U-Pb Ages of Lunar Apatites

Apatite is one of the minerals that is rarely utilized in U-Pb geochronology, compared to some other U-rich accessory phases. Relatively low U concentration, commonly high proportion of common Pb and low closure temperature of U-Pb system of apatite inhibit its application as geochronological tool when other minerals such as zircon are widely available. However, zircon appear to be restricted to certain type of lunar rocks, carrying so called KREEP signature, whereas apatite (and whitlockite) is a common accessory mineral in the lunar samples. Therefore, utilizing apatite for lunar chronology may increase the pool of rocks that are available for U-Pb dating. The low stability of U-Pb systematics of apatite may also result in the resetting of the system during meteoritic bombardment, in which case apatite may provide an additional tool for the study of the impact history of the Moon. In order to investigate these possibilities, we have analysed apatites and zircons from two breccia samples collected during the Apollo 14 mission. Both samples were collected within the Fra Mauro formation, which is interpreted as a material ejected during the impact that formed the Imbrium Basin

Heavy Isotope Composition of Oxygen in Zircon from Soil Sample 14163: Lunar Perspective of an Early Ocean on the Earth

Thirty oxygen analyses of a large (sub-millimetre) zircon grain from the lunar soil sample 14163 have been determined using CAMECA 1270 ion microprobe. The sample 14163 was returned form the Fra Mauro region by Apollo 14 mission. Zircon grain of 0.6-0.8 mm in size extracted from the sample was imaged using CL detector fitted to the Philips Electron Microscope in order to reveal internal structure. Oxygen isotopes have been analysed during two sessions. The first set of data was collected using the original mount where the grain was set in the resin attached to the glass slide. This resulted in the two complications: (i) standard zircon has to be analysed from the separate mount and (ii) the lunar zircon grain was rased in the holder compared to the standard. In order to investigate, if the elevated oxygen compositions observed during this session could have resulted from this difference in geometric configuration during the standard and sample analyses, the lunar zircon was extracted from the original mount, remounted with the standard chip in the new resin disk and reanalysed during the second session. All analyses made during the first session show delta O-18 values heavier than 6.0%. The second set of data has a wider spread of delta O-18 values with some values as low as 5.6%. Nevertheless, a half of observed delta O-18 values in this set is also higher than 6.0%. Slightly lighter oxygen compositions observed during the second session indicate possible dependence of measured delta O-18 values on the geometry of analysed samples. Presence of zircons with similar heavy oxygen isotope compositions on the Moon, which neither had liquid water or felic crust similar to that on the Earth nor ever developed regime similar to plate tectonics, suggests that other mechanisms can be responsible for elevated delta O-18 values in zircons. This implies that there is no support for the presence of an ocean on the surface of the early Earth and as the ocean appears to be an essential ingredient for the plate tectonics, there is no basis for belief that this mechanism was operating in the early history of the planet

Constraining the Flux of Impactors Postdating Heavy Bombardment Using U-Pb Ages of Impact Glasses

Spherules of glass varying in size from a few micrometres to a few millimetres are common in the lunar regolith. While some of these glass beads are products of pyroclastic fire fountains others originate as impact melt ejected from the target that breaks into small droplets and solidifies as spherical particles while raining back to the lunar surface. These glasses preserve information about the chemical composition of the target and often contain sufficient amount of radioactive nuclides such as 40K to enable Ar-40-Ar-39 dating of individual beads. Studies measuring the age of glass beads have been used in attempts to establish variations in the flux of impactors hitting the Moon, particularly during the period that postdates the formation of major impact basins [1,2]. These studies proposed a possibility of spike in the impact flux about 800 Ma [2] and over the last 400 Ma [1]. More recently U-Th-Pb isotopic systems have been also utilized to determine the age of impact glasses from the Apollo 17 regolith [3]. Our aim is to extend the application of the U-Pb system in impact glasses to spherules isolated from Apollo 14 soil 14163 in an attempt to further investigate the applicability of this isotopic system to the chronology of impact glass beads and gain additional information on the impact flux in the inner Solar system

Lunar basalt chronology, mantle differentiation and implications for determining the age of the Moon

