close to the mean of feldspathic lunar meteorites and Apollo FAN‐suite rocks. The low concentration of incompatible trace elements (0.39 ppm Th, 0.13 ppm U) reflects the absence of a significant KREEP component. Plagioclase is highly anorthitic with a mean of An_96.9Ab_3.0Or_0.1. Bulk rock Mg# is 63 and molar FeO/MnO is 76. The terrestrial age of the meteorite is 33.4 ± 5.2 kyr. AaU 012 contains a ~1.4 × 1.5 mm² exotic clast different from the lithic clast population which is dominated by clasts of anorthosite breccias. Bulk composition and presence of relatively large vesicles indicate that the clast was most probably formed by an impact into a precursor having nonmare igneous origin most likely related to the rare alkali‐suite rocks. The IM clast is mainly composed of clinopyroxenes, contains a significant amount of cristobalite (9.0 vol%), and has a microcrystalline mesostasis. Although the clast shows similarities in texture and modal mineral abundances with some Apollo pigeonite basalts, it has lower FeO and higher SiO₂ than any mare basalt. It also has higher FeO and lower Al₂O₃ than rocks from the FAN‐ or Mg‐suite. Its lower Mg# (59) compared to Mg‐suite rocks also excludes a relationship with these types of lunar material.     

The origin of young mare basalts inferred from lunar meteorites Northwest Africa 4734, 032, and LaPaz Icefield 02205

Northwest Africa (NWA) 4734 is an unbrecciated basaltic lunar meteorite that is nearly identical in chemical composition to basaltic lunar meteorites NWA 032 and LaPaz Icefield (LAP) 02205. We have conducted a geochemical, petrologic, mineralogic, and Sm‐Nd, Rb‐Sr, and Ar‐Ar isotopic study of these meteorites to constrain their petrologic relationships and the origin of young mare basalts. NWA 4734 is a low‐Ti mare basalt with a low Mg* (36.5) and elevated abundances of incompatible trace elements (e.g., 2.00 ppm Th). The Sm‐Nd isotope system dates NWA 4734 with an isochron age of 3024 ± 27 Ma, an initial ε_Nd of +0.88 ± 0.20, and a source region ¹⁴⁷Sm/¹⁴⁴Nd of 0.201 ± 0.001. The crystallization age of NWA 4734 is concordant with those of LAP 02205 and NWA 032. NWA 4734 and LAP 02205 have very similar bulk compositions, mineral compositions, textures, and ages. Their source region ¹⁴⁷Sm/¹⁴⁴Nd values indicate that they are derived from similar, but distinct, source materials. They probably do not sample the same lava flow, but rather are similarly sourced, but isotopically distinct, lavas that probably originate from the same volcanic complex. They may have experienced slightly different assimilation histories in route to eruption, but can be source‐crater paired. NWA 032 remains enigmatic, as its source region ¹⁴⁷Sm/¹⁴⁴Nd definitively precludes a simple relationship with NWA 4734 and LAP 02205, despite a similar bulk composition. Their high Ti/Sm, low (La/Yb)_N, and Cl‐poor apatite compositions rule out the direct involvement of KREEP. Rather, they are consistent with low‐degree partial melting of late‐formed LMO cumulates, and indicate that the geochemical characteristics attributed to urKREEP are not unique to that reservoir. These and other basaltic meteorites indicate that the youngest mare basalts originate from multiple sources, and suggest that KREEP is not a prerequisite for the most recent known melting in the Moon.     

Chemistry of the Calcalong Creek lunar meteorite and its relationship to lunar terranes

Abstract— The Calcalong Creek lunar meteorite is a polymict breccia that contains clasts of both highlands and mare affinity. Reported here is a compilation of major, minor, and trace element data for bulk, clast, and matrix samples determined by instrumental neutron activation analysis (INAA). Petrographic information and results of electron microprobe analyses are included. The relationship of Calcalong Creek to lunar terranes, especially the Procellarum KREEP Terrane and Feldspathic Highlands Terrane, is established by the abundance of thorium, incompatible elements and their KREEP‐like CI chondrite normalized pattern, FeO, and TiO₂. The highlands component is associated with Apollo 15 KREEP basalt but represents a variant of the KREEP‐derived material widely found on the moon. Sources of Calcalong Creek's mare basalt components may be related to low‐titanium (LT) and very low‐titanium (VLT) basalts seen in other lunar meteorites but do not sample the same source. The content of some components of Calcalong Creek are found to display similarities to the composition of the South Pole‐Aitken Terrane. What appear to be VLT relationships could represent new high aluminum, low titanium basalt types.     

