Cayley Plains Research Articles

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.

The Global Distribution of Lunar Light Plains From the Lunar Reconnaissance Orbiter Camera

AbstractWe present the first globally consistent map of lunar light plains from Lunar Reconnaissance Orbiter Camera mosaics 100 m/pixel. Based on this map, ~70% of light plains are likely related to the Orientale and Imbrium basins; however, many light plains deposits originated with local‐ and regional‐scale impacts, nonmare volcanics (i.e., the Apennine Bench Formation), as well as ancient impact melt deposits that lack diagnostic small‐scale features. Compositional evidence suggests that the light plains associated with basins (i.e., Cayley plains) form through substantial mixing with local materials, implying that ballistic sedimentation is a primary formation mechanism. Based on the distribution of light plains with respect to the Orientale and Imbrium basins, the stratigraphic extent of an individual basin extends to at least four basin radii. As such, the South Pole‐Aitken basin modified at least 80% of the lunar surface. This work suggests that the entire lunar surface was modified to varying degrees at the time of large basin formation. This has implications for sample interpretations because any highland site within four radii of a large basin could potentially contain primary basin material or more local material that was redeposited by secondaries, neither of which is necessarily representative of the terrain upon which they are found.

Volatile distributions in and on the Moon revealed by Cu and Fe isotopes in the ‘Rusty Rock’ 66095

The Apollo 16 ‘Rusty Rock’ impact melt breccia 66095 is a volatile-rich sample, with the volatiles inherited through vapor condensation from an internal lunar source formed during thermo-magmatic evolution of the Moon. We report Cu and Fe isotope data for 66095 and find that bulk-rocks, residues and acid leaches span a relatively limited range of compositions (3.0 ± 1.3 wt.% FeO [range = 2.0–4.8 wt.%], 5.4 ± 3.1 ppm Cu [range = 3–12 ppm], average δ56Fe of 0.15 ± 0.05‰ [weighted mean = 0.156‰] and δ65Cu of 0.72 ± 0.14‰ [weighted mean = 0.78‰]). In contrast to the extreme enrichment of the light isotopes of Zn and heavy isotopes of Cl in 66095, δ65Cu and δ56Fe in the sample lie within the previously reported range for lunar mare basalts (0.92 ± 0.16‰ and 0.12 ± 0.02‰, respectively). The lack of extreme isotopic fractionation for Cu and Fe isotopes reflects compositions inherent to 66095, with condensation of a cooling gas from impact-generated fumarolic activity at temperatures too low to lead to the condensation of Cu and Fe in the sample, but higher than required to condense Zn. Together with thermodynamic models, these constraints suggest that the gas condensed within 66095 between 700 and 900 °C (assuming a pressure of 10−6 and an fO2 of IW-2). That the Cu and Fe isotopic compositions of sample 66095 are within the range of mare basalts removes the need for an exotic, volatile-enriched source. The enrichment in Tl, Br, Cd, Sn, Zn, Pb, Rb, Cs, Ga, B, Cl, Li relative to Bi, Se, Te, Ge, Cu, Ag, Sb, Mn, P, Cr and Fe in the ‘Rusty Rock’ is consistent with volcanic outgassing models and indicates that 66095 likely formed distal from the original source of the gas. The volatile-rich character of 66095 is consistent with impact-generated fumarolic activity in the region of the Cayley Plains, demonstrating that volatile-rich rocks can occur on the lunar surface from outgassing of a volatile-poor lunar interior. The ‘Rusty Rock’ indicates that the lunar interior is significantly depleted in volatile elements and compounds and that volatile-rich surface rocks likely formed through vapor condensation. Remote sensing studies have detected volatiles on the lunar surface, attributing them dominantly to solar wind. Based on the ‘Rusty Rock’, some of these surface volatiles may also originate from the Moon’s interior.

Small lunar craters at the Apollo 16 and 17 landing sites - morphology and degradation

