Lunar Crust Research Articles

Global Distribution and Geological Context of Co‐Existing Occurrences of Olivine‐Rich and Plagioclase‐Rich Materials on the Lunar Surface

AbstractWe examined the global distribution and geological context of lunar sites where olivine‐ and plagioclase‐rich materials co‐exist. These sites are areas showing plagioclase‐rich spectra adjacent to areas showing olivine‐rich spectra, and they extend over several hundreds of meters on the lunar surface. From an analysis of the high‐spatial‐resolution data obtained from the SELENE (Kaguya) multiband imager, we identified 14 co‐existing occurrences among 49 olivine‐rich sites located around large impact basins. We found that the co‐existing occurrences are limited to a few sites among the olivine‐rich sites in the Moscoviense, Crisium, and Imbrium basins, while six of seven olivine‐rich sites in the Schrödinger basin show co‐existing occurrence. The geological features between the co‐existing occurrences and the olivine‐rich sites without plagioclase‐rich materials showed no clear difference, but both types of site are found at fresh geological features located around the peak rings of impact basins. These results may suggest that the co‐existing occurrences are closely related to the material heterogeneity of the peak ring regions. The heterogeneity could be formed by mixing of the olivine‐rich mantle and the anorthosite‐rich crust during impact basin formation, or it might be a reflection of the compositional heterogeneity of the lunar lower crust. For future sample return missions, the detailed information on the co‐existing occurrences presented here might be useful for a sampling strategy to select a “one‐stop” site that provides important information for the overall characterization of lunar mantle and crustal materials.

Towards geochemical alternatives to geophysical models of the internal structure of the lunar mantle and core

Based on the Markov Chain Monte Carlo method, we present a reconciliation analysis of the three most important but not directly related sets of seismic, selenodetic, and geochemical data (Apollo seismic travel times, mean mass (M) and moment of inertia (MOI), tidal Love number (k2), quality factors, bulk silicate Moon (BSM) models) to determine the composition and internal structure of the four-layer mantle, deep-seated transition layer (LVZ), liquid outer and solid inner Fe-based core. A distinctive feature of this approach is the inclusion of geochemical parameters as “observed” values when calculating the likelihood function in combination with phase-equilibrium computations. We consider the effect of two bulk composition models with different refractory oxide contents (models E with terrestrial values of Al2O3 and models M with a higher Al2O3 content) and variations in temperature profiles on the geochemical and geophysical parameters of the mantle, LVZ, and core. The inversion results show that all successful E and M models are grouped around bulk FeO 11–13 wt% and Mg# 79–81, indicating a significant difference in the composition of the silicate Earth (BSE) and its satellite. A slight SiO2 enrichment is necessary for the pyroxenite upper mantle, where low-Ca orthopyroxene, not olivine, is the dominant mineral. The primordial lower mantle (=BSM) is enriched in Al compared to the overlying differentiated layers: ∼4.5% Al2O3 (1 × BSE) for models E and ∼6 wt% Al2O3 (1.5 × BSE) for models M. Comparison of measured high-pressure density and sound velocity data for liquid Fe(Ni)-C-S-Si alloys with inverted models constrains the relatively narrow range of physical parameters for the lunar core and favors the presence of Fe(Ni)-S liquid outer core with 3–10 wt% S. The results correspond to inverted P-wave velocities in the range of 3600–4100 m/s and densities in the range of 6200–7000 kg/m3 for the outer core with a radius of 300–350 km and are consistent with the speed of sound (but not density) and the radius of the lunar outer core by Weber et al. (2011).

The initial lunar mantle structure constrained by thermodynamic simulation

The initial lunar mantle structure constrained by thermodynamic simulation

Yutu-2 Radar Sounding Evidence of a Buried Crater at Chang’E-4 Landing Site

Buried craters within tens of meters of lunar regolith are rarely studied but are significant for understanding the evolution of surface processes on the Moon. Here, we first report the evidence of an intact buried crater within the layered strata at Chang’E-4 (CE-4) landing site revealed by the lunar penetrating radar (LPR). The time–frequency comparative analysis method based on the variational mode decomposition (VMD) and the rock quantitative analysis method based on the local unit correlation (LUC) are proposed and applied to the processing and analysis of LPR data within 15 lunar days. The results presented by the two methods provide evidence of a buried crater at the CE-4 landing site and simultaneously reveal the rock-concentrated structure within the buried crater. According to the results, it is considered that the filling materials within the buried crater have survived the impaction and gardening during the formation of the overlying fine-grained regolith. Recent works have proposed that the near-surface material at the CE-4 landing site is mainly the lunar mantle materials excavated from the nearby Finsen crater. Therefore, the buried crater probably preserves the initial lunar mantle materials.

