Lunar Chronology Research Articles

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

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

New crater calibrations for the lunar crater-age chronology

Measuring the spatial density of craters emplaced on geologic units is the primary method used for remotely estimating ages of solar system surfaces. The calibration for this method, which enables conversion of crater density to absolute age, comes from Apollo and Luna lunar samples for which absolute radiometric ages have been determined. Researchers throughout the 1970s worked to establish the lunar chronology system based on that calibration, correlating crater densities with absolute ages. However, no uniform crater study has been conducted on all calibration terrains, a limitation that was previously unaddressed until this study. The latest lunar images from the Lunar Reconnaissance Orbiter were used here to re-map the eleven main sampling sites and new crater counts of those surfaces were conducted. These show significant differences for many sites' crater counts, in many cases having more craters than previously identified. These results, calibrated to the radiometric ages, show a revised lunar crater chronology that changes previously established crater-based ages by up about 1 billion years: Surfaces younger than ∼3.6 Gyr and older than ∼3.9 Gyr under the classic chronology are younger in this system, and those in-between are older in this new system.

A sawtooth-like timeline for the first billion years of lunar bombardment

We revisit the early evolution of the Moon's bombardment. Our work combines modeling (based on plausible projectile sources and their dynamical decay rates) with constraints from the lunar crater record, radiometric ages of the youngest lunar basins, and the abundance of highly siderophile elements in the lunar crust and mantle. We deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history, but also there was no prominent and narrow impact spike ∼3.9Gy ago, unlike that typically envisioned in the lunar cataclysm scenario. Instead, we show the timeline of the lunar bombardment has a sawtooth-like profile, with an uptick in the impact flux near ∼4.1Gy ago. The impact flux at the beginning of this weaker cataclysm was 5–10 times higher than the immediately preceding period. The Nectaris basin should have been one of the first basins formed at the sawtooth. We predict the bombardment rate since ∼4.1Gy ago declined slowly and adhered relatively close to classic crater chronology models (Neukum and Ivanov, 1994). Overall we expect that the sawtooth event accounted for about one-fourth of the total bombardment suffered by the Moon since its formation. Consequently, considering that ∼12–14 basins formed during the sawtooth event, we expect that the net number of basins formed on the Moon was ∼45–50. From our expected bombardment timeline, we derived a new and improved lunar chronology suitable for use on pre-Nectarian surface units. According to this chronology, a significant portion of the oldest lunar cratered terrains has an age of 4.38–4.42Gyr. Moreover, the largest lunar basin, South Pole Aitken, is older than 4.3Gy, and therefore was not produced during the lunar cataclysm.

Stratigraphic sequence and ages of volcanic units in the Gruithuisen region of the Moon

Several domes indicative of non‐mare volcanism, morphologically as well as spectrally distinct from surrounding mare and highland units, are located in the Gruithuisen region in northern Oceanus Procellarum. The determination of the nature and age of non‐mare volcanism helps to put constraints on lunar crustal evolution. The ages of the domes were previously thought to be mid‐Imbrian or possibly even Eratosthenian. In order to assess more closely the stratigraphic relationships in this region, we carried out geologic mapping and crater size‐frequency measurements (1) on the Gruithuisen volcanic domes, (2) on the adjacent mare units, and (3) in the highland areas. (4) The nearby Iridum impact event was also dated by means of crater counts. A recently updated and improved version of the lunar production function polynomial was used to fit measured crater size‐frequency distributions of geologic units, and a cratering chronology model was applied in order to assign these units to the periods and epochs of the lunar chronology system. Oceanus Procellarum mare materials in the study area were found to range in age from Late Imbrian to Eratosthenian and show peak cratering model ages at 3.55 Gyr (1 Gyr = 1 billion years), 3.2–3.3 Gyr, and about 2.4 Gyr. Some mare units show evidence for at least two events in their crater size‐frequency distributions; these are most likely due to the superposition of later thin lava flows. Mare volcanism, hence, has lasted over a period of several hundred million years in the Gruithuisen region. The Gruithuisen domes show a much more restricted age range; they were found to range in age from 3.85 to 3.72 Gyr and thus were formed in the early part of the Late Imbrian, an older age than previously thought. Iridum crater, stratigraphically older than the Gruithuisen domes, was formed about 3.7–3.8 Gyr ago. The Iridum impact event and the formation of the two Gruithuisen domes cannot be well separated by means of crater counts. A few stratigraphically older highland units and some mare areas show ages of less than 2.0 Gyr, but these are less reliable due to (1) the crater distributions being close to saturation, (2) less effective crater retention on slopes, and (3) erosional removal of craters.

