Feldspathic Fragmental Breccias Research Articles

The bombardment history of the Moon as recorded by 40Ar‐39Ar chronology

AbstractNew petrography and 40Ar‐39Ar ages have been obtained for 1–3 mm sized rock fragments from Apollo 16 Station 13 soil 63503 (North Ray crater ejecta) and chips from three rocks collected by Apollo 16 and Apollo 17 missions. Selection of these samples was aimed at the old 40Ar‐39Ar ages to understand the early history of the lunar magnetic field and impact flux. Fifteen samples were studied including crustal material, polymict feldspathic fragmental breccias, and impact melts. The impact ages obtained range between approximately 3.3 and 4.3 billion years (Ga). Polymict fragmental breccia 63503,1 exhibits the lowest signs of recrystallization observed and a probable old relic age of 4.547 ± 0.027. The plateau age of 4.293 ± 0.044 Ga obtained for impact melt rock 63503,13 represents the oldest known age for such a lithology. Possibly, this age represents the minimum age for the South Pole‐Aitken (SPA) Basin. In agreement with literature data, these results show that impact ages >3.9 Ga are found in lunar rocks, especially within soil 63503. Impact exhumation of deep‐seated warm crustal material onto the lunar surface is considered to explain the common 4.2 Ga ages obtained for weakly shocked samples from soil 63503 and Apollo 17. This would directly imply that one or more basin‐forming events occurred at that time. Some rock fragments showing none to limited petrologic features indicate thermal annealing. These rocks may have lost Ar while resident within the hot‐ejecta of a large basin. Concurrent with previous studies, these results lead us to advocate for a complex impact flux in the inner solar system during the initial approximately 1.3 Ga.

Imbrium provenance for the Apollo 16 Descartes terrain: Argon ages and geochemistry of lunar breccias 67016 and 67455

In order to improve our understanding of impact history and surface geology on the Moon, we obtained 40Ar– 39Ar incremental heating age data and major + trace element compositions of anorthositic and melt breccia clasts from Apollo 16 feldspathic fragmental breccias 67016 and 67455. These breccias represent the Descartes terrain, a regional unit often proposed to be ejecta from the nearby Nectaris basin. The goal of this work is to better constrain the emplacement age and provenance of the Descartes breccias. Four anorthositic clasts from 67016 yielded well-defined 40Ar– 39Ar plateau ages ranging from 3842 ± 19 to 3875 ± 20 Ma. Replicate analyses of these clasts all agree within measurement error, with only slight evidence for either inheritance or younger disturbance. In contrast, fragment-laden melt breccia clasts from 67016 yielded apparent plateau ages of 4.0–4.2 Ga with indications of even older material (to 4.5 Ga) in the high-T fractions. Argon release spectra of the 67455 clasts are more variable with evidence for reheating at 2.0-2.5 Ga. We obtained plateau ages of 3801 ± 29 to 4012 ± 21 Ma for three anorthositic clasts, and 3987 ± 21 Ma for one melt breccia clast. The anorthositic clasts from these breccias and fragments extracted from North Ray crater regolith ( Maurer et al., 1978) define a combined age of 3866 ± 9 Ma, which we interpret as the assembly age of the feldspathic fragmental breccia unit sampled at North Ray crater. Systematic variations in diagnostic trace element ratios (Sr/Ba, Ti/Sm, Sc/Sm) with incompatible element abundances show that ferroan anorthositic rocks and KREEP-bearing lithologies contributed to the clast population. The Descartes breccias likely were deposited as a coherent lithologic unit in a single event. Their regional distribution suggests emplacement as basin ejecta. An assembly age of 3866 ± 9 Ma would be identical with the accepted age of the Imbrium basin, and trace element compositions are consistent with a provenance in the Procellarum-KREEP Terrane. The combination of age and provenance constraints points toward deposition of the Descartes breccias as ejecta from the Imbrium basin rather than Nectaris. Diffusion modeling shows that the older apparent plateau ages of the melt brecia clasts plausibly result from incomplete degassing of ancient crust during emplacement of the Descartes breccias. Heating steps in the melt breccia clasts that approach the primary crystallization ages of lunar anorthosites show that earlier impact events did not completely outgas the upper crust.

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.

Lunar soil evolution processes and Apollo 16 core 60013/60014.