Impact history of the Apollo 17 landing site revealed by U‐Pb SIMS ages

Secondary ion mass spectrometry (SIMS) U‐Pb ages of Ca‐phosphates from four texturally distinct breccia samples (72255, 76055, 76015, 76215) collected at the Apollo 17 landing site were obtained in an attempt to identify whether they represent a single or several impact event(s). The determined ages, combined with inferences from petrologic relationships, may indicate two or possibly three different impact events at 3920 ± 3 Ma, 3922 ± 5 Ma, and 3930 ± 5 Ma (all errors 2σ). Searching for possible sources of the breccias by calculating the continuous ejecta radii of impact basins and large craters as well as their expected ejecta thicknesses, we conclude that Nectaris, Crisium, Serenitatis, and Imbrium are likely candidates. If the previous interpretation that the micropoikilitic breccias collected at the North Massif represent Serenitatis ejecta is correct, then the average ²⁰⁷Pb/²⁰⁶Pb age of 3930 ± 5 Ma (2σ) dates the formation of the Serenitatis basin. The occurrence of zircon in the breccias sampled at the South Massif, which contain Ca‐phosphates yielding an age of 3922 ± 5 Ma (2σ), may indicate that the breccia originated from within the Procellarum KREEP terrane (PKT) and the Imbrium basin appears to be the only basin that could have sourced them. However, this interpretation implies that all basins suggested to fall stratigraphically between Serenitatis and Imbrium formed within a short (<11 Ma) time interval, highlighting serious contradictions between global stratigraphic constraints, sample interpretation, and chronological data. Alternatively, the slightly older age of the two micropoikilitic breccias may be a result of incomplete resetting of the U‐Pb system preserved in some phosphate grains. Based on the currently available data set this possibility cannot be excluded.     

Annealing of radiation damage in zircons from Apollo 14 impact breccia 14311: Implications for the thermal history of the breccia

Impact breccia 14311, was collected from the Apollo 14 landing site as a potential sample of the underlying Fra Mauro Formation. Published zircon U‐Pb ages of >4000 Ma date the source material of the breccia and the apatite U‐Pb age of ~3940 Ma is interpreted as dating thermal resetting of the apatite U‐Pb systems. In this contribution we present new age information on the late stage thermal history of the breccia based on the annealing of radiation damage in the zircons. From Raman spectroscopic determination of the radiation damage within SIMS analytical spots on the zircons and the U and Th concentrations determined on these spots, we demonstrate that the radiation damage in the zircons has been annealed and we estimate the age of annealing at 3410 ± 80 Ma. This age is interpreted as a cooling age following heating of the breccia to above the annealing temperature of ~230 °C for stage 1 radiation damage in zircon, but below the temperature needed to reset the U‐Pb system of apatite (~500 °C). It is proposed that this thermal event was associated with the prolonged period of Mare volcanism, from 3150 to 3750 Ma, that generated massive basalt flows in the vicinity of the sample location.     

40Ar‐39Ar age determinations of lunar basalt meteorites Asuka 881757, Yamato 793169, Miller Range 05035, La Paz Icefield 02205, Northwest Africa 479, and basaltic breccia Elephant Moraine 96008

Abstract— ⁴⁰Ar‐³⁹Ar data are presented for the unbrecciated lunar basaltic meteorites Asuka (A‐) 881757, Yamato (Y‐) 793169, Miller Range (MIL) 05035, LaPaz Icefield (LAP) 02205, Northwest Africa (NWA) 479 (paired with NWA 032), and basaltic fragmental breccia Elephant Moraine (EET) 96008. Stepped heating ⁴⁰Ar‐³⁹Ar analyses of several bulk fragments of related meteorites A‐881757, Y‐793169 and MIL 05035 give crystallization ages of 3.763 ± 0.046 Ga, 3.811 ± 0.098 Ga and 3.845 ± 0.014 Ga, which are comparable with previous age determinations by Sm‐Nd, U‐Pb Th‐Pb, Pb‐Pb, and Rb‐Sr methods. These three meteorites differ in the degree of secondary ⁴⁰Ar loss with Y‐793169 showing relatively high Ar loss probably during an impact event ?200 Ma ago, lower Ar loss in MIL 05035 and no loss in A‐881757. Bulk and impact melt glass‐bearing samples of LAP 02205 gave similar ages (2.985 ± 0.016 Ga and 2.874 ± 0.056 Ga) and are consistent with ages previously determined using other isotope pairs. The basaltic portion of EET 96008 gives an age of 2.650 ± 0.086 Ga which is considered to be the crystallization age of the basalt in this meteorite. The Ar release for fragmental basaltic breccia EET 96008 shows evidence of an impact event at 631 ± 20 Ma. The crystallization age of 2.721 ± 0.040 Ga determined for NWA 479 is indistinguishable from the weighted mean age obtained from three samples of NWA 032 supporting the proposal that these meteorites are paired. The similarity of ⁴⁰Ar‐³⁹Ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the K‐Ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime.     

