Lunar Magma Ocean Research Articles

Revisiting the formation of lunar anorthosites via the Rb[sbnd]Sr isotope systematics

The incorporation of KREEP (potassium, rare-earth element, and phosphorus), mantle-derived mafic melts and trapped liquid into the lunar ferroan anorthosite (FAN) suite plays a pivotal role in generating their geochemical and isotopic variations. Nonetheless, the specific involvement and distinct roles of these different components remain controversial. This study presents in-situ Sr isotopic data for 28 anorthositic clasts found within lunar feldspathic meteorites to trace their sources and post-modification processes. We find that these plagioclases exhibit substantial variations in their measured 87Sr/86Sr values (0.69978–0.70357), in contrast to the relatively narrow range observed in Apollo 16 FANs, thereby likely reflecting a diverse chemical composition of lunar crustal rocks. In contrast, the analyzed plagioclases have consistently low 87Rb/86Sr ratios (0.00185–0.03962), similar to those of Apollo samples, reflecting impact-induced loss of Rb. Detailed investigations indicate that certain elevated 87Sr/86Sr ratios are probably not caused by terrestrial contamination or instrumental analysis, but most likely result from the decay of 87Rb from sources with initial 87Rb/86Sr higher than 0.0119–0.1380. However, such elevated 87Rb/86Sr values cannot solely result from crystallization of the lunar magma ocean (LMO) and likely involve KREEP components. Combined with trace element data, we estimate the maximum proportion of KREEP melt in the formation of lunar anorthosites. Future analyses of lunar anorthosites collected by China's Chang'e-5 and Chang'e-6 missions will be crucial for validating the observed Sr isotopic heterogeneity.

Chandrayaan-3 APXS elemental abundance measurements at lunar high latitude.

The elemental composition of the lunar surface provides insights into mechanisms of the formation and evolution of the Moon1,2. The chemical composition of lunar regolith have so far been precisely measured using the samples collected by the Apollo, Luna and Chang'e 5 missions, which are from equatorial to mid-latitude regions3,4; lunar meteorites, whose location of origin on the Moon is unknown5,6; and the in situ measurement from the Chang'e 3 and Chang'e 4 missions7-9, which are from the mid-latitude regions of the Moon. Here we report the first in situ measurements of the elemental abundances in the lunar southern high-latitude regions by the Alpha Particle X-ray Spectrometer (APXS) experiment10 aboard the Pragyan rover of India's Chandrayaan-3 mission. The 23 measurements in the vicinity of the Chandrayaan-3 landing site show that the local lunar terrain in this region is fairly uniform and primarily composed of ferroan anorthosite (FAN), a product of the lunar magma ocean (LMO) crystallization. However, observation of relatively higher magnesium abundance with respect to calcium in APXS measurements suggests the mixing of further mafic material. The compositional uniformity over a few tens of metres around the Chandrayaan-3 landing site provides an excellent ground truth for remote-sensing observations.

Early terrestrial and lunar anorthosites: Comparative geochemistry and evolutionary processes

In a paper in 1970, Brian Windley first recognised that early terrestrial and lunar anorthosites both have calcic plagioclase, and low TiO2and high CaO and Al2O3contents. Despite these similarities, the geochemistry of early terrestrial and lunar anorthosites has not been rigorously compared and contrasted. To this end, we compiled 425 analyses from 212 early terrestrial anorthosite occurrences and 306 analyses from 16 lunar anorthosite occurrences. This was supplemented by a compilation of plagioclase anorthite (An) contents and pyroxene Mg# from early terrestrial and lunar anorthosites. Early terrestrial anorthosites have lower whole-rock An contents but similar Mg# to lunar anorthosites. The CaO contents of lunar anorthosites are higher than those of early terrestrial anorthosites for a given MgO and Al2O3content, early terrestrial anorthosites have higher SiO2contents than lunar anorthosites at a given MgO content, and lunar anorthosites have higher Eu/Eu* anomaly ratios yet broadly similar La/Yb and Nd/Sm ratios than early terrestrial anorthosites. Some early terrestrial anorthosites have less fractionated chondrite-normalised rare earth element (REE) patterns and less prominent positive Eu anomalies than lunar anorthosites. Lunar anorthosites have higher plagioclase An contents, yet a similar range of pyroxene Mg# compared to their early terrestrial counterparts. Some early terrestrial anorthositesare more fractionated than some lunar anorthosites. Our interpretations imply that most early terrestrial anorthosites crystallised from basaltic parental magmas that were generated by high-degree partial melting of sub-arc asthenosphere mantle wedge sources that were hydrated by slab-derived fluids, with the remainder being associated with mid-ocean ridge and mantle plume settings. Some of the arc-related early terrestrial anorthosites were influenced by crustal contamination. In addition, early terrestrial anorthosites originated from partial melting of the mantle at various depths with variable garnet residua, whereas lunar anorthosites formed without any significant garnet residua. Lower plagioclase CaO contents and pyroxene Mg# in early terrestrial anorthosites can be explained by higher proportions of clinopyroxene and olivine fractionation in terrestrial magma chambers than in the lunar magma ocean where orthopyroxene and olivine fractionation occurred. Low TiO2contents in both terrestrial and lunar anorthosites reflect rutile and/or ilmenite fractionation.