Soils of the 62-cm deep Apollo 16 double drive tube 60013/14 are mature at the top and submature at the bottom. Modal analyses of 5529 grains from the 90-150 micrometers and the 500-1000 micrometers fractions from 12 levels of the core show that, in general, agglutinate abundance increases somewhat monotonically to the top and mimics the Is/FeO profile. There is a general decrease in the modal abundance of monomineralic fragments towards the top, suggesting that agglutinates were formed in part at the expense of monomineralic grains, especially feldspars, which are by far the most abundant mineral in these soils. In detail, the top 27 cm of the core differs from the bottom 21 cm, and the middle 14 cm is intermediate in its properties. In the upper segment, variations in the abundances of feldspars correspond with those of feldspathic fragmental breccias and cataclastic anorthosites; in the bottom segment, a similar but weak correspondence between feldspars and crystalline matrix breccias is observed. Mixing of the comminuted products of these three rock types likely produced the bulk of the core material. Many single feldspars in all size fractions are remarkably fresh, show no damage from shock, and are similar in appearance to the large feldspars in anorthosites and feldspathic fragmental breccias, which we consider to be the primary sources of single feldspars in this core. Major (Na, Al, Si, K, Ca) and minor (Fe, Ba) element analyses of 198 single feldspar grains indicate the presence of only one population of feldspars, which is consistent with our interpretation of feldspar provenance. Classification of 890 monomineralic feldspar, olivine, pyroxene, and glass spherules on the basis of the presence or absence of thin brownish coating--related to reworking at the surface--shows that coated grains are much more abundant in the top segment than in the bottom segment. A comparison with the mixing and maturation model (McKay et al., 1977) of soils in the core 60009/10, some 60 m away from 60013/14, shows that mixtures of an immature, nearly pure plagioclase soil (dominant in 60009/10) and another immature, crystalline breccia-rich soil (dominant in 60013/14) may have matured through in situ reworking to produce the soils under investigation. We conclude that the soils in this core are products of mixing along soil evolution Path 2 of McKay et al. (1974). Superimposed on that soil column is the reworking of the upper part, which has evolved more recently along Path 1. This core thus represents a consanguineous column of the lunar regolith with an upper reworked segment.

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.

Geochemistry and petrography of the MacAlpine Hills lunar meteorites

MacAlpine Hills 88104 and 88105 are anorthositic lunar meteorites recovered from the same area in Antarctica which are demonstrated to be paired samples of the same fall. Petrographic studies of matrix and clasts show that MAC88104/5 is a polymict breccia dominated by impact melt clasts. It contains a small amount of highland impact glass and very few regolith glass spherules. It is better classified as a fragmental breccia than a regolith breccia. The bulk composition is ferroan ( mg' = 63) and highly aluminous ( Al 2 O 3 = 28%). REE and other incompatible element concentrations are very low. Compositions of anorthositic, granulitic, and impact melt breccia clasts are very similar to, but distinct from, matrix composition. MAC88104/5 is very similar in composition to the other anorthositic lunar meteorites, but each of the meteorites is distinct. The anorthositic lunar meteorites, especially MAC88104/5 and Y82192/3/86032, are very similar to North Ray Crater feldspathic fragmental breccias in compositon and petrography. These fragmental lunar meteorites may have been ancient, deeply buried breccias from the megaregolith. Comparisons of chemical-petrographic features and exposure histories suggest that the twelve lunar meteorites were derived from two to nine, but probably four to seven, impacts onto the lunar surface. Discrepancies between the proportions of lunar meteorites and results of Apollo missions may be due to unrepresentative sampling. Lunar meteorites provide valuable new information on the nature of the lunar crust.

Petrology and precursors of lithic clasts from feldspathic fragmental breccia 67975

Fourteen lithic clasts were extracted from feldspathic fragmental breccia 67975 and analyzed by instrumental neutron activation analysis (INAA) by Lindstrom (1984), who classified the clasts into three granulite groups based upon color and grain size. The INAA data demonstrated that there are significant differences in the chemistry among these three groups, with both magnesian and ferroan rocks present in the clast suite. Evaluation of the textures and mineral chemistries of five of these lithic clasts indicates that several rock types are present, including feldspathic microporphyritic melt breccia, metamorphosed mafic‐rich melt breccia with zoned and mantled clasts, “typical” reequilibrated lunar feldspathic granulites with granoblastic texture and homogeneous mineral chemistry, hornfels, and a recrystallized mafic‐rich granulite with relict igneous texture and the composition of a ferroan anorthositic norite. These five lithic clasts were derived from both ferroan‐anorthosite suite and Mg‐suite precursors, with most of them containing more than one precursor. The ferroan anorthositic norite clast represents an unusual but important precursor that is a possible component of many of these clasts. The two light gray clasts (Lindstrom, 1984) examined in this study differ greatly; one is a plagioclase‐rich, possibly monomict rock with ferroan anorthositic precursor(s), and the other represents a mixture of feldspathic granulite and ferroan anorthositic norite lithologies. Lindstrom's dark gray group is a suite of feldspathic microporphyritic melt breccias and the light green group consists of clasts of metamorphosed polymict mafic melt breccias that have distinctive texture, mineralogy, and REE contents.

