The case for an Imbrium origin of the Apollo thorium-rich impact-melt breccias

Abstract— Mafic, Th-rich impact-melt breccias, most of which are identified with the composition known as low-K Fra Mauro (LKFM), are the most common rock type in the nonmare regoliths of the Apollo lunar landing sites. The origin of mafic impact-melt breccias bears on many lunar problems: the nature of the late meteoroid bombardment (cataclysm); the spatial distribution of KREEP, both near the surface and at depth; the ages of the major basins; and the composition of the early crust of the nearside lunar highlands. Thus, it is crucial that the origin of mafic impact-melt breccias be accurately understood. Because of both intra- and intersite differences in compositions of mafic impact-melt breccia samples, apparent differences in crystallization age, and differences in siderophile-element ratios, previous studies have argued that either (1) most mafic impact-melt breccias are the products of several large craters local to the site at which they were found but that some are of basin origin or that (2) they are all from the Imbrium (Apollos 14 and 15), Nectaris (Apollo 16), and Serenitatis (Apollo 17) basins. Here, we reconsider the hypothesis that virtually all of the Th-rich, mafic impact-melt breccias from the Apollo missions are products of the Imbrium impact. Ejecta deposit modeling based on modern crater scaling indicates that the Imbrium event produced ejecta deposits that average hundreds of meters thick or more at all Apollo highland sites, which is thicker than some previous estimates. Substantial amounts of Imbrium ejecta should have been sampled at every Apollo highland site. We suggest that the mafic impact-melt breccias may be the principal form of those ejecta. The Imbrium projectile impacted into Th-rich material that we regard as part of a unique, mafic, lunar geochemical province we call the High-Th Oval Region. Based on the surface distribution of Th, only basins within the High-Th Oval Region excavated Th-rich material; the Th concentrations of the highlands as observed by the Apollo orbiting γ-ray experiments are consistent with the estimates from ejecta modeling. Of the younger basin-forming impacts, only Imbrium was large enough to produce the copious amount of melt required by the ubiquitous presence of mafic impact-melt breccias in the Apollo-sampled regolith. The High-Th Oval Region still may have been molten or hot at shallow depths ～4 Ga ago when the Imbrium projectile struck. We reason that compositional heterogeneity of ejected melt breccia is to be expected under these circumstances. We argue that siderophile-element “fingerprints” of mafic impact-melt breccias are not inconsistent with production of all common types by a single projectile. We suggest that the narrow range of ages of 3.7–4.0 Ga for all successfully dated mafic impact-melt breccias may reflect a single event whose age is difficult to measure precisely, rather than a number of discrete impact events closely spaced in time, such that reported age variations among mafic impact-melt breccias reflect the ability to measure ⁴⁰Ar/³⁹Ar ages with greater precision than the accuracy with which measured portions of mafic impact-melt breccias have recorded the time of their formation.     

Origin of lunar fragmental matrix breccias—Highly siderophile element constraints

Ejecta at North Ray crater (Apollo 16) sampled a unique section of the lunar highlands not accessible at most other landing sites and provide important constraints on the composition of late accreted materials. New data on multiple aliquots of four fragmental matrix breccias and a fragment‐laden melt breccia from this site display a variety of highly siderophile element patterns which may represent the signatures of volatile element‐depleted carbonaceous chondrite‐like material, primitive achondrite, differentiated metal, and an impactor component that cannot be related to known meteoritic material. The latter component is prevalent in these rocks besides characterized by depletions in Re and Os compared to Ir, Ru and Pt, chondritic Re/Os, and a gradual depletion of Pd and Au. The observed characteristics are more consistent with fractionations by nebular processes, like incomplete condensation or evaporation, than with lunar crustal processes, like partial melting or volatilization. The impactor signature preserved in these breccias may stem from primitive meteorites with a refractory element composition moderately different from known chondrites. The presence of distinct impactor components within the North Ray crater breccias together with observed correlations of characteristic element ratios (e.g., Re/Os, Ru/Pt, Pd/Ir) in different impact lithologies of four Apollo landing sites constrains physical mixing processes ranging from the scale of gram‐sized samples to the area covered by the Apollo missions.     

