Lunar Zircon Research Articles

Impact origin of lunar zircon melt inclusions in Apollo impact melt breccia 14311

AbstractInvestigations of trapped melt inclusions in minerals can yield insights into the compositions and conditions of parent magmas. These insights are particularly important for detrital grains like many of the lunar zircons found in samples returned by the Apollo missions. However, unlike their terrestrial counterparts, lunar zircons have potentially been exposed to billions of years of impact bombardment. Samples from terrestrial impact structures and impact shock experiments have revealed that deformation during an impact event produces melt and glass blebs that can mimic igneous melt inclusions in both morphology and composition. We have undertaken a geochemical and textural investigation of zircons from Apollo impact melt breccia 14311 to assess their formation mechanisms. The association of trapped melts with shock microtwins and monomineralic melt compositions suggests some inclusions formed as a result of the high pressures and temperatures of impact shock. All other inclusions in this study are associated with curviplanar features, planar features, crystal plastic deformation, or embayments (large regions in contact with adjacent melts or minerals) suggesting that they are not igneous melt inclusions. While these textures can be produced in tectonic environments, impacts are a likely formation mechanism since impacts are the main driver of tectonics on the Moon. The results of this study demonstrate that a combination of textural and compositional analyses can be employed distinguish between igneous melt inclusions and melt blebs in zircons from impact environments.

Methodologies for 176Lu-176Hf Analysis of Zircon Grains from the Moon and Beyond.

Zircons are found in extraterrestrial rocks from the Moon, Mars, and some differentiated meteorite parent-bodies. These zircons are rare, often of small size, and have been affected by neutron capture induced by cosmic ray exposure. The application of the 176Lu-176Hf decay system to zircons from planetary bodies such as the Moon can help establish the chronology of large-scale differentiation processes such as the crystallization of the lunar magma ocean. Here, we present methods to measure the isotopic composition of Hf of extraterrestrial zircons dated using ID-TIMS U-Pb after chemical abrasion. We introduce a 2-stage elution scheme to separate Hf from Zr while preserving the unused Zr fraction for future isotopic analysis. The effect of neutron capture is also re-examined using the latest thermal neutron capture cross sections and epithermal resonance integrals. Our tests show that the precision of Hf isotopic analyses is close to what is theoretically attainable. We have tested this method to a limited set of zircon grains from lunar rocks returned by the Apollo missions (lunar soil 14163, fragmental polymict breccia 72275, and clast-rich breccia 14321). The model ages align with previously reported values, but further work is needed to assess the chronology of lunar magma ocean crystallization as only a handful of small zircons (5 zircons from 3 samples) were analyzed, and the precision of the analyses can be improved by measuring more and larger lunar zircon grains.

The youngest lunar zircon reveals an extremely fractionated nature of Chang’e-5 basalt

The U(Pb)-Pb age of zircon is commonly used to represent the crystallization age for igneous rocks due to its high closure temperature and robust resistance to impact disturbance. However, no zircon crystallization age has yet been reported for Chang'e-5 (CE-5) basalt due to their limited occurrence in mare basalt. In this study, rare zircon grains from CE-5 lunar samples were investigated by the in-situ Pb isotopic analysis, and a precise zircon crystallization age of 2036 ± 19 Ma was determined from Pb-Pb isochron. This is hitherto the youngest reported crystallization age of lunar zircon, similar to the ages of CE-5 lunar basalts obtained by zirconium (Zr)-bearing minerals such as baddeleyite, tranquillityite, and zirconolite. Petrographic evidence and rare-earth element geochemistry indicate that zircon in the CE-5 lunar basalts were formed by the reaction of early-formed baddeleyite with SiO2 melt within the latest residue of extreme fractionation of a non-KREEP (an acronym for potassium, REE, and phosphorus) basaltic magma. In contrast to the prevailing view that lunar zircon have formed in late-stage enriched melts resulting from extensive fractional crystallization of the Lunar Magma Ocean, this study shows that zircon could be derived from extreme fractionation of non-KREEP basaltic magma unrelated to Lunar Magma Ocean.