Theoretical calculation of tidal Love numbers of the Moon with a new spectral element method

The tidal Love numbers of the Moon are a set of nondimensional parameters that describe the deformation responses of the Moon to the tidal forces of external celestial bodies. They play an important role in the theoretical calculation of the Moon’s tidal deformation and the inversion of its internal structure. In this study, we introduce the basic theory for the theoretical calculation of the tidal Love numbers and propose a new method of solving the tidal Love numbers: the spectral element method. Moreover, we explain the mathematical theory and advantages of this method. On the basis of this new method, using 10 published lunar internal structure reference models, the lunar surface and lunar internal tidal Love numbers were calculated, and the influence of different lunar models on the calculated Love numbers was analyzed. Results of the calculation showed that the difference in the second-degree lunar surface Love numbers among different lunar models was within 8.5%, the influence on the maximum vertical displacement on the lunar surface could reach ±8.5 mm, and the influence on the maximum gravity change could reach ±6 μGal. Regarding the influence on the Love numbers inside the Moon, different lunar models had a greater impact on the Love numbers<i> h</i>₂ and<i> l</i>₂ than on <i>k</i>₂ in the lower lunar mantle and core.

Chronological and compositional mapping of the Mare Orientale basin using Chandrayaan-1 – M3 and LRO datasets

The lunar multiring mare basins have different ages, compositions and are associated with the diverse unit of mare deposits, providing evidence for the origin and evolution of the lunar crust. The multiring basins offer the opportunity to assess many unknown questions that may reveal the lunar geologic standpoint. The Mare Orientale basin is one of the youngest impact multiring basins on the Moon, which covers about 930 km in diameter and coordinates centered at 200S 950W. In the present study, the compositional, topographical, and chronological studies have been done by using Moon Mineralogy Mapper (M3) from Chandrayaan-1 orbiter and Lunar Reconnaissance Orbiter Camera-Wide Angle Camera (LROC-WAC) image from Lunar Reconnaissance Orbiter (LRO). The M3 data used to map the Optical maturity (OMAT), FeO & TiO2 concentrations of the basin. The Standard Band Ratio (SBR) of the Orientale basin has been prepared to discriminate the different mare and highland lithologies. The 1 μm and 2 μm band depths have been mapped to demarcate the mafic minerals such as olivine and pyroxenes. The age of the Orientale basin mare units was mapped as Orientale event (3.72 Ga - Upper Imbrian), Orientale South West (3.7 Ga), Orientale West unit (3.37 Ga), Lacus Veris North unit- 1, 2, and 3 (3.1 Ga, 2.63 Ga, and 3.3 Ga), Lacus Veris East (2.9 Ga), Lacus Autumni - North (3.1 Ga), Middle (2.01 Ga), South (2.27 Ga) and Kopff crater (mare) (2.92 Ga) by using LROC-WAC data through Crater Size Frequency Distribution (CSFD) technique. Characterization of M3 reflectance spectra profiles was done to map the Orientale basin lithologies like olivine, pyroxene (low calcic and high calcic), plagioclase, ilmenite, Mg-spinel, and olivine – pyroxene mixtures that are present in the anorthositic, basaltic, noritic, and gabbroic rocks. It is revealed that the Orientale basin could have been formed either by global thermal evolution and basin mare volcanic evolution or pressure release melting and associated with secondary convection.