Ages of Globally Distributed Lunar Paleoregoliths and Soils from 3.9 Ga to the Present

This study determines the ages of 191 discrete lunar regolith samples from the Apollo, Luna, and meteorite collections. Model closure ages (for lithified breccias) and appearance ages (for unconsolidated soils) are calculated using the trapped ⁴⁰Ar and ³⁶Ar abundances of each sample, determined from published Ar data. Model closure ages of regolith breccias span ~3.9 to 0.01 Ga and appearance ages of soils range from ~3.6 to 0.03 Ga; 169 of these ages are published here for the first time, while 22 are recalculated ages. The regolith breccias with the oldest closure ages originate from the ancient highlands and oldest mare surfaces sampled by the Apollo missions. Soils generally have similar ages to each other, regardless of location and collection depth, with most model ages <2.0 Ga. Together, the soils and regolith breccias represent a record of regolith processes over the past 3.9 Ga. The data illustrate that individual landing sites can provide a diversity of ages, which has implications for planning future missions. Differences in maturity between older and younger regolith samples may reflect a change in collisional regimes over time. We note, too, that the closure ages published here are critical data needed for selecting temporally appropriate regolith samples used to decipher the diversity of impactors hitting the lunar surface over time and how the Sun has changed in time.     

Some things we can infer about the Moon from the composition of the Apollo 16 regolith

Abstract— Characteristics of the regolith of Cayley plains as sampled at the Apollo 16 lunar landing site are reviewed and new compositional data are presented for samples of <1 mm fines (“soils”) and 1–2 mm regolith particles. As a means of determining which of the many primary (igneous) and secondary (crystalline breccias) lithologic components that have been identified in the soil are volumetrically important and providing an estimate of their relative abundances, more than 3 × 10⁶ combinations of components representing nearly every lithology that has been observed in the Apollo 16 regolith were systematically tested to determine which combinations best account for the composition of the soils. Conclusions drawn from the modeling include the following. At the site, mature soil from the Cayley plains consists of 64.5% ± 2.7% components representing “prebasin” materials: anorthosites, feldspathic breccias, and a small amount (2.6% ± 1.5% of total soil) of nonmare, mafic plutonic rocks, mostly gabbronorites. On average, these components are highly feldspathic, with average concentrations of 31–32% Al₂O₃ and 2–3% FeO and a molar Mg/(Mg + Fe) ratio of 0.68. The remaining 36% of the regolith is syn- and postbasin material: 28.8% ± 2.4% mafic impact-melt breccias (MIMBs, i.e., “LKFM” and “VHA basalts”) created at the time of basin formation, 6.0% ± 1.4% mare-derived material (impact and volcanic glass, crystalline basalt) with an average TiO₂ concentration of 2.4%, and 1% postbasin meteoritic material. The MIMBs are the principal (80–90%) carrier of incompatible trace elements (rare earths, Th, etc.) and the carrier of about one-half of the siderophile elements and elements associated with mafic mineral phases (Fe, Mg, Mn, Cr, Sc). Most (71%) of the Fe in the present regolith derives from syn- and postbasin sources (MIMBs, mare-derived material, and meteorites). Thus, although the bulk composition of the Apollo 16 regolith is nominally that of noritic anorthosite, the noritic part (the MIMBs) and the anorthositic part (the prebasin components) are largely unrelated. There is compositional evidence that 3–4% of the soil is Th-rich material such as that occurring at the Apollo 14 site, and one fragment of this type was found among the small regolith particles studied here. If regolith such as that represented by the Apollo 16 ancient regolith breccias was a protolith of the present regolith, such regolith cannot exceed ～71% of the present regolith; the rest must be material added or redistributed since closure of the ancient regolith breccias. The postclosure material includes the mare-derived material and the Apollo-14-like component. Compositions of all mature surface soils from Apollo 16, even those collected 4 km apart on the Cayley plains, are very similar, which is in stark contrast to the wide compositional range of the lithologies of which the soil is composed. This uniformity indicates that the ratio of MIMBs to feldspathic prebasin components is not highly variable in the megaregolith over distances of a few kilometers, that there are no large, subsurface concentrations of “pure” mafic impact-melt breccia, and that the intimate mixing is inherent to the Cayley plains at a gross scale. Thus, the mixing of mafic impact-melt breccias and feldspathic prebasin components must have occurred during formation and deposition of the Cayley plains; such uniformity could not have been achieved by small postdeposition impacts into a stratified megaregolith. Using this conclusion as one constraint, and the known distribution of Th on the lunar surface as another, and the assumption that the Imbrium impact is primarily responsible for formation of the Cayley plains, arguments are presented that the Apollo 16 MIMBs derive from the Imbrium region, and, consequently, that one-fourth of the Apollo 16 regolith is primary Imbrium ejecta in the form of mafic impact-melt breccias.     