Experimental Constraints on the Origin of the Lunar High‐Ti Basalts

AbstractHigh‐pressure and high‐temperature experiments were conducted to simulate melting of a hybrid cumulate lunar mantle. The experimental results show that intermediate to high‐Ti lunar pyroclastic glasses (>6 wt% TiO2) can be produced by partial melting of lunar cumulates. High‐Ti basalts are generated when the ilmenite/clinopyroxene ratios in the lunar mantle cumulates are between 1/1 and 4/1, depending on the degree of melting. The presence of an urKREEP component in the mantle cumulate strongly influences Al2O3/CaO of the melts. The experiments provide strong evidence for the model that the compositional diversity of lunar basalts is a consequence of a gravitational overturn of the lunar interior after the lunar magma ocean had solidified. Ilmenite/clinopyroxene in the cumulate mantle, which generates high‐Ti melts at partial melting, do not comprise the ratios in ilmenite‐bearing cumulates (IBC), which crystallized after ∼90% solidification of the lunar magma ocean and indicate local accumulation of ilmenite in the overturned lunar mantle. However, to fully match the natural composition of the most primitive lunar samples, secondary processes such as assimilation are still required.

Sound velocities in lunar mantle aggregates at simultaneous high pressures and temperatures: Implications for the presence of garnet in the deep lunar interior

Recent experimental and theoretical studies on lunar magma ocean crystallisation have suggested the presence of significant proportions of garnet in the deep lunar interior. While phase relation studies indicate a deep lunar mantle consisting of olivine, pyroxene, and garnet, the compatibility of such an assemblage with seismic models of the lunar interior is yet untested. In this study we report compressional and shear wave velocities in an iron-rich assemblage consisting of olivine, orthopyroxene, clinopyroxene, and garnet up to ∼8 GPa and 1300 K, by means of ultrasonic interferometry measurements combined with synchrotron techniques using the multi-anvil press apparatus. Sound velocity and density models of lunar mantle rocks along a selenotherm based on our experimental results find good agreement with the seismic and density profiles at lunar interior depths of 740–1260 km. Further models are constructed, allowing for the variation of chemical composition, phase proportion, and temperature; these suggest that a garnet-rich deep lunar mantle is compatible with present-day lower lunar mantle temperatures of between 1400–1800 K. Our results show that lunar mantle rocks with up to 33 wt.% garnet may provide an explanation for the observed high velocities of the lower lunar mantle. The presence of garnet in the lowermost part of the Moon's mantle has significant implications for the depth and temperature of the Moon's magma ocean as well as the composition, structure and internal dynamics of the solid Moon.