Nano- and micro-structures in lunar zircon from Apollo 15 and 16 impactites: implications for age interpretations

Meteorite impact processes are ubiquitous on the surfaces of rocky and icy bodies in the Solar System, including the Moon. One of the most common accessory minerals, zircon, when shocked, produces specific micro-structures that may become indicative of the age and shock conditions of these impact processes. To better understand the shock mechanisms in zircon from Apollo 15 and 16 impact breccias, we applied transmission electron microscopy (TEM) and studied nano-structures in eight lunar zircons displaying four different morphologies from breccias 15455, 67915, and 67955. Our observations revealed a range of shock-related features in zircon: (1) planar and non-planar fractures, (2) “columnar” zircon rims around baddeleyite cores, (3) granular textured zircon, in most cases with sub-µm-size inclusions of monoclinic ZrO2 (baddeleyite) and cubic ZrO2 (zirconia), (4) silica-rich glass and metal inclusions of FeS and FeNi present at triple junctions in granular zircon and in baddeleyite, (5) inclusions of rutile in shocked baddeleyite, (6) amorphous domains, (7) recrystallized domains. In many grain aggregates, shock-related micro-structures overprint each other, indicating either different stages of a single impact process or multiple impact events. During shock, some zircons were transformed to diaplectic glass (6), and others (7) were completely decomposed into SiO2 and Zr-oxide, evident from the observed round shapes of cubic zirconia and silica-rich glass filling triple junctions of zircon granules. Despite the highly variable effect on textures and Zr phases, shock-related features show no correlation with relatively homogeneous U–Pb or 207Pb/206Pb ages of zircons. Either the shock events occurred very soon after the solidification or recrystallization of the different Zr phases, or the shock events were too brief to result in noticeable Pb loss during shock metamorphism.

Radiogenic Pb mobilization induced by shock metamorphism of zircons in the Apollo 72255 Civet Cat norite clast

In situ U-Pb radiometric dating of zircons is regarded as one of the most widely used and reliable methods to acquire geochronologic ages. However, it has been recently reported that radiogenic Pb (Pb*) mobilization within zircon may, in some cases, cause inaccurate age determinations with no geological significance. Such Pb* mobilization can be caused by deformation, α-coil damage, fluid-assisted annealing, and recrystallization. In this study, we report an investigation of Pb* mobilization in shock metamorphosed lunar zircons. NanoSIMS (nanoscale secondary ion mass spectrometry) and IMS 1280HR ion microprobe dating, EBSD (electron backscatter diffraction) and CL (cathodoluminescence) mapping, and scanning ion imaging (SII) were applied to micro-zircon grains from the Apollo 72255 Civet Cat norite clast. Based on the large number of grains with similarities in internal zoning, habit and trace element geochemistry, and host mineral context, the Civet Cat norite zircons are interpreted to be primary, igneous grains. The chronology obtained for three consecutive surfaces (at different depths) by NanoSIMS, SII, and IMS 1280HR, respectively, indicates that the radiogenic Pb distribution of the Civet Cat norite zircons is heterogeneous among different polished or sputtering surfaces. Forty-two NanoSIMS U-Pb ages (beam size of 5 μm) are concordant on a Wetherill Concordia diagram, and their corresponding 207Pb/206Pb ages spread from 4015 Ma to 4459 Ma. More notably, the six oldest spots of the 42 define a concordant U-Pb age of 4460 ± 31 Ma (2σ, MSWD = 0.47, P = 0.92) and a weighted mean 207Pb/206Pb age of 4453 ± 34 Ma (MSWD = 0.056, P = 0.998). These dates are among the oldest in the lunar highland rocks. However, the 207Pb/206Pb ages of repolished surfaces of these zircons by IMS 1280HR (beam size of 5 μm) do not reproduce the NanoSIMS results (up to 300 Ma younger). The SII (spatial resolution of 2 μm) confirms a heterogeneous distribution of radiogenic Pb within single grains. The EBSD mapping of these zircon grains shows that they have 3–20° of cumulative lattice misorientation. It is proposed that shock-related deformation has facilitated Pb* migration after primordial crystallization. With currently available data, we cannot preclude the possibility that the large errors of the U-Pb ages obscure reverse discordance that would bias our oldest 207Pb/206Pb ages to older values. Conversely, our data could be explained by mixing of Pb-retention and Pb-loss nanodomains as seen in shocked terrestrial zircon such that U-Pb date of 4460 ± 31 Ma approximates the norite formation.

