Fra Mauro Formation Research Articles

Geological mapping and chronology of lunar landing sites: Apollo 14

The lunar cratering chronology function is based on the careful and systematic correlation of crater size-frequency distributions (CSFDs) with radioisotopic ages of samples from the Apollo, Luna, and Chang'e landing sites. The link between the samples and the CSFDs requires a detailed understanding of the landing site geology. Therefore, in this study, we have created a new detailed regional geological map of the Apollo 14 landing site using LRO, M3, and SELENE/Kaguya data. The geologic map allows us to define homogeneous areas for new CSFD measurements, from which updated N(1) values (i.e., cumulative number of craters with diameter ≥ 1 km) can be determined for the sampled geologic units. In particular, at the Apollo 14 landing site we measured the N(1) of the Imbrian Fra Mauro Formation (Ifm), which is interpreted to be ejecta from the Imbrium basin, and can be correlated with the Apollo 14 Imbrium ejecta samples. This new calibration point can be compared with the lunar chronology of Neukum (1983). We find that the new calibration point is consistent with the lunar cratering chronology of Neukum (1983). Together with our previous studies, this result supports the validity of this chronology.

Morphological and Chronological Mapping of Manilius Crater Region Using Chandrayaan-1 Data Sets

Fine-resolution morphological mapping aided by ortho-images and digital elevation model from Chandrayaan-1 Terrain Mapping Camera and 3D GIS visualization has revealed scientifically diverse characteristics of lunar surface features, due to unique topographical significance of morphological features, i.e., highlands, basaltic plains and craters, which are very well manifested in 3D GIS environment. The distribution of various morphological features provides insights into the sequential evolution and surface process of the study area. The highland region represented by the Fra Mauro formation in the study area exhibits high albedo with distinct topography. The northern part of the study area falls in the southern part of major basin Serenitatis, and exhibits the dark mantling material with low albedo. The morphological features, i.e., wrinkle ridges and rilles, indicate volcanic flow events consequence to the loading of basaltic materials in the interior of the Serenitatis and Imbrium Basins and related extensional failure. The Manilius crater, which occupies the central part of the study area, is a complex crater with a central peak and asymmetric ejecta deposit. The ages of the major surficial features were determined based on size, frequency and distribution pattern of craters using crater size-frequency distribution model. Age of the Fra Mauro highland, Manilius crater, Mare Serenitatis and Mare Vaporum is, respectively, 3.9, 3.5, 2.8 and 1.7 Ga years, indicating that the lunar surface of this region evolved in Imbrian to Eratosthenian age of lunar selenological timescale.

Annealing history of zircons from Apollo 14083 and 14303 impact breccias

AbstractBreccia boulders scattered around the 25 Ma Cone Crater near the Apollo 14 landing site provide a potential source of material ejected from the underlying Fra Mauro formation, which is interpreted to form from the ejecta blanket of the ~3.9 Ga Imbrium impact. However, questions remain as to whether all or some of the rocks collected are from the Fra Mauro Formation. In this contribution, we present new Raman measurements on zircons from impact breccias 14083 and 14303 that indicate a zircon radiation damage age of the breccias of 3900 ± 120 (2σ) Ma. These ages are compatible with a history of thermal shock during their emplacement as part of the Imbrium impact ejecta blanket. In contrast, previously published Raman analyses of zircons from a sample of breccia 14311 have a younger radiation damage age of 3410 ± 80 (2σ) Ma, confirming that this breccia had a different thermal history and was possibly unrelated to the Fra Mauro Formation. The radiation damage ages of breccias 14083 and 14303, in combination with previously published apatite U‐Pb ages, place constraints on the thermal conditions within the Imbrium ejecta blanket.