Large impacts and their contribution to the water budget of the Early Moon

The Earth/Moon system likely results from a giant impact between a Mars-size object and the proto-Earth. This high-energy impact leads to extreme conditions under which volatile elements would not normally be preserved in the protolunar disk. However, recent measurements of lunar samples highlight the presence of a non-negligible amount of water in the Moon’s interior (from 1.2 to 74 ppm). The aim of the present work is to quantify the water contribution of the late accretion on the early Moon. Here, we use a 2D axisymmetric model with the hydrocode iSALE-Dellen to study the fate of a large impactor on a target body similar to the early Moon with a crust, a magma ocean, and a mantle. For this purpose, we compute different models to monitor the depth to which the impacted material is buried at the end of the impact event and the degree of devolatilisation of the impactor. Three parameters are explored: the crustal thickness (ranging from 10 to 80 km), the impactor radius (ranging from 25 to 200 km) and the impactor velocity (ranging from 1 to 4 times the target escape velocity). Our models show that impactors with a radius greater than 25 km impacting a partially molten lunar body with a crust thinner than 40 km could significantly contribute to the water content of the lunar mantle even for impact velocities close to the lunar escape velocity. For impact velocities greater than 3 times the target escape velocity, the impactor material is significantly molten and its water content is devolatilised within the lunar atmosphere. Depending on the water content of the impactor material and the ability of the lunar magma ocean to maintain chemical heterogeneities, the late lunar accretion following the Moon-forming giant impact could explain the differences in water content among the lunar samples.

Non-Henrian behavior of hydrogen in plagioclase – basaltic melt partitioning

Plagioclase crystals in lunar anorthosite may have formed directly from the lunar magma ocean (LMO), with their elemental and isotopic compositions providing unique probes of LMO composition, age, and evolution. Low hydrogen concentrations have been reported in lunar plagioclase, but translating this to estimates of LMO hydrogen contents is complicated by incomplete knowledge of hydrogen partitioning between plagioclase and melt at low hydrogen contents. We conducted a series of high-temperature experiments to better quantify the plagioclase-melt partition coefficient of hydrogen in the Moon at low melt H abundances. Hydrogen contents of plagioclase and coexisting melt, reported in terms of water equivalent concentrations, were analyzed using Fourier Transform Infrared Spectroscopy. Resulting plagioclase-melt partition coefficients of water, Dplag-melt water, at melt water contents below 300 ppm, are 0.10 ± 0.05 to 0.36 ± 0.19. the highest Dplag-melt water reported to date. Our results, in conjunction with literature data, indicate a strong dependence of Dplag-melt water on water concentration in the melt (x in ppm): Dplag-melt water=20±5·x−0.88±0.03. This strong non-Henrian behavior is likely related to changes in the hydrogen activity in basaltic melt due to variations in H2O/OH speciation as a function of melt hydrogen content, implying that hydrogen partitioning in other mineral-melt systems may also be non-Henrian. At the low water contents in the lunar interior, Dplag-melt water is approximately 2 orders of magnitude higher than previously assumed. If hydrogen contents measured in lunar plagioclase crystals reflect pristine plagioclase-LMO equilibrium without subsequent modification, we conclude that <14 ppm H2O equivalent would have been present in the Moon when 95% of the initial LMO had crystallized.

Trace element partitioning in the lunar magma ocean: an experimental study

Modeling the behavior of trace elements during lunar magma ocean solidification is important to further our understanding of the chemical evolution of the Moon. Lunar magma ocean evolution models rely on consistent datasets on how trace elements partition between a lunar silicate melt and coexisting minerals at different pressures, temperatures, and redox conditions. Here we report new experimental trace element partition coefficients (D) between clinopyroxene (cpx), pigeonite, orthopyroxene, plagioclase, olivine (ol), and silicate melt at conditions relevant for the lunar magma ocean. The data include Dcpx−melt at ambient and high pressures (1.5 GPa and 1310 °C), and partition coefficients at ambient pressure for pig, opx, ol, and pl. Overall, clinopyroxene is a phase that may control the fractionation of key geochemical trace element ratios, such as Lu/Hf and Sm/Nd, during the evolution of the lunar magma ocean. We explore the impact of the new silicate Dmineral−melt on the trace element evolution of the lunar magma ocean and we find that accessory phosphate minerals, such as apatite or whitlockite are of critical importance to explain the observed trace element and isotopic signature of the KREEP reservoir on the Moon. The new partition coefficients were applied to calculate the trace element evolution of the residual melts of the crystallizing lunar magma ocean and we propose a new trace element composition for the urKREEP reservoir. The new data will be useful for future thermo-chemical models in order to adequately predict the duration of the lunar magma ocean and the age of the Moon.