Xenon systematics of individual lunar zircons, a new window on the history of the lunar surface

We demonstrate a new way of investigating the processing of the lunar surface (and other planetary regoliths) that combines XeS-XeN ages (based on uranium fission) in individual zircons with their xenon isotopic record of solar wind and cosmic ray exposure. We report the first xenon isotopic analyses of individual lunar zircons (from Apollo 14 soil and breccias samples). Parallel analyses of a suite of zircons from the Vredefort impact structure in South Africa revealed XeS-XeN ages that agree well with U-Pb systematics, suggesting that the diffusion kinetics of xenon and lead in zircon are similar in the pressure-temperature environment of sub-basin floors. In contrast, all Apollo 14 zircons examined exhibit XeS-XeN ages markedly younger than the associated U-Pb and 207Pb-206Pb ages, and soil zircons with 207Pb-206Pb ages greater than 3900 Ma produced an abundance of XeS-XeN ages <1000 Ma. The young ages cannot be explained by thermal neutron irradiation on the lunar surface, and diurnal heating is unlikely to cause preferential loss of xenon. As such these young soil zircon ages likely record regolith gardening processes. The breccia zircons typically record older ages, >2400 Ma, suggesting that these samples may be useful for investigating ancient events and regolith processing at an earlier epoch. However, none of the zircons contain xenon from now-extinct 244Pu implying that either the samples have completely degassed since ∼3900 Ma or that the initial Pu/U ratio of the Moon is lower than that on Earth. We also describe a methodology for conducting component deconvolution that can be applied to multi-isotopes systems beyond xenon. We have also determined new xenon isotopic yields from rare earth element spallation in the lunar environment and high precision yields for neutron induced fission of 235U in geologic samples.

A Nanoscale Record of Impact-Induced Pb Mobility in Lunar Zircon

A Nanoscale Record of Impact-Induced Pb Mobility in Lunar Zircon

Shock metamorphic history of &gt;4 Ga Apollo 14 and 15 zircons

AbstractDuring impact events, zircons develop a wide range of shock metamorphic features that depend on the pressure and temperature conditions experienced by the zircon. These conditions vary with original distance from impact center and whether the zircon grains are incorporated into ejecta or remain within the target crust. We have employed the range of shock metamorphic features preserved in &gt;4 Ga lunar zircons separated from Apollo 14 and 15 breccias and soils in order to gain insights into the impact shock histories of these areas of the Moon. We report microstructural characteristics of 31 zircons analyzed using electron beam methods including electron backscatter pattern (EBSP) and diffraction (EBSD). The major results of this survey are as follows. (1) The abundance of curviplanar features hosting secondary impact melt inclusions suggests that most of the zircons have experienced shock pressures between 3 and 20 GPa; (2) the scarcity of recrystallization or decomposition textures and the absence of the high‐pressure polymorph, reidite, suggests that few grains have been shocked to over 40 GPa or heated above 1000 °C in ejecta settings; (3) one grain exhibits narrow, arc‐shaped bands of twinned zircon, which map out as spherical shells, and represent a novel shock microstructure. Overall, most of the Apollo 14 and 15 zircons exhibit shock features similar to those of terrestrial zircon grains originating from continental crust below large (~200 km) impact craters (e.g., Vredefort impact basin), suggesting derivation from central uplifts or uplifted rims of large basins or craters on the Moon and not high‐temperature and ‐pressure ejecta deposits.