An investigation of the Gregorian chronology of early Islamic history : A Pilot Study

Abstract This pilot study presents a lunar and Gregorian chronology of the Sira, that is the biography of the prophet Muhammad. The paper discusses the problems surrounding the chronology of very early Islamic history, namely, the pre‐Hijra era. This history is not covered by the Hijra calendar, and therefore, historians dated its events using other primitive dating systems, resulting in considerable difficulties and inaccuracies when comparing Gregorian dates of this early Islamic history. In order to overcome this problem and establish an accurate chronology of the early Islamic history, the introduction of a new lunar calendar (the Muhammadi calendar) which starts with the birth month and year of the prophet Muhammad, and covers all Islamic history including the pre‐Hijra era, is suggested. The Muhammadi calendar was used to study the earliest three versions of the Sira. All dated events of the Sira were compiled from these sources and presented, for the first time, according to their historical order....

Kimberlites, mineralogy, geochemistry, and petrology

The Chang'E-5 (CE-5) mission has successfully collected lunar regolith samples from the northern Oceanus Procellarum. The Eratosthenian-aged mare units where the CE-5 landed on, might have been contaminated by distant ejecta delivered by several craters, such as Pythagoras, Sharp B, Harpalus, Copernicus, and other craters. Laboratory dating of distant ejecta in the CE-5 regolith can provide the formation ages of these craters. Therefore, these ejecta materials, the same as local young mare basalts, would serve as new calibration points for lunar chronology curve. Among above craters, the absolute model ages (AMAs) of Pythagoras, Sharp B and Harpalus are not well constrained. Here we used the Crater Size Frequency distribution (CSFD) method to yield their formation ages and the crater density values (N(1)/(km−2)). Using high resolution images, craters on the selected counting areas of their floors, ejecta blankets were identified. Our results indicate that Pythagoras is an Eratosthenian-age crater with an AMA of 2.13 ± 0.16 Ga and a N(1) =1.79 × 10−3 km−2. The Sharp B and Harpalus are both Copernican. The AMA of Sharp B crater is 1.01 ± 0.12 Ga with a N(1) of 8.46 × 10−4 km−2 while the AMA of Harpalus is 1.10 ± 0.04 Ga with a N(1) of 9.22 × 10−4 km−2. The ejecta from Sharp B and Harpalus as well as local mare basalt of the CE-5 site may be served as calibration points for lunar chronology curve, which will fill the age gaps of lunar chronology curve. Based on the estimated ages, we interpreted that Pythagoras ejecta might be beneath the young CE-5 mare unit, while the Sharp B and Harpalus ejecta should overlie the surface of the CE-5 site.