Pb-Pb ages of feldspathic clasts in two Apollo 14 breccia samples

Pb-Pb isochron ages of ca. 3.92Ga for three K-feldspar-rich clasts from Apollo 14 breccias 14303 and 14083 were determined using Secondary Ion Mass Spectrometry (SIMS). These ages are interpreted to represent the resetting of the U-Pb system in the clasts as a result of brecciation during the Imbrium impact. One of the clasts contains zircon grains that record a significantly older crystallization age (ca. 4.33–4.35Ga) for the rock represented by that clast. Initial Pb compositions determined for the clasts, combined with the previously measured Pb isotopic compositions of K-feldspar grains from several Apollo 14 breccia samples, constrain a range of initial Pb compositions in the ca. 3.9Ga Fra Mauro formation at the Apollo 14 landing site. This range in initial Pb compositions indicates that the rocks represented by these clasts, or the sources of those rocks, evolved with a high 238U/204Pb (μ-value) for substantial periods of time, although the precise crystallization ages of the rocks represented by at least two of the clasts investigated here are unknown.

Origin and transportation history of lunar breccia 14311

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

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

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

The Chandrayaan-1 X-ray Spectrometer: First results

We present X-ray fluorescence observations of the lunar surface, made by the Chandrayaan-1 X-ray Spectrometer during two solar flare events early in the mission (12th December 2008 and 10th January 2009). Modelling of the X-ray spectra with an abundance algorithm allows quantitative estimates of the MgO/SiO2 and Al2O3/SiO2 ratios to be made for the two regions, which are in mainly basaltic areas of the lunar nearside. One of these ground tracks includes the Apollo 14 landing site on the Fra Mauro Formation. Within the 1σ errors provided, the results are inside the range of basaltic samples from the Apollo and Luna collections. The Apollo 14 soil composition is in agreement with the results from the January flare at the 1σ uncertainty level. Discrepancies are observed between our results and compositions derived for the same areas by the Lunar Prospector gamma-ray spectrometer; some possible reasons for this are discussed.

The comparative behavior of apatite‐zircon U‐Pb systems in Apollo 14 breccias: Implications for the thermal history of the Fra Mauro Formation

Abstract— We report secondary ion mass spectrometry (SIMS) U‐Pb analyses of zircon and apatite from four breccia samples from the Apollo 14 landing site. The zircon and apatite grains occur as cogenetic minerals in lithic clasts in two of the breccias and as unrelated mineral clasts in the matrices of the other two. SIMS U‐Pb analyses show that the ages of zircon grains range from 4023 ± 24 Ma to 4342 ± 5 Ma, whereas all apatite grains define an isochron corresponding to an age of 3926 ± 3 Ma. The disparity in the ages of cogenetic apatite and zircon demonstrates that the apatite U‐Pb systems have been completely reset at 3926 ± 3 Ma, whereas the U‐Pb system of zircon has not been noticeably disturbed at this time. The apatite U‐Pb age is slightly older than the ages determined by other methods on Apollo 14 materials highlighting need to reconcile decay constants used for the U‐Pb, Ar‐Ar and Rb‐Sr systems. We interpret the apatite age as a time of formation of the Fra Mauro Formation. If the interpretation of this Formation as an Imbrium ejecta is correct, apatite also determines the timing of Imbrium impact. The contrast in the Pb loss behavior of apatite and zircon places constraints on the temperature history of the Apollo 14 breccias and we estimate, from the experimentally determined Pb diffusion constants and an approximation of the original depth of the excavated samples in the Fra Mauro Formation, that the breccias experienced an initial temperature of about 1300–1100 °C, but cooled within the first five to ten years.

CAS-1 lunar soil simulant

Lunar soil simulant is a geochemical reproduction of lunar regolith, and is needed for lunar science and engineering researches. This paper describes a new lunar soil simulant, CAS-1, prepared by the Chinese Academy of Sciences, to support lunar orbiter, soft-landing mission and sample return missions of China’s Lunar Exploration Program, which is scheduled for 2004–2020. Such simulants should match the samples returned from the Moon, all collected from the lunar regolith rather than outcrops. The average mineral and chemical composition of lunar soil sample returned from the Apollo 14 mission, which landed on the Fra Mauro Formation, is chosen as the model for the CAS-1 simulant. Source material for this simulant was a low-Ti basaltic scoria dated at 1600 years from the late Quaternary volcanic area in the Changbai Mountains of northeast China. The main minerals of this rock are pyroxene, olivine, and minor plagioclase, and about 20–40% modal glass. The scoria was analyzed by XRF and found to be chemically similar to Apollo 14 lunar sample 14163. It was crushed in an impact mill with a resulting median particle size 85.9 μm, similar to Apollo soils. Bulk density, shear resistance, complex permittivity, and reflectance spectra were also similar to Apollo 14 soil. We conclude that CAS-1 is an ideal lunar soil simulant for science and engineering research of future lunar exploration program.