“New” lunar meteorites: Impact melt and regolith breccias and large‐scale heterogeneities of the upper lunar crust

Abstract— We have analyzed nine highland lunar meteorites (lunaites) using mainly INAA. Several of these rocks are difficult to classify. Dhofar 081 is basically a fragmental breccia, but much of its groundmass features a glassy‐fluidized texture that is indicative of localized shock melting. Also, much of the matrix glass is swirly‐brown, suggesting a possible regolith derivation. We interpret Dar al Gani (DaG) 400 as an extremely immature regolith breccia consisting mainly of impact‐melt breccia clasts; we interpret Dhofar 026 as an unusually complex anorthositic impact‐melt breccia with scattered ovoid globules that formed as clasts of mafic, subophitic impact melt. The presence of mafic crystalline globules in a lunar material, even one so clearly impact‐heated, suggests that it may have originated as a regolith. Our new data and a synthesis of literature data suggest a contrast in Al₂O₃‐incompatible element systematics between impact melts from the central nearside highlands, where Apollo sampling occurred, and those from the general highland surface of the Moon. Impact melts from the general highland surface tend to have systematically lower incompatible element concentration at any given Al₂O₃ concentration than those from Apollo 16. In the case of Dhofar 026, both the bulk rock and a comparatively Al‐poor composition (14 wt% Al₂O₃, 7 μg/g Sm) extrapolated for the globules, manifest incompatible element contents well below the Apollo 16 trend. Impact melts from Luna 20 (57°E) distribute more along the general highland trend than along the Apollo 16 trend. Siderophile elements also show a distinctive composition for Apollo 16 impact melts: Ni/Ir averaging ?1.8x chondritic. In contrast, lunaite impact‐melt breccias have consistently chondritic Ni/Ir. Impact melts from Luna 20 and other Apollo sites show average Ni/Ir almost as high as those from Apollo 16. The prevalence of this distinctive Ni/Ir ratio at such widely separated nearside sites suggests that debris from one extraordinarily large impact may dominate the megaregolith siderophile component of a nearside region 2300 km or more across. Highland polymict breccia lunaites and other KREEP‐poor highland regolith samples manifest a strong anticorrelation between Al₂O₃ and mg. The magnesian component probably represents the chemical signature of the Mg‐suite of pristine nonmare rocks in its most “pure” form, unaltered by the major KREEP‐assimilation that is so common among Apollo Mg‐suite samples. The average composition of the ferroan anorthositic component is now well constrained at Al₂O₃ ?29–30 wt% (implying about 17–19 wt% modal mafic silicates), in good agreement with the composition predicted for flotation crust over a “ferroan” magma ocean (Warren 1990).     

Petrography and geochemistry of five new Apollo 16 mare basalts and evidence for post‐basin deposition of basaltic material at the site

Abstract— We present the petrography and geochemistry of five 2–4 mm basalt fragments from the Apollo 16 regolith. These fragments are 1) a high‐Ti vitrophyric basalt compositionally similar to Apollo 17 high‐Ti mare basalts, 2) a very high‐Ti vitrophyric basalt compositionally similar to Apollos 12 and 14 red‐black pyroclastic glass, 3) a coarsely crystalline high‐Al basalt compositionally similar to group 5 Apollo 14 high‐Al mare basalts, 4) a very low‐Ti (VLT) crystalline basalt compositionally similar to Luna 24 VLT basalts, and 5) a VLT basaltic glass fragment compositionally similar to Apollo 17 VLT basalts. High‐Ti basalt has been reported previously at the Apollo 16 site; the other basalt types have not been reported previously. As there are no known cryptomaria or pyroclastic deposits in the highlands near the Apollo 16 site (ruling out a local origin), and scant evidence for basaltic material in the Apollo 16 ancient regolith breccias or Apollo 16 soils collected near North Ray Crater (ruling out a basin ejecta origin), we infer that the basaltic material in the Apollo 16 regolith originated in maria near the Apollo 16 site and was transported laterally to the site by small‐ to medium‐sized post‐basin impacts. On the basis of TiO₂ concentrations derived from the Clementine UVVIS data, Mare Tranquillitatis (?300 km north) is the most likely source for the high‐Ti basaltic material at the Apollo 16 site (craters Ross, Arago, Dionysius, Maskelyne, Moltke, Sosigenes, Schmidt), Mare Nectaris/Sinus Asperitatis (?220 km east) is the most likely source for the low‐Ti and VLT basaltic material (craters Theophilus, Madler, Torricelli), and a large regional pyroclastic deposit near Mare Vaporum (?600 km northwest) is the most likely source region for pyroclastic material (although no source craters are apparent in the region).     