Sm-Nd isotopic evolution of chondrites

The 143Nd/ 144Nd and 147Sm/ 144Nd ratios have been measured in five chondrites and the Juvinas achondrite. The range in 143Nd/ 144Nd for the analyzed meteorite samples is 5.3 ε-units (0.511673–0.511944) normalized to 150Nd/ 142Nd= 0.2096 . This is correlated with the variation of 4.2% in 147Sm/ 144Nd (0.1920–0.2000). Much of this spread is due to small-scale heterogeneities in the chondrites and does not appear to reflect the large-scale volumetric averages. It is shown that none of the samples deviate more than 0.5 ε-units from a 4.6-AE reference isochron and define an initial 143Nd/ 144Nd ratio at 4.6 AE of 0.505828 ± 9. Insofar as there is a range of values of 147Sm/ 144Nd there is no unique way of picking solar or average chondritic values. From these data we have selected a new set of self-consistent present-day reference values for CHUR (“chondritic uniform reservoir”) of ( 143Nd/ 144Nd) CHUR 0 = 0.511836and( 147Sm/ 144Nd) CHUR 0 = 0.1967 . The new 147Sm/ 144Nd value is 1.6% higher than the previous value assigned to CHUR using the Juvinas data of Lugmair. This will cause a small but significant change in the CHUR evolution curve. Some terrestrial samples of Archean age show clear deviations from the new CHUR curve. If the CHUR curve is representative of undifferentiated mantle then it demonstrates that depleted sources were also tapped early in the Archean. Such a depleted layer may represent the early evolution of the source of present-day mid-ocean ridge basalts. There exists a variety of discrepancies with most earlier meteorite data which includes determination of all Nd isotopes and Sm/Nd ratios. These discrepancies require clarification in order to permit reliable interlaboratory comparisons. The new CHUR curve implies substantial changes in model ages for lunar rocks and thus also in the interpretation of early lunar chronology.

Limited response of the K–Ar system to the Nordlinger Ries giant meteorite impact

FOR the proper interpretation of radiometric rock ages it is important to know which processes are able to reset radioactive clocks. In particular, the resetting by intense shock effects is significant in lunar chronology because most lunar highland rocks are impact breccias produced in the course of multiple large meteorite impacts. For the important K–Ar scheme, we have dated variously shocked samples from ejecta and from a drill core through one of the best documented terrestrial meteorite craters the Nordlinger Ries (Southern Germany)1. From K–Ar and fission track dating of thoroughly melted impact glasses the 24 km diameter Ries crater was found to have formed 14.7±0.7 Myrago by meteorite impact into the variscian bedrock (for summary of ages see ref. 2). The modern version of K–Ar dating (39Ar–40Ar technique) can reveal quantitative information about partial losses of radiogenic argon (fractional resetting) in addition to yielding ages eventually not affected by these gas losses3,4. We have applied this dating technique to mineral separates (hornblende, biotite, chlorite) from ejecta, fall-back breccias and underlying bedrock of the Ries Crater, including drill core samples taken at depths up to 1,201 m(ref. l).The samples exhibit various degrees of mechanical strain (from ∼ 10 kbar up to >400 kbar shock pressure). Apart from the thoroughly molten glasses which reproduced the impact age of ∼ 14.7 Myr, we found that all other rocks involved in the impact irrespective of the degree of shock metamorphism and within the limits of error, yield the same 39Ar–4oAr age of 313±3 Myr, probably the age of the bedrock. Apparently, rocks exposed to the very intense shock effects frequent on the Moon give reliable K–Ar ages when measured as K–Ar plateau ages. Age resetting, if present, probably results from heating associated with such impacts. In the case of the Ries, we found from the diffusion characteristics of argon that the post-shock temperature of the crystalline fragments within the suevite layer has not exceeded 600 °C.

Lunar chronology as determined from the radiometric ages of returned lunar sample