The gallium isotopic composition of the Moon

In this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The δ71Ga values of these samples range from -0.10 to +0.66‰ (where δ71Ga is the relative difference between the 71Ga/69Ga ratio of a sample and the Ga-IPGP standard), which is an order of magnitude more heterogeneous than δ71Ga values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk δ71Ga value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have δ71Ga values that are lower than high-Ti basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher δ71Ga values suggests that Ga isotopes are fractionated by processes that operate within the chemically evolving LMO, rather than localized degassing or volatile redistribution.Correlations between indices of plagioclase removal from the LMO (e.g. Eu/Eu*) with Ga isotope ratios suggest that a Δ71Gaplagioclase-melt of -0.3‰, (where Δ71Gaplagioclase-melt is the isotopic fractionation associated with crystallization of plagioclase from a melt), could drive the observed isotopic fractionation in high-Ti mare basalts and KREEP-rich rocks. This would be consistent with the observation that FAS rocks have δ71Ga values that are lower than mare basalts. However, the addition of KREEP-like material into the mare basalt source regions would not contribute enough Ga to perturb the isotopic composition outside of analytical uncertainty. Thus, basalts derived from early formed LMO cumulates such as those from Apollo 15, would preserve light Ga isotopic compositions despite containing modest amounts of urKREEP.We estimate that the δ71Ga value of the LMO was ∼0.14‰ prior to the onset of plagioclase crystallization and extraction. Whether this δ71Ga value is representative of the initial BSM cannot be ascertained from the current dataset. It remains plausible that the Moon accreted with a heavier Ga isotopic composition than the Earth. Alternatively, the Moon and Earth could have accreted with similar isotopic compositions (BSE = 0.00 ± 0.06‰, Kato et al., 2017) and volatile loss drove the LMO to higher δ71Ga values prior to formation of the lunar crust.

Employing magma ocean crystallization models to constrain structure and composition of the lunar interior

The process of lunar magma ocean solidification provides crucial constraints on the composition and extent of distinct chemical reservoirs in the lunar mantle that formed during the early evolution of the Moon. We use a combination of phase equilibria models consistent with recent experimental results on fractional crystallization of the lunar magma ocean to study the effect of bulk silicate Moon composition on the properties of lunar mantle reservoirs.We find that the densities and relative proportions of these mantle reservoirs, in particular of the late forming ilmenite bearing cumulates (IBC), strongly depend on the FeO content of the bulk silicate Moon. This relation has implications for post-magma ocean mantle dynamics and the resulting mass distribution in the lunar interior, because the IBC form at shallow depths but tend to sink towards the core mantle boundary due to their high density. We quantify the relations between bulk silicate Moon FeO content, IBC thickness and bulk Moon density as well as mantle stratigraphy and bulk silicate Moon moment of inertia in order to constrain the bulk silicate Moon FeO content and the efficiency of IBC sinking.In combination with seismic and selenodetic constraints on mantle stratigraphy, core radius and extent of the possibly IBC bearing low velocity zone at the core mantle boundary as well as considerations about the present day selenotherm and the effects of reservoir mixing by convection our model indicates that the bulk silicate Moon is only moderately enriched in FeO compared to the Earth's mantle and contains most likely about 9.4–10.9 wt% FeO (with a lowermost limit of 8.3 wt% and an uppermost limit of 11.9 wt%). We further conclude that the observed bulk silicate Moon moment of inertia requires that sinking of the IBC layer by mantle convection was incomplete: only ~20–60% of the IBC material might have reached the core mantle boundary, while the rest either remained at the depth of its formation right beneath the crust or was mixed into the middle mantle.

Chemical heterogeneities reveal early rapid cooling of Apollo Troctolite 76535

The evolution of the lunar interior is constrained by samples of the magnesian suite of rocks returned by the Apollo missions. Reconciling the paradoxical geochemical features of this suite constitutes a feasibility test of lunar differentiation models. Here we present the results of a microanalytical examination of the archetypal specimen, troctolite 76535, previously thought to have cooled slowly from a large magma body. We report a degree of intra-crystalline compositional heterogeneity (phosphorus in olivine and sodium in plagioclase) fundamentally inconsistent with prolonged residence at high temperature. Diffusion chronometry shows these heterogeneities could not have survived magmatic temperatures for >~20 My, i.e., far less than the previous estimated cooling duration of >100 My. Quantitative modeling provides a constraint on the thermal history of the lower lunar crust, and the textural evidence of dissolution and reprecipitation in olivine grains supports reactive melt infiltration as the mechanism by which the magnesian suite formed.