Petrography,mineralogy, and geochemistry of lunar meteorite Sayh al Uhaymir 300

Abstract— We report here the petrography, mineralogy, and geochemistry of lunar meteorite Sayh al Uhaymir 300 (SaU 300). SaU 300 is dominated by a fine‐grained crystalline matrix surrounding mineral fragments (plagioclase, pyroxene, olivine, and ilmenite) and lithic clasts (mainly feldspathic to noritic). Mare basalt and KREEPy rocks are absent. Glass melt veins and impact melts are present, indicating that the rock has been subjected to a second impact event. FeNi metal and troilite grains were observed in the matrix. Major element concentrations of SaU 300 (Al₂O₃ 21.6 wt% and FeO 8.16 wt%) are very similar to those of two basalt‐bearing feldspathic regolith breccias: Calcalong Creek and Yamato (Y‐) 983885. However, the rare earth element (REE) abundances and pattern of SaU 300 resemble the patterns of feldspathic highlands meteorites (e.g., Queen Alexandra Range (QUE) 93069 and Dar al Gani (DaG) 400), and the average lunar highlands crust. It has a relatively LREE‐enriched (7 to 10 x CI) pattern with a positive Eu anomaly (?11 x CI). Values of Fe/Mn ratios of olivine, pyroxene, and the bulk sample are essentially consistent with a lunar origin. SaU 300 also contains high siderophile abundances with a chondritic Ni/Ir ratio. SaU 300 has experienced moderate terrestrial weathering as its bulk Sr concentration is elevated compared to other lunar meteorites and Apollo and Luna samples. Mineral chemistry and trace element abundances of SaU 300 fall within the ranges of lunar feldspathic meteorites and FAN rocks. SaU 300 is a feldspathic impact‐melt breccia predominantly composed of feldspathic highlands rocks with a small amount of mafic component. With a bulk Mg# of 0.67, it is the most mafic of the feldspathic meteorites and represents a lunar surface composition distinct from any other known lunar meteorites. On the basis of its low Th concentration (0.46 ppm) and its lack of KREEPy and mare basaltic components, the source region of SaU 300 could have been within a highland terrain, a great distance from the Imbrium impact basin, probably on the far side of the Moon.     

Impact processing of chondritic planetesimals: Siderophile and volatile element fractionation in the Chico L chondrite

Abstract— A large impact event 500 Ma ago shocked and melted portions of the L‐chondrite parent body. Chico is an impact melt breccia produced by this event. Sawn surfaces of this 105 kg meteorite reveal a dike of fine‐grained, clast‐poor impact melt cutting shocked host chondrite. Coarse (1–2 cm diameter) globules of FeNi metal + sulfide are concentrated along the axis of the dike from metal‐poor regions toward the margins. Refractory lithophile element abundance patterns in the melt rock are parallel to average L chondrites, demonstrating near‐total fusion of the L‐chondrite target by the impact and negligible crystal‐liquid fractionation during emplacement and cooling of the dike. Significant geochemical effects of the impact melting event include fractionation of siderophile and chalcophile elements with increasing metal‐silicate heterogeneity, and mobilization of moderately to highly volatile elements. Siderophile and chalcophile elements ratios such as Ni/Co, Cu/Ga, and Ir/Au vary systematically with decreasing metal content of the melt. Surprisingly small (?10²) effective metal/silicate‐melt distribution coefficients for highly siderophile elements probably reflect inefficient segregation of metal despite the large degrees of melting. Moderately volatile lithophile elements such K and Rb were mobilized and heterogeneously distributed in the L‐chondrite impact breccias whereas highly volatile elements such as Cs and Pb were profoundly depleted in the region of the parent body sampled by Chico. Volatile element variations in Chico and other L chondrites are more consistent with a mechanism related to impact heating rather than condensation from a solar nebula. Impact processing can significantly alter the primary distributions of siderophile and volatile elements in chondritic planetesimals.     

Indigenous abundances of siderophile elements in the lunar highlands: Implications for the origin of the moon

Substantial indigenous abundances of siderophile elements have been found to be present in the lunar highlands. The abundances of 13 siderophile elements in the parental magma of the highlands crust were estimated by using a simple model whereby the Apollo 16 highlands were regarded as being a mixture of three components (i.e. cumulus plagioclase + intercumulus magma that was parentel to the highlands crust + meteoritic contamination by ordinary chondrites). The parental magma of the highlands was found to possess abundances of siderophile elements that were generally similar to the abundances of the unequivocally indigenous siderophile elements in primitive, low-Ti mare basalts. This striking similarity implies that these estimated abundances in the parental highlands magma are truly indigenous, and also supports the basic validity of our simple model.It is shown that metal/silicate fractionation within the Moon cannot have been the cause of the siderophile element abundances in the parental highlands magma and primitive, low-Ti mare basalts. The relative abundances of the indigenous siderophile elements in highland and mare samples seem, instead, to be the result of complex processes which operatedprior to the Moon's accretion.The abundances of the relatively involatile, siderophile elements in the parental highlands magma are strikingly similar to the abundances observed in terrestrial oceanic tholeiites. Furthermore, the abundances of the relatively volatile, siderophile elements in the parental highlands magma are also systematically related to the corresponding abundances in terrestrial oceanic tholeiites. In fact, the parental magma of the lunar highlands can be essentially regarded as having been a volatile-depleted, terrestrial oceanic tholeiite.The complex, siderophile element fractionations in the Earth's upper mantle are thought to be the result of core segregation. However, it is well-known that the siderophile element abundances do not correspond to expectations based solely upon equilibration of metal/silicate at low-pressures, as evidenced by the over-abundances of Au, Re, Ni, Co and Cu. Ringwood (1977a) has suggested that the siderophile element abundances in the Earth's upper mantle are the product of equilibration at very high-pressures between the mantle and a segregating core that contained substantial quantities of an element with a low atomic weight, such as oxygen. Comparable processes cannot have operated within the Moon due to its small internal pressures and the very small size of its possible core. Therefore, the fact that the Moon exhibits a systematic resemblance to the Earth's upper mantle is highly significant.The origin of the Moon is discussed in the context of these results. The possibility that depletion of siderophile elements occurred in an earlier generation of differentiated planetesimals similar to those which formed the basaltic achondrites, stony-irons, and irons is examined but can be dismissed on several grounds. It seems that the uniquely terrestrial siderophile signature within the Moon can be explained only if the Moon was derived from the Earth's mantle subsequent to core-formation.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