New analysis and modeling approaches are applied to high-resolution images and topography of the Apollo 16 and 17 landing sites to investigate the morphology and estimate degradation of small lunar craters (SLCs; 35 to 250 m diameter). We find SLCs at the two sites are mostly degraded with an average depth-diameter ratio (dD)<0.1, resulting in a landscape dominated by shallow, inverted cone-shaped craters. An improved standardized morphological classification and a novel set of quantitative shape indicators are defined and used to compare SLCs between the two sites. Our classification methodology allows morphological class populations to be designated with minimal (and measurable) ambiguity simplifying the study of SLC degradation at different target regions. SLC shape indicators are computationally obtained from topography, further facilitating a quantitative and repeatable comparison across study areas. Our results indicate that the interior slopes of SLCs evolve faster and through different processes relative to larger craters ( > 500 m). Assuming SLCs are formed with large initial depth-to-diameter ratio (dD≥0.2), our observation that even the fresher SLCs are relatively shallow imply that a faster mass wasting process post-formation stabilizes the crater walls and eventually slows down degradation. We also found that the Apollo 16 Cayley plains have a higher percentage of fresh craters than the Apollo 17 Taurus Littrow (TL) plains. A combination of a less-cohesive target material and/or seismic shaking resulting from moonquakes or the impact of Tycho crater secondaries was likely responsible for a higher degradation rate in the TL-plains compared to the Cayley plains. This study explores the relationship between the symmetry and probability densities of key morphological traits like dD, mean wall slope and rate of degradation. We show that the shape of dD probability density function of SLCs in a study area encodes their rate of degradation. Comparison of power-law fitting and probabilistic modeling of depth-diameter relations shows that probabilistic methods complement regression models and are necessary for robust prediction of SLC depths from diameter (and vice versa) for different geological targets.

A record of impacts preserved in the lunar regolith

The absolute chronology of meteoroid impacts on the Moon is largely quantified by only a few 40 Ar/ 39 Ar “plateau ages” of rocks thought to be associated with specific impact events ( Stöffler et al., 2006). We demonstrate a more broadly applicable approach by using high-resolution 40 Ar/ 39 Ar thermochronometry to investigate the physical conditions responsible for partially reset K-Ar systems in lunar rocks. Seven rocks from Apollo 16 regolith sample 63503 have plateau ages of either 3.9 billion yr (Ga) or 4.2 Ga and all experienced varying degrees of partial resetting. Concordance between diffusion kinetics and the degree of resetting among all samples shows that these observations are best explained by a heating event 3.3 Ga ago that lasted between ∼ 10 3 s (at ∼ 600 °C) and ∼ 20 yr (at ∼ 300 °C). We conclude that partial resetting of the K-Ar systems in these samples record an impact event ∼ 3.3 Ga ago that mixed several preexisting ejecta units in the Cayley Plains. If partially reset 40 Ar/ 39 Ar ages of other lunar highland samples also constrain the timing of late-stage reheating associated with impact events, they constitute an additional record of impacts preserved in the lunar regolith. A review of existing datasets from this perspective reveals that episodic pulses in the impactor flux in the inner system are common, and most likely related to dynamical events in the asteroid belt or outer Solar System.

Remote sensing studies of the Dionysius region of the Moon

The Dionysius region is located near the western edge of Mare Tranquillitatis and is centered on Dionysius crater, which exhibits a well‐developed dark ray system. Proposed origins for these dark rays included impact melt deposits and dark primary ejecta. The region also contains extensive deposits of Cayley‐type light plains. Clementine multispectral images and a variety of spacecraft photography were utilized to investigate the composition and origin of geologic units in the Dionysius region. The portions of the dark rays for which spectral and chemical data were obtained are composed of mare debris contaminated with minor amounts of highland material. Both five‐point spectra and values of the optical maturity (OMAT) parameter indicate that the dark rays are dominated by mare basalts, not glassy impact melts. The high‐albedo rays associated with Dionysius exhibit FeO and TiO2 values that are lower than those of the adjacent dark ray surfaces and OMAT values that indicate that bright ray surfaces are not fully mature. The high‐albedo rays are bright largely because of the contrast in albedo between ray material containing highlands‐rich ejecta and the adjacent mare‐rich surfaces. The mafic debris ejected by Dionysius was derived from a dark, iron‐rich unit exposed high on the inner wall of the crater. This layer probably represents a mare deposit that was present at the surface of the preimpact target site. With one possible exception, there is no evidence for buried mare basalts associated with Cayley plains in the region.