We have heard earlier in this Discussion Meeting that from the systematics of Sr and Pb isotopes in lunar samples, it is possible to ascertain that the Moon had a solid crust about 4.6 Ga ago, that is, very soon after the formation of the solar system. In addition, it would seem that the major ring-basins on the Earth side of the Moon were all formed before 3.8 Ga ago. After the formation of the basins by impact, there was extensive magmatic activity in the form of basalt flows, expecially in the major ring-basins. About 3.1 Ga ago, all major lava-flow activity on the Moon had ceased. This outline of lunar chronology is accepted in practically all proposed interpretations of the radiometric ages. The first 600 Ma of lunar chronology is not as clear. During this time the multi-ring basins were formed. It has been proposed by G. Turner and by G. J. Wasserburg at this meeting, that the multi-ring basins formed in a rather short interval of time, perhaps as short as 100 Ma. This would imply an intense cratering rate some 3.9 Ga ago and a rapid decline in cratering rate thereafter. Such an event would have probably greatly altered the other bodies in the solar system, especially the Earth, and as such is of no small significance. It has been pointed out previously by Tera et al. (1974) that an alternative interpretation of the radiometric data is that widespread, simultaneous metamorphism (which the systematics of the lead isotopes in the highland rocks seems to imply) could as well result from a single widespread event such as Imbrium as from a multiple basin-forming sequence in a short period of time. In that case, the chronology of the multi-ring basins is an open question.

Outline of a lunar chronology

We present here an outline of lunar chronology and evolution based on analyses of the isotopic parent-daughter systems 87Rb-87Sr, U-Th-206Pb-207Pb-208Pb and 40K-40Ar. An overview of the chronology will first be given, followed by an outline of the observational basis. A more complete discussion of 40K-40Ar results and their interpretation is presented in the paper by G. Turner in this volume. While the body of data on lunar materials is limited, the chronology for lunar evolution appears to be rather well defined. The samples which have been investigated represent mare basalts [returned by the Apollo missions (11, 12, 15 and 17) and by the Soviet Luna 16 mission] and terra rocks, which include non-mare basalts, anorthosites, troctolites and norites but are predominantly comprised of complex breccias [returned by Apollo 12, 14, 16 and 17 and Luna 20]. The mare basalts are associated with the late stage lava flows which covered the mare basins. These flood basalts have been broken up by impact processes but for the most part are associated with the local areas and have not been subject to major transport or metamorphism by impact. The highland rocks predate the mare lava flows but are not clearly associated with a particular magmatic or impact process. They may have been excavated from considerable depths and transported over wide distances. Impact metamorphism is certainly one of the critical stages in their development.

Chronology of terrestrial planet evolution: The evidence from Mercury

It is premature to establish a chronology for Mars and Mercury, relative to the known lunar chronology, to better than an order of magnitude. Lunar evidence neither requires nor excludes a “cataclysmic” episode of bombardment about 4.0 b.y. ago. Such a cataclysm might have resulted naturally from tidal disruption by a planet or collisional fragmentation in the asteroid belt of either a Uranus/Neptune-scattered planetesimal or a large asteroid, in which case any lunar cataclysm would have occurred as well on other planets. There is no independent evidence in Mariner 10 imagery for (or against) an early episodic bombardment on Mercury. Crater densities on plains units of the Moon, Mars, and Mercury have not been shown to be “strikingly similar” and do not imply, in the absence of definitive dynamical calculations of planetary impact rates of plausible populations of planetesimals, any similarity in the geological chronologies for those planets. Photogeological studies alone cannot determine absolute chronologies for planets. In combination with dynamical analyses, they can help us date to no better than a factor of 3 to 10 the formation of the Caloris Basin or the epoch when the Martian rivers ran.

Lunar Chronology and Evolution Inferred from the Radiometric Age Data of Appollo and Luna Rocks and Soils

Lunar Chronology and Evolution Inferred from the Radiometric Age Data of Appollo and Luna Rocks and Soils

Mechanical erasure of particle tracks: A tool for lunar microstratigraphic chronology

Mechanical erasure of particle tracks caused by impact deformation is shown to be a common feature of lunar soil grains. Etching of lunar pyroxenes reveals deformation markings that are evidence of prior impact events. The soil microstratigraphic chronology can be derived if it is assumed that the minimum track density in a thin soil layer is commonly found in grains where such erasure was caused by the impact that deposited the soil. With Apollo 12 core samples as examples, the minimum track density found in each layer is used to calculate the longest exposure that layer could have received since it was last disturbed. By summing the upper limits on the exposure times of the different layers, a lower limit on the average deposition rate was estimated to be 0.35 cm/m.y.