The atomic arrangement of merrillite from the Fra Mauro Formation, Apollo 14 lunar mission: The first structure of merrillite from the Moon

The atomic arrangement of lunar merrillite has been refined to R = 0.0452 in R 3 c using X-ray diffraction data recorded on a CCD detector; previous attempts at structure solution using a point detector were not successful because of the poorly crystallized nature of the lunar material. The atomic arrangement of merrillite has a structural unit of [(Mg,Fe)(PO 4 ) 6 ] 16− 2 that forms a “bracelet-and-pinwheel” unit that is common in hexagonal-closest-packed layers. The individual structural units are not polymerized and exist in layers at z = 1/6, 1/3, 1/2, 2/3, and 5/6. In lunar merrillite, the [(Mg,Fe)(PO 4 ) 6 ] 16− 2 structural units are linked by a [(Ca,REE) 18 Na 2 (PO 4 ) 2 ] 32+ interstitial complex, formed of Ca1O 8 , Ca2O 8 , Ca3O 8 , NaO 6 , and P1O 4 polyhedra. There has long been speculation regarding the relationship between merrillite and terrestrial whit-lockite, and the solution of the Fra Mauro merrillite atomic arrangement allows the characterization of the lunar phase. Lunar merrillite and terrestrial whitlockite have largely similar atomic arrangements, but the phases differ due to the presence or absence of hydrogen. In whitlockite, H is an essential element and allows the charge balance. Hydrogen is incorporated into the whitlockite atomic arrangement by disordering one of the phosphate tetrahedra and forming a PO 3 (OH) group. Lunar merrillite is devoid of hydrogen, and thus no disordered tetrahedral groups exist. Charge balance for substituents Y and REE (for Ca) is maintained by Si ↔ P tetrahedral substitution and □ ↔ Na at the Na site. The structure solution demonstrates the effectiveness of the CCD detector in unraveling previously intractable diffraction data and urges that previously analyzed lunar material be reexamined using this instrumentation.

The case for a younger Imbrium basin: New 40Ar- 39Ar ages of Apollo 14 rocks

Petrographically and chemically well-characterized rocks of the Apollo 14 landing site (12 impact melt breccias, 3 clast-free impact melt rocks, 2 granulitic rocks, 1 cataclastic noritic anorthosite, 1 shocked gabbronorite, 1 mare basalt, and 1 plagioclase single crystal) have been dated by the 40Ar- 39Ar technique. The samples represent two different formations at the Apollo 14 site: (1) small lithic clasts (10 specimens) from fragmental breccia 14063 ejected from the 26 Ma old Cone crater and one clast (mare basalt) from Cone crater soil 14140, and (2) large rock samples (10 specimens) collected throughout the regolith of the landing site. The first group of samples has exposure ages close to 26 Ma and 40Ar- 39Ar ages ranging from 3.86 to 4.09 Ga, which indicates that breccia 14063 was assembled 3.86 Ga ago. The second group of samples displays lower 40Ar- 39Ar ages ranging from 3.73 to 3.85 Ga. We interpret breccia 14063 as part of a Nectarian megabreccia unit which was incorporated into the Fra Mauro Formation as a megablock by secondary mass wasting of the ejecta blanket of a local pre-Imbrian crater. The remaining samples are thought to represent clasts of the Apollo 14 subregolith basement megabreccia defined as the Fra Mauro Formation. Consequently, the reported ages of these clasts confirm previous suggestions that the Fra Mauro Formation and therefore the Imbrium basin is only 3.75 Ga old and not 3.85 Ga as commonly assumed.

