Lunar primitive mantle olivine returned by Chang’e-6

Domain: earth_science
Published: 2025-04-23
Editorial outcome: minor_revision

Abstract
The lunar mantle is important for unraveling the Moon’s formation and early differentiation processes. Here, we identify primitive lunar olivines in soils returned by the Chang’e-6 mission. These olivines have oxygen isotopic compositions plotting along the terrestrial fractionation line, and are characterized by high forsterite contents up to 95.6, and a broad range of nickel abundances from zero to 682 ppm. While the low-nickel (zero to 251 ppm), forsteritic olivines align with a Mg-suite origin, the most primitive, high-nickel olivines (337 to 682 ppm) have a different origin. They could be either the first olivine crystallized from the Lunar Magma Ocean (LMO) with an Earth-like initial composition, or crystallized from a hitherto unrecognized ultra-magnesian lava produced by extensive melting of the early LMO cumulate. The exposure of these mantle olivines was facilitated by their entrainment in ascending high-Mg lavas and conveyed to the surface at the South Pole-Aitken Basin.

Introduction
Constraining the nature of the lunar mantle is fundamental to understanding the Moon’s formation, differentiation and evolution1. Lunar scientists have long been searching for mantle materials ever since the first samples were brought back by the Apollo mission in year of 1969. A possible piece of the lunar mantle, as represented by a dunite fragment, has recently been identified in a lunar meteorite2. However, no definitive confirmed mantle materials have been found from the sample-return missions3,4. Our understanding of the ancient and modern lunar mantle is largely based on the physio-chemical models, laboratory experiments, geophysical and remote sensing observations, and coordinated analyses of volcanic products5–17. These sources of knowledge require further validation and are likely to be revised once the lunar mantle materials become accessible for direct analyses.

The Lunar Magma Ocean (LMO) paradigm posits a stratified mantle, where the lower mantle is dominated by magnesian olivine cumulates and transitions into a more ferroan orthopyroxene-dominated upper mantle6,18. However, as the Moon evolved, this initial cumulate stratigraphy was disrupted: the less dense olivine cumulates would have ascended upon the overlying high-density mantle via gravitational restructuring19. There is currently no consensus on what lithologies of the lunar upper mantle would have been1,5,20.

The South Pole-Aitken Basin (SPA) on the Moon’s farside is believed to be the most promising location for mantle materials to be exposed on the lunar surface1. On June 25, 2024, the Chang’e-6 (CE-6) mission successfully retrieved the first sample from the SPA (41.625 °S, 153.978 °W). Here, we identify highly magnesian olivines (Fo from 86.8 to 95.6; Fo=molar Mg/(Mg + Fe) * 100) in the CE-6 soils, and provide elemental and oxygen isotopic evidence that the nickel (Ni)-poor, forsteritic olivines are of Mg-suite origin, whereas the most primitive, Ni-rich olivines are likely of mantle origin and compositionally similar to the first olivines crystallized from the LMO.

Results
Olivine grains are generally rare in the CE-6 soils21. By employing Tescan Integrated Mineral Analysis (TIMA) techniques, we identified 63 olivine grains in approximately 12 milligrams of CE-6 soils (CE6C0400YJFM005) (Fig. 1 and Supplementary Fig. 1). Half of the forsteritic olivine grains either occur as single anhedral mineral fragments, or are embedded together with anorthite in impact glasses (Fig. 1a–c and Supplementary Fig. 1). One particular lithic fragment (approximately 100 μm in width and 250 μm in length) contains numerous anhedral to euhedral olivines of approximately 5–50 μm in size in a groundmass composed of plagioclase, olivine, and spinel (Fig. 1d). The mineral phases in the groundmass are unfortunately too fine-grained for accurate analysis. The bulk compositions of the groundmass, obtained by electron microprobe analysis (EPMA, with a spot size of 6 μm) and semi-quantitative scanning electron microscopy (SEM) analysis, differ significantly from the impact glasses in the CE-6 soils but show similarities to the proposed parental melts of Mg-suite troctolite (Supplementary Fig. 2a; Supplementary Data 1)22–26. The fine-grained and spinifex-like texture of the groundmass suggests that it is a Mg-rich lava that underwent rapid cooling, equivalent to the extrusive Mg-suite magmatism27,28. The high temperature expected for such extrusive Mg-rich lavas would be capable of assimilating the anorthositic crust and producing Mg-Al spinel23,29–31. Crystal fractionation simulations applied to the groundmass successfully reproduce the Mg-suite trend, with a crystallization sequence of dunite, troctolite to norite (Supplementary Fig. 2b, c).

The micro-olivines in the groundmass are generally <3 μm in size and exhibit a narrow Fo range of 91.6 ± 1.2 (n = 9, 1 sd) (Figs. 1, 2), which seems in Fe-Mg equilibrium with the melts of the groundmass (Supplementary Fig. 3). Large olivine crystals generally display skeletal and erosional morphologies, consisting of a wide compositional interior zone (Fo=89.7–95.6) and a narrow overgrowth rim (mostly <1 μm in width) with Fo=91.6 ± 0.9 (n = 8, 1 sd), which is similar to the micro-olivines in the groundmass. The olivine rim is too narrow to obtain accurate data. Three olivine populations were distinguished based on the texture and chemical composition (Fig. 2 and Supplementary Fig. 4): Group I olivine is represented by the largest polygonal crystal (~50 μm) with a dissolution feature at the margin. Its interior is homogenous, with relatively low Fo (89.9–90.8) and low Ni (0–55 ppm); Group II olivine is predominantly hopper-shaped, containing an irregular, relic core featured by low Fo (89.7–92.4) and low Ni (0–149 ppm), and a more magnesian mantle with higher Fo (92.9–95.4) but similar Ni contents (8–188 ppm); Group III olivine is polyhedral to hopper in shape, characterized by the highest Fo (92.9–95.6) and the highest Ni (337–682 ppm) among all CE-6 olivines. Only a few olivine grains are large enough for precise measurements of Ni abundance using laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS). The EPMA data generally agree with the LA-ICPMS data (Supplementary Fig. 5 and Supplementary Data 1), i.e., bimodal distribution of Ni for olivine.

Oxygen isotopic compositions of the CE-6 olivines (mainly Group I and III), measured by using in-situ secondary ion mass spectrometry (SIMS), fall within the range defined by lunar rocks32–34, plotting along the terrestrial fractionation line (TFL) with a homogenous δ18O value of 5.28 ± 0.19‰ (1 sd) (Fig. 3a and Supplementary Data 2).

Forsteritic olivines of exogenous origin have been reported in the lunar collection35. These olivines were characterized by high FeO/MnO (i.e., MnO below detection limit) and extremely low abundances of siderophile elements (e.g., <0.008 wt% Ni), which were interpreted as remnants of primitive chondritic meteorites impacted onto the lunar surface35. In contrast, the CE-6 olivines have a narrow FeO/MnO range (53–123; Fig. 3b), consistent with those of other lunar olivines35,36, and a wide range of Ni abundances (from zero to 682 ppm; Supplementary Data 1). The lack of mass-independent oxygen isotope fractionation in CE-6 olivines further supports their indigenous lunar origin. Fagan et al.37 found that impact-derived olivines in Apollo 14 impact melts have higher Fo values compared to the pristine olivines from Apollo 14 and 12 high-alumina basalts. The lack of difference in Fo between olivine grains that coexist with the impact glass and those that do not (Supplementary Fig. 6), suggests that the high Fo of the CE-6 olivines is a pristine feature.

The most forsteritic pristine olivines in lunar crustal rocks are commonly associated with ultramafics and troctolites38. Their petrogenesis may be linked to the pressure-release melting of early LMO cumulates during mantle overturn, followed by crystallization at crustal depths22,24,39,40. A prominent feature of these forsteritic olivines in lunar crustal rocks is their Ni depletion compared to the less magnesian olivines in mare basalts7,26,40 (Fig. 4). Longhi et al.26 investigated the Ni abundance in Mg-suite olivines and found that the most forsteritic olivines in lunar ultramafics and troctolites have a maximum Ni content of approximately 300 ppm (Fig. 4). Several models have been proposed to explain the Ni depletion in Mg-suite olivines, such as remelting of LMO cumulate pile composed of primary dunite, norite and gabbronorite fluxed in part by KREEP (an acronym for the incompatibles K, rare-earth elements (REE), and P)26 and small amounts of metal fractionation from the Mg-suite parental magmas prior to olivine crystallization39. Despite of different models, Ni depletion is considered to be a typical signature of olivines of Mg-suite origin.