The composition and history of breccia 67015 from North Ray Crater

We have examined feldspathic fragmental breccia 67015 from the rim of North Ray Crater and found it to be the most diverse and complex of the Apollo 16 FFBs. It contains a wider range of clast types than any of the others and is not dominated by any single lithology. We have identified several crystalline lithologies, occurring in minor amounts, that we believe represent highland plutonic rocks. Plagioclase fragments are the most abundant breccia constituent, followed by clasts of impact melt rocks and granulites. The impact melts are of two main types: feldspathic fragment‐laden melts and mafic, KREEPy melt rocks of VHA basalt composition. Textural evidence indicates that some of the VHA melts were hot and plastic when they accumulated into the breccia. 40Ar‐39Ar dates show that both types of melts solidified about 3.9 × 109 years ago. Magnetic measurements on clasts show a scatter of NMR directions, making it impossible to read a consistent imprint of an ancient dipolar magnetic field. Previous studies of Apollo 16 stratigraphy suggested that KREEPy melt rocks were introduced to the site as Imbrium impact ejecta, about 3.8 × 106 years ago. Our results indicate that the highlands crust contained KREEP components as early as 3.9 × 109 years ago.

Alkali gabbronorite, ultra‐KREEPy melt rock and the diverse suite of clasts in North Ray Crater feldspathic fragmental breccia 67975

Lunar breccia 67975 is a feldspathic fragmental breccia from North Ray crater, Apollo 16. It contains clasts of alkali gabbronorite and ultra‐KREEPy mafic fragment‐laden melt breccias, which are unique among Apollo 16 samples. Both are alkali‐ and iron‐rich rocks with moderate to high REE concentrations. They more strongly resemble Apollo 14 gabbronorites and alkali anorthosites and KREEP‐rich rocks than they do other Apollo 16 samples. The other clasts in 67975 are the ferroan anorthosites, feldspathic melt rocks, and magnesian granulites, which are typical of other feldspathic fragmental breccias. Examination of bulk and mineral compositions of other breccias and melt rocks suggests that alkali gabbronorite may be a minor component in other North Ray crater breccias and feldspathic melt rocks. This implies that alkali gabbronorite was a fairly early (4.0 b.y.) crustal component in the North Ray crater region.

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.

Rock 67015: A feldspathic fragmental breccia with KREEP‐rich melt clasts

Breccia 67015, one of the feldspathic fragmental breccias (FFBs) from the rim of North Ray Crater, contains a wider range of clast lithologies than has been described previously in FFB's. Our petrologic and INAA studies have revealed two components of major petrologic and geologic significance. One of these is a dark, vesicular, KREEP‐rich melt rock of VHA (very high alumina) basalt composition, which occurs in clasts up to 6 cm long. Melt rocks of this type are common at the Apollo 16 site, but in previous years had not been discovered as prominent constituents of the feldspathic fragmental breccias. Their apparent absence led to hypotheses, no longer valid, that the KREEPy melt rocks were younger than the anorthositic breccia matrices and that the two lithologies might represent Imbrian and Nectaris ejecta, respectively. The second lithology we discovered is a large clast of medium‐grained ferroan anorthositic norite with a composition that falls among the most magnesian members of the ferroan anorthosite suite of pristine cumulate rocks. This material constitutes a previously ‘missing’ member of certain mixing models designed to account for the North Ray Crater melt rocks and bulk breccias.

Geochemical studies of feldspathic fragmental breccias and the nature of North Ray Crater ejecta

Major and trace element analyses of clasts and matrix from feldspathic fragmental breccias show that these breccias are not all alike but vary widely in their bulk compositions and clast assemblages. These studies, together with analyses of other North Ray Crater large rocks, reveal the major components to be anorthosites, ferroan anorthositic norites, magnesian granulites, feldspathic melt rocks, and VHA impact melts. The ferroan anorthositic norites are important lunar crustal rocks identified here for the first time. The KREEPy VHA melts are found to be much more important in North Ray Crater ejecta than was previously assumed. Sodic ferrogabbro is an important minor component. The compositional characteristics of bulk breccias and feldspathic melt rocks are modeled quantitatively as mixtures of two to five components: anorthosites, ferroan anorthositic norites, magnesian granulites, sodic ferrogabbros, and VHA melt rocks. Breccia 67455 is unique in consisting wholly of ferroan, KREEP‐free components.

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.