Despite more than 40 years of studying Apollo samples, the age and early evolution of the Moon remain contentious. Following the formation of the Moon in the aftermath of a giant impact, the resulting Lunar Magma Ocean (LMO) is predicted to have generated major geochemically distinct silicate reservoirs, including the sources of lunar basalts. Samples of these basalts, therefore, provide a unique opportunity to characterize these reservoirs. However, the precise timing and extent of geochemical fractionation is poorly constrained, not least due to the difficulty in determining accurate ages and initial Pb isotopic compositions of lunar basalts. Application of an in situ ion microprobe approach to Pb isotope analysis has allowed us to obtain precise crystallization ages from six lunar basalts, typically with an uncertainty of about ±10Ma, as well as constrain their initial Pb-isotopic compositions. This has enabled construction of a two-stage model for the Pb-isotopic evolution of lunar silicate reservoirs, which necessitates the prolonged existence of high-μ reservoirs in order to explain the very radiogenic compositions of the samples. Further, once firm constraints on U and Pb partitioning behaviour are established, this model has the potential to help distinguish between conflicting estimates for the age of the Moon. Nonetheless, we are able to constrain the timing of a lunar mantle reservoir differentiation event at 4376±18Ma, which is consistent with that derived from the Sm–Nd and Lu–Hf isotopic systems, and is interpreted as an average estimate of the time at which the high-μ urKREEP reservoir was established and the Ferroan Anorthosite (FAN) suite was formed

A Zircon U-Pb Study of the Evolution of Lunar KREEP

SIMS U-Pb analyses show that zircons from breccias from Apollo 14 and Apollo 17 have essentially identical age distributions in the range 4350 to 4200 Ma but, whereas Apollo 14 zircons additionally show ages from 4200 to 3900 Ma, the Apollo 17 samples have no zircons with ages <4200 Ma. The zircon results also show an uneven distribution with distinct peaks of magmatic activity. In explaining these observations we propose that periodic episodes of KREEP magmatism were generated from a primary reservoir of KREEP magma, which contracted over time towards the centre of Procellarum KREEP terrane

The phases of the Moon : modelling crystallisation of the lunar magma ocean through equilibrium thermodynamics

Funding: TEJ acknowledges support from the State Key Laboratory for Geological Processes and Mineral Resources, China University of Geosciences, Wuhan (Open Fund GPMR201903 ). We thank J. B. Balta and T. Prissel for their challenging reviews that led to significant improvements in the final version.Despite some 50 years of intense research on samples returned from the Apollo missions and lunar meteorites, along with remote-sensing and Earth-based observations, many questions regarding the formation and evolution of the Moon persist. These include the detailed compositional and density structure of the lunar mantle and the source and petrogenesis of the diverse suite of extrusive and intrusive igneous rocks. There is broad agreement that the primary internal structure of the Moon reflects crystallisation of a lunar magma ocean (LMO), and that an inverted density gradient within the mantle cumulates led to some reorganisation of layers by partial convective overturn. Experimental studies have provided invaluable constraints on crystallisation of the LMO, but are limited by the relatively small number of experiments that can practically be undertaken. Here we use recently-developed thermodynamic models for minerals and melt in the K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–TiO2–Cr2O3 system to model crystallisation of a full-moon LMO based on two existing end-member bulk compositions—Taylor Whole-Moon (TWM), and Lunar Primitive Upper Mantle, LPUM—on which many experimental studies have been based. We follow several recent studies in considering equilibrium crystallisation of the first 50 vol.% and fractional crystallisation thereafter. Our results match well with experimental studies, and provide detailed constraints on the major oxide composition, mineralogy and density structure based on the two starting compositions that, while exhibiting some similarities, show important differences. The more fertile TWM composition contains significant quantities of garnet in the deep mantle, whereas the LPUM composition has none. By contrast, prior to any gravitational overturn, the uppermost mantle cumulates for TWM are strongly silica-undersaturated and contain abundant aluminous spinel, whereas those for LPUM are silica-saturated. For both starting compositions, with the exception of TiO2 and Na2O, our modelled compositions of the final dregs of fractionated melt show a reasonable match with existing estimates on the composition of urKREEP. Modelled partial melts of the upper-mantle cumulates at low to moderate melt fractions have major oxide compositions that match well with low- and intermediate-Ti lunar basalts. The correspondence is particularly good for picritic (green) glasses that likely represent melts derived from deeper levels within the upper mantle. The wide spread in TiO2 concentrations in lunar basalts and basaltic glasses is consistent with density-driven reorganisation involving ilmenite. Our simulations provide thermodynamically-robust estimates of the compositional, mineralogical and density structure of the lunar interior that are unprecedented in their detail, and which provide the foundation for several lines of future research addressing the origin and secular evolution of the Moon.PostprintPeer reviewe

Where are the Shocked Grains in the Hadean Zircon Record? Insights on the Preservation of Shocked Zircon and Their U-Pb Systematics

While the earliest history of many planetary bodies within the inner Solar System is dominated by intense bombardment, this record is missing from Earth due to active tectonics and erosion. Where-as rocks from the earliest history of Earth are absent, mineral relics, such as ancient detrital zircon concentrated in sediments within the Jack Hills, Narryer, Illara and Maynard Hills greenstone belts of the Yilgarn Craton in Western Australia preserve a record of this time.Shock in zircon: During shock deformation, resulting from hyper-velocity impact, zircon can be modified in crystallographically-controlled ways. This includes the development of planar and subplanar low-angle grain boundaries, the formation of mechanical twins, transformation to the high pressure polymorph reidite, development of polycrystalline microtexture, and dissociation to its dioxide constituents SiO2 and ZrO2