The Group I and Group II olivines in our CE-6 collection have Ni contents below 300 ppm, falling within the range defined by Mg-suite olivines (Fig. 4). We tentatively assign these olivines to a Mg-suite origin. The Group II olivine exhibits a reverse zoning composed of a relic, Group I–like olivine core and a more magnesian mantle, suggesting a possible genetic relationship between the two groups. The reverse zoning can be formed by reaction of a pre-existing Group I-like olivine with a new pulse of hotter and more magnesian magma, such that the entrained olivines partially resorbed and become more Mg-rich41. The Ni abundances of the mantles are only slightly elevated, indicating that the introduced magma is compositionally similar to the eruptive, Ni-depleted Mg-suite parent magma, but more magnesian. Overall, the Group I and Group II olivines may represent early formed Mg-suite olivine cumulates at the crustal level, which were later entrained into more Mg-rich eruptive lavas and transported to the surface. This formation mechanism, however, cannot explain the origin of the Group III olivine.

The Group III olivine, characterized by the highest Fo (Fo 92.9-95.6) and highest Ni contents (337–682 ppm), cannot be easily reconciled with typical Mg-suite origin. They may crystallize from an ultra-magnesian lava with Ni contents at least twice as high as those of typical Mg-suite parent magmas. However, no such ultra-magnesian lavas or basalts containing such high-Ni forsteritic olivines have been found in previous lunar collections14,38. Given our still limited understanding of lunar rock diversity, we cannot simply rule out the existence of such new type of ultra-magnesian and Ni-rich lavas. If present, the ultra-magnesian lava would have a similar genesis to the terrestrial komatiites42, likely being produced by extensive melting of early mantle cumulates to achieve the most primitive signature of the Group III olivine (Supplementary Fig. 7). The hypothetical Ni-depletion event accounting for the origin of Mg-suite magmas would not have exerted an effect, in order to prevent the ultra-magnesian lava from Ni-depletion. Further in-depth study of the CE-6 returned samples and future lunar exploration would help test the possible existence of this new type of lava on the Moon. However, we propose below an alternative possibility for the origin of the Group III olivine: the first olivine crystallized from the LMO.

Laboratory experiments and numerical simulations of LMO solidification suggest that the Mg-rich olivine is the earliest crystallized mineral from the cooling magma ocean, with Fo varying from 96.0 to 87.6, corresponding to 0 to ~50% solidification6–9,15–17. Our simulation utilizes the Earth-like Lunar Primitive Upper Mantle (LPUM) composition13 as the initial composition of the LMO. The Taylor Whole Moon (TWM) composition43 has an apparently low Mg-number of 84 that is unable to produce olivine composition with Fo > 947,9,15. We also model the Ni content of olivine during the LMO solidification following a well-established approach26. The empirical Beattie-Jones model44,45 was employed to understand the partition of Ni between olivine and liquid: D Ol/L=3.346*D Ol/L–3.66544, in which the D Ol/L=MgOOl/MgOL can be calculated at each step of LMO crystallization. Several studies estimated the initial Ni content of the LMO (also the Bulk Silicate Moon) to be depleted by a factor of about 3~4 compared to the terrestrial upper mantle, around 415 ± 105 ppm14,46. Consequently, our simulation on LMO crystallization is in accordance with previous attempts7,26, demonstrating that the first crystallized olivine has a Fo value of 96.2 with Ni content of 509 ± 131 ppm (Fig. 4). The Ni content of olivine increases with ongoing LMO solidification and reaches a maximum at Fo of ~92.4, after which the Ni content of olivine drops with decreasing Fo (Fig. 4). Compositions of the Group III olivines correspond well with the modeling results and thus could be of mantle origin. This inference is also supported by our additional modeling on Ni/Co evolution of olivines during LMO differentiation (Supplementary Fig. 8).

We acknowledge that the above two hypotheses for the origin of Group III olivine are not mutually exclusive. Given the equilibrium crystallization of olivine at the early stage of LMO solidification and the formation of the ultra-magnesian magma through high-degree partial melting of those olivine cumulates, the olivine crystallized from the ultra-magnesian magma is expected to have similar compositions to its source component, i.e., the early LMO cumulate (Supplementary Fig. 7). That is, the Group III olivine compositionally resembles the first olivine crystallized from the LMO.

Olivines are generally rare in the CE-6 soils (<0.5%)21. Orbital data of the SPA basin suggest that the upper mantle in that area is likely dominated by low-Ca pyroxene47–49. A possible explanation for the exposure of mantle olivines at the SPA basin would be that these olivines were entrained into the Mg-suite erupted lavas and brought to the surface (Fig. 5). At the onset of gravitational instability in the early history of the Moon, the less dense olivine cumulates of the LMO ascended and invaded into the upper mantle and experienced decompression melting to produce Mg-suite primary melts22,24,39,40. These Mg-suite primary melts are generally denser than the anorthositic crust and are thus predominately intrusive, forming deep-seated Mg-suite intrusions50. The Group I olivine and the relic core of Group II olivine formed at this stage. Subsequent SPA basin-forming impact should have removed the upper anorthositic crust and excavated deep-seated mafic lithologies, making the major mafic compositional anomaly associated with the vast SPA basin51,52. The GRAIL data show that the crustal density at the SPA basin is higher than the Mg-suite primary melts27,50, making it possible for the eruption of late ultramafic lavas. These high-degree, more magnesian eruptive lavas would have entrained mantle olivines (Group III) and crustal olivines (Group I), resulting in a mixed cargo of olivines formed at variable depths and conveyed them to the surface. Reaction of this Mg-rich magma with the less magnesian crustal olivines formed the Group II olivine. Future searching for potential exposure of mantle materials on the Moon should be focused on regions where Mg-suite extrusives are developed.

Methods
Tescan integrated mineral analysis
Major elemental mappings of the CE-6 soils were determined by TIMA at the Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University. The TIMA system comprises of a Tescan Mira Schottky field-emission scanning electron microscope with four silicon-drift energy-dispersive (EDS) detectors arranged at 90° intervals around the chamber. The measurements were performed in the high-resolution liberation analysis mode, and the backscatter electron image was obtained to identify individual particles and boundaries between distinct preliminary phases. A rectangular mesh of measurements on each distinct phase was obtained with X-ray spectra. TIMA was performed at 25 kV using a spot size of ~1 μm, a working distance of 15 mm, and a field size set at 1500 μm.

Scanning electron microscopy and energy dispersive spectroscopy analyses
The fine backscatter electron (BSE) images and energy dispersive spectroscopy (EDS) mapping with semi-quantitative elemental analyses were performed using a Zeiss Supra 55 field-emission scanning electron microscope (FESEM) with an X-ray energy dispersive spectrometer, at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing (CUGB). The EDS mappings with semi-quantitative elemental analyses were performed at an accelerating voltage of 20 kV and a working distance of 15 mm. Minerals as well as synthetic phases (MINM25-53) were used as standards. Data are reported in Supplementary Data 1.

Electron microprobe analysis
The major and trace element compositions of olivines were determined using a JOEL JXA-iHP200F field emission EPMA electron microprobe at the Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS). The olivines were analyzed at an acceleration voltage of 15 kV a probe current of 20 nA, and a focused beam diameter of ~1 μm. The peak counting time was 100 s for Ni, 60 s for Ca, Al, Cr, Mn, Co and 10 s for Si, Mg, Fe, Ti, Na, K. The background time was half of peak time. The detection limit was 0.006 wt.% for Ni, 0.028 wt.% for Co, 0.031 wt.% for Cr and 0.006–0.03 wt.% for other elements. A random set of olivine grains covering the entire range of Fo was further analyzed by using EPMA 1720 electron microprobe at the EPMA Lab, CUGB. Both labs in CAGS and CUGB yield consistent data, further confirming the data quality (Supplementary Fig. 9). Data are reported in Supplementary Data 1.

In situ trace element analysis
Siderophile elements including Ni, Co and Cr and Mn of olivines were further determined using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICPMS) employing an Element XR HR-ICP-MS instrument coupled to a GeolasHD 193 nm ArF excimer LA system at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Helium was used as the ablation gas to improve the transport efficiency of the ablated aerosols. All measurements were conducted at 2 Hz frequency. The laser energy density was approximately 4.0 J cm-2. The analyses were conducted using a laser diameter of 10 μm. ARM-153 reference glass was used for external calibration (Si was used as a standard for data reduction). Olivine MongOLSh11-254 and silicate glass GOR132-G55 were used for quality control monitoring. Each set of standard samples, including ARM-1, GOR132-G and MongOLSh11-2, was analyzed once every 2 unknown samples were interspersed. The data obtained during ablation runs were processed using the Iolite 3.4 software with an in-house-built data reduction scheme mode56 with the bulk normalization as 100 wt.% strategy. Data are reported in Supplementary Data 1. The analyses of reference materials MongOLSh11-2 and GOR132-G are given in Supplementary Data 3 and Supplementary Fig. 10, agreeing with published data. Their quoted uncertainties are defined as two times the standard error (2 SE).