Thermal and irradiation history of lunar meteorite Dhofar 280

Dhofar 280 recorded a complex history on the Moon revealed by high‐resolution ⁴⁰Ar‐³⁹Ar dating. Thermal resetting occurred less than 1 Ga ago, and the rock was exposed to several impact events before and afterwards. The cosmic ray exposure (CRE) age spectrum indicates a 400 ± 40 Ma CRE on the lunar surface. A unique feature of this lunar sample is a partial loss of cosmogenic ³⁸Ar, resulting in a (low‐temperature) CRE age plateau of about 1 Ma. This was likely caused by the same recent impact event that reset the (low‐temperature) ⁴⁰Ar‐³⁹Ar age spectrum and preceded the short transit phase to Earth of ≤1 Ma. Dhofar 280 may be derived from KREEP‐rich lunar frontside terrains, possibly associated with the Copernicus crater or with a recent impact event on the deposits of the South Pole–Aitken basin. Although Dhofar 280 is paired with Dhofar 081, their irradiation and thermal histories on the Moon were different. An important trapped Ar component in Dhofar 280 is “orphan” Ar with a low ⁴⁰Ar/³⁶Ar ratio. It is apparently a mixture of two components, one endmember with ⁴⁰Ar/³⁶Ar = 17.5 ± 0.2 and a second less well‐constrained endmember with ⁴⁰Ar/³⁶Ar ≤10. The presence of two endmembers of trapped Ar, their compositions, and the breccia ages seem to be incompatible with a previously suggested correlation between age or antiquity and the (⁴⁰Ar/³⁶Ar)_trapped ratio (Eugster et al. 2001; Joy et al. 2011a). Alternatively, “orphan” Ar of this impact melt breccia may have an impact origin.     

Evidence for multiple 4.0–3.7 Ga impact events within the Apollo 16 collection

In a histogram of lunar impact ages from the Apollo 16 site, there is a spike circa 3.9 Ga that has been interpreted to represent either a large number of nearly synchronous events or an abundance of samples that were affected slightly differently by the event that produced the Imbrium basin. To further scrutinize those age relationships, we extracted six centimeter‐sized clasts of impact melt from ancient regolith breccia 60016 and performed petrological and geochronological (⁴⁰Ar‐³⁹Ar) analyses. Three clasts have similar poikilitic textures, while others have porphyritic, aphanitic, or intergranular textures. Compositions and abundances of relict minerals are different in all six clasts and variously imply Mg‐suite and ferroan anorthosite target sequences. Estimated bulk compositions of four clasts are similar to previously defined group 1 Apollo 16 impact melt rocks, while the other two have higher Al₂O₃ and lower FeO+MgO compositions. All six clasts have similar K₂O and P₂O₅ concentrations, which could have been derived from a KREEP‐bearing component among target sequences. Eighteen ⁴⁰Ar/³⁹Ar analyses of the six clasts produced an age range from 3823 ± 75 to 4000 ± 23 Ma, consistent with estimates for the proposed late heavy bombardment. Four clasts have multiple temperature steps that define plateau ages. These ages are distinct, so they cannot be explained by a single impact event, such as the one that produced the Imbrium impact basin. The conclusion that these represent distinct ages remains after considering the possibility of artifacts in defining plateaus.     

An integrated approach to understanding Apollo 16 impact glasses: Chemistry,isotopes, and shape

Abstract The major‐ and minor‐element abundances were determined by electron microprobe in 1039 glasses from regoliths and regolith breccias to define the compositional topology of lunar glasses at the Apollo 16 landing site in the central highlands of the Moon. While impact glasses with chemical compositions similar to local materials (i.e., Apollo 16 rocks and regoliths) are abundant, glasses with exotic compositions (i.e., transported from other areas of the Moon) account for up to ?30% of the population. A higher proportion of compositionally exotic, angular glass fragments exists when compared to compositionally exotic glass spherules. Ratios of non‐volatile lithophile elements (i.e., Al, Ti, Mg) have been used to constrain the original source materials of the impact glasses. This approach is immune to the effects of open‐system losses of volatile elements (e.g., Si, Na, K). Four impact glasses from one compositionally exotic group (low‐Mg high‐K Fra Mauro; lmHKFM) were selected for ⁴⁰Ar/³⁹ Ar dating. The individual fragments of lmHKFM glass all yielded ages of ?3750 ± 50 Ma for the time of the impact event. Based on the petrography of these individual glasses, we conclude that the likely age of the impact event that formed these 4 glasses, as well as the possible time of their ballistic arrival at the Apollo 16 site from a large and distant cratering event (perhaps in the Procellarum KREEP terrain) (Zeigler et al. 2004), is 3730 ± 40 Ma, close to the accepted age for Imbrium.     