Chronology of dimict breccias and the age of South Ray crater at the Apollo 16 site

Abstract—We report the noble gas isotopic abundances of five dimict breccias and one cataclastic anorthosite that were collected at the Apollo 16 landing site. Orbital and surface photographs indicate that rays from South Ray crater, an almost 1 km wide young crater in the Cayley plains, extend several kilometers from their source into the area that was sampled by the Apollo 16 mission. Previous studies have shown that South Ray crater formed 2 Ma ago and that a large number of rocks might originate from this cratering event. On the basis of cosmic‐ray produced nuclei, we find that the six rocks investigated in this work yield the same lunar surface exposure age. Using literature data, we recalculate the exposure ages of additional 16 rocks with suspected South Ray crater origin and obtain an average exposure age of 2.01 ± 0.10 Ma. In particular, all nine dimict breccias (a type of rock essentially restricted to the Apollo 16 area consisting of anorthosite and breccia phases) dated until now yield an average ejection age of 2.06 ± 0.17 Ma. We conclude that they must originate from the Cayley formation or from bedrock underlying the Cayley plain. We determined the gas retention ages for the dimict breccias based on the40K‐40Ar and U,Th‐136Xe dating methods: rock 64425 yields a40K‐40Ar age of 3.96 Ga and rock 61016 a U,Th‐136Xe age of 3.97 Ga. These results, together with39Ar‐40Ar ages obtained by other workers for rocks 64535 (3.98 Ga) and 64536 (3.97 Ga), show that the dimict breccias formed 3.97 Ga ago.

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 &lt;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.

Lithological variation with depth and decoupling of maturity parameters in Apollo 16 regolith core 68001/2

Using ferromagnetic resonance (FMR) and instrumental neutron activation analysis (INAA), we have determined the maturity (surface exposure) parameter I s FeO and concentrations of twenty-five chemical elements on samples taken every half centimeter down the 61-cm length of the 68001/2 regolith core (double drive tube) collected at station 8 on the Apollo 16 mission to the Moon. Contrary to premission expectations, no ejecta or other influence from South Ray crater is evident in the core, although a small inflection in the I s FeO profile at 3 cm depth may be related the South Ray crater impact. Regolith maturity generally decreases with depth, as in several previously studied cores. We recognize five compositionally distinct units in the core, which we designate A through E, although all are similar in composition to each other and to other soils from the Cayley plains at the Apollo 16 site. Unit A (0–33 cm) is mature to submature throughout ( I s FeO : 34–89 units) and is indistinguishable in composition from surface soils collected at station 8. Unit B (33–37 cm) is enriched slightly in a component of anorthositic norite composition. Unit D (42–53 cm) is compositionally equivalent to 80 wt% Unit-A soil plus 20 wt% Apollo-16-type dimict breccia consisting of subequal parts anorthosite and impact-melt breccia. Compared to Unit A, Unit E (53–61 cm) contains a small proportion (up to 4%) of some component compositionally similar to Apollo 14 sample 14321. Unit C (37–42 cm) is unusual. For lithophile and siderophile elements, it is similar to Units A and D. However, I s FeO is low throughout the unit (<30 units) and in a bluish-gray zone at 41 cm depth I s FeO drops to 1.6 units, the lowest value that we have observed in several hundred Apollo 16 soil samples. Samples from the bluishgray zone also have low Zn concentrations, <10 μg/g, compared to 20–30 μg/g for the rest of the core. Although both values are consistent with fragmented rock material that has received virtually no surface exposure, the abundance of agglutinates in the bluish-gray soil of Unit C is moderately high, typical of a submature soil that would ordinarily have I s FeO ≈ 30 . We believe that the anomalously low values of I s FeO and Zn concentration result because the soil was heated to ∼800–1000 °C, probably during an impact. This temperature range is sufficient to volatize the surface-correlated Zn and agglomerate the nanophase metal giving rise to the FMR signal but is not great enough to sinter the soil. Alternatively, the unusual soil interval may represent a disaggregated or incipient regolith breccia, although there is no significant difference in the texture or clast-matrix relationships between Unit C and adjacent units.

On the relationship between the Apollo 16 ancient regolith breccias and feldspathic fragmental breccias, and the composition of the prebasin crust in the Central Highlands of the Moon