The formation of large neoblasts in shocked zircon and their utility in dating impacts

Uranium-lead (U-Pb) geochronology of individual shocked zircon grains has unique potential for dating bolide impact events. Neoblasts in granular-textured zircon have been recognized as the shock-related feature most effective at recording the impact age. Here we report the discovery of large neoblasts (5-100 mu m in dimension) in shocked zircon at the Sudbury impact structure, Canada-the first report of in situ coarsely granular zircon from a terrestrial impact site other than the Vredefort structure, South Africa. The neoblast-bearing sample was taken from a heterogeneous, lithic clast-rich igneous unit associated with the roof rocks of the impact melt sheet, making this the first time a crater has been dated using neoblastic zircon from the upper part of its stratigraphy. Previous in situ discoveries of coarsely granular zircon at Vredefort were all in impact-generated mafic melt emplaced beneath the impact melt sheet. Electron backscatter diffraction analysis of the impact-aged neoblasts indicates that the high-pressure conditions inferred in the formation of many small neoblasts were not necessarily involved in the formation of these large ones. Their large size, internal zonation, and occurrence in a slowly cooling environment collectively suggest that large neoblasts at Sudbury formed by relatively protracted, post-impact growth in shocked zircon incorporated into impact-related melt. Based on insight from large neoblast growth in terrestrial settings, we suggest that the ca. 4.33 Ga neoblasts recently reported in lunar zircon may imply a major basin-forming event on the Moon at that time. New knowledge of the cratering environments in which large neoblasts form also raises the prospect of possibly linking ex situ granular zircon in lunar breccias with specific impact structures-and thus better calibrating the lunar cratering record with radiometric ages.

Hf isotope evidence for effective impact melt homogenisation at the Sudbury impact crater, Ontario, Canada

We report on the first zircon hafnium-oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets. The differentiated melt sheet at the 1.85Ga, originally ca. 200km in diameter Sudbury impact crater, Ontario, Canada, yields a tight range of uniform zircon Hf isotope compositions (εHf(1850) of ca. −9 to −12). This is consistent with its well-established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised. We propose that the heterogeneity in other isotopic systems, such as Pb, in early-emplaced impact melt at Sudbury is associated with volatility-related depletion during the impact cratering process. This depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post-impact assimilation of country rock, whereas the systems of more refractory elements preserve initial homogeneities. Zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks. However, they display a marked offset approximately one-third up the melt sheet stratigraphy that is interpreted to be a result of post-impact assimilation of 18O-enirched rocks into the base of the cooling impact melt.Given that impact cratering was a more dominant process in the early history of the inner Solar System than it is today, and the possibility that impact melt sheets were sources of ex situ Hadean zircon grains, these findings may have significance for the interpretation of the early zircon Hf record. We speculate that apparent εHf-time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust.We also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at Sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself.

Early formation of the Moon 4.51 billion years ago.

Establishing the age of the Moon is critical to understanding solar system evolution and the formation of rocky planets, including Earth. However, despite its importance, the age of the Moon has never been accurately determined. We present uranium-lead dating of Apollo 14 zircon fragments that yield highly precise, concordant ages, demonstrating that they are robust against postcrystallization isotopic disturbances. Hafnium isotopic analyses of the same fragments show extremely low initial 176Hf/177Hf ratios corrected for cosmic ray exposure that are near the solar system initial value. Our data indicate differentiation of the lunar crust by 4.51 billion years, indicating the formation of the Moon within the first ~60 million years after the birth of the solar system.

Coordinated U–Pb geochronology, trace element, Ti-in-zircon thermometry and microstructural analysis of Apollo zircons

We present the results of a coordinated SIMS U–Pb, trace element, Ti-in-zircon thermometry, and microstructural study of 155 lunar zircons separated from Apollo 14, 15, and 17 breccia and soil samples that help resolve discrepancies between the zircon data, the lunar whole rock history and lunar magma ocean crystallization models. The majority of lunar grains are detrital fragments, some nearly 1mm in length, of large parent crystals suggesting that they crystallized in highly enriched KREEP magmas. The zircon age distributions for all three landing sites exhibit an abundance of ages at ∼4.33Ga, however they differ in that only Apollo 14 samples have a population of zircons with ages between 4.1 and 3.9Ga. These younger grains comprise only 10% of all dated lunar zircons and are usually small and highly shocked making them more susceptible to Pb-loss. These observations suggest that the majority of zircons crystallized before 4.1Ga and that KREEP magmatism had predominantly ceased by this time. We also observed that trace element analyses are easily affected by contributions from inclusions (typically injected impact melt) within SIMS analyses spots. After filtering for these effects, rare-earth element (REE) abundances of pristine zircon are consistent with one pattern characterized by a negative Eu anomaly and no positive Ce anomaly, implying that the zircons formed in a reducing environment. This inference is consistent with crystallization temperatures based on measured Ti concentrations and new estimates of oxide activities which imply temperatures ranging between 958±57 and 1321±100°C, suggesting that zircon parent magmas were anhydrous. Together, the lunar zircon ages and trace elements are consistent with a ⩽300My duration of KREEP magmatism under anhydrous, reducing conditions. We also report two granular texture zircons that contain baddeleyite cores, which both yield 207Pb–206Pb ages of 4.33Ga. These grains are our best constraints on impact ages within our sample population, and suggest at least one large impact is contemporaneous with the most common time of magmatic zircon formation on the Moon’s crust visited by the Apollo missions.