Petrography and geochemistry of lunar meteorites Dhofar 1673, 1983, and 1984

The Dhofar 1673, Dhofar 1983, and Dhofar 1984 meteorites are three lunar regolith breccias classified based on their petrography, mineralogy, oxygen isotopes, and bulk chemistry. All three meteorites are dominated by feldspathic lithic clasts; however, impact melt rock clasts and spherules are also found in each meteorite. The bulk chemistry of these samples is similar to other feldspathic highland meteorites with the Al₂O₃ content only slightly lower than average. Within the lithic clasts, the Mg # of mafic phases versus the anorthite content of feldspars is similar to other highland meteorites and is found to plot intermediate of the ferroan‐anorthositic suite and magnesian suite. The samples lack any KREEPy signature and have only minor indications of a mare basalt component, suggesting that the source region of all three meteorites would have been distal from the Procellarum KREEP Terrane and could have possibly been the Feldspathic Highland Terrane. All three meteorites were found within 500 m of each other in the Dhofar region of Oman. This, together with their similar petrography, stable isotope chemistry, and geochemistry indicates the possibility of a pairing.     

Magnesian anorthositic granulites in lunar meteorites Allan Hills A81005 and Dhofar 309: Geochemistry and global significance

Abstract– Fragments of magnesian anorthositic granulite are found in the lunar highlands meteorites Allan Hills (ALH) A81005 and Dhofar (Dho) 309. Five analyzed clasts of meteoritic magnesian anorthositic granulite have Mg′ [molar Mg/(Mg + Fe)] = 81–87; FeO ≈ 5% wt; Al₂O₃ ≈ 22% wt; rare earth elements abundances ≈ 0.5–2 × CI (except Eu ≈ 10 × CI); and low Ni and Co in a non‐chondritic ratio. The clasts have nearly identical chemical compositions, even though their host meteorites formed at different places on the Moon. These magnesian anorthositic granulites are distinct from other highlands materials in their unique combination of mineral proportions, Mg′, REE abundances and patterns, Ti/Sm ratio, and Sc/Sm ratio. Their Mg′ is too high for a close relationship to ferroan anorthosites, or to have formed as flotation cumulates from the lunar magma ocean. Compositions of these magnesian anorthositic granulites cannot be modeled as mixtures of, or fractionates from, known lunar rocks. However, compositions of lunar highlands meteorites can be represented as mixtures of magnesian anorthositic granulite, ferroan anorthosite, mare basalt, and KREEP. Meteoritic magnesian anorthositic granulite is a good candidate for the magnesian highlands component inferred from Apollo highland impactites: magnesian, feldspathic, and REE‐poor. Bulk compositions of meteorite magnesian anorthositic granulites are comparable to those inferred for parts of the lunar farside (the Feldspathic Highlands Terrane): ～4.5 wt% FeO; ～28 wt% Al₂O₃; and Th <1 ppm. Thus, magnesian anorthositic granulite may be a widespread and abundant component of the lunar highlands.     

Argon‐40‐argon‐39 chronology and petrogenesis along the eastern limb of the Moon from Luna 16, 20 and 24 samples

Abstract— Eighteen new lithic fragments from the Soviet Luna missions have been analyzed with electron microprobe and ⁴⁰Ar‐³⁹Ar methods. Luna 16 basalt fragments have aluminous compositions consistent with previous analyses, but have two distinct sets of well‐constrained ages (3347 ± 24 Ma, 3421 ± 30 Ma). These data, combined with other Luna 16 basalt ages, imply that there were multiple volcanic events filling Mare Fecunditatis. The returned basalt fragments have relatively old cosmicray exposure (CRE) ages and may have been recovered from the ejecta blanket of a young (1 Ga), nearby crater. A suite of highlands rocks (troctolites and gabbros) is represented in the new Luna 20 fragments. One fragment is the most compositionally primitive (Mg# = 91–92) spinel troctolite yet found. Both troctolites have apparent crystallization ages of 4.19 Ga; other rocks in the suite have progressively younger ages and lower Mg#s. The age and composition progression suggests that these rocks may have crystallized from a single source magma, or from similar sources mobilized at the same time. Within the new Luna 24 basalt fragments is a quench‐textured olivine vitrophyre with the most primitive composition yet analyzed for a Luna 24 basalt, and several much more evolved olivine‐bearing basalts. Both new and previously studied Luna 24 very low‐Ti (VLT) basalt fragments have a unimodal age distribution (3273 ± 83 Ma), indicating that most returned samples come from a single extrusive episode within Mare Crisium much later than the Apollo 17 VLT basalts (3.6–3.7 Ga).