In situ SIMS oxygen isotope analysis
Oxygen isotopic ratios of forsteritic olivines were analyzed using a CAMECA IMS-1280 multi collector ion probe at the IGGCAS. The Cs+ primary beam was accelerated at 10 kV with an intensity of ~2 nA. The spot is approximately 15 μm in diameter. An electron gun was used to compensate for sample charging during the analysis. Secondary ions were extracted at a potential of -10 kV. Oxygen isotopes were measured in multi-collector mode with two off-axis Faraday cups. Each analysis consisted of 200 cycles with 2-s counting time. We analyzed the reference materials San Carlos olivine and NIST SRM 610 after analyzing every three unknown samples in the experiment to monitor analytical precision and calibrate instrumental mass fractionation. The average δ18O value for San Carlos olivine is 5.25‰ in this study, which is similar to the reported certified value of 5.3‰57. The external reproducibility of the reference materials was 0.11–0.19‰ and 0.16–0.22‰ for δ18O and δ17O (1 sd) over two days of analyses, respectively. Data are reported in Supplementary Data 2.

Modeling of Ni and Co content of olivines during LMO solidification
We modeled the solidification of the LMO following a two-stage crystallization strategy6,7,15,17,58. The first 50 vol.% solidification process was assumed to be the equilibrium crystallization, followed by the fractional crystallization at >50 vol.% in steps with each representing 1/10th of the total magma ocean volume58. The initial depth of the LMO was assumed to be 1000 km based on geophysical constraints11,59. For every step, input pressure of the calculation was set to correspond to the middle depth of the LMO58. The oxygen fugacity was set at the iron-wüstite (IW) buffer. The first 50 vol.% solidification process was modeled by GeoPS60 using the thermodynamic database HP63361 and solution models for melt, olivine, orthopyroxene, clinopyroxene and feldspar from ref. 62. Calculated step was set as 300. The following fractional crystallization was modeled by the menu-driven alpha-MELTS interface (version 2.1) to run the subroutine version of rhyolite-MELTS63,64. Phase equilibrium calculations from liquidus to solidus in 1°C increments were performed.

For Ni and Co, the concentration of the early LMO during equilibrium crystallization at step m, Cl m, is given by:
Cl m = C0 * W0 / (Wl m + D Ol m * W Ol m + D Opx m * W Opx m) (1)
Where C0 and W0 stands for initial composition and mass of the LMO, respectively;
while the Ni and Co concentration of the early LMO during fractional crystallization is given by:
Cl m = (Cl m-1 * Wl m-1) / (Wl m + D Ol m * W Ol m + D Opx m * W Opx m) (2)
Where Cl m-1 and Wl m-1 stands for composition and remaining mass of the LMO at step m −1, respectively; D i and W i stands for partition coefficient and mass of phase i respectively. The initial Ni and Co content of the LMO were taken from ref. 46. Previous studies have revealed that the olivine-melt and orthopyroxene-melt partition coefficients of Ni and Co should be functions of D Mg44,45. Thus, those partition coefficients would be functions of percent solidified (PCS) of the LMO. The Ni and Co contents of mantle olivines are then given by:
C Ol m = D Ol m * Cl m (3)

Equilibrium melting modeling of the LMO cumulate pile at 50 percents solidified
We chose the GeOPS to model the olivine composition during equilibrium melting model of the LMO cumulate pile at 50 PCS7, from a LMO with an initial composition of the LPUM13 (LPUMcp). Input pressure of the calculation was set to correspond to the depth of the remaining LMO at 50 PCS. The oxygen fugacity was set at the iron-wüstite (IW) buffer. The major elemental compositions of the LPUMcp were calculated from LMO crystallization modeling result in this study with 3% trapped instantaneous residual liquid (TIRL) at 50 PCS, and taken from ref. 7, respectively. The equilibrium melting was modeled using the thermodynamic database HP63361 and solution models for melt, olivine, orthopyroxene, clinopyroxene and feldspar from ref. 62. Calculated step was set as 300.

The Ni concentration of the equilibrated melt during remelting of early LMO cumulates at step m, Cl m, is given by:
Cl m = (C0 s * W0) / (Wl m + D Ol m * W Ol m + D Opx m * W Opx m) (4)
Where C0 s and W0 stands for initial Ni content and mass of LPUMcp, respectively; D i and W i stands for partition coefficient and mass of phase i, respectively. The initial Ni content of the LPUMcp in this study was obtained by performing a Ni mass balance of the minerals and TIRL at 50 PCS. The initial Ni content of the LPUMcp from ref. 7 was obtained by first calculating the proportion of olivine, orthopyroxene and the remaining LMO liquid using the average mineral compositions at 50 PCS from their experiments and composition of the LPUMcp given in their study7. The Ni content of the remaining LMO liquid at 50 PCS can be calculated with initial Ni content of the LMO from ref. 46 through a mass balance calculation of the minerals and TIRL.

The Ni content of olivine equilibrated with the melt is then given by:
C Ol m = D Ol m * Cl m (5)

Model results are shown in Supplementary Fig. 7.