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.     

Siderophile elements in Earth's upper mantle and lunar breccias: Data synthesis suggests manifestations of the same late influx

Abstract— The platinum group elements (PGE; Ru, Rh, Pd, Os, Ir, Pt), Re and Au comprise the highly siderophile elements (HSE). We reexamine selected isotopic and abundance data sets for HSE in upper mantle peridotites to resolve a longstanding dichotomy. Re‐Os and Pt‐Os isotope systematics, and approximately chondritic proportions of PGE in these rocks, suggest the presence in undepleted mantle of a chondrite‐like component, which is parsimoniously explained by late influx of large planetisimals after formation of the Earth's core and the Moon. But some suites of xenolithic and orogenic spinel lherzolites, and abyssal peridotites, have a CI‐normalized PGE pattern with enhanced Pd that is sometimes termed “non‐chondritic”. We find that this observation is consistent with other evidence of a late influx of material more closely resembling enstatite, rather than ordinary or carbonaceous, chondrites. Regional variations in HSE patterns may be a consequence of a late influx of very large objects of variable composition. Studies of many ancient (>3.8 Ga) lunar breccias show regional variations in Au/Ir and suggest that “graininess” existed during the early bombardment of the Earth and Moon. Reliable Pd values are available only for Apollo 17 breccias 73215 and 73255, however. Differences in HSE patterns between the aphanitic and anorthositic lithologies in these breccias show fractionation between a refractory group (Re, Os and Ir) and a normal (Pd, Ni, and Au) group and may reflect the compositions of the impacting bodies. Similar fractionation is apparent between the EH and EL chondrites, whose PGE patterns resemble those of the aphanitic and anorthositic lithologies, respectively. The striking resemblance of HSE and chalcogen (S, Se) patterns in the Apollo aphanites and high‐Pd terrestrial peridotites suggest that the “non‐chondritic” abundance ratios in the latter may be reflected in the composition of planetisimals striking the Moon in the first 700 Ma of Earth–Moon history. Most notably, high Pd may be part of a general enhancement of HSE more volatile than Fe suggesting that the Au abundance in at least parts of the upper mantle may be 1.5 to 2x higher than previously estimated. The early lunar influx may be estimated from observed basin‐sized craters. Comparison of relative influx to Earth and Moon suggests that the enrichment of HSE is limited to the upper mantle above 670 km. To infer enrichment of the whole mantle would require several large lunar impacts not yet identified.     

The regolith portion of the lunar meteorite Sayh al Uhaymir 169

Abstract— –Sayh al Uhaymir (SaU) 169 is a composite lunar meteorite from Oman that consists of polymict regolith breccia (8.44 ppm Th), adhering to impact‐melt breccia (IMB; 32.7 ppm Th). In this contribution we consider the regolith breccia portion of SaU 169, and demonstrate that it is composed of two generations representing two formation stages, labeled II and III. The regolith breccia also contains the following clasts: Ti‐poor to Ti‐rich basalts, gabbros to granulites, and incorporated regolith breccias. The average SaU 169 regolith breccia bulk composition lies within the range of Apollo 12 and 14 soil and regolith breccias, with the closest correspondence being with that of Apollo 14, but Sc contents indicate a higher portion of mare basalts. This is supported by relations between Sm‐Al₂O₃, FeO‐Cr₂O₃‐TiO₂, Sm/Eu and Th‐K₂O. The composition can best be modeled as a mixture of high‐K KREEP, mare basalt and norite/troctolite, consistent with the rareness of anorthositic rocks. The largest KREEP breccia clast in the regolith is identical in its chemical composition and total REE content to the incompatible trace‐element (ITE)‐ rich high‐K KREEP rocks of the Apollo 14 landing site, pointing to a similar source. In contrast to Apollo 14 soil, SaU 169 IMB and SaU 169 KREEP breccia clast, the SaU 169 regolith is not depleted in K/Th, indicating a low contribution of high‐Th IMB such as the SaU 169 main lithology in the regolith. The data presented here indicate the SaU 169 regolith breccia is from the lunar front side, and has a strong Procellarum KREEP Terrane signature.     