Pre-4.2 AE mare-basalt volcanism in the lunar highlands

The concept that the plutonism of the lunar highlands and the mare-type volcanism are two separate problems in both time (> 4.4 AE versus < 3.95 AE) and space is seriously questioned by the discovery of a 4.23-AE low-Ti mare basalt from Fra Mauro Formation. Apollo 14 breccia 14305 contains a clast (,122) which is an olivine gabbronorite that is texturally and mineralogically similar to several Apollo 12 basalts (e.g., 12005, 12035, 12040). It consists of cumulus olivine (40 modal %; Fo 62–70) and Ti-chromite (2.5 modal %); post-cumulus phases include low-Ca pyroxene (29 modal %; Wo 7–13 En 68–75), augite (10 modal %; Wo 31–40 En 47–50), plagioclase (15 modal %, An 82–93), and ilmenite (4 modal %, 5–7 MgO). The TiO 2 content of this rock = 4.3%; CaO/Al 2O 3 ⋍ 1.0, CaO = 5.1%; MgO/FeO ⋍ 1.0, MgO = 21.9%. The REE pattern, normalized to chondritic abundances, is approximately 30 × Ch and “hump-shaped” with a pronounced Eu depletion and a non-KREEPy signature. A four-point Rb-Sr isochron reveals an age of 4.23 ± 0.05 AE. The sample has a low initial 87Sr/ 86Sr= 0.69911 ± 3. The data presented here show that non-KREEPy, mare-type volcanism commenced at least as early as 4.2 AE in the Fra Mauro region and probably across much of the lunar surface. Massive bombardment during the “terminal cataclysm” and the subsequent veneer of younger mare basalts has obliturated most of the evidence for these ancient volcanic events. These old, mare-type volcanics may be related to basin-forming events such as made Procellarum (i.e., impact-triggered igneous activity).

Lunar magnetic anomalies detected by the Apollo substatellite magnetometers

Properties of lunar crustal magnetization thus far deduced from Apollo subsatellite magnetometer data are reviewed using two of the most accurate presently available magnetic anomaly maps — one covering a portion of the lunar near side and the other a part of the far side. The largest single anomaly found within the region of coverage on the near-side map correlates exactly with a conspicuous, light-colored marking in western Oceanus Procellarum called Reiner Gamma. This feature is interpreted as an unusual deposit of ejecta from secondary craters of the large nearby primary impact crater Cavalerius. An age for Cavalerius (and, by implication, for Reiner Gamma) of 3.2 ± 0.2 × 10 9 y is estimated. The main (30 × 60 km) Reiner Gamma deposit is nearly uniformly magnetized in a single direction, with a minimum mean magnetization intensity of ∼7 × 10 −2 G cm 3/g (assuming a density of 3 g/cm 3), or about 700 times the stable magnetization component of the most magnetic returned samples. Additional medium-amplitude anomalies exist over the Fra Mauro Formation (Imbrium basin ejecta emplaced ∼3.9 × 10 9 y ago) where it has not been flooded by mare basalt flows, but are nearly absent over the maria and over the craters Copernicus, Kepler, and Reiner and their encircling ejecta mantles. The mean altitude of the far-side anomaly gap is much higher than that of the near-side map and the surface geology is more complex, so individual anomaly sources have not yet been identified. However, it is clear that a concentration of especially strong sources exists in the vicinity of the craters Van de Graaff and Aitken. Numerical modeling of the associated fields reveals that the source locations do not correspond with the larger primary impact craters of the region and, by analogy with Reiner Gamma, may be less conspicuous secondary crater ejecta deposits. The reason for a special concentration of strong sources in the Van de Graaff-Aitken region is unknown, but may be indirectly related to the existence of strongly modified crustal terrain which also occurs in the same region. The inferred directions of magnetization for the several sources of the largest anomalies are highly inclined with respect to one another, but are generally depleted in the north-south direction. The north-south depletion of magnetization intensity appears to continue across the far-side within the region of coverage. The mechanism of magnetization and the origin of the magnetizing field remain unresolved, but the uniformity with which the Reiner Gamma deposit is apparently magnetized, and the north-south depletion of magnetization intensity across a substantial portion of the far side, seem to require the existence of an ambient field, perhaps of global or larger extent. The very different inferred directions of magnetization possessed by nearly adjacent sources of the Van de Graaff-Aitken anomalies, and the depletion in their north-south component of magnetization, do not favor an internally generated dipolar field oriented parallel to the present spin axis. A variably oriented interplanetary magnetizing field that was intrinsically strong or locally amplified by unknown surface processes is least inconsistent with the data.