Geomorphology, Mineralogy, and Geochronology of Mare Basalts and Non-Mare Materials around the Lunar Crisium Basin

The Nectarian-aged Crisium basin exhibits an extremely thin crust and complicated lunar geological history. This large multi-ring impact basin is characterized by prolonged lunar volcanism ranging from the Imbrian age to the Eratosthenian period, forming the high-Ti mare unit, low-Ti mare basalts, and very low-Ti mare unit. We produced an updated geological map of the Crisium basin and defined four mare units (Im1: 3.74 Ga; Im2: 3.49 Ga; Im3: 3.56 Ga; EIm: 2.49 Ga) in terms of distinct composition and mineralogy. Olivine was widely determined in the Ti-rich Im1, implying the hybridization source in the lunar mantle with the occurrence of small-scale convective overturn. The major phase of low-Ti basaltic volcanism occurred c.a. 3.5 Ga, forming Im2 and Im3 in the western area. The youngest mare unit (EIm) has slight variations of pyroxene compositions, implying a decrease of calcic content of basaltic volcanisms with time. Later, distal material transports from large impact events in highlands could complicate the mixing of local mare basalts in the Copernicus age, especially the Im3 unit. The identified olivine-bearing outcrops and widely Mg-rich materials (Mg# &gt; 70, where Mg# = molar 100 × Mg/(Mg + Fe)) in the western highlands, assumed to be the occurrence of the Mg-suite candidates, require future lunar exploration missions to validate.

Lunar Megaregolith Structure Revealed by GRAIL Gravity Data.

We use gravity data from NASA's GRAIL mission to characterize the porosity structure of the upper lunar crust. We analyze the gravitational anomalies produced by the porosity of craters with diameters D between 10 and 30 km. We find that the gravitational signature of craters changes significantly at D=16.4−0.6+1.4 km, which is related to a discrete change in porosity at a depth ∼3–5 km. We propose that this discrete porosity change reveals the location of the boundary between large‐scale basin ejecta and the deeper less porous portion of the megaregolith, known as the structurally disturbed crust. The ejecta thickness can help constrain models of material transport and mixing on the Moon and, because the ejecta layer acts as an insulating blanket, models of heat flow and magmatism.

Physical Properties and Internal Structure of the Central Region of the Moon

One of the pivoting problems of the geochemistry and geophysics of the Moon is the structure of its central region, i.e., its core and adjacent transition layer located at the boundary between the solid mantle and liquid or partially molten core. The chemical composition of the mantle and the internal structure of the central region of the Moon were simulated based on the joint inversion of seismic, selenophysical, and geochemical parameters that are not directly interrelated. The solution of the inverse problem is based on the Bayesian approach and the use of the Markov chain Monte Carlo algorithm in combination with the method of Gibbs free energy minimization. The results show that the radius of the Moon’s central region is about 500–550 km. The thickness of the transition layer and the radii of the outer and inner cores relatively weakly depend on the composition models of the bulk silicate Moon with different contents of refractory oxides. The silicate portion of the Moon is enriched in FeO (12–13 wt %, FeO ~ 1.5 × BSE) and depleted in MgO (Mg# 79–81) relative to the bulk composition of the silicate Earth (BSE), which is in conflict with the possibility of the formation of the Moon from the Earth’s primitive mantle and does not find an adequate explanation in the current canonical and non-canonical models of the origin of the Moon. SiO2 concentrations in all zones of the lunar mantle vary insignificantly and amount to 52–53 wt %, and the predominant mineral of the upper mantle is low-Ca orthopyroxene but not olivine. With respect to Al2O3, the lunar mantle is stratified, with a Al2O3 content higher in the lower mantle than in all overlying shells. The partially molten transition layer surrounding the core is about 200–250 km thick. The radii of the solid inner core are within 50–250 km, and the most probable radii of the liquid outer core are ~300–350 km. The physical characteristics of the lunar core are compared with experimental measurements of the density and speed of sound of liquid Fe(Ni)–S–C–Si alloys. If the seismic model of the liquid outer core with VP = 4100 ± 200 m/s (Weber et al., 2011) is reasonably reliable, then this uncertainty range is in the best agreement with the VP values of 3900–4100 m/s of liquid Fe(Ni)–S alloys, with sulfur content up to ~10 wt % and a density of 6200–7000 kg/m3, as well as with the inverted values of density and velocity of the outer core. The VP values of liquid Fe–Ni–C and Fe–N–Si alloys at 5 GPa exceed seismic estimates of the speed of sound of the outer lunar core, which indicates that carbon and silicon can hardly be dominant light elements of the lunar core. The inner Fe(Ni) core (possibly with an insignificant content of light elements: sulfur and carbon) is presumably solid and has a density of 7500–7700 kg/m3. The difference in density between the inner and outer cores Δρ ~ 500–1000 kg/m3 can be explained by the difference in their composition.