Abstract Two types of texturally and compositionally similar breccias that consist largely of fragmental debris from meteorite impacts occur at the Apollo 16 lunar site: Feldspathic fragmental breccias (FFBs) and ancient regolith breccias (ARBs). Both types of breccia are composed of a suite of mostly feldspathic components derived from the early crust of the Moon and mafic impact‐melt breccias produced during the time of basin formation. The ARBs also contain components, such as agglutinates and glass spherules, indicating that the material of which they are composed occurred at the surface of the Moon as fine‐grained regolith prior to lithification of the breccias. These components are absent from the FFBs, suggesting that the FFBs might be the protolith of the ARBs. However, several compositional differences exist between the two types of breccia, making any simple genetic relationship implausible. First, clasts of mafic impact‐melt breccia occurring in the FFBs are of a different composition than those in the ARBs. Also the feldspathic “prebasin” components of the FFBs have a lower average Mg/Fe ratio than the corresponding components of the ARBs; the average composition of the plagioclase in the FFBs is more sodic than that of the ARBs; and there are differences in relative abundances of rare earth elements. The two breccia types also have different provenances: the FFBs occur primarily in ejecta from North Ray crater and presumably derive from the Descartes Formation, while the ARBs are restricted to the Cayley plains. Together these observations suggest that although some type of fragmental breccia may have been a precursor to the ARBs, the FFBs of North Ray crater are not a significant component of the ARBs and, by inference, the Cayley plains. The average compositions of the prebasin components of the two types of fragmental breccia are generally similar to the composition of the feldspathic lunar meteorites. With 30–31% Al2O3, however, they are slightly richer in plagioclase than the most feldspathic lunar meteorites (∼29% Al2O3), implying that the crust of the early central nearside of the Moon contained a higher abundance of highly feldspathic anorthosite than typical lunar highlands, as inferred from the lunar meteorites. The ancient regolith breccias, as well as the current surface regolith of the Cayley plains, are more mafic than (1) prebasin regoliths in the Central Highlands and (2) regions of highlands presently distant from nearside basins because they contain a high abundance (∼30%) of mafic impact‐melt breccias produced during the time of basin formation that is absent from other regoliths.

Composition and exposure age of the Apollo 16 Cayley and Descartes regions from Clementine data: Normalizing the optical effects of space weathering

In order to quantitatively assess the composition of the lunar surface from reflectance spectroscopy, it is necessary to be able to differentiate between the optical effects due to composition and those due to exposure to space weathering processes. Laboratory analyses of returned lunar soils are used to show that limited quantitative compositional information and the exposure age of the lunar surface can be determined using straightforward calculations from remotely acquired reflectance data sampled with a small number of spectral channels. Exposure age, approximated by Is/FeO (where Is is the characteristic ferromagnetic signal from single‐domain iron metal), is estimated using a ratio of the reflectance at a wavelength outside of the 1‐μm ferrous iron absorption band to the reflectance at a wavelength within the band. The iron plus titanium content of a soil can then be calculated from its Is/FeO and reflectance. These combined techniques are applied to Clementine UVVIS multispectral data of the lunar highlands in the vicinity of the Apollo 16 landing site. The Is/FeO and FeO + TiO2 contents are calculated for this region. Materials associated with recently formed impact craters and recent and/or ongoing downslope movement are easily detected as nonmature. The FeO + TiO2 content of the Descartes Mountains is found to be 1–2 wt% less than the Cayley plains west of the Apollo 16 landing site. Localized areas of compositional heterogeneity associated with some small impact events are also revealed. A model for normalizing the optical contribution due to exposure to space weathering processes is applied to the Clementine UVVIS data in order to bring the data to an equivalent mature exposure state. This approach allows residual spectral differences between the observed materials to be confidently interpreted as being due to compositional differences alone rather than to differences both of composition and of exposure. The exposure‐normalized data reveal that this region of the highlands is characterized by limited large‐scale compositional heterogeneity that is detectable by a small number of spectral bandpasses. The Descartes Mountains are identified as being more anorthositic than the Cayley plains units. Most small impacts in the region excavated material compositionally similar to material surrounding them, with the notable exceptions of the impacts that created South Ray Crater and an unnamed crater on the floor of Abulfeda.

Compositional variation in Apollo 16 impact-melt breccias and inferences for the geology and bombardment history of the Central Highlands of the Moon