Lunar polymict breccia 14321: a petrographic study

Seven petrographic thin sections of lunar rock sample 14321, ‘Big Bertha’, have been examined. It is a complex rock incorporating diverse lithic and single crystal fragments and represents a sampling of the heterogeneous Fra Mauro formation, considered by the writers to be lithified debris from the Imbrium impact event. Electron probe microanalysis and microscopic study of textures reveal the assembly history of this breccia which in turn allows some interpretation of the nature of the pre-Imbrium crust and the effect of the Imbrium impact and the subsequent transportation to the Apollo 14 site. The present-day polymict breccia 14321 is composed of basaltic clasts originating from the fragmentation of a single or closely related set of lava cooling units, a set of fragmental clasts designated as microbreccia 3 (themselves polymict microbreccias), and a light colored matrix which formed rock 14321 by cementing the two major groups of clasts. The light colored matrix material is derived from the fragmentation and mutual abrasion of the basalt and microbreccia 3. On the basis of consistent textural relations two older sets of microbreccias have been identified within microbreccia 3. Microbreccia 1 clasts are well-rounded, relatively light colored, and noritic. They are always completely enclosed within microbreccia 3, most often forming the central cores of rounded accretionary lapilli structures which we have designated as microbreccia 2. Microbreccias 1, 2, 3, and macrobreccia 14321 represent a chronological series of fragmentation and lithification events. Each of these events involved some thermal and/or shock metamorphism as evidenced by mineralogical and textural criteria, and the chronological order of formation of the breccias also corresponds to a decreasing intensity of associated thermal effects. The petrology and mineralogy of 14321 are described in detail in this paper. A more general interpretation of the combined petrographic and chemical data is given in Duncan et al. (1975a).

Analyses of the returned lunar surface fines for porphyrins.

In the present studies, no porphyrins were found in the lunar surface fines collected from the Sea of Tranquillity by the Apollo 11 mission, from the Ocean of Storms by the Apollo 12 mission, and from the nonmare Fra Mauro Formation by the Apollo 14 expedition under the conditions in which porphyrins would have been detected had they been present in amounts as small as 10−14 mole.

Lunar materials: Their mineralogy, petrology and chemistry

The manned Apollo 11, 12, 14 and 15 and the automated Luna 16 lunar missions have provided us with lunar rock and regolith (soil) samples from a number of geologically distinct sites. The mare regions were sampled by Apollo 11, 12 and Luna 16, whereas Apollo 14 landed on a terrain with more relief, the Fra Mauro Formation which represents an ejecta blanket from the Imbrian Basin, and Apollo 15 touched down near the lunar highlands. The samples collected consist of a mixture, mainly of basalt, breccia and regolith (soil-particulate matter, generally < 1 cm in size). The basalts show considerable variation in texture, mineralogy and chemistry and probably represent fragments from various parts of relatively thin and extensive lava flows in the maria. The breccias represent regolith material which was indurated to varying degrees by impact events. The regolith is a product of the breakdown, again by impact, of coherent rock masses of basalt and breccia. The composition (major as well as trace elements) of the regolith at a specific site does not generally represent the average composition of the basaltic rock types collected at that same site. The regolith contains small amounts of anorthositic and noritic fragments, a component rich in KREEP (potassium, rare earths and phosphorus) and a small meteoritic component which appears generally to be chondritic or a mixture of chondrite and iron meteorite in composition. The chemistry of the lunar basalts shows a depletion in the generally more volatile elements (e.g., alkalis) with respect to terrestrial basalts. Some refractory elements such as titanium and rare earths, however, show a considerably higher concentration in lunar basalts than in their average terrestrial counterparts. Three new minerals have been recognized in the basalts, breccias and regolith. These are: pyroxferroite (Ca 0.15 Fe 0.85 Sio 3), armalcolite [Fe,Mg)Ti 2O 5] and tranquillityite [Fe 8 2+ (Zr + Y) 2Ti 3Si 3O 24]. New compositional varieties of several terrestrially known minerals have also been found. H 2O and Fe 3+ are completely lacking and organic compounds are also absent from the lunar materials. Impact craters ranging from several hundred kilometers to several microns in diameter (microcraters) attest to the importance of meteorite impact as an erosional mechanism on the Moon. The age of the Moon appears to be approximately 4.6 aeons (1 aeon= 1,000 million years) and the ages of the basalts indicate that they originated between 3.9 and 3.3 aeons ago. This age span may represent the period during which the mare were flooded with basaltic flows or the specific dates may represent the last major flooding at each site. Regolith ages from all sites lie between 4.3 and 4.6 aeons.