Composition of the lunar mantle for lower mantle high-velocity seismic model

We investigated models of the internal structure of initially homogeneous Moon differentiated as a result of partial melting, using data on seismic velocities according to the seismic models assume the zonal structure of the lunar mantle is a model of the Moon which was obtained with using the array processing methods of high velocities in the lower mantle. As a result of inversion of gravity (mass, moment of inertia), seismic (P- and S-waves velocities) and petrological (balance ratios) data, the Monte Carlo method was used to reconstruct the chemical composition and internal structure of the Moon. The phase composition and physical properties of the mantle were obtained with Gibbs free energy minimization method and equations of state in the five-component system CaO-FeO-MgO-Al2O3-SiO2. For all models, possible values of seismic velocities and concentrations of the main oxides in three zones of the mantle were obtained, satisfying the geochemical and geophysical constraints and the possible sizes of the Fe-10%S core were determined. It was found that the lunar mantle chemical composition (concentration of FeO, Al2O3 and CaO) differs depending on the mantle zone. Constraints on the values of seismic velocities in the lower mantle and the most probable size of the lunar core were determined: VP ≤ 8.45 km/s; Fe-10%S core radius is ∼360 km.

Viscous Dissipation in the Fluid Core of the Moon

AbstractThe spin axes of the mantle, fluid core, and solid inner core of the Moon precess at frequency Ωp = 2π/18.6 years−1 though with different orientations, leading to viscous friction at the core‐mantle boundary (CMB) and inner core boundary (ICB). Here, we use a rotational model of the Moon and consider a range of inner core and outer core radii to investigate the relative importance of viscous dissipation at the CMB and ICB, and to show how this dissipation is connected to the phase lead angle (ϕp) of the mantle ahead of its Cassini state. We show that when the inner core radius is km and the free inner core nutation frequency Ωficn approaches Ωp, viscous dissipation at the ICB can be comparable to that at the CMB, and in the most extreme cases exceed it by as much as a factor of 10. If so, the viscous dissipation in the lunar core projected back in time depends on how Ωficn has evolved relative to Ωp. We further show that constraints on the CMB and ICB radii of the lunar core can in principle be extracted by matching the observed phase lead of ϕp = 0.27 arcsec; this requires an improved estimate of tidal dissipation and an accurate model of the turbulent viscous torque. Finally, when our rotational model is constrained to match ϕp = 0.27 arcsec, our results suggest that the viscous dissipation at the ICB is likely insufficient to have ever been above the threshold to power a thermally driven dynamo.

The Global Conductivity Structure of the Lunar Upper and Midmantle

AbstractMagnetic sounding is a powerful tool to explore the interior of planetary bodies through the electrical conductivity structure. The electrical conductivity structure of the lunar mantle has previously been derived from surface magnetic field measurements as part of the Apollo 12 mission and concurrent magnetometer data acquired from orbit through the Explorer 35 satellite. Here, we derive the first global conductivity structure of the upper and midmantle using only satellite magnetometer data collected by the recent Lunar Prospector and Kaguya Selene satellite missions. We show that the field in the geomagnetic tail exhibits a simple geometrical structure and can be well described by a single spherical harmonic of degree and order one. Employing this information about the inducing field geometry and assuming a potential representation of the field in the geomagnetic tail, we derive a frequency‐dependent transfer function and invert it for a one‐dimensional (1‐D) electrical conductivity profile of the lunar upper and midmantle. Our global transfer function shows striking similarity with the local one obtained from joint analysis of Apollo 12 and Explorer 35 magnetometer data. This indicates the lack of local variations at the Apollo 12 landing site compared to the globally averaged upper to midmantle electrical conductivity structure.