High-precision data for the concentrations of a number of lithophile and siderophile elements were obtained on multiple subsamples from 109 impact-melt rocks and breccias (mostly crystalline) from the Apollo 16 site. Compositions of nearly all Apollo 16 melt rocks fall on one of two trends of increasing Sm concentration with increasing Sc concentration. The Eastern trend (lower Sm Sc , Mg Fe , and Sm Yb ratios) consists of compositional groups 3 and 4 of previous classification schemes. These melt rocks are feldspathic, poor in incompatible and siderophile elements, and appear to have provenance in the Descartes formation to the east of the site. The Western trend (higher Sm Sc , Mg Fe , and Sm Yb ratios) consists of compositional groups 1 and 2. These relatively mafic, KREEP-bearing breccias are a major component (~35%) of the Cayley plains west of the site and are unusual, compared to otherwise similar melt breccias from other sites, in having high concentrations of Fe-Ni metal (1–2%). The metal is the carrier of the low-Ir/Au (~0.3 × chondritic) siderophile-element signature that is characteristic of the Apollo 16 site. Four compositionally distinct groups (1M, 1F, 2DB, and 2NR) of Western-trend melt breccias occur that are each represented by at least six samples. Compositional group 1 of previous classification schemes (the “poikilitic” or “LKFM” melt breccias) can be subdivided into two groups. Group 1M (represented by six samples, including 60315) is characterized by lower Al 2O 3 concentrations, higher MgO and alkali concentrations, and higher Mg Fe and Cr Sc ratios than group 1F (represented by fifteen samples, including 65015). Group 1M also has siderophile-element concentrations averaging about twice those of group 1F and Ir Au and Ir Ni ratios that are even lower than those of other Western-trend melt rocks ( Ir Au = 0.24 ± 0.03 , CI-normalized). At the mafic extreme of group 2 (“VHA” melt breccias), the melt lithology occurring as clasts in feldspathic fragmental breccias from North Ray crater (group 2NR) is compositionally distinct from the melt lithology of dimict breccias from the Cayley plains (group 2DB) in having higher concentrations of Sc, Cr, and heavy rare earth elements and lower concentrations of siderophile elements. The distinct siderophile-element signature (high absolute abundances, low Ir Au ratio) suggest that the four groups of mafic melt breccia are all somehow related. Ratios of some lithophile elements also suggest that they are more closely related to each other than they are to melt breccias from other Apollo sites. However, none of the breccia compositions can be related to any of the others by any simple process of igneous fractionation or mixing involving common lunar materials. Thus, the origin of the four groups of mafic melt breccia is enigmatic. If they were produced in only one or two impacts, then a mechanism exists for generating regimes of impact-melt breccia in a single impact that are substantially different from each other in composition. For various reasons, including the problem of delivering large volumes of four different types of melt to the Apollo 16 site, it is unlikely that any of these breccias were produced in basin-forming impacts. If they were produced in as many as four crater-forming impacts, then the unusual siderophile-element signature is difficult to explain. Possible explanations are 1. (1) the four groups of melt breccia all contain metal from a single, earlier impact, 2. (2) they were each formed by related metal-rich meteoroids, or 3. (3) some common postimpact process has resulted in metal of similar composition in each of four melt pools. Within a compositional group, most intrasample and intersample variation in lithophile element concentrations is caused by differences among samples in the proportion of a component of normative anorthosite or noritic anorthosite. In most cases, this compositional variation probably reflects variation in clast abundance. For group 2DB (and probably 2NR), differences in abundance of a component of ferroan anorthosite (estimated Al 2O 3 ≈ 32%) accounts for the compositional variation. For groups 1M and 1F, the anorthositic component is more mafic (estimated Al 2O 3 ≈ 26%). Some group-2 samples may be related by a troctolitic component of varying abundance.

Composition and maturity of Apollo 16 regolith core 60013/14

Samples from every half-centimeter dissection interval of double drive tube 60013/14 (sections 60013 and 60014) were analyzed by magnetic techniques for Fe° concentration and surface maturity parameter I s FeO , and by neutron activation for concentrations of 25 lithophile and siderophile elements. Core 60013/14 is one of three regolith cores taken in a triangular array 40–50 m apart on the Cayley plains during Apollo 16 mission to the Moon. The core can be divided into three zones based both on I s FeO and composition. Unit A (0–44 cm depth) is compositionally similar to other soils from the surface of the central region of the site and is mature throughout, although maturity decreases with depth. Unit B (44–59 cm) is submature and compositionally more feldspathic than Unit A. Regions of lowest maturity in Unit B are characterized by lower Sm Sc ratios than any soil obtained from the Cayley plains as a result of some unidentified lithologic component with low surface maturity. The component is probably some type of mafic anorthosite that does not occur in such high abundance in any of the other returned soils. Unit C (59–62 cm) is more mature than Unit B and compositionally equivalent to an 87: 13 mixture of soil such as that from Unit A and plagioclase such as found in ferroan anorthosite. Similar soils, but containing greater abundances of anorthosite (plagioclase), are found at depth in the other two cores of the array. These units of immature to submature soil enriched to varying degrees (compared to the mature surface soil) in ferroan anorthosite consisting of ~99% plagioclase are the only compositionally distinct subsurface similarities among the three cores. Each of the cores contains other units that are compositionally dissimilar to any soil unit in the other two cores. These compositionally distinct units probably derive from local subsurface blocks deposited by the event(s) that formed the Cayley plains. The ferroan anorthosite with ~99% plagioclase, however, must represent some subsurface lithology that is significant on the scale of tens of meters. The compositional uniformity of the surface soil (0–10 cm depth) over distances of kilometers reflects the large-scale uniformity of the plains deposits; the fine-structure reflects small-scale nonuniformity and the inefficiency of the impact-mixing process at depths as shallow as even one meter.