Procrustean science: Indigenous siderophiles in the lunar highlands,according to Delano and Ringwood

The best estimate of indigenous lunar siderophiles comes from 29 pristine lunar rocks, characterized by low siderophile abundances, plutonic textures, and high age. Delano and Ringwood's blanket rejection of these rocks, on the contention that they are impact melts, is not justified by the petrologic evidence. Contrary to their claims, gold in highland breccias is largely meteoritic and is unaffected by fumarolic volcanism, as shown by its correlation with Ir and noncorrelation with fumarolic T1 (r=0.896 and 0.272). Delano and Ringwood's approach, involving subtraction of an H-chondrite meteoritic component from highland breccias, ignores the variation of Ir/Au ratios in modern and ancient meteorites, and hence leads to spurious excesses of Au, Ni, and volatiles, and in some cases to physically meaningless, negative residuals. Their excess volatiles in highland crust relative to mare basalts disappear when the highland composition is based on pristine lunar rocks rather than under-corrected breccias. Contrary to claims by Delano and Ringwood, the Ni/Co trend in Apollo 16 samples cannot be explained by an indigenous component rich in Ni (150–200 ppm) and Co (30–45ppm); mixing lines show that much lower Ni and Co contents are required (e.g., 7 ppm each).Chondrites and lunar highland breccias show essentially parallel fractionation trends for the siderophile-element ratios Re/Ir, Au/Ir, Ni/Ir, Ni/Pd, and Os/Ir. Because the chondritic ratios were established in the solar nebula, it appears that the lunar ratios also reflect nebular processes, and have not been modified by planetary processes.Properly derived abundances for the lunar highlands show large, systematic depletions relative to terrestrial oceanic tholeiites, by the following factors: Ge 270, Re 230, Sb170, Zn150, Au60, Tl 50, Ag 48, Ni 42, Se 12. It would seem that the resemblance to the Earth's mantle is not quite as striking as claimed by Delano and Ringwood.     

Lithologic distribution and geologic history of the Apollo 17 site: The record in soils and small rock particles from the highland massifs

Abstract— Through analysis by instrumental neutron activation (INAA) of 789 individual lithic fragments from the 2 mm–4 mm grain-size fractions of five Apollo 17 soil samples (72443, 72503, 73243, 76283, and 76503) and petrographic examination of a subset, we have determined the diversity and proportions of rock types recorded within soils from the highland massifs. The distribution of rock types at the site, as recorded by lithic fragments in the soils, is an alternative to the distribution inferred from the limited number of large rock samples. The compositions and proportions of 2 mm–4 mm fragments provide a bridge between compositions of <1 mm fines, and types and proportions of rocks observed in large collected breccias and their clasts. The 2 mm–4 mm fraction of soil from South Massif, represented by an unbiased set of lithic fragments from station-2 samples 72443 and 72503, consists of 71% noritic impact-melt breccia, 7% incompatible-trace-element-(ITE)-poor highland rock types (mainly granulitic breccias), 19% agglutinates and regolith breccias, 1% high-Ti mare basalt, and 2% others (very-low-Ti (VLT) basalt, monzogabbro breccia, and metal). In contrast, the 2 mm–4 mm fraction of a soil from the North Massif, represented by an unbiased set of lithic fragments from station-6 sample 76503, has a greater proportion of ITE-poor highland rock types and mare-basalt fragments: it consists of 29% ITE-poor highland rock types (mainly granulitic breccias and troctolitic anorthosite), 25% impact-melt breccia, 13% high-Ti mare basalt, 31% agglutinates and regolith breccias, 1% orange glass and related breccia, and 1% others. Based on a comparison of mass-weighted mean compositions of the lithic fragments with compositions of soil fines from all Apollo 17 highland stations, differences between the station-2 and station-6 samples are representative of differences between available samples from the two massifs. From the distribution of different rock types and their compositions, we conclude the following: (1) North-Massif and South-Massif soil samples differ significantly in types and proportions of ITE-poor highland components and ITE-rich impact-melt-breccia components. These differences reflect crudely layered massifs and known local geology. The greater percentage of impact-melt breccia in the South-Massif light-mantle soil stems from derivation of the light mantle from the top of the massif, which apparently is richer in noritic impact-melt breccia than are lower parts of the massifs. (2) At station 2, the 2 mm–4 mm grain-size fraction is enriched in impact-melt breccias compared to the <1 mm fraction, suggesting that the <1 mm fraction within the light mantle has a greater proportion of lithologies such as granulitic breccias which are more prevalent lower in the massifs and which we infer to be older (pre-basin) highland components. (3) Soil from station 6, North Massif, contains magnesian troctolitic anorthosite, which is a component that is rare in station-2 South-Massif soils. (4) Compositional differences between poikilitic impact-melt breccias from the two massifs suggest broad-scale heterogeneity in impact-melt breccia interpreted by most investigators to be ejecta from the Serenitatis basin. We have found rock types not previously recognized or uncommon at the Apollo 17 site. These include (1) ITE-rich impact-melt breccias that are compositionally distinct from previously recognized “aphanitic” and “poikilitic” groups at Apollo 17; (2) regolith breccias that are free of mare components and poor in impact melt of the types associated with the main melt-breccia groups, and that, if those groups derive from the Serenitatis impact, may represent the pre-Serenitatis surface; (3) several VLT basalts, including an unusual very-high-K basaltic breccia; (4) orange-glass regolith breccias; (5) aphanitic-matrix melt breccias at station 6; (6) fragments of alkali-rich composition, including alkali anorthosite, and monzogabbro; (7) one fragment of 72275-type KREEP basalt from station 3; (8) seven lithic fragments of ferroan-anorthositic-suite rocks; and (9) a fragment of metal, possibly from an L chondrite. Some of these lithologies have been found only as lithic fragments in the soils and not among the large rock samples. In contrast, we have not found among the 2 mm–4 mm lithic fragments individual samples of certain lithologies that have been recognized as clasts in breccias (e.g., dunite and spinel troctolite). The diversity of lithologic information contained in the lithic fragments of these soils nearly equals that found among large rock samples, and most information bearing on petrographic relationships is maintained, even in such small samples. Given a small number of large samples for “petrologic ground truth,” small lithic fragments contained in soil “scoop” samples can provide the basis for interpreting the diversity of rock types and their proportions in remotely sensed geologic units. They should be considered essential targets for future automated sample-analysis and sample-return missions.     

