A more reduced mantle beneath the lunar South Pole–Aitken basin

Domain: earth_science
Published: 2025-07-30
Editorial outcome: accept

Abstract
The oxygen fugacity (fO2) of the lunar mantle is of pivotal significance in comprehending the formation and evolution of the Moon. However, the fO2 of the lunar farside mantle remains unknown due to the lack of samples returned from the farside. Here, we determine the oxygen fugacity of 23 basaltic fragments from the Chang’e-6 (CE6) soil, the first farside sample collected from the South Pole–Aitken (SPA) basin. The spinel V oxybarometer and pyroxene Eu oxybarometer yield an average fO2 of ΔIW–1.93 ± 0.58 (2σ), indicating a more reduced state compared to the nearside Apollo and Chang’e-5 (CE5) basalts, which have an average fO2 of ΔIW–0.80 ± 0.64 (2σ). Such asymmetry in oxygen fugacity of the lunar mantle can be attributed to two processes: nearside mantle oxidation by a larger amount of Fe sinking into the core triggered by asymmetric crystallisation of the lunar magma ocean, and/or farside mantle reduction caused by S and CO2 degassing during the SPA massive impact. Nevertheless, the reduced nature of the underlying mantle beneath the SPA basin reveals another aspect of lunar asymmetry.

Introduction
The redox conditions, determined by oxygen fugacity (fO2), for basaltic magmas originating from the melting of planetary mantles provide significant insights for determining the compositions of the precursor materials that contributed to planetary formation and the subsequent evolution of their interiors1–3. For example, the oxygen fugacity is crucial for ascertaining the stability and composition of crystallizing phases, the oxidation state of multivalent elements, and the composition and stability of volatile phases in a magma4,5. Therefore, the oxygen fugacity of the planetary mantle is crucial for comprehending its origin and evolution. The Moon displays a global asymmetry encompassing basalt distribution6, geomorphology7, chemical composition8,9 and crustal thickness10. However, the question of whether there is an asymmetry of oxygen fugacity between the lunar nearside and farside remains unanswered. This is attributable to the fact that all studied lunar samples were collected from the nearside of the Moon11,12. The Apollo lunar mare basalts and pyroclastic glasses yielded an oxygen fugacity of ΔIW –1 using olivine/spinel V oxybarometers4,13,14. A recent study indicates that the lunar mantle fO2 derived from CE5 basalts is ΔIW–0.84 using the same oxybarometers15. These results indicate that the fO2 of the lunar nearside mantle is nearly identical15.

China’s Chang’e-6 (CE6) mission, which successfully returned the first lunar farside samples (1935.3 g) from the SPA basin (41.625°S, 153.978°W), provides a unique opportunity to investigate the potential variation in fO2 between the lunar nearside and farside. Recent studies have revealed that the majority of the basaltic fragments in the CE6 soil are low-Ti basalt formed 2.8 billion years ago (Ga) (refs. 9,16,17), which are characterized by ultra-depleted Sr–Nd isotopic compositions18 and low water content19. However, the fO2 of this basalt has not been determined.

In this work, we conducted comprehensive analyses of the spinel V oxybarometer and pyroxene Eu oxybarometer on 23 CE6 basaltic fragments, with the objective of constraining the oxygen fugacity of the lunar farside mantle. The result indicates that the fO2 of CE6 basalts is more reduced than that of the nearside Apollo and CE5 basalts. The findings will facilitate a more comprehensive understanding of the oxygen fugacity between the nearside and farside and offer new insights for the origin of lunar asymmetry.

Results
Petrography and mineralogy of the CE6 basalts
The CE6 basaltic fragments examined in this study exhibit a diverse range of textures, including poikilitic, subophitic, and coarse-grained textures (Supplementary Fig. 1 and Supplementary Table 1). The primary minerals include clinopyroxene, plagioclase, and ilmenite, while the secondary minerals consist of olivine, silica, spinel, troilite, and apatite. The clinopyroxene exbibits notable chemical variability, with compositions of En 0.2–60.7 Fs 20.8–91.1 Wo 7.4–39.5 (Supplementary Fig. 2 and Supplementary Data 1). The rare earth element (REE) concentrations of clinopyroxene are significantly lower than those of the CE5 basalts (Supplementary Fig. 3 and Supplementary Data 2; ref. 15). The plagioclase has a relatively more homogeneous composition (An 82.2–92.5) and lower TiO2 contents than those in the CE5 basalts (Supplementary Fig. 4 and Supplementary Data 1; ref. 15). The modal abundance of olivine in the CE6 basalts is less than that observed in the CE5 basalts15,20. The majority of olivine grains are fayalite and occur in mesostasis, exhibiting a Fo [molar (Mg/Mg+Fe)] ranging from 0.3–2.1, indicative of a late-stage crystallization product. The mineral chemistry of the CE6 basaltic fragments studied here aligns with the low-Ti basaltic fragments previously reported18, which yielded an Pb-Pb age of ca. 2.8 Ga (refs. 9,16,17).

The spinel in the CE6 basalts manifests in two forms, which are Cr-spinel and Ti-spinel. The Cr-spinel that crystallized in the early stage is less abundant than the Ti-spinel (Supplementary Fig. 5). The V contents in the spinel (2372–6621 μg g−1; Supplementary Data 3) are significantly higher than those of the CE5 basalts15. The occurrence of Fe-Ni metal grains in the CE6 basalts is commonly observed. They occur at the grain boundaries of Cr-spinel in the early phases, at the boundaries of late-formed plagioclase and pyroxene, or within the troilite formed in the late-stage crystallization (Supplementary Fig. 