A scanning ion imaging investigation into the micron-scale U-Pb systematics in a complex lunar zircon

The full U-Pb isotopic systematics in a complex lunar zircon ‘Pomegranate’ from lunar impact breccia 73235 have been investigated by the development of a novel Secondary Ion Mass Spectrometry (SIMS) scanning ion imaging (SII) technique. This technique offers at least a four-fold increase in analytical spatial resolution over traditional SIMS analyses in zircon. Results from this study confirm the hypothesis that the Pomegranate zircon crystallized at 4.302±0.013Ga and experienced an impact that formed, U-enriched zircon around primary zircon cores at 4.184±0.007Ga (2σ, all uncertainties). The increase in spatial resolution offered by this technique has facilitated targeting of primary zircon that was previously inaccessible to conventional spot analyses. This approach has yielded results indicating that individual grains with a diffusive distance of less than ~4μm have been reset to the young impact age, while individual grains with a diffusive distance larger than ~6μm have retained the old crystallization age. Assuming a broad range in cooling rate of 0.5–50°C/year, which has been observed in a suite of similar lunar breccias, a maximum localized temperature generated by the impact that reset small primary zircon and created new, high-U zircon is estimated to be between 1100 and 1280°C.

Constraints on formation and evolution of the lunar crust from feldspathic granulitic breccias NWA 3163 and 4881

Lunar granulitic meteorites provide new constraints on the composition and evolution of the lunar crust as they are potentially derived from outside the Apollo and Luna landing sites. Northwest Africa (NWA) 3163, the focus of this study, and its paired stones NWA 4881 and NWA 4483, are shocked granulitic noritic anorthosites. They are petrographically and compositionally distinct from the Apollo granulites and noritic anorthosites. Northwest Africa 3163 is REE-depleted by an order of magnitude compared to Apollo granulites and is one of the most trace element depleted lunar samples studied to date. New in-situ mineral compositional data and Rb–Sr, Ar–Ar isotopic systematics are used to evaluate the petrogenetic history of NWA 3163 (and its paired stones) within the context of early lunar evolution and the bulk composition of the lunar highlands crust. The NWA 3163 protolith was the likely product of reworked lunar crust with a previous history of heavy REE depletion. The bulk feldspathic and pyroxene-rich fragments have 87Sr/86Sr that are indistinguishable and average 0.699282±0.000007(2σ). A calculated source model Sr TRD age of 4.340±0.057Ga is consistent with (1) the recently determined young FAS (Ferroan Anorthosite) age of 4.360±0.003Ga for FAS 60025, (2) 142Nd model ages for the closure of the Sm–Nd system for the mantle source reservoirs of the Apollo mare basalts (4.355–4.314Ga) and (3) a prominent age peak in the Apollo lunar zircon record (c. 4.345Ga). These ages are ∼100Myr younger than predicted timescales for complete LMO crystallization (∼10Myrs after Moon formation, Elkins-Tanton et al., 2011). This supports a later, major event during lunar evolution associated with crustal reworking due to magma ocean cumulate overturn, serial magmatism, or a large impact event leading to localized or global crustal melting and/or exhumation. The Ar–Ar isotopic systematics on aliquots of paired stone NWA 4881 are consistent with an impact event at ⩾3.5Ga. This is inferred to record the event that induced granularization of NWA 3163 (and paired rocks). A later event is also recorded at ∼2Ga by Ar–Ar isotopes is consistent with an increase in the number of impacts on the lunar surface at this time (Fernandes et al., 2013). Northwest Africa 3163 and its paired stones therefore record a c. 2.4Gyr record of lunar crustal production, metamorphism, brecciation, impacts and eventual ejection from the lunar surface.