Consortium study of the unusual H chondrite regolith breccia,Noblesville

Abstract— The Noblesville meteorite is a genomict, regolith breccia (H6 clasts in H4 matrix). Mössbauer analysis confirms that Noblesville is unusually fresh, not surprising in view of its recovery immediately after its fall. It resembles “normal” H4–6 chondrites in its chemical composition and induced thermoluminescence (TL) levels. Thus, at least in its contents of volatile trace elements, Noblesville differs from other H chondrite, class A regolith breccias. Noblesville's small pre-atmospheric mass and fall near Solar maximum and/or its peculiar orbit (with perihelion <0.8 AU as shown by natural TL intensity) may partly explain its levels of cosmogenic radionuclides. Its cosmic ray exposure age of ～ 44 Ma, is long, is equalled or exceeded by <3% of all H chondrites, and also differs from the 33 ± 3 Ma mean exposure age peak of other H chondrite regolith breccias. One whole-rock aliquot has a high, but not unmatched, ¹²⁹Xe/¹³²Xe of 1.88. While Noblesville is now among the chondritic regolithic breccias richest in solar gases, elemental ratios indicate some loss, especially of He, perhaps b; impacts in the regolith that heated individual grains. While general shock-loading levels in Noblesville did not exceed 4 GPa, individual clasts record shock levels of 5–10 GPa, doubtless acquired prior to lithification of the whole-rock meteoroid.     

Petrology and provenance of a very‐low‐titanium picrite clast in lunar highland regolith breccia 15295

Clast 100 in regolith breccia 15295 could be a key to resolving the relationship(s) between mare basalts and lunar picritic glasses. The clast is basaltic, with texture, mineralogy, mineral compositions, and calculated bulk composition suggesting that it crystallized in a thick lava flow or shallow intrusive body from a very‐low‐titanium (VLT) basaltic magma. The estimated bulk composition of clast 15295,100 is primitive (i.e., magnesian) compared to those of known VLT basalts, and is very close to those of VLT picritic green glasses, especially the Apollo 14 A green glass. From these similarities, we infer that clast 15295,100 is a crystalline product of a picritic magma similar to the Apollo 14 A glass. Clementine and M³ remotely sensed data of the lunar surface were used to find areas that have chemical compositions consistent with those of clast 15295,100, not only near the Apollo 15 site, but in a broad region surrounding the site. Two regions are consistent with clast's 15295,100 compositional data. The larger region is in southern Mare Imbrium, and a smaller region is in the eastern half of Sinus Aestuum. These locations should be considered as candidates for future missions focusing on sample science.     