Fractional crystallization modeling of the groundmass
PETROLOG program65 was applied to perform crystal fractionation simulation on the groundmass. Our modeling was set as pure (100%) crystal fractionation at a constant pressure (1 bar). The calculation step was set as 1%. Mineral-melt equilibrium models used here are as follows: olivine66, plagioclase67, orthopyroxene68, clinopyroxene69 and spinel70. The fO2 was set at the iron-wüstite (IW) buffer. The crystal fractionation modeling ceased when Cr2O3 < 0 wt.%. The crystallization trend of the groundmass in terms of Mg# of mafic minerals versus An of plagioclase is shown in Supplementary Fig. 2b; the crystallized mineral assemblages are consistent with petrological characteristics of the Mg-suite rocks and shown in Supplementary Fig. 2c.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "(1) Summary of strengths, weaknesses, and overall contribution\nThis study focuses on the identification and potential origin of forsteritic (Fo up to 95.6) olivine with high Ni abundance (374-657 ppm) in newly returned Chang’e-6 soils using quantitative (SEM, EPMA, LA-ICP-MS, SIMS) and qualitative (TIMA, SEM) analyses and geochemical modelling (pMELTS, PETROLOG). The identified Chang’e-6 olivine are lunar in origin as they fall along the δ18O/ δ17O terrestrial fractionation line (TFL). There are two populations of high Fo olivine with low Ni and high Ni abundance. High-Ni forsterite olivine are hypothesized to be fragments of early, lunar magma ocean (LMO) cumulates (i.e., fragments of the lunar mantle) entrained in a lunar Mg-suite like extrusive magma that brought the olivine to the surface within Chang’e-6 landing site. This hypothesis is supported by fractional crystallization and equilibrium geochemical modelling of the LMO at 20 kbar using lunar primitive upper mantle (LPUM) compositions. The origin for the low-Ni forsteritic olivine may be lunar Mg-suite in origin.\nThis research in this study is on a topic of relevance in the search for lunar mantle material, constraining the composition of the bulk-silicate Moon, and LMO crystallization. However, this paper as some major weaknesses that needs to be addressed.\nOne major weakness is in the discussion of High-Ni versus Low-Ni forsterite olivine. It is unclear in Figure 1 which olivine phenocrysts or mineral fragments contain high-Ni abundance versus low-Ni abundance, which would change the interpretation of the results of this work. For example, in Figure 1D, some of the olivine show inverse zoning of rounded, relict olivine cores (Mg#=90) to more magnesian mantles (Mg#=93) to thin, bright outer rims that are more than likely enriched in Fe. Furthermore, some olivine Figure 1D show embayment and dissolution in the cores of the minerals. Both inverse zoning and dissolution are evidence for early processes that would have altered the chemistry and history of the olivine minerals. The remaining olivine in Figure 1D that show only high Fo cores (Mg# > 93) could be the result of cutting effects in the thin section and are not true cores. Looking at Extended Data Table 1, olivine- Ol01 has a core with average Mg# 90.1 and average low-Ni abundance of 16ppm but is mantled by Mg# 91 and Ni abundance 212. If the remaining high Fo, high-Ni olivine datapoints are of the mantle regions of the olivine, this would be indicative that the origin for this particular chemical signature is not early-LMO cumulates and more than likely the result of a secondary magma or geologic process. Additionally, if one focused on the olivine with relict, earlier formed cores that have Mg#s =~90 and low-Ni abundances, they are more in line with the Mg-Suite olivines discussed in this work. However, this paper does not address or discuss the inverse zoned olivine.\nA second major weakness is not considering or discussing an impact origin for the olivine single mineral fragments that are often observed associated with glass in the BSE images (Figure 1, Extended Data Figure 1). Fagan et al. (2014) (DOI: https://doi.org/10.1016/j.gca.2012.12.032) discusses how different olivine chemical signatures can be used to discern impact-derived olivine versus endogenous. Typically, remelting of olivine would increase the Mg#, as observed in lunar impact melts. Additionally, the Chang-e’6 soils were collected from a mare basalt region of ~2.8 Ga in age that are similar to the mare basaltic meteorites with 2.8 Ga ages (Elardo et al., 2014; DOI: https://doi.org/10.1111/maps.12239) and have the current, highest Ni abundances when compared to Apollo basalts and Feldspathic samples. There may be a correlation between the two and should be discussed further.\nA third major weakness is the geochemical modelling used a definitive support for early LMO cumulate origin for the High Fo, High-Ni olivines. In geochemical modelling, there are multiple degrees of freedom that are not highlighted in both the discussion of the results and the methods. Additionally, for LMO modelling, the fractional crystallization at 20 kbar matched best with the results. However, LMO solidification may have had two-steps of crystallization, early equilibrium crystallization followed by fractional (Elkins-Tanton et al., 2011; DOI: https://doi.org/10.1016/j.epsl.2011.02.004). If the olivine identified in this work are products of early-LMO crystallization (0-50%), then the geochemical modelling should better reflect equilibrium crystallization as a potential crystallization pathway of the high-Fo olivine, but in Figure 3 and Extended Data Figure 4, this is not well represented.\nI recommend the authors address the above major weaknesses before publication, especially major weakness 1, as this would change the interpretation of the identified olivine from lunar mantle origin to lunar Mg-Suite origin.\n(2) Major Comments:\n• Line 84: Add the Olivine ID’s for the olivine in a-c similar to the Extended Data Fig. 1\n• Line 93: It needs to be explained further why the UMMFs, with Olivines (Fo up to 98) are too high to be products of the LMO but not the olivines from this study (Fo up to 96).\n• Line 114: This paragraph introduces the different lunar rock suites that have primitive olivine (Fo >90) but does not discuss in detail the high forsteritic olivine in the lunar Mg-suite whose olivine overlap in Mg# with the olivine of this work. This should be introduced in this paragraph.\n• Line 142-144: Some low-Ti mare basalts, in-particular, the low-Ti mare basaltic meteorites that have high Ni abundances, may be the product of late-stage LMO cumulates (85-96% LMO Crystallization) and thus, the comparison made in this sentence is misleading. Elaborating which low-Ti mare basalts are being extrapolated for Ni abundances needs to be discussed here.\n• Line 217: This is a little confusing since it is not clearly distinguished in the BSE images which olivine are low-Ni versus high-Ni. Therefore, the readers cannot compare texture.\n• Line 301: References are missing here to refer to which previous studies used 20 kbar\n(3) Minor Comments:\n• Line 36: Lunar mantle is a fundamental part in the puzzle of Moons formation and differentiation.\no Insert “Constraining the lunar mantle is fundamental to understanding the Moon’s formation, differentiation, and evolution.”\no Reference used is Moriarty III et al. (2021) which focuses on the search for the lunar mantle in remote sensing.\n• Line 38: insert “the” in front of “Apollo-mission samples”\n• Line 38: Change “Unfortunately” to “However”\n• Line 38: Change “definitively” to “definitive”\n• Line 40: Our understanding of the lunar mantle is also based on coordinated analyses of volcanic products (i.e., mare basalts, volcanic glass beads) whom partial melts are products of mantle cumulates. This sentence needs to be restructured to reflect ways the lunar mantle is studied.\n• Line 40: Change “Physico-chemical” to “Physio-chemical”\n• Line 41: Remove “however”\n• Line 42: Add “are” before “subject to change”\n• Line 44: “Classic lunar magma ocean (LMO) concept” can be changed to “The Lunar Magma Ocean (LMO) paradigm”\n• Line 45: “Magnesian olivine-dominated lower mantle” are mentioned here but then rephrased as “dunite cumulates” in line 47. To reflect consistency, either add the type of magnesian olivine lithologies predicted in by models, such as: (i.e., dunite cumulates, ect.) or change dunite cumulates in line 47 to less dense mafic cumulates.\n• Line 48: Shallow mantle here might not be the right phrase. Consider rephrasing to include density instability alongside gravitational restructuring.\n• Line 49: This sentence needs references\n• Line 49: There isn’t a consensus to what mineral assemblages are in the lunar upper mantle but examples could be provided, for example: Elkins-Tanton et al., 2011 discusses and compares the mineral assemblages for the lunar mantle in different LMO models.\n• Line 50: Rephrase to reflect that potential excavated upper lunar mantle material has been identified in the SPA basin. References: Moriarty III et al., 2021\n• Line 76: Remove “, and” after “textures, and”\n• Line 93: “Olivines with Fo high” should be “Olivines with high Fo”\n• Line 128: Change “Now that the question is” to “It is unclear”\n• Line 128: Remove “,” from “LMO cumulates,”\n• Line 130: Remove “We will tackle this issue by using Ni content in olivine”\n• Line 135: Need references for the lunar DNi diffusion coefficients\n• Line 152: Figure 3: The “Apollo Mg-Suite” Field is missing “Mg-Suite”\n• Line 165-167: This isn’t the only hypothesis for the formation of the Mg-suite troctolites. Rephrasing “whose petrogenesis is linked” to “whose petrogenesis may be linked” would better fit this sentence.\n• Line 168: Remove “,” from “Mg-suite olivines,”\n• Line 172: Remove “,” from “by KREEP55,”\n• Line 173: Change “Despite the debate on the Ni depletion problem” to “Despite competing hypotheses on Ni depletion in Mg-suite olivines,”\n• Line 196: Remove “,” from “upper mantle,”\n• Line 209: Change “unambiguous” to “ambiguous”\n• Line 215: Remove “,” from “primitive melt,”", "reviewer_2": "Review of Nature Communications submission 559489: “Lunar primitive mantle olivine returned by Chang’e-6” by Si-Zhang Sheng et al.