Rb-Sr-analyses of Apollo 16 melt rocks and a new age estimate for the Imbrium basin: Lunar basin chronology and the early heavy bombardment of the moon

Rb-Sr-model ages on 7 impact “glass-bombs” and internal Rb-Sr isochrons for two crystalline impact melt rocks from the Apollo 16 collection have been determined. The post-Cayley “glass-bombs” with model ages between 4.75 ± 0.45 AE and 3.97 ± 0.08 AE can be classified according to their calculated single stage (87Rb86Sr)I-ratios: 67728, 67946, and 67627.8 point to a KREEP-free precursor terrain—the Descartes highlands; whereas 63566, 67567, 67627.10 and 67629 are derived from the more heterogeneous Cayley plains. The very feldspar-rich impact melt rock 65795, which is compositionally similar to the group of feldspathic microporphyritic melt breccias (FM-suite), yields a crystallization age of 3.81 ± 0.04 AE (2σ; λ87Rb = 1.42−11yr−) and ISr of .69929 ± 3. In the subophitic impact melt rock 60635.5 two ages were recorded: One isochron with 3.87 ± 0.02AE and ISr of 0.69920 ± 3 gives the crystallization age of a fine-grained internal clast. At 3.75 ± 0.03 AE this fragment was incorporated into the host lithology of 60635 which is a member of the Anorthositic Noritic Melt Rock (ANMR) group. This group has a crystallization age of about 3.75 AE. The ANMR rocks and several samples of Apollo 14 crystalline impact melt rocks belong to a suite of “young crystalline impact melt rocks” ranging in age from 3.71 to 3.81 AE. These ages are in conflict with the canonical age of the Imbrium event (3.85 AE). It is argued that the young Apollo 14 and 16 melt rocks cannot represent solid exotic fragments or melt splashes ejected from post-Imbrium local or distant craters. Instead, these rocks are derived from coherent impact melt sheets of pre-Imbrian craters with diameters larger than 5–7 km. They are interpreted as clasts of the Fra Mauro and Cayley formation breccia deposits, and therefore their ages are used as an upper limit for the age of the Imbrium event. We suggest that the Imbrium basin and the related Fra Mauro and Cayley formations were formed 3.77 ± 0.02 AE ago and could be even as young as 3.75 AE. As a consequence, we adopt 3.92 ± 0.03 AE, 3.87 ± 0.03 AE, and 3.84 ± 0.04 AE as ages for the Nectaris, Serenitatis, and Crisium basins, respectively, in agreement with the relative crater densities measured on the ejecta blankets of these basins (Wilhelms, 1984). The proposed age sequence leads to an average formation interval for the observed 12–13 Nectarian basins of 7 to 14 m.y. leaving ~30 pre-Nectarian basins of unknown age. These facts suggest that there is no “late terminal lunar cataclysm” in the sense of a culmination of the lunar impact rate at ~3.8 AE ago. Rather, the observations are compatible with a steeply and steadily decreasing flux of impactors (Hartmann, 1980) in the sense of an “early heavy bombardment” which started at the time of the moon's accretion and terminated around 3.75 AE ago.

Composition and evolution of the lunar crust in the Descartes Highlands, Apollo 16