A terrestrial perspective on usingex situshocked zircons to date lunar impacts

Deformed lunar zircons yielding U-Pb ages from 4333 Ma to 1407 Ma have been interpreted as dating discrete impacts on the Moon. However, the cause of age resetting in lunar zircons is equivocal; as ex situ grains in breccias, they lack lithologic context and most do not contain microstructures diagnostic of shock that are found in terrestrial zircons. Detrital shocked zircons provide a terrestrial analog to ex situ lunar grains, for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons. Electron backscatter diffraction and sensitive high-resolution ion microprobe U-Pb analysis of zircons eroded from the ca. 2020 Ma Vredefort impact structure (South Africa) show that complete impact-age resetting did not occur in microstructural domains characterized by microtwins, planar fractures, and low-angle boundaries, which record ages from 2890 Ma to 2645 Ma. An impact age of 1975 ± 39 Ma was detected in neoblasts within a granular zircon that also contains shock microtwins, which link neoblast formation to the impact. However, we show that granular texture can form during regional metamorphism, and thus is not unique to impact environments. These results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance, and then targeting specific age-reset microstructures. With the recognition that zircon can deform plastically in both impact and magmatic environments, age-resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts. Identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon-based impact chronologies for the Moon, Earth, or other planetary bodies.

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.

A 4.2 billion year old impact basin on the Moon: U–Pb dating of zirconolite and apatite in lunar melt rock 67955

A sharp rise in the flux of asteroid-size bodies traversing the inner Solar System at 3.9 Ga has become a central tenet of recent models describing planetary dynamics and the potential habitability of early terrestrial environments. The prevalence of ∼3.9 Ga crystallization ages for lunar impact-melt breccias and U–Pb isotopic compositions of lunar crustal rocks provide the primary evidence for a short-lived, cataclysmic episode of late heavy bombardment at that time. Here we report U–Pb isotopic compositions of zirconolite and apatite in coarse-grained lunar melt rock 67955, measured by ion microprobe, that date a basin-scale impact melting event on the Moon at 4.22±0.01 Ga followed by entrainment within lower grade ejecta from a younger basin approximately 300 million yr later. Significant impacts prior to 3.9 Ga are also recorded by lunar zircons although the magnitudes of those events are difficult to establish. Other isotopic evidence such as 40Ar–39Ar ages of granulitic lunar breccias, regolith fragments, and clasts extracted from fragmental breccias, and Re–Os isotopic compositions of lunar metal is also suggestive of impact-related thermal events in the lunar crust during the period 4.1–4.3 Ga. We conclude that numerous large impactors hit the Moon prior to the canonical 3.9 Ga cataclysm, that some of those pre-cataclysm impacts were similar in size to the younger lunar basins, and that the oldest preserved lunar basins are likely to be significantly older than 3.9 Ga. This provides sample-based support for dynamical models capable of producing older basins on the Moon and discrete populations of impactors. An extended period of basin formation implies a less intense cataclysm at 3.9 Ga, and therefore a better opportunity for preservation of early habitable niches and Hadean crust on the Earth. A diminished cataclysm at 3.9 Ga suggests that the similarity in the age of the oldest terrestrial continental crust with the canonical lunar cataclysm is likely to be coincidental with no genetic significance.

Correlated δ18O and [Ti] in lunar zircons: a terrestrial perspective for magma temperatures and water content on the Moon