\nI recommend that the manuscript would be acceptable for publication, after revisions. I’ve attached a version of the submitted manuscript with a few edits to the text, a few additional comments, and some suggestions of additional references.\nThis manuscript reports intriguing high-Mg olivine crystals from the regolith returned by the Chang’e-6 spacecraft from the Moon’s South-Pole Aitkin Basin. The data are an important contribution to understanding the Moon, but I am not convinced that the high-Mg olivines are necessarily samples of the Moon’s mantle. The authors come to this inference because the olivine grains are very magnesian, Mg* = molar Mg/(Mg+Fe) up to almost 96%, and have high Ni abundances, up to ~650 ppm. No other known lunar olivines have both these characteristics, neither basalts nor basaltic plutonic rocks. The authors show that such olivine compositions could have formed in cumulate rocks from the early Lunar Magma Ocean.\nThis argument is reasonable, and I have no serious problems with it. However, there is another possible interpretation that ought to be mentioned in the manuscript; an interpretation which (sadly) changes the focus and apparent importance of the olivines. It is reasonable that the magnesian nickeloan olivines crystallized from ultrabasic lavas unlike those discovered so far. This might seem far-fetched, but no more so than finding fragments of mantle olivine.\nFirst, mantle rocks on Earth, and in the only recognized sample of lunar mantle material (Treiman and Semprich, 2023), do not contain euhedral or subhedral olivines, i.e. the edges of their olivine grains are irregular and not related to the orientations of the grains’ crystal structures. However, the manuscript claims the opposite for the Chang’e-6 magnesian olivines: “These forsteritic olivine grains are euhedral to subhedral … .”. So, I do not think that the Chang’e-6 magnesian olivines are pieces of mantle material – they are more likely phenocrysts from magnesian basalts. It is possible that there is a translation issue here, as the olivine grains of Figures 1A-C are anhedral, but those of Figure 1D are euhedral to subhedral.\nSecond, the Ni contents of the Chang’e-6 magnesian olivines are similar to those in lunar basalts, as shown in this graph, which is Figure 1a of Longhi et al. (2010) with data for the manuscript’s olivines as smaller symbols. One might reasonably expect that mantle olivine would have Ni abundances as high or higher than those from basalts. So, Ni itself does not prove a mantle origin.\nThird is the question of the high Mg* of the Chang’e-6 magnesian olivines. No known lunar basalts contain olivines so magnesian, but part of the point of Chang’e-6 was to explore an area of unknown geology, and extend our understanding of the diversity of lunar rocks. Is it possible that a lunar basalt could be so magnesian as to crystallize such olivines as phenocrysts? I think the answer is yes. I base that answer on the existence of comparable lavas on Earth, the ultra-magnesian komatiite lavas of the Commondale area, South Africa (Wilson et al. 2019). These lavas contain olivine phenocrysts with Mg* to 96% (comparable to those reported here) and with up to ~2000 ppm Ni. As the Moon’s mantle is inferred to contain 1/3 to 1/4 of the Ni as the Earth’s mantle1, this would suggest the possibility of a lunar ultrabasic lava with Mg* of ~96% and 500-700 ppm Ni. None has been reported so far, but I think it is possible that the high-Mg olivines reported here could be from a magma like this.\nREFERENCES\nLonghi, J., Durand, S. R., & Walker, D. (2010). The pattern of Ni and Co abundances in lunar olivines. Geochimica et Cosmochimica Acta 74, 784-798.\nTreiman, A. H., & Semprich, J. (2023). A dunite fragment in meteorite Northwest Africa (NWA) 11421: A piece of the Moon’s mantle. American Mineralogist, 108(12), 2182-2192.\nWilson, A. H. (2019). The late-Paleoarchean ultra-depleted Commondale komatiites: Earth's hottest lavas and consequences for eruption. Journal of Petrology 60(8), 1575-1620.\nEND – Allan H. Treiman"}, "author_response": "Response to comments\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\n(1) Summary of strengths, weaknesses, and overall contribution\nThis study focuses on the identification and potential origin of forsteritic (Fo up to 95.6) olivine with\nhigh Ni abundance (374-657 ppm) in newly returned Chang’e-6 soils using quantitative (SEM, EPMA,\nLA-ICP-MS, SIMS) and qualitative (TIMA, SEM) analyses and geochemical modelling (pMELTS,\nPETROLOG). The identified Chang’e-6 olivine are lunar in origin as they fall along the δ18O/ δ17O\nterrestrial fractionation line (TFL). There are two populations of high Fo olivine with low Ni and high\nNi abundance. High-Ni forsterite olivine are hypothesized to be fragments of early, lunar magma ocean\n(LMO) cumulates (i.e., fragments of the lunar mantle) entrained in a lunar Mg-suite like extrusive\nmagma that brought the olivine to the surface within Chang’e-6 landing site. This hypothesis is\nsupported by fractional crystallization and equilibrium geochemical modelling of the LMO at 20 kbar\nusing lunar primitive upper mantle (LPUM) compositions. The origin for the low-Ni forsteritic olivine\nmay be lunar Mg-suite in origin.\nThis research in this study is on a topic of relevance in the search for lunar mantle material,\nconstraining the composition of the bulk-silicate Moon, and LMO crystallization. However, this paper\nas some major weaknesses that needs to be addressed.\nOne major weakness is in the discussion of High-Ni versus Low-Ni forsterite olivine. It is unclear in\nFigure 1 which olivine phenocrysts or mineral fragments contain high-Ni abundance versus low-Ni\nabundance, which would change the interpretation of the results of this work. For example, in Figure\n1D, some of the olivine show inverse zoning of rounded, relict olivine cores (Mg#=90) to more\nmagnesian mantles (Mg#=93) to thin, bright outer rims that are more than likely enriched in Fe.\nFurthermore, some olivine Figure 1D show embayment and dissolution in the cores of the minerals.\nBoth inverse zoning and dissolution are evidence for early processes that would have altered the\nchemistry and history of the olivine minerals. The remaining olivine in Figure 1D that show only high\nFo cores (Mg# > 93) could be the result of cutting effects in the thin section and are not true cores.\nLooking at Extended Data Table 1, olivine- Ol01 has a core with average Mg# 90.1 and average low-\nNi abundance of 16 ppm but is mantled by Mg# 91 and Ni abundance 212. If the remaining high Fo,\nhigh-Ni olivine datapoints are of the mantle regions of the olivine, this would be indicative that the\norigin for this particular chemical signature is not early-LMO cumulates and more than likely the result\nof a secondary magma or geologic process. Additionally, if one focused on the olivine with relict,\nearlier formed cores that have Mg#s =~90 and low-Ni abundances, they are more in line with the Mg-\nSuite olivines discussed in this work. However, this paper does not address or discuss the inverse zoned\nolivine.\nReply: Thanks for this useful comment. We acknowledge that the olivines in the CE-6 soils are\ncompositionally and texturally complicated. Following this suggestion, we did additional rim-core-rim\nEPMA analyses of olivines, hoping to have a better classification and understanding of the olivine\npopulations in the CE-6 soils. Based on morphological and geochemical characteristics, we\ncategorized the large olivine grains in the lithic fragment into three types (Fig.1 below): “Group I\nolivine is represented by the largest polygonal crystal (~50 µm) with a dissolution feature at the margin.\nIts interior is homogenous, with relatively low Fo (89.9-90.8) and low Ni (0-55 ppm); Group II olivine\nis predominantly hopper-shaped, containing an irregular, relic core featured by low Fo (89.7-92.4)\nand low Ni (0-149 ppm), and a more magnesian mantle with higher Fo (92.9-95.4) but similar Ni\ncontents (8-188 ppm); Group III olivine is polyhedral to hopper in shape, characterized by the highest\nFo (92.9-95.6) and highest Ni (337-682 ppm) amongst all CE-6 olivines.”\nFig. 1. Three populations of large olivine grains in the lithic fragment. (a) Schematic zoning profiles show the\nFo and Ni patterns. (b) Violin plots show the Fo and Ni of the olivine interior. The compositions of micro-olivine are\nshown as grey bars. Detail electron probe traverses across olivine crystals are reported in the Supplementary Table 1\nand shown in Supplementary Fig. 4.\nThe compositions of Group I and Group II olivines fall within the range defined by Mg-suite olivines,\nindicating a Mg-suite origin. The reverse zoning in Group II olivines can be formed by reaction of a\npre-existing Group I-like olivine with a more magnesian magma. For details, please see our revision\nin Lines 165 to 177:\n“The Group I and Group II olivines in our CE-6 collection have Ni contents below 300 ppm, falling\nwithin the range defined by Mg-suite olivines (Fig. 4). We tentatively assign these olivines to a Mg-\nsuite origin. The Group II olivine exhibits a reverse zoning composed of a relic, Group I – like olivine\ncore and a more magnesian mantle, suggesting a possible genetic relationship between the two groups.\nThe reverse zoning can be formed by reaction of a pre-existing Group I -like olivine with a new pulse\nof hotter and more magnesian magma, such that the entrained olivines partially resorbed and become\nmore Mg-rich41. The Ni abundances of the mantles are only slightly elevated, indicating that the\nintroduced magma is compositionally similar to the eruptive, Ni-depleted Mg-suite parent magma, but\nmore magnesian. Overall, the Group I and Group II olivines may represent early formed Mg-suite\nolivine cumulates at the crustal level, which were later entrained into more Mg-rich eruptive lavas and\ntransported to the surface.”