Thorium anomaly on the lunar surface and its indicative meaning

The Moon has been divided into three terranes: Procellarum KREEP Terrane (PKT), Feldspathic Highland Terrane (FHT), and South Pole-Aitken Terrane (SPAT), using globally measured Th and FeO. Many lunar evolution models have predicted that a lunar magma ocean will produce a residual layer enriched in incompatible elements such as K, REE, and P (i.e., KREEP) in the late age of crystallization; and that the distribution of thorium can be used as a proxy for determining the global distribution of KREEP. The thorium distribution in these three terranes is inhomogeneous. The highest concentration of thorium is in PKT, the medium concentration of thorium is in SPAT, and almost none in FHT. Then what is the specific distribution in each of the terrane and what enlightenment can it tell us? Here we present and describe the detailed thorium distribution in PKT, SPAT, and FHT and provide some information for the origin of asymmetries on the lunar surface.

Small‐Scale Overturn of High‐Ti Cumulates Promoted by the Long Lifetime of the Lunar Magma Ocean

AbstractThe fractional crystallization of the lunar magma ocean (LMO) results in a gravitationally unstable layering, with dense Fe‐ and Ti‐oxides overlying lighter mafic cumulates. Due to their high density, these are prone to overturn via Rayleigh‐Taylor instability. However, this instability competes with the downward growth of the cold and stiff stagnant lid, which tends to trap the cumulates preventing their overturn. The fate of high‐Ti cumulates (HTC) plays a major role in several aspects of the Moon's history, including mare volcanism, early magnetism, and the presence of a partially molten layer at the core‐mantle boundary (CMB). To assess the extent of the overturn of HTC, we use 2D simulations of thermo‐chemical mantle convection in the presence of a solidifying LMO. The long lifetime of the magma ocean caused by the insulating effect of the plagioclase crust delays the growth of the stagnant lid and promotes the onset of solid‐state convection during magma ocean solidification. Both phenomena favor a high degree of mobilization of the HTC. Independent of the rheology of the mafic and HTC, the overturn is always characterized by small‐scale instabilities and is completed within ∼300–600 Myr. High‐Ti material accumulates at the CMB where it undergoes partial melting until present‐day, in agreement with the existence of a deep, partially molten layer inferred from geodetic data. Part of the overturned cumulates is entrained by mantle flow and can participate in secondary melting until ∼1 Ga, in agreement with the age of high‐Ti mare basalts.

Sulfide compositions of young Chang’e-5 basalts and implications for sulfur isotopes in lunar basalt sources

Sulfides are accessory phases in lunar rocks butare important forunderstanding lunar interior processes as well as impacts on the lunar surface.Whether or not the lunar mantle had achieved sulfide saturation during magma ocean evolution and displays homogeneous sulfur isotopes remains under debate. TheChang’e-5 (CE-5) mission returned young (2.0 Ga) basalts from a mare terrain in thenorthern Oceanus Procellarum.Here we study chemical and sulfur isotopic compositions (δ34SV-CDT) of sulfides from CE-5 basaltic fragments and combine them with δ34S of other young (3.1–3.0 Ga) lunar low-Ti basalt (NWA 10597 and NWA 4734) and gabbro meteorites (NWA 6950) to compare them with Apollo low-Ti and high-Ti mare basalts. The sulfides in basaltic fragments of CE-5 are troilites (FeS) with lowabundances ofNi, Co, and Cu(e.g., Ni < 0.04 wt% and Ni/Co < 0.3). Textures and chemical compositions indicate that most troilites arelate-stage crystallization products fromthe highly evolvedCE-5 basalts. Several troilites occur in the matrices of impactite clasts and are intergrown with Fe–Ni metal(12–36 wt% Ni, Ni/Co of 12–39). These troilites are distinct from the major population of troilites withnoticeably higher Ni abundances (mostly > 0.2 wt% with Ni/Co of 1–3) andreconcile with the addition of meteoritic materials into the impact melts.The δ34SV-CDTof large troilite grains (>10 μm) from the CE-5 basaltic fragments and lunar meteorites were obtained by high-precision, high-spatial-resolution femtosecond laser ablation MC-ICP-MS which achieved external uncertainty (0.65 ‰, 2SD at 8-μm laser spots) like nano-SIMS. Sulfur degassing during surficial effusive lava flow likely led to a slightdecrease inδ34S (by ∼ 1 ‰) for some basaltic fragments; however, such effects were limited to the scale of bulk rock samples, consistent with previous results. The mean δ34Sof troilites in CE-5 basaltic fragments (0.35 ± 0.25 ‰, 2SE, n = 45) is similar to those of ancient (3.8–3.1 Ga old) Apollo low-Ti and high-Ti mare basalts and the young gabbro cumulate NWA 6950 (0.56 ± 0.21 ‰, 2SE, n = 10). The paired NWA 10597 and NWA 4734 show consistent δ34S, lower than most values by ∼ 0.5 ‰. Current data thus indicate that most mantle sources of lunar basalts would be homogeneous for δ34S(0.6 ± 0.3 ‰) and minor regions may be different. The overall homogenous δ34S from different mantle sources with variably low sulfur contents supports sulfide-undersaturated accumulation of the lunar magma ocean, which was inherited from strong volatile loss and evaporative fractionation during the formation of the Moon.