Samples from the North Ray crater ejecta blanket, Apollo 16, were investigated by petrographic microscope, electron microprobe, instrumental neutron activation and Xray fluorescence analyses, and 40Ar‐39Ar and Rb‐Sr dating techniques. Nine major groups of monomict and polymict breccias were defined on the basis of microscopic texture and these were further subdivided into chemical subgroups on the basis of characteristic elements such as Al, Mg, Fe, Cr, REE, Ni, and Co. The polymict breccias — fragmental breccias, granulitic breccias, and impact melt breccias — are the result of multiple impact‐induced mechanical mixing and melting, and of thermal and impact metamorphism of rock and mineral clasts derived from primordial igneous crustal rocks. For calculations of mixing models it was found that end‐members consisting of the pristine igneous rock components present as discrete samples at the Apollo 16 site and supplemented by KREEP, dunite, and a meteoritic component yield the best fits for the composition of polymict breccias. The end‐member rocks a re: ferroan anorthosite, various magnesian gabbronorites including “sodic ferrogabbro” and “feldspathic lherzolite,” and spinei troctolite. The following model is proposed for the composition and stratigraphy of the target for North Ray crater. The lower section of the stratigraphy is composed of a megabreccia with clasts of highly feldspathic polymict breccias (KREEP‐free “Old Eastern Highland Rock Suite”) interpreted as Nectaris ejecta (Descartes formation). The top section contains KREEP‐bearing polymict breccias (KREEP‐bearing “Young Western Highland Rock Suite”) and appears to be similar to the lithologies found in the Cayley plains. This material interpreted as Cayley formation may be a distant facies of Imbrium basin ejecta deposits of the Imbrium basin. The petrographic differences between these two major selenographic units (the Descartes and the Cayley formations) in the Apollo 16 area are more distinct than the chemical differences. The petrographic and chemical composition of the primordial igneous upper crust in the regions of the Nectaris and Imbrium basins has been calculated by subtraction of the KREEP and meteoritic components from the bulk composition of the Descartes and Cayley materials. The Nectaris, crust which is better constrained, consists of 86‐87% a northosite, 4% sodic ferrogabbro, 0.5‐1.3% feldspathic lherzolite, 6‐8% regular magnesian gabbronorite, 1.8‐2.8% dunite, and 0.1% spinel troctolite. A model for the evolution of the upper lunar crust in the Descartes highlands is proposed on the basis of isotope ages and clast‐matrix relationships of polymict breccias. Essential features of this model in sequenial order are: (1) development of multiple layers of KREEP‐free “early feldspathic fragmental megabreccias” and impact melt sheets on the primordial crust in the time period from 4.4 aeons to the time of the Nectaris impact, which could have occurred as late as 3.85 aeons ago, (2) excavation of these megabreccias by the Nectaris event and deposition of a “Nectarian feldspathic fragmental megabreccia” in the Descartes area, (3) introduction of KREEP‐basaltic material into the upper crust, mainly west and northwest of the Descartes site and formation of KREEPy impact melt sheets (most probably from 4.05 to 3.80 aeons ago), (4) Imbrium impact at 3.8 aeons ago and deposition of KREEP‐bearing “Imbrian feldspathic fragmental breccias” at the Descartes site which form the Cayley formation, and (5) redeposition , mixing, and melting of all preexisting breccias yielding KREEP‐bearing and KREEP‐poor post‐Cayley fragmental breccias and melt ejecta (“glass bombs”) as the result of small impacts (craters &lt; 1.5 km in diameter) in the Cayley plains and the Descartes mountains, respectively.

Apollo 16: Impact melt sheets, contrasting nature of the Cayley Plains and Descartes Mountains, and geologic history

Apollo 16 stations 4 and 5 rake samples were examined petrographically and by electron microprobe and INAA. Results were combined with published information (major and trace element compositions, petrologic information, and age determinations) for nonrake samples from stations 4 and 5 and some other Apollo 16 sample localities. Lithologic abundances support the idea (Korotev, 1981) that the variation of soil compositions at Apollo 16 is a result of mixing between a component represented by station 5 soil and components much like either the dimict breccias (station 4 soils) or feldspathic fragmental breccias (stations 1, 2, and 11) in composition. Dimict breccias, because of their restricted occurrence, are apparently not a constituent of either the Cayley or Descartes formations: they were contributed to the surface materials by South Ray crater and were probably excavated from beneath both formations by this event. Pyroxene, olivine, and coexisting plagioclase compositions from within the anorthosite portions of dimict breccias bridge the gap between the Mg‐rich and ferroan anorthosite fields. Analyses from associated cumulate and granulitic clasts indicate that they are the source of the intermediate material; dimict breccia anorthosites are thus polymict rocks, not pristine ferroan anorthosite. Compositions of Apollo 16 impact melt rocks suggest a number of compositional clusters. Discriminant and cluster analyses of compositional data confirm the existence and strength of these clusters, as do age determinations and Ir/Au ratios within each group. Each of three clusters represents an impact event in the Apollo 16 region. Petrologic information and age determinations indicate that dimict breccias formed approximately 3.92 b.y. ago. The Nectaris event occured between 3.84 and 3.92 b.y. ago, and the Cayley plains were deposited as a result of the Imbrium event sometime later than 3.84 b.y.