\nHowever, Group III olivines, with the highest Fo and highest Ni abundance, have a different origin\nagainst Group I and Group II olivines. We propose that the compositions of Group III olivines\nresemble the first olivine crystallized from the Lunar Magma Ocean. For details, please see our\nrevision in Lines 178 to 221:\n“The Group III olivine, characterized by the highest Fo (Fo92.9-95.6) and highest Ni contents\n(337-682 ppm), cannot be easily reconcealed with typical Mg-suite origin. They may crystallize from\nan ultra-magnesian lava with Ni contents at least twice as high as those of typical Mg-suite parent\nmagmas. However, no such ultra-magnesian lavas or basalts containing such high-Ni forsteritic\nolivines have been found in previous lunar collections14,38. Given our still limited understanding of\nlunar rock diversity, we cannot simply rule out the existence of such new type of ultra-magnesian and\nNi-rich lavas. If present, the ultra-magnesian lava would have a similar genesis to the terrestrial\nkomatiites42, likely being produced by extensive melting of early mantle cumulates to achieve the most\nprimitive signature of the Group III olivine (Supplementary Fig. 7). The hypothetical Ni-depletion\nevent accounting for the origin of Mg-suite magmas would not have exerted an effect, in order to\nprevent the ultra-magnesian lava from Ni-depletion. Further in-depth study of the CE-6 returned\nsamples and future lunar exploration would help test the possible existence of this new type of lava on\nthe Moon. However, we propose below an alternative possibility for the origin of the Group III olivine:\nthe first olivine crystallized from the LMO… We acknowledge that the above two hypotheses for the\norigin of Group III olivine are not mutually exclusive. Given the equilibrium crystallization of olivine\nat the early stage of LMO solidification and the formation of the ultra-magnesian magma through\nhigh-degree partial melting of those olivine cumulates, the olivine crystallized from the ultra-\nmagnesian magma is expected to have similar compositions to its source component, i.e., the early\nLMO cumulate (Supplementary Fig. 7). That is, the Group III olivine resembles the first olivine\ncrystallized from the LMO in terms of composition.”\nA second major weakness is not considering or discussing an impact origin for the olivine single\nmineral fragments that are often observed associated with glass in the BSE images (Figure 1, Extended\nData Figure 1). Fagan et al. (2014) (DOI: https://doi.org/10.1016/j.gca.2012.12.032) discusses how\ndifferent olivine chemical signatures can be used to discern impact-derived olivine versus endogenous.\nTypically, remelting of olivine would increase the Mg#, as observed in lunar impact melts. Additionally,\nthe Chang-e’6 soils were collected from a mare basalt region of ~2.8 Ga in age that are similar to the\nmare basaltic meteorites with 2.8 Ga ages (Elardo et al., 2014;\nDOI: https://doi.org/10.1111/maps.12239) and have the current, highest Ni abundances when\ncompared to Apollo basalts and Feldspathic samples. There may be a correlation between the two and\nshould be discussed further.\nReply: Thanks for bringing us Fagan et al. paper. Fagan et al. (2013) found that olivines in Apollo 14\nimpact melts have higher Fo values compared to the pristine olivines from Apollo 14 and 12 high-\nalumina basalts. Nonetheless, the lack of difference in Fo between olivine grains that coexist with the\nimpact glass and those that do not, suggests that the high Fo of the CE-6 olivines is a pristine feature\n(Fig. 2 below). Please see our revision in Lines 138 to 142:\n“Fagan et al.37 found that impact-derived olivines in Apollo 14 impact melts have higher Fo values\ncompared to the pristine olivines from Apollo 14 and 12 high-alumina basalts. The lack of difference\nin Fo between olivine grains that coexist with the impact glass and those that do not (Supplementary\nFig. 6), suggests that the high Fo of the CE-6 olivines is a pristine feature.”\nFig. 2 Fo versus Ni content of the single olivine fragments coexist with the impact glass (filled) compared with those\ndo not (unfilled).\nWith respect to the basaltic meteorites with 2.8 Ga ages, we total agree that they may be potentially\nlinked to the Chang'e 6 landing site. Although these 2.8 Ga basaltic meteorites have the highest Ni\nabundance among lunar basalts and meteorites, olivines therein have significantly lower Fo (34-65)\ncomparing with the high Fo and high Ni olivines (92.9-95.6). Therefore, we consider that they have a\ndifferent origin with the high Fo and high Ni olivines in this study.\nA third major weakness is the geochemical modelling used a definitive support for early LMO\ncumulate origin for the High Fo, High-Ni olivines. In geochemical modelling, there are multiple\ndegrees of freedom that are not highlighted in both the discussion of the results and the methods.\nAdditionally, for LMO modelling, the fractional crystallization at 20 kbar matched best with the results.\nHowever, LMO solidification may have had two-steps of crystallization, early equilibrium\ncrystallization followed by fractional (Elkins-Tanton et al., 2011;\nDOI: https://doi.org/10.1016/j.epsl.2011.02.004). If the olivine identified in this work are products of\nearly-LMO crystallization (0-50%), then the geochemical modelling should better reflect equilibrium\ncrystallization as a potential crystallization pathway of the high-Fo olivine, but in Figure 3 and\nExtended Data Figure 4, this is not well represented.\nReply: Thanks for this comment. Following your suggestion, we have revised the model to a two-\nstage LMO solidification: The first 50 vol.% solidification process is assumed to be the equilibrium\ncrystallization, followed by the fractional crystallization at >50 vol.%. The initial depth of the LMO is\nassumed to be 1000km based on geophysical constraints (Zhao et al., 2012; Harada et al., 2014). The\noxygen fugacity was set at the iron-wüstite (IW) buffer. Our revised modeling results align well with\nthe high-Fo, high-Ni olivines, indicating that the compositions of Group III olivines resemble those\nequilibrated and crystallized from the early LMO (Fig. 3 below). For the modeling details please see\nthe Methods in Lines 330 to 341:\n“We modeled the solidification of the LMO following a two-stage crystallization strategy6,7,15,17,58. The\nfirst 50 vol.% solidification process was assumed to be the equilibrium crystallization, followed by the\nfractional crystallization at >50 vol.% in steps with each representing 1/10th of the total magma ocean\nvolume58. The initial depth of the LMO is assumed to be 1000km based on geophysical constraints11,59.\nFor every step, input pressure of the calculation was set to correspond to the middle depth of the LMO58.\nThe oxygen fugacity was set at the iron-wüstite (IW) buffer. The first 50 vol.% solidification process\nwas modeled by GeoPS60 using the thermodynamic database HP63361 and solution models for melt,\nolivine, orthopyroxene, clinopyroxene and feldspar from ref. 62. Calculated step was set as 300. The\nfollowing fractional crystallization was modeled by the menu-driven alphaMELTS interface (version\n2.1) to run the subroutine version of rhyolite-MELTS63,64. Phase equilibrium calculations from liquidus\nto solidus in 1°C increments were performed.”\nFig. 3 Nickel content versus Fo value of the CE-6 olivines. (b) magnified view of the panel (a) with a histogram\nshowing the bimodal distribution of Fo and Ni in CE-6 olivines. Modeled compositions of olivine crystallized from\nthe LMO with initial depth of 1000 km is shown in the solid line. The squares stand for the proportion of remaining\nLMO (%).\nI recommend the authors address the above major weaknesses before publication, especially major\nweakness 1, as this would change the interpretation of the identified olivine from lunar mantle origin\nto lunar Mg-Suite origin.\nReply: We appreciate the valuable comments and suggestions, and have revised the manuscript\naccordingly.\n(2) Major Comments:\n• Line 84: Add the Olivine ID’s for the olivine in a-c similar to the Extended Data Fig. 1.\nReply: Thanks. Done.\n• Line 93: It needs to be explained further why the UMMFs, with Olivines (Fo up to 98) are too high\nto be products of the LMO but not the olivines from this study (Fo up to 96).\nReply: We have now revised the discussion of UMMFs. Please see Lines 130 to 137 for the revision:\n“Forsteritic olivines of exogenous origin have been reported in the lunar collection35. These olivines\nwere characterized by high FeO/MnO (i.e., MnO below detection limit) and extremely low abundances\nof siderophile elements (e.g., <0.008 wt% Ni), which were interpreted as remnants of primitive\nchondritic meteorites impacted onto the lunar surface35. In contrast, the CE-6 olivines have a narrow\nFeO/MnO range, consistent with those of other lunar olivines35,36 (53 to 123; Fig. 3b), and a wide\nrange of Ni abundances (from zero to 682 ppm; Supplementary Table 1). The lack of mass-independent\noxygen isotope fractionation in CE-6 olivines further supports their indigenous lunar origin.”\n• Line 114: This paragraph introduces the different lunar rock suites that have primitive olivine (Fo >90)\nbut does not discuss in detail the high forsteritic olivine in the lunar Mg-suite whose olivine overlap in\nMg# with the olivine of this work. This should be introduced in this paragraph.