Determining the Effect of Varying Magmatic Volatile Content on Lunar Magma Ascent Dynamics

AbstractThe Moon is not volcanically active at present, therefore, we rely on data from lunar samples, remote sensing, and numerical modeling to understand past lunar volcanism. The role of different volatile species in propelling lunar magma ascent and eruption remains unclear. We adapt a terrestrial magma ascent model for lunar magma ascent, considering different compositions of picritic magmas and various abundances of H2, H2O, and CO (measured and estimated) for these magmas. We also conduct a sensitivity analysis to investigate the relationship between selected input parameters (pre‐eruptive pressure, temperature, conduit radius, and volatile content) and given outputs (exit gas volume fraction, velocity, pressure, and mass eruption rate). We find that, for the model simulations containing H2O and CO, CO was more significant than H2O in driving lunar magma ascent, for the range of volatile contents considered here. For the simulations containing H2 and CO, H2 had a similar or slightly greater control than CO on magma ascent dynamics. Our results showed that initial H2 and CO content has a strong control on exit velocity and pressure, two factors that strongly influence the formation of an eruption plume, pyroclast ejection, and overall deposit morphology. Our results highlight the importance of (a) quantifying and determining the origin of CO, and (b) understanding the abundance of different H‐species present within the lunar mantle. Quantifying the role of volatiles in driving lunar volcanism provides an important link between the interior volatile content of the Moon and the formation of volcanic deposits on the lunar surface.

Thermal metamorphism on the Moon as recorded by the granulite suite

Thermally metamorphosed rocks on the Moon are an important, yet under-studied, suite of lithologies that have been identified within the Apollo and lunar meteorite collections. These rocks, with granoblastic and poikilitic textures, are generally referred to as granulites. However, unlike their terrestrial counterparts, which are the metamorphic products of both high temperatures and high pressures, lunar granulites are thought to be the products of only high-temperature (&gt;1000°C) thermal metamorphism, which completely recrystallized their protoliths. We summarize the range of textures and chemical systematics reported from lunar granulites. These data enable constraints to be placed on the thermal conditions in the lunar crust required for high-temperature metamorphism to have taken place. Most studies indicate that impact melt sheets have the relevant thermal properties to sustain high temperatures over the timescales required to fully recrystallize the surrounding crustal lithologies. However, the roles of alternative heat sources, such as magmatic intrusions into the crust, have not been extensively investigated and, as such, cannot be ruled out. In addition, the chemical data yield important insights into the protoliths of the granulite suite. By identifying the protoliths, we greatly enhance our understanding of the range of lithologies that make up the primary lunar crust. In turn, this enables crustal formation models to be better constrained. Supplementary material: All the geochemical data discussed within this review are available at: https://doi.org/10.6084/m9.figshare.c.5623326

Patched Local Lunar Gravity Solutions Using GRAIL Data.

We present a method to determine local gravity fields for the Moon using Gravity Recovery and Interior Laboratory (GRAIL) data. We express gravity as gridded gravity anomalies on a sphere, and we estimate adjustments to a background global start model expressed in spherical harmonics. We processed GRAIL Ka‐band range‐rate data with a short‐arc approach, using only data over the area of interest. We determine our gravity solutions using neighbor smoothing constraints. We divided the entire Moon into 12 regions and 2 polar caps, with a resolution of 0.15°×0.15° (which is equivalent to degree and order 1199 in spherical harmonics), and determined the optimal smoothing parameter for each area by comparing localized correlations between gravity and topography for each solution set. Our selected areas share nodes with surrounding areas and they are overlapping. To mitigate boundary effects, we patch the solutions together by symmetrically omitting the boundary parts of overlapping solutions. Our new solution has been iterated, and it has improved correlations with topography when compared to a fully iterated global model. Our method requires fewer resources, and can easily handle regionally varying resolution or constraints. The smooth model describes small‐scale features clearly, and can be used in local studies of the structure of the lunar crust.