Of Time and the Moon

Considerable information concerning lunar chronology has been obtained by the study of rocks and soil returned by the Apollo 11 and Apollo 12 missions. It has been shown that at the time the moon, earth, and solar system were formed, approximately 4.6 approximately 10(9) years ago, a severe chemical fractionation took place, resulting in depletion of relatively volatile elements such as Rb and Pb from the sources of the lunar rocks studied. It is very likely that much of this material was lost to interplanetary space, although some of the loss may be associated with internal chemical differentiation of the moon. It has also been shown that igneous processes have enriched some regions of the moon in lithophile elements such as Rb, U, and Ba, very early in lunar history, within 100 million years of its formation. Subsequent igneous and metamorphic activity occurred over a long period of time; mare volcanism of the Apollo 11 and Apollo 12 sites occurred at distinctly different times, 3.6 approximately 10(9) and 3.3 approximately 10(9) years ago, respectively. Consequently, lunar magmatism and remanent magnetism cannot be explained in terms of a unique event, such as a close approach to the earth at a time of lunar capture. It is likely that these phenomena will require explanation in terms of internal lunar processes, operative to a considerable depth in the moon, over a long period of time. These data, together with the low present internal temperatures of the moon, inferred from measurements of lunar electrical conductivity, impose severe constraints on acceptable thermal histories of the moon. Progress is being made toward understanding lunar surface properties by use of the effects of particle bombardment of the lunar surface (solar wind, solar flare particles, galactic cosmic rays). It has been shown that the rate of micrometeorite erosion is very low (angstroms per year) and that lunar rocks and soil have been within approximately a meter of the lunar surface for hundreds of millions of years. Future work will require sampling distinctly different regions of the moon in order to provide data concerning other important lunar events, such as the time of formation of the highland regions and of the mare basins, and of the extent to which lunar volcanism has persisted subsequent to the first third of lunar history. This work will require a sufficient number of Apollo landings, and any further cancellation of Apollo missions will jeopardize this unique opportunity to study the development of a planetary body from its beginning. Such a study is fundamental to our understanding of the earth and other planets.

Lunar chronology and evolution from Rb [sbnd]Sr studies of Apollo 11 and 12 samples

Rb-Sr internal isochrons for a total of eight Apollo 12 crystalline rocks yield ages of 3.36 to 3.16 AE. The initial Sr compositions ( I) are relatively primitive and range from 0.69918 to 0.69957 as compared to BABI= 0.69898 ± 3. No clear groupings in I are observed, however, the wide range indicates that at least four different rock bodies were sampled. An Apollo 11 basalt (10024) yielded an age of 3.61 ± 0.07AE andI = 0.69935 ∓ 8 in agreement with previous results on other Apollo 11 high K rocks. Several Apollo 12 soil samples yield model ages which range from 4.4 to 4.6 AE and indicate that the special nature and older “age” of the lunar soil determined at the Apollo 11 site is a widespread phenomenon. Initial Sr compositions from Apollo 11 and Apollo 12 support our previous conclusions that the moon as a whole has a Rb/Sr much lower ( Rb/Sr≈ 0.008) than found in chondrites. We summarize the current status of Rb Sr lunar chronology and some implications regarding the melting and differentiation history of the moon.

Our Astronomical Column

THE DATE OF EASTER.—It seems a curious anachronism that our calendar in the twentieth century should still be largely influenced by the lunar chronology which passed out of direct use nearly two thousand years ago. That was the most obvious system to employ at the dawn of astronomy, the moon's rapid motion and the ease of locating its position in the heavens making it far superior to the sun as a time-measurer. But as time went on the inconvenience of having a variable number of months in the year, and of having events like the equinoxes and solstices occurring on variable dates, caused the system to be abandoned and a purely solar calendar substituted.