\nReply: Thanks. We have now revised the discussion of the olivine with Mg-suite origin. Please see the\nrevision in Lines 152 to 164:\n“The most forsteritic pristine olivines in lunar crustal rocks are commonly associated with ultramafics\nand troctolites38. Their petrogenesis may be linked to the pressure-release melting of early LMO\ncumulates during mantle overturn, followed by crystallization at crustal depths22,24,39,40. A prominent\nfeature of these forsteritic olivines in lunar crustal rocks is their Ni depletion compared to the less\nmagnesian olivines in mare basalts7,26,40 (Fig. 4). Longhi et al.26 investigated the Ni abundance in Mg-\nsuite olivines and found that the most forsteritic olivines in lunar ultramafics and troctolites have a\nmaximum Ni content of approximately 300 ppm (Fig. 4). Several models have been proposed to explain\nthe Ni depletion in Mg-suite olivines, such as remelting of LMO cumulate pile composed of primary\ndunite, norite and gabbronorite fluxed in part by KREEP26 and small amounts of metal fractionation\nfrom the Mg-suite parental magmas prior to olivine crystallization38. Despite of different models, Ni\ndepletion is considered to be a typical signature of olivines of Mg-suite origin.”\n• Line 142-144: Some low-Ti mare basalts, in-particular, the low-Ti mare basaltic meteorites that have\nhigh Ni abundances, may be the product of late-stage LMO cumulates (85-96% LMO Crystallization)\nand thus, the comparison made in this sentence is misleading. Elaborating which low-Ti mare basalts\nare being extrapolated for Ni abundances needs to be discussed here.\nReply: Thanks. We have now focused on the discussion on the origin of the three groups of olivine in\nthe revision, and removed the low-Ti mare basalt part.\n• Line 217: This is a little confusing since it is not clearly distinguished in the BSE images which\nolivine are low-Ni versus high-Ni. Therefore, the readers cannot compare texture.\nReply: Thanks for this comment. We have now added the Ni content of the olivines in Fig. 1 (Fig. 4\nbelow) as well as in Supplementary Fig. 1. We also made new rim-core-rim EPMA analyses on the\nolivine grains and show in Fig. 2 in the main text (Fig. 1 above) and Supplementary Fig. 4.\nFig. 4. Backscattered electron (BSE) images of representative olivine fragments (a-c) and olivine aggregates in\nthe troctolitic groundmass (d). Numbers are the Fo values (white) and Ni contents (orange) of olivines (* stands\nfor average values from multiple analyses on a single olivine grain). LA-ICPMS data were shown in parentheses if\nanalyzed. G1-3 are representative energy dispersive spectroscopy (EDS) mappings of the groundmass (outline in\ndash lines). The inclusions in olivines have compositions align with the groundmass.\n• Line 301: References are missing here to refer to which previous studies used 20 kbar.\nReply: We have now modeled LMO crystallization assuming an initial LMO depth of 1000km based\non the geophysical constraints (Zhao et al., 2012; Harada et al., 2014). The input pressure was set to\ncorrespond to the middle depth of the LMO (Arai et al., 2017).\n(3) Minor Comments:\n• Line 36: Lunar mantle is a fundamental part in the puzzle of Moons formation and differentiation.\no Insert “Constraining the lunar mantle is fundamental to understanding the Moon’s formation,\ndifferentiation, and evolution.”\no Reference used is Moriarty III et al. (2021) which focuses on the search for the lunar mantle in remote\nsensing.\nReply: Done.\n• Line 38: insert “the” in front of “Apollo-mission samples”\n• Line 38: Change “Unfortunately” to “However”\n• Line 38: Change “definitively” to “definitive”\nReply: Done.\n• Line 40: Our understanding of the lunar mantle is also based on coordinated analyses of volcanic\nproducts (i.e., mare basalts, volcanic glass beads) whom partial melts are products of mantle cumulates.\nThis sentence needs to be restructured to reflect ways the lunar mantle is studied.\n• Line 40: Change “Physico-chemical” to “Physio-chemical”\nReply: Thanks. Done. Please see our revision in Lines 47 to 49:\n“Our understanding of the ancient and modern lunar mantle is largely based on the physio-chemical\nmodels, laboratory experiments, geophysical and remote sensing observations, and coordinated\nanalyses of volcanic products5-17.”\n• Line 41: Remove “however”\nReply: Done.\n• Line 42: Add “are” before “subject to change”\nReply: Done.\n• Line 44: “Classic lunar magma ocean (LMO) concept” can be changed to “The Lunar Magma Ocean\n(LMO) paradigm”\nReply: Done.\n• Line 45: “Magnesian olivine-dominated lower mantle” are mentioned here but then rephrased as\n“dunite cumulates” in line 47. To reflect consistency, either add the type of magnesian olivine\nlithologies predicted in by models, such as: (i.e., dunite cumulates, ect.) or change dunite cumulates in\nline 47 to less dense mafic cumulates.\nReply: Done.\n• Line 48: Shallow mantle here might not be the right phrase. Consider rephrasing to include density\ninstability alongside gravitational restructuring.\nReply: Thanks. Done. Please see our revision in Lines 54 to 56:\n“However, as the Moon evolved, this initial cumulate stratigraphy was disrupted: the less dense olivine\ncumulates would have ascended upon the overlying high-density mantle via gravitational\nrestructuring19.”\n• Line 49: This sentence needs references\n• Line 49: There isn’t a consensus to what mineral assemblages are in the lunar upper mantle but\nexamples could be provided, for example: Elkins-Tanton et al., 2011 discusses and compares the\nmineral assemblages for the lunar mantle in different LMO models.\nReply: Thanks. We have now added several references focusing on the mineral assemblages for the\nlunar mantle. Please see the revision in Lines 56 to 58:\n“There is currently no consensus on what lithologies of the lunar upper mantle would have been1,5,20.”\n• Line 50: Rephrase to reflect that potential excavated upper lunar mantle material has been identified\nin the SPA basin. References: Moriarty III et al., 2021\nReply: Thanks. Please see our revision in Lines 59 to 60:\n“The South Pole-Aitken Basin (SPA) on the Moon’s farside is believed to be the most promising\nlocation for mantle materials to be exposed on the lunar surface1.”.\n• Line 76: Remove “, and” after “textures, and”\nReply: Done.\n• Line 93: “Olivines with Fo high” should be “Olivines with high Fo”\nReply: We have removed this sentence.\n• Line 128: Change “Now that the question is” to “It is unclear”\n• Line 128: Remove “,” from “LMO cumulates,”\nReply: We have removed this sentence.\n• Line 130: Remove “We will tackle this issue by using Ni content in olivine”\nReply: Done.\n• Line 135: Need references for the lunar DNi diffusion coefficients\nReply: Thanks. Done. Please see the revision in Lines 201 to 204:\n“The empirical Beattie-Jones model44,45 was employed to understand the partition of Ni between\nolivine and liquid: D Ol/L = 3.346 * D Ol/L – 3.66544, in which the D Ol/L = MgOOl/MgOL can be\nNi MgO MgO\ncalculated at each step of LMO crystallization.”\n• Line 152: Figure 3: The “Apollo Mg-Suite” Field is missing “Mg-Suite”\nReply: Thanks, Done.\n• Line 165-167: This isn’t the only hypothesis for the formation of the Mg-suite troctolites. Rephrasing\n“whose petrogenesis is linked” to “whose petrogenesis may be linked” would better fit this sentence.\nReply: Thanks. Done.\n• Line 168: Remove “,” from “Mg-suite olivines,”\nReply: Done.\n• Line 172: Remove “,” from “by KREEP55,”\nReply: Done.\n• Line 173: Change “Despite the debate on the Ni depletion problem” to “Despite competing\nhypotheses on Ni depletion in Mg-suite olivines,”\nReply: Thanks. Please see Lines 163 to 164 for the revision:\n“Despite of different models, Ni depletion is considered to be a typical signature of olivines of Mg-\nsuite origin.”\n• Line 196: Remove “,” from “upper mantle,”\nReply: Done.\n• Line 209: Change “unambiguous” to “ambiguous”\nReply: We have removed this sentence.\n• Line 215: Remove “,” from “primitive melt,”\nReply: We have removed this sentence.\nReviewer #2 (Remarks to the Author):\nReview of Nature Communications submission 559489: “Lunar primitive mantle olivine returned by\nChang’e-6” by Si-Zhang Sheng et al.\nI recommend that the manuscript would be acceptable for publication, after revisions. I’ve attached a\nversion of the submitted manuscript with a few edits to the text, a few additional comments, and some\nsuggestions of additional references.\nReply: We appreciate the valuable comments and suggestions provided by the reviewer.\nThis manuscript reports intriguing high-Mg olivine crystals from the regolith returned by the Chang’e-\n6 spacecraft from the Moon’s South-Pole Aitkin Basin. The data are an important contribution to\nunderstanding the Moon, but I am not convinced that the high-Mg olivines are necessarily samples of\nthe Moon’s mantle. The authors come to this inference because the olivine grains are very magnesian,\nMg* = molar Mg/(Mg+Fe) up to almost 96%, and have high Ni abundances, up to ~650 ppm. No other\nknown lunar olivines have both these characteristics, neither basalts nor basaltic plutonic rocks. The\nauthors show that such olivine compositions could have formed in cumulate rocks from the early Lunar\nMagma Ocean.\nThis argument is reasonable, and I have no serious problems with it. However, there is another possible\ninterpretation that ought to be mentioned in the manuscript; an interpretation which (sadly) changes\nthe focus and apparent importance of the olivines. It is reasonable that the magnesian nickeloan\nolivines crystallized from ultrabasic lavas unlike those discovered so far. This might seem far-fetched,\nbut no more so than finding fragments of mantle olivine.