Zircon grains were separated from lunar regolith and rocks returned from four Apollo landing sites, and analyzed in situ by secondary ion mass spectrometry. Many regolith zircons preserve magmatic δ18O and trace element compositions and, although out of petrologic context, represent a relatively unexplored resource for study of the Moon and possibly other bodies in the solar system. The combination of oxygen isotope ratios and [Ti] provides a unique geochemical signature that identifies zircons from the Moon. The oxygen isotope ratios of lunar zircons are remarkably constant and unexpectedly higher in δ18O (5.61 ± 0.07 ‰ VSMOW) than zircons from Earth’s oceanic crust (5.20 ± 0.03 ‰) even though mare basalt whole-rock samples are nearly the same in δ18O as oceanic basalts on Earth (~5.6 ‰). Thus, the average fractionation of oxygen isotopes between primitive basalt and zircon is smaller on the Moon [Δ18O(WR-Zrc) = 0.08 ± 0.09 ‰] than on Earth (0.37 ± 0.04 ‰). The smaller fractionations on the Moon suggest higher temperatures of zircon crystallization in lunar magmas and are consistent with higher [Ti] in lunar zircons. Phase equilibria estimates also indicate high temperatures for lunar magmas, but not specifically for evolved zircon-forming melts. If the solidus temperature of a given magma is a function of its water content, then so is the crystallization temperature of any zircon forming in that melt. The systematic nature of O and Ti data for lunar zircons suggests a model based on the following observations. Many of the analyzed lunar zircons are likely from K, rare earth elements, P (KREEP)-Zr-rich magmas. Zircon does not saturate in normal mafic magmas; igneous zircons in mafic rocks are typically late and formed in the last most evolved portion of melts. Even if initial bulk water content is moderately low, the late zircon-forming melt can concentrate water locally. In general, water lowers crystallization temperatures, in which case late igneous zircon can form at significantly lower temperatures than the solidus inferred for a bulk-rock composition. Although lunar basalts could readily lose H2 to space during eruption, lowering water fugacity; the morphology, large size, and presence in plutonic rocks suggest that many zircons crystallized at depths that retarded degassing. In this case, the crystallization temperatures of zircons are a sensitive monitor of the water content of the parental magma as well as the evolved zircon-forming melt. If the smaller Δ18O(zircon–mare basalt) values reported here are characteristic of the Moon, then that would suggest that even highly evolved zircon-forming magmas on the Moon crystallized at higher temperature than similar magmas on Earth and that magmas, though not necessarily water-free, were generally drier on the Moon.

Interpreting U–Pb data from primary and secondary features in lunar zircon

In this paper, we describe primary and secondary microstructures and textural characteristics found in lunar zircon and discuss the relationships between these features and the zircon U–Pb isotopic systems and the significance of these features for understanding lunar processes. Lunar zircons can be classified according to: (i) textural relationships between zircon and surrounding minerals in the host breccias, (ii) the internal microstructures of the zircon grains as identified by optical microscopy, cathodoluminescence (CL) imaging and electron backscattered diffraction (EBSD) mapping and (iii) results of in situ ion microprobe analyses of the Th–U–Pb isotopic systems. Primary zircon can occur as part of a cogenetic mineral assemblage (lithic clast) or as an individual mineral clast and is unzoned, or has sector and/or oscillatory zoning. The age of primary zircon is obtained when multiple ion microprobe analyses across the polished surface of the grain give reproducible and essentially concordant data. A secondary set of microstructures, superimposed on primary zircon, include localised recrystallised domains, localised amorphous domains, crystal–plastic deformation, planar deformation features and fractures, and are associated with impact processes. The first two secondary microstructures often yield internally consistent and close to concordant U–Pb ages that we interpret as dating impact events. Others secondary microstructures such as planar deformation features, crystal–plastic deformation and micro-fractures can provide channels for Pb diffusion and result in partial resetting of the U–Pb isotopic systems.

Lunar zirconology

Compared with zircons found in a variety of rocks on Earth, lunar zircons have received relatively little attention over the last four decades since samples were delivered by the Apollo missions. Nevertheless, the comparatively small number of studies carried out following these missions and in particular made over the last five years has demonstrated enormous potential of zircon research for understanding of lunar magmatism, impact history and provenance of lunar breccias. These studies have identified zircon age patterns that shed new light on the history of the Moon and raise new questions related to our understanding of lunar evolution. In particular, (i) the youngest limit for Lunar Magma Ocean (LMO) crystallisation was determined as 4417 ± 6 Ma; (ii) several periods of intensified magmatic activity at about 4.34, 4.20 and 4.00 Ga were identified in the post-LMO history of the Moon; and (iii) several pre-3.9 Ga impacts have been identified on the Moon. Here we discuss some of these results, questions and new directions, and propose an approach for further lunar zircon research.