Detection of apatite in ferroan anorthosite indicative of a volatile-rich early lunar crust

Determination of the systematics of volatile elements (for example, H, Cl, S) of the early Moon is one of the main objectives of lunar science. This has been hindered by the lack of the main volatile-bearing mineral, apatite, in ferroan anorthosites (FANs), which are thought to represent the primary products of the lunar magma ocean and the earliest lunar crust. Due to the absence of apatite, plagioclase and bulk samples of the FAN suite have been previously utilized for the studies of volatiles in samples representing the earliest-formed lunar crust. Here we provide evidence of apatite in a FAN clast in the lunar meteorite Arabian Peninsula 007. We report that Arabian Peninsula 007 has an ancient age, comparable to those of Apollo FAN samples, with lighter hydrogen (δD = −45‰) and heavier chlorine (δ37Cl = +44‰) isotopic compositions than FAN bulk and plagioclase data. These results suggest that the early lunar crust was significantly more volatile rich than previously thought.

Fractionation of iron and titanium isotopes by ilmenite and the isotopic compositions of lunar magma ocean cumulates

Basaltic volcanism on the Moon produced low- and high-Ti mare basalt suites that are also distinct with respect to their iron, titanium, and magnesium isotopic compositions. Here, the equilibrium fractionation of Fe and Ti isotopes between ilmenite and melt was experimentally investigated in order to evaluate the role of ilmenite in generating the isotopic compositional variability among the lunar mare basalts. Ilmenite crystallization experiments were conducted using two bulk compositions: an ilmenite-saturated basaltic andesite and an ilmenite-saturated Apollo 14 black glass, and the Fe and Ti isotopic compositions of the experimental ilmenites and glass (quenched melt) were analyzed using solution MC-ICPMS after hand-picking. Additionally, Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements on synthetic ilmenite were conducted and compared to previous NRIXS measurements on synthetic lunar glasses in order to derive temperature-dependent equilibrium ilmenite-melt Fe isotopic fractionations. Experimentally determined ilmenite-melt fractionations were then incorporated into a lunar magma ocean crystallization model that tracks the major element and isotopic compositional evolution of lunar magma ocean cumulates and residual liquid. There is good agreement between the Fe equilibrium isotopic fractionation measured by NRIXS and the laboratory equilibration experiments, and we find that the isotopic fractionation is sensitive to ilmenite compositional differences (0 vs. 10% Fe3+). Further, the light Ti isotopic composition of ilmenite relative to the melt (Δ49Ti=ilmenite-melt−0.09±0.03‰ at 1100∘C) is consistent with the higher coordination of Ti in ilmenite relative to melts and results of previous studies. The modeled Ti isotopic compositions for lunar magma ocean cumulates display Ti isotopic variability sufficient to explain the low- and high-Ti mare basalt sources. However, the difference in Fe isotopic composition between the low- and high-Ti mare basalts cannot be attributed solely to ilmenite fractionation. Instead, Fe isotopic fractionation by additional products of lunar magma ocean crystallization, such as clinopyroxene, is required to generate the inferred Fe and Mg isotopic variability in the lunar mantle. Alternatively, the Fe and Mg isotopic compositions of the lunar mare basalts may indicate Fe-Mg interdiffusion has occurred in the Ti-rich component of the mare basalt source regions via reaction between ilmenite cumulates and the olivine- and pyroxene-rich lunar mantle.