Occurrence of chromian, hercynitic spinel (“pleonaste”) in Apollo-14 samples and its petrologic implications

Many isolated grains of a reddish pleonaste-type spinel occur in fines and metabreccia samples, particularly 14 319. Electron microprobe analyses (104) of spinels and their associated phases include 58 of pleonaste which show Mg/(Mg + Fe) 0.44–0.62 and Cr/(Cr + Al) 0.017–0.134 (atomic), plus minor amounts of other ions, and differ greatly from almost all previously recorded lunar spinels; almost no spinels of intermediate composition were found. Two types of compositional zoning exist: a diffuse primary one with cores lower in Ti, and a narrow secondary one from reaction with matrix yielding rims higher in Cr, Ti, and Mn. At contacts with breccia matrix there is a narrow corona of almost pure plagioclase (An 80-An 94), free of opaque minerals and pyroxene. Two types of solid inclusions found in the pleonaste are calcic plagioclase, and tiny spherical masses of nickel-rich sulfide. Similar pleonaste occurs in crystalline rock clasts, mainly with plagioclase; one clast (A) consists only of coarse olivine, plagioclase, and pleonaste, with granulated grain boundaries suggestive of deformation. From composition and texture, this clast is one possible candidate for the mafic cumulate counterpart of the “anorthositic” crust. Another clast (B), also made solely of olivine, plagioclase and pleonaste, is itself a breccia. These data suggest a two-stage brecciation process: 1) disruption (probably pre-Imbrian) of a deep-seated pleonaste-bearing source rock like A and reconsolidation to form a breccia without addition of pyroxene, ilmenite or other minerals; and 2) disruption of this breccia to yield breccia clast B which was then incorporated into the Fra Mauro formation.

A 4.3 Aeon Pre-Imbrium Event

HERE we point out that published Rb-Sr data for Apollo 14 whole rock samples define an isochron age of 4.31 ± 0.04 aeon, which is interpreted as their time of igneous crystallization. Previously reported internal isochron ages of these rocks range from 3.87 to 3.96 aeon1, in excellent agreement with their 40Ar/39Ar ages2. We think that these approximate to the time of the Imbrium impact event and ejection of the material of the Fra Mauro Formation.

Plutonium-244 Fission Tracks: Evidence in a Lunar Rock 3.95 Billion Years Old

Tracks attributed to the spontaneous fission of plutonium-244 and of uranium-238 were detected in a large whitlockite crystal in the lunar breccia 14321 from the Fra Mauro formation. For a track-retention age of 3.95 x 10(9) years the number of plutonium tracks relative to the number of uranium tracks is 0.51 +/- 0.15, provided that the rock was not heavily neutron-irradiated 3.95 X 10(9) years ago.

Apollo 14 Active Seismic Experiment

Explosion seismic refraction data indicate that the lunar near-surface rocks at the Apollo 14 site consist of a regolith 8.5 meters thick and characterized by a compressional wave velocity of 104 meters per second. The regolith is underlain by a layer with a compressional wave velocity of 299 meters per second. The thickness of this layer, which we interpret to be the Fra Mauro Formation, is between 16 and 76 meters. The layer immediately beneath this has a velocity greater than 370 meters per second. We found no evidence of permafrost.