A dry lunar mantle reservoir for young mare basalts of Chang\u2019e-5

The distribution of water in the Moon’s interior carries implications for the origin of the Moon1, the crystallization of the lunar magma ocean2 and the duration of lunar volcanism2. The Chang’e-5 mission returned some of the youngest mare basalt samples reported so far, dated at 2.0 billion years ago (Ga)3, from the northwestern Procellarum KREEP Terrane, providing a probe into the spatiotemporal evolution of lunar water. Here we report the water abundances and hydrogen isotope compositions of apatite and ilmenite-hosted melt inclusions from the Chang’e-5 basalts. We derive a maximum water abundance of 283 ± 22 μg g−1 and a deuterium/hydrogen ratio of (1.06 ± 0.25) × 10–4 for the parent magma. Accounting for low-degree partial melting of the depleted mantle followed by extensive magma fractional crystallization4, we estimate a maximum mantle water abundance of 1–5 μg g−1, suggesting that the Moon’s youngest volcanism was not driven by abundant water in its mantle source. Such a modest water content for the Chang’e-5 basalt mantle source region is at the low end of the range estimated from mare basalts that erupted from around 4.0 Ga to 2.8 Ga (refs. 5,6), suggesting that the mantle source of the Chang’e-5 basalts had become dehydrated by 2.0 Ga through previous melt extraction from the Procellarum KREEP Terrane mantle during prolonged volcanic activity.

High-temperature inter-mineral potassium isotope fractionation: implications for K-Ca-Ar chronology.

Recent advances in high-precision potassium (K) isotopic analysis have found considerable isotopic variation in rock samples of the Earth's continental and oceanic crusts; however, it is still uncertain whether there is any resolvable inter-mineral and mineral-melt K isotopic fractionation during igneous and metamorphic processes. Here, we report K isotope compositions of mineral separates from three extremely well preserved igneous rocks (intrusive/extrusive and mafic/intermediate/felsic) in order to investigate possible inter-mineral and mineral-melt K isotopic fractionation at magmatic temperatures. For the first time, we found large inter-mineral fractionation of K isotopes in natural samples (up to 1.072‰), where plagioclase displays a significant enrichment of heavier K isotopes compared to potassium feldspar and biotite in a granite. In addition, we also observed smaller but measurable K isotope fractionation (0.280‰±0.030‰) between ternary feldspar phenocrysts and matrix in a trachyandesite, as well as a comparable isotope fractionation (0.331‰±0.010‰) between plagioclase and the bulk in a gabbroic intrusive rock. We also evaluated such results by comparing the theoretically calculated equilibrium K isotope fractionation factors between relevant igneous minerals in literature and this study. In general, the measured inter-mineral fractionations are consistent with the theoretical calculations (i.e., plagioclase is enriched in heavier isotopes compared to potassium feldspar). Specifically, the measured K isotope fractionation between phenocryst rim and matrix in the trachyandesite agrees well with the calculated equilibrium isotope fractionation. However, the measured K isotope fractionations between phenocryst core and matrix as well as between plagioclase and K-feldspar are significantly larger (by a factor of ~2-3) than the calculated isotope fractionations, which suggest isotopic disequilibrium due to kinetic processes. Using a range of plagioclase-melt isotope fractionation factors inferred from the theoretical calculations in this study, we modeled the K isotopic fractionation during the formation of lunar anorthositic crust, and the result shows a negligible effect on the K isotopic compositions in both lunar crust and mantle. The K isotopic difference between Earth and Moon, therefore, cannot be the result of Lunar Magma Ocean differentiation. Finally, we evaluate the effect of observed inter-mineral fractionations on K-Ar and 40Ar-39Ar dating. This study indicates the variation of 40K/K ratio would contribute a maximum 0.08% error to the K-Ar and 40Ar-39Ar age uncertainties. We propose a refined 40K/total K ratio as 0.00011664±0.00000011 (116.64±0.11ppm) instead of the conventional value, 0.0001167(2) for the present Earth. Because some minerals fractionate K isotopes, ultrahigh precision age dating with the K-Ca-Ar dating systems must measure the K isotope fractionation in the same mineral fractions used for age dating.