The spatial and temporal distribution of lunar mare basalts as deduced from analysis of data for lunar meteorites

In this work we analyze data for lunar meteorites with emphasis on the spatial and temporal distribution of lunar mare basalts. The data are mostly from the Lunar Meteorite Compendium (http://www-curator.jsc.nasa.gov/antmet/lmc/contents.cfm cited hereafter as Compendium) compiled by Kevin Righter, NASA Johnson Space Center, and from the associated literature. Analysis of the data showed that (i) a significant part of the lunar meteorite source craters are not larger than hundreds of meters in diameter; (ii) cryptomaria seem to be rather abundant in lunar highlands; (iii) the ratios of lunar meteorites belonging to three broad petrologic groups (mare basalt/gabbro, feldspatic highland breccias, and mingled breccias which are a mixture of mare and highland components) seem to be roughly proportional to the areal distribution of these rocks on the lunar surface; and (iv) the meteorite mare basalt ages show a range from ～2.5 to 4.3 Ga and fill the gaps in the Apollo/Luna basalt age distribution. The ages of mare basalt clasts from mingled breccias seem to be systematically higher than those of “normal” mare basalts, which supports the suggestion that mingled breccias originated mostly from cryptomaria.     

Particle track measurements in lunar regolith breccias

Particle track measurements have been reported for 25 (5%) of the regolith breccias in the collection; they have been reported for 16 breccias (30%) in the reference suite. The most frequently reported measurement for these 25 breccias is the maximum surface exposure age of the compacted rock (48% of the published breccia measurements). Information on the nature of the precompaction regolith is given for 9 rocks (36%) and on the nature of the compaction event for 6 rocks (24%). Most of the breccias appear to have simple post-compaction surface exposure histories (89%). From the few track density frequency distributions (7) that are available and inferring from the low exposure ages of these rocks (75% < 10⁶ yr), it appears that most of these breccias are amenable to studies which separate the contemporary surface exposure age from information about the precompaction regolith. If the number of immature-submature precompaction soils (6 out of 10 of the breccias for which appropriate data are available) represents many regolith breccias, then we can infer that regolith breccias may sample the deeper, less reworked materials in the lunar soil and compliment the samples available from the returned cores.     

Meteoritic material on the moon

Three types of meteoritic material are found on the Moon: micrometeorites, ancien planetesimal debris from the ‘early intense bombardment’, and debris of recent, crater-forming projectiles. Their amounts and compositions have been determined from trace element studies. The micrometeorite component is uniformly distributed over the entire lunar surface, but is seen most clearly in mare soils. It has a primitive, C1-chondrite-like composition, and comprises 1-1.5% of mature soils. Apparently it represents cometary debris. The mean annual influx rate is 2.4 × 10^?9 g cm^?2 yr^?1. It shows no detectable time variation or dependence on selenographic position. The ancient component is seen in highland breccias and soils more than 3.9 AE old. It has a fractionated composition, with volatiles depleted relative to siderophiles. The abundance pattern does not match that of any known meteorite class. At least two varieties exist (LN and DN, with Ir/Au, Re/Au 0.25-0.5 and > 0.5 the C1 value). Both seem to represent the debris of planetesimals that produced the mare basins and highland craters during the first 700 Myr of the Moon's history. It appears that the LN and DN objects impacted at less then 10 km s^?1, had diameters less than 100 km, contained more than 15% Fe, and were not internally differentiated. Both were depleted in volatiles; the LN objects also in refractories (Ir, Re). This makes it unlikely that the LN bodies served as important building blocks of the Moon. The crater-forming component has remained elusive. Only a possible hint of this component has been seen, in ejecta from Dune Crater and Apollo 12 KREEP glasses of Copernican (?) origin.     

The magnetic characteristics of returned lunar samples and 5heir implications for regolith processes

Magnetic observations yield information about the amount and nature of the magnetic phases present in a sample. They reveal that the predominant magnetic phase in the lunar samples is metallic iron which is sometimes alloyed with nickel and cobalt. In the mare basalts less than 0.1% of metallic iron is present, whereas in the non-mare crystalline rocks several percent of iron has been found in some samples. The soils have approximately 0.5% of iron, which is fine grain, rather pure iron occurring in impact glass. In the recrystallized breccias and the igneous rocks the iron is coarser. Systematic minor variations in metallic iron content in the soils reveal soil maturity trends. Mixing between highland and mare soils can be traced with the Fe²⁺ content. Mare soils differ from highland soils in having a higher value of reduced remanence. The magnetic characteristics of the Apollo 14 breccias are not consistent with the progressive metamorphism of a common starting material. Shock welding in the range of tens of kbs can account for the characteristics of some of the ‘unmetamorphosed’ breccias. Greater shock accompanied by recovery can account for the magnetic characteristics of the ‘recrystallized’ breccias.