\nReply: Thank you for this comment. We acknowledge that the high Fo, high Ni olivines could have\ncrystallized from an unrecognized ultra-magnesian magma. We have made additional rim-core-rim\nanalyses of the large olivines in the lithic fragment, in order to have a better understanding of the\nolivine populations and their origins. We classified the olivines in the lithic fragment into three groups\n(please see Fig. 1 above), and attributed the Group I and Group II olivines to a Mg-suite origin,\nwhereas the Group III olivine has a different origin. We have expanded the discussion in this revision\naccordingly. Please see Lines 178 to 221:\n“The Group III olivine, characterized by the highest Fo (Fo ) and highest Ni contents (337-\n92.9-95.6\n682 ppm), cannot be easily reconcealed with typical Mg-suite origin. They may crystallize from an\nultra-magnesian lava with Ni contents at least twice as high as those of typical Mg-suite parent\nmagmas. However, no such ultra-magnesian lavas or basalts containing such high-Ni forsteritic\nolivines have been found in previous lunar collections14,38. Given our still limited understanding of\nlunar rock diversity, we cannot simply rule out the existence of such new type of ultra-magnesian and\nNi-rich lavas. If present, the ultra-magnesian lava would have a similar genesis to the terrestrial\nkomatiites42, likely being produced by extensive melting of early mantle cumulates to achieve the most\nprimitive signature of the Group III olivine (Supplementary Fig. 7). The hypothetical Ni-depletion\nevent accounting for the origin of Mg-suite magmas would not have exerted an effect, in order to\nprevent the ultra-magnesian lava from Ni-depletion. Further in-depth study of the CE-6 returned\nsamples and future lunar exploration would help test the possible existence of this new type of lava on\nthe Moon. However, we propose below an alternative possibility for the origin of the Group III olivine:\nthe first olivine crystallized from the LMO.\nLaboratory experiments and numerical simulations of LMO solidification suggest that the Mg-rich\nolivine is the earliest crystallized mineral from the cooling magma ocean, with Fo varying from 96.0\nto 87.6, corresponding to 0 to ~50 % solidification6-9,15,16. Our simulation utilizes the Earth-like Lunar\nPrimitive Upper Mantle (LPUM) composition13 as the initial composition of the LMO. The Taylor\nWhole Moon (TWM) composition43 has an apparently low Mg-number of 84 that is unable to produce\nolivine composition with Fo > 947,9,15. We also model the Ni content of olivine during LMO\nsolidification following a well-established approach26. The empirical Beattie-Jones model44,45 was\nemployed to understand the partition of Ni between olivine and liquid: D Ol/L = 3.346 * D Ol/L –\nNi MgO\n3.66544, in which the D Ol/L = MgOOl/MgOL can be calculated at each step of LMO crystallization.\nMgO\nSeveral studies estimated the initial Ni content of the LMO (also the Bulk Silicate Moon) to be depleted\nby a factor of about 3 ~ 4 compared to the terrestrial upper mantle, around 415 ± 105 ppm14,46.\nConsequently, our simulation on LMO crystallization is in accordance with previous attempts7,26 and\ndemonstrates that the first crystallized olivine has a Fo value of 96.2 with Ni content of 509 ± 131 ppm\n(Fig. 4). The Ni content of olivine increases with ongoing LMO solidification and reaches a maximum\nat Fo of ~92.4, after which the Ni content of olivine drops with decreasing Fo (Fig. 4). Compositions\nof the Group III olivines correspond well with the model results and thus could be of mantle origin.\nThis inference is also supported by our additional modellings on evolution of Ni/Co ratios of olivines\nduring LMO differentiation (Supplementary Fig. 8).\nWe acknowledge that the above two hypotheses for the origin of Group III olivine are not mutually\nexclusive. Given the equilibrium crystallization of olivine at the early stage of LMO solidification and\nthe formation of the ultra-magnesian magma through high-degree partial melting of those olivine\ncumulates, the olivine crystallized from the ultra-magnesian magma is expected to have similar\ncompositions to its source component, i.e., the early LMO cumulate (Supplementary Fig. 7). That is,\nthe Group III olivine resembles the first olivine crystallized from the LMO in terms of composition.”\nFirst, mantle rocks on Earth, and in the only recognized sample of lunar mantle material (Treiman and\nSemprich, 2023), do not contain euhedral or subhedral olivines, i.e. the edges of their olivine grains\nare irregular and not related to the orientations of the grains’ crystal structures. However, the\nmanuscript claims the opposite for the Chang’e-6 magnesian olivines:\n“These forsteritic olivine grains are euhedral to subhedral … .”. So, I do not think that the Chang’e-6\nmagnesian olivines are pieces of mantle material – they are more likely phenocrysts from magnesian\nbasalts. It is possible that there is a translation issue here, as the olivine grains of Figures 1A-C are\nanhedral, but those of Figure 1D are euhedral to subhedral.\nReply: Thank you for this comment. As we have mentioned in the above replies, it is possible that the\nhigh Fo, high Ni olivine could have crystallized from hitherto unrecognized lunar ultra-magmasian\nlavas. Nevertheless, we provide an alternative explanation for the origin of Group III olivine: the first\nolivine crystallized from the LMO. However, as we have revised in line 178-221, these two hypotheses\non the origin of Group III are not mutually exclusive.\nWe agree with the reviewer that terrestrial mantle olivines and the one reported by the reviewer do not\nshow perfect euhedral or subhedral structure. However, we do not necessarily agree with that early\nlunar cumulate olivines show similar textures as the modern terrestrial mantle olivines. Olivines\ncrystallized during the equilibrium crystallization of the early Lunar Magma Ocean are likely to be\neuhedral, similar to the dunite cumulate. Later melt-related metasomatism is able to alter the olivine,\nthough.\nSecond, the Ni contents of the Chang’e-6 magnesian olivines are similar to those in lunar basalts, as\nshown in this graph, which is Figure 1a of Longhi et al. (2010) with data for the manuscript’s olivines\nas smaller symbols. One might reasonably expect that mantle olivine would have Ni abundances as\nhigh or higher than those from basalts. So, Ni itself does not prove a mantle origin.\nReply: Thank you for the comment. We acknowledge that the high Ni content does not necessarily\nprove that these olivines originated from the lunar mantle. Our simulations show that the high Fo, high\nNi olivines are consistent with the composition of olivines crystallized during the early stages of LMO\nsolidification, in agreement with the model results of Longhi et al. (2010). Nonetheless, we agree with\nthe other possibility as you suggest: these olivines may have crystallized from an unrecognized ultra-\nmagnesian magma possibly formed by the extensive remelting of the early LMO cumulates. We also\ndid the modelling of extensive melting of early LMO cumulates and crystallization of olivine from this\nultra-magnesian magma in Supplementary Fig. 7. Please see Fig. 5 below.\nOverall, the two hypotheses on the origin of the high-Fo and high-Ni olivines are not mutually\nexclusive. Given the high degree of melting, the compositions of olivine crystallized from the ultra-\nmagnesian magma would resemble the lunar mantle olivine, comparable with olivine from terrestrial\nkomatiite lavas versus peridotites. Therefore, regardless of the scenario, the high-Fo, high-Ni olivines\nin our study can represent the composition of the early lunar mantle olivine cumulates.\nThird is the question of the high Mg* of the Chang’e-6 magnesian olivines. No known lunar basalts\ncontain olivines so magnesian, but part of the point of Chang’e-6 was to explore an area of unknown\ngeology, and extend our understanding of the diversity of lunar rocks. Is it possible that a lunar basalt\ncould be so magnesian as to crystallize such olivines as phenocrysts? I think the answer is yes. I base\nthat answer on the existence of comparable lavas on Earth, the ultra-magnesian komatiite lavas of the\nCommondale area, South Africa (Wilson et al. 2019). These lavas contain olivine phenocrysts with\nMg* to 96% (comparable to those reported here) and with up to ~2000 ppm Ni. As the Moon’s mantle\nis inferred to contain 1/3 to 1/4 of the Ni as the Earth’s mantle1, this would suggest the possibility of a\nlunar ultrabasic lava with Mg* of ~96% and 500-700 ppm Ni. None has been reported so far, but I\nthink it is possible that the high-Mg olivines reported here could be from a magma like this.\nReply: Thank you for the comment. We agree with you and make an effort to discuss this possibility\nin the revision. However, no such magmas have been identified on the Moon to date (Delano, 1986;\nShearer et al., 2015). If present, the ultra-magnesian lava would have a similar genesis to the terrestrial\nkomatiites, likely being produced by extensive melting of early mantle cumulates to achieve the most\nprimitive signature of the high Fo, high Ni olivine (Fig. 5 below). Additionally, given the high degree\nof melting, the compositions of olivine crystallized from the ultra-magnesian magma would resemble\nthe lunar mantle olivine, comparable with olivine from terrestrial komatiite lavas versus peridotites.\nTherefore, regardless of the scenario, the high-Fo, high-Ni olivines in our study can represent the\ncomposition of the early lunar mantle olivine cumulates.\nTherefore, regardless of the scenario, the high-Fo, high-Ni olivines in our study can represent the\ncomposition of the early lunar mantle olivine cumulates.\nPlease see our revisions in Lines 178 to 221 for the detailed discussion.\nFig. 5 Thermodynamic modeling of Fo versus Ni content of olivine during equilibrium melting of the LMO cumulate\npile (LPUMcp) with an initial composition of the LPUM (Longhi et al., 2006) at 50 per cent solidified from this study\n(blue) and Elardo et al. (2011) (orange). The squares stand for the melting degree (%). The EPMA/LA-ICPS analyses\nof Group III olivines are shown as the blue circles/red diamonds. Error bars represent 2se of LA-ICPMS analysis."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thank you to the authors for addressing all comments. I especially appreciate the additional analyses, figures, and\nmodelling to further justify their argument.\nI recommend this manuscript for publication."}, "author_response": ""}]

Decision letter