The timing of basaltic volcanism at the Apollo landing sites

Precise crystallisation ages have been determined for a range of Apollo basalts from Pb-Pb isochrons generated using Secondary Ion Mass Spectrometry (SIMS) analyses of multiple accessory phases including K-feldspar, K-rich glass and phosphates. The samples analysed in this study include five Apollo 11 high-Ti basalts, one Apollo 14 high-Al basalt, seven Apollo 15 low-Ti basalts, and five Apollo 17 high-Ti basalts. Together with the samples analysed in two previous similar studies, Pb-Pb isochron ages have been determined for all of the major basaltic suites sampled during the Apollo missions. The accuracy of these ages has been assessed as part of a thorough review of existing age determinations for Apollo basalts, which reveals a good agreement with previous studies of the same samples, as well as with average ages that have been calculated for the emplacement of the different basaltic suites at the Apollo landing sites. Furthermore, the precision of the new age determinations helps to resolve distinctions between the ages of different basaltic suites in more detail than was previously possible. The proposed ages for the basaltic surface flows at the Apollo landing sites have been reviewed in light of these new sample ages. Finally, the data presented here have also been used to constrain the initial Pb isotopic compositions of the mare basalts, which indicate a significant degree of heterogeneity in the lunar mantle source regions, even among the basalts collected at individual landing sites

Ancient volcanism on the Moon: Insights from Pb isotopes in the MIL 13317 and Kalahari 009 lunar meteorites

Lunar meteorites provide a potential opportunity to expand the study of ancient (>4000 Ma) basaltic volcanism on the Moon, of which there are only a few examples in the Apollo sample collection. Secondary Ion Mass Spectrometry (SIMS) was used to determine the Pb isotopic compositions of multiple mineral phases (Ca-phosphates, baddeleyite K-feldspar, K-rich glass and plagioclase) in two lunar meteorites, Miller Range (MIL) 13317 and Kalahari (Kal) 009. These data were used to calculate crystallisation ages of 4332 ±2Ma (95% confidence level) for basaltic clasts in MIL 13317, and 4369 ±7Ma (95% confidence level) for the monomict basaltic breccia Kal 009. From the analyses of the MIL 13317 basaltic clasts, it was possible to determine an initial Pb isotopic composition of the protolith from which the clasts originated, and infer a 238U/204Pb ratio (μ-value) of 850 ±130(2σ uncertainty) for the magmatic source of this basalt. This is lower than μ-values determined previously for KREEP-rich (an acronym for K, Rare Earth Elements and P) basalts, although analyses of other lithological components in the meteorite suggest the presence of a KREEP component in the regolith from which the breccia was formed and, therefore, a more probable origin for the meteorite on the lunar nearside. It was not possible to determine a similar initial Pb isotopic composition from the Kal 009 data, but previous studies of the meteorite have highlighted the very low concentrations of incompatible trace elements and proposed an origin on the farside of the Moon. Taken together, the data from these two meteorites provide more compelling evidence for widespread ancient volcanism on the Moon. Furthermore, the compositional differences between the basaltic materials in the meteorites provide evidence that this volcanism was not an isolated or localised occurrence, but happened in multiple locations on the Moon and at distinct times. In light of previous studies into early lunar magmatic evolution, these data also imply that basaltic volcanism commenced almost immediately after Lunar Magma Ocean (LMO) crystallisation, as defined by Nd, Hf and Pb model ages at about 4370Ma

Timing of crystallization of the lunar magma ocean constrained by the oldest zircon

The Moon is thought to have formed through the consolidationof debris from the collision of a Mars-sized body with the Earthmore than 4,500 million years ago. The primitive Moon wascovered with a thick layer of melt known as the lunar magmaocean1, the crystallization of which resulted in the Moon?ssurface as it is observed today. There is considerable debate,however, over the precise timing and duration of the processof magma ocean crystallization. Here we date a zircon fromlunar breccias to an age of 4,4176 million years. This dateprovides a precise younger age limit for the solidification ofthe lunar magma ocean. We propose a model that suggestsan exponential rate of lunar crystallization, based on acombination of this oldest known lunar zircon and the age of theMoon-forming giant impact. We conclude that the formationof the Moon?s anorthositic crust followed the solidification of80?85% of the original melt, within about 100 million years ofthe collision. The existence of younger zircons2 is indicative ofthe continued solidification of a small percentage of melt for anextra 200?400 million years