Subophitic lithologies in KREEP-rich poikilitic impact melt rocks from Cayley Plains, Apollo 16 — remnants of a volcanic highland crust?

KREEP-rich poikilitic impact melt rocks 65777,11, 65015,88, and 62235,66 are the only mafic impact melt rocks from Cayley Plains stations, Apollo 16, from which areas of subophitic texture can be reported. The bulk chemistry of these unique subophitic areas and the surrounding poikilitic matrices, as well as mineral compositions (olivine, plagioclase, pyroxene, Fe-Ni metal) were determined by electron microprobe analysis. All subophitic areas could be undoubtedly identified as impact melt rock clasts. Inclusion 65777,11 II is of uniquely KREEP-rich composition, 62235,66 II can be classified as anorthositic. Therefore our attempt to identify pristine volcanic basement rocks of the Cayley regions among these inclusions of basaltic texture failed. However, the absence of pristine volcanic target rock fragments and the existence of KREEP-rich and anorthositic impact melt clasts in KREEP-rich impact melt rocks from Cayley Plains favors the theory that the Cayley Plains formation is part of the ejecta blanket from a large basin-type impact crater (Imbrium?), which is underlain by anorthositic material (Nectaris ejecta?), and has been reworked by local impacts in post-Imbrian times.

Petrologic variations in Apollo 16 surface soils

Detailed modal analyses of fifteen surface soils were compared with rake and rock sample data to provide information about source rocks, maturation history, and intrasite variations in Apollo 16 regolith. Triangular plots of source rock components [defined as anorthosites, crystalline (melt) matrix breccias, and metamorphosed breccias] show that, with the possible exception of soils 64501 and 68501, Apollo 16 soils have similar source rocks that are well homogenized throughout the site. Modal abundance variations in submillimeter size fractions tend to reflect those of rake and rock samples. Maturity plots in agglutinate‐monomineralic fragment‐lithic and crystalline breccia space show Apollo 16 soils to be generally mature except for those collected from trenches (61221 and 61241) and from locations near North Ray Crater (stations 11 and 13). Data produced by other workers is combined with data from this study and used to divide the Apollo 16 site into three soil petrographic provinces. Central site soils are mature, well homogenized, and enriched in glass, but patches of immature and/or inhomogenous material are also present. They are probably the most typical Cayley Plains materials present at the site. North Ray soils are immature to submature, containing North Ray ejecta in the form of feldspathic fragmental breccia and plagioclase. South Ray soils are mature, but contain small amounts of fresh impact melts and plagioclase, possibly from breakdown of blocky South Ray ejecta. South Ray and North Ray ejecta differ in composition and physical properties, supporting the hypothesis that the South Ray event excavated Cayley material while the North Ray event excavated Descartes material.

Different ages of lunar light plains

The crater populations of 18 lunar light plains (Cayley plains) show a variation in relative ages by a factor of about 4 in crater frequency of regions in the surroundings of the Orientale resp. Imbrium basin, and by a factor of greater than 25 for more distant sites. Thus the idea of a Moonwide synchronism in the emplacement of the lunar light plains with the formation of the basins Imbrium or Orientale cannot be supported. Some light plains are younger than the youngest basin Orientale. Since these plains cannot have been emplaced by any other basin-forming event and local impact-derived origin can certainly be excluded, an endogenic (magmatic) origin is proposed for these plains. Age determination data (D L -values) by Soderblom and Lebofsky (1972) and Soderblom and Boyce (1972) are shown to be correlated with own cumulative crater frequency data (N) for surfaces younger than ≃ 3.8 × 109 years; we findD L × N 0.6, different from the originally proposed relationD L × N. For ages > 3.8 × 109 years, theD L data by those authors, especially their light plains data, are incompatible with our crater frequency data.

Apollo 16 Exploration of Descartes: A Geologic Summary

The Cayley Plains at the Apollo 16 landing site consist of crudely stratified breccias to a depth of at least 200 meters, overlain by a regolith 10 to 15 meters thick. Samples, photographs, and observations by the astronauts indicate that most of the rocks are impact breccias derived from an anorthositegabbro complex. The least brecciated members of the suite include coarse-grained anorthosite and finer-grained, more mafic rocks, some with igneous and some with metamorphic textures. Much of the traverse area is covered by ejecta from North Ray and South Ray craters, but the abundance of rock fragments increases to the south toward the younger South Ray crater. The Descartes highlands, a distinct morphologic entity, differ from the adjacent Cayley formation more in physiographic expression than in lithologic character.