Isotopic Geochronological Constraints on the Formation and Evolution of the Moon

One major task of studying the formation and evolution of the Moon is to construct a timeline of the important events with precise isotopic ages. Here, we review recent major isotopic geochronological progress in the past decade and the unsolved problems in isotopic geochronology. The Moon probably formed between 4.52 and 4.42 Ga. Recent high-precision whole-rock and mineral Sm-Nd isotopic dating results suggested that ferroan anorthosite and highlands magnesian suite rocks formed contemporarily around 4.37 to 4.33 Ga. Although the major mare basaltic volcanism took place from 3.85 to 2.93 Ga, new geochronological data from lunar meteorites and Chang’e-5 basalts suggested that lunar basaltic volcanism took place as old as up to 4.37 Ga and at least as young as 2.0 Ga, respectively. Impact events older than 3.9 Ga have also been revealed based on U-bearing minerals Pb/Pb ages and Ar-Ar ages and can provide important clues to understand the late heavy bombardment hypothesis. However, the reliable isotopic ages for the important events on the Moon are still far from conclusive, due to lack of pristine samples that directly crystallized from Lunar Magma Ocean and samples from impact melt sheets in large impact basins (e.g., the South Pole-Aitken basin). In the future, collection and return of pristine samples of ferroan anorthosite and highlands magnesian suite rocks from the farside, cryptomare basalts and late-stage basalts, quartz monzogabbros, granites/felsites, and rocks from impact melt sheets in large impact basins are required for better understanding the formation and evolution of the Moon.

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.

Fluorine abundance of the lunar magma ocean constrained by experimentally determined mineral-melt F partitioning

To quantify fluorine (F) evolution during lunar magma ocean (LMO) crystallization, high-pressure, high-temperature experiments have been conducted to determine mineral-melt partitioning of F for lunar minerals (plagioclase, orthopyroxene and ilmenite). Results constrain the F abundance in the magma ocean to 21–41 ppm at the time crust-forming plagioclase started crystallizing. Forward modeling shows that 352–703 ppm F would remain in the final 1 % of magma toward the end of magma ocean solidification. This range overlaps that inferred for the urKREEP reservoir (660 ppm). Taking into account model uncertainties, from the perspective of F abundances the urKREEP reservoir can be formed at 98.9–99.5% LMO solidification, with negligible loss of F from the Moon since the onset of crust formation. Backward modeling from initial crust-forming plagioclase, an initial LMO would contain 4.2–8.5 ppm F, which is consistent with estimates of the lunar primitive mantle F content derived from melt inclusions in Apollo samples. This finding is consistent with previous suggestions that the bulk silicate Moon is depleted in F relative to the bulk silicate Earth (which contains ∼25 ppm F). A BSE-like initial LMO would yield a magma containing 122 ppm F at the onset of crust formation, significantly higher than our calculated 21–41 ppm F. Fluorine depletion could have occurred by degassing during the early LMO stages (between the onset of LMO crystallization and first crust formation), and/or prior to the LMO stage (e.g., depletion during the giant impact or vapor drainage in the protolunar disk), but seems to have ended by the time the crust started forming.

Trace element partitioning between apatite and silicate melts: Effects of major element composition, temperature, and oxygen fugacity, and implications for the volatile element budget of the lunar magma ocean

Apatite, as an accessory phase in igneous and metamorphic rocks, has important petrological significance due to its capacity to accommodate appreciable amounts of many trace elements in its mineral structure. To better constrain trace element partitioning between apatite and silicate melts, we conducted experiments that produced apatites approaching fluorapatite (FlAp), hydroxylapatite (OHAp) and chlorapatite (ClAp) endmembers separately at 1050 and 1100 °C, 1 GPa pressure, under oxygen fugacity (fO2) about one log unit below iron-wüstite buffer to four log unit above fayalite-magnetite-quartz buffer. We report the results of 12 experiments which demonstrate that ClAp exhibits lower trace element partition coefficients compared with FlAp and OHAp, especially for Rare Earth Elements (REEs) under all run conditions explored, suggesting trace element partitioning is sensitive to anion site occupancy. Divalent cations are less sensitive to anion occupancy. Positive Eu partitioning anomalies (DEu/DEu*, where Eu is the chondrite normalized abundance and Eu* is the interpolated value from neighboring elements ordered by atomic number) are observed in ClAp experiments under relatively low fO2s, whereas negative Eu anomalies are exhibited by FlAp and OHAp under the same fO2 conditions. We infer that anionic occupancies have a direct impact on the substitution mechanisms of trace elements in apatite, thereby influencing their partition coefficients. Beyond the anions, correlations of apatite compositional components (XCa, XNa, XP and XSi) with partition coefficients suggest they exert crystal chemical controls on trace element partitioning. Based on these observations, we developed parameterized lattice strain models to predict the partitioning of divalent and trivalent elements as a function of temperature and apatite composition, and an fO2-dependent apatite-melt Eu partitioning model and oxybarometer. We further developed a Eu in apatite-plagioclase oxybarometer that enables us to calculate the fO2 of apatite and plagioclase-bearing magmatic and subsolidus systems, and evaluated the influence of subsolidus reequilibration on the new oxybarometer. Applied to one of our experiments, winonaite HaH193, and samples from Sept Iles layered intrusion, the oxybarometer recovers their anticipated fO2s, ranging from about two log units below the iron-wüstite buffer to the fayalite-magnetite-quartz buffer. Using the new REE and fO2-dependent Eu partitioning models, we constrained the petrogenesis of lunar KREEP basalt and estimated the relative volatile content in the late lunar magma ocean (LMO) cumulates. The model suggests a relative depletion of Cl in the LMO cumulates, consistent with Cl isotopic analyses and volatile abundance measurements in previous work, suggesting that differential loss of volatiles occurred before or during the late-stage evolution of the LMO.

Long-lived lunar volcanism sustained by precession-driven core-mantle friction.

Core-mantle friction induced by the precession of the Moon's spin axis is a strong heat source in the deep lunar mantle during the early phase of a satellite's evolution, but its influence on the long-term thermal evolution still remains poorly explored. Using a one-dimensional thermal evolution model, we show that core-mantle friction can sustain global-scale partial melting in the upper lunar mantle until ∼3.1 Ga, thus accounting for the intense volcanic activity on the Moon before ∼3.0 Ga. Besides, core-mantle friction tends to suppress the secular cooling of the lunar core and is unlikely to be an energy source for the long-lived lunar core dynamo. Our model also favours the transition of the Cassini state before the end of the lunar magma ocean phase (∼4.2 Ga), which implies a decreasing lunar obliquity over time after the solidification of the lunar magma ocean. Such a trend of lunar obliquity evolution may allow volcanically released water to be buried in the lunar regolith of the polar regions. As a consequence, local water ice could be more abundant than previously thought when considering only its accumulation caused by solar wind and comet spreading.

Moon’s high-energy giant-impact origin and differentiation timeline inferred from Ca and Mg stable isotopes

Mass-dependent stable isotopic variations recorded in lunar samples provide novel resolution to the formation and differentiation history of the Moon. In this study, we report new high-precision Ca-isotope measurements for lunar rocks and minerals. Ca-isotope data and modeling of the lunar magma ocean together demonstrate indistinguishable mass-dependent Ca isotopic compositions of the bulk silicate Earth and Moon. This implied Earth-Moon isotope equilibration is consistent with the Moon’s high-energy giant-impact (Synestia) origin and not readily compatible with the traditional giant-impact models. Moreover, a cross-comparison between Ca and Mg isotopic data for an important anorthosite sample (60025) consistently clarifies its formation near the completion of the lunar magma ocean crystallization. Therefore, the various existing radiometric dating for 60025 sets the lunar magma ocean to have fully solidified by either 4.51 or 4.38 billion years ago, constraining the two respective lunar differentiation timescales to <30 (short-lived) or ~130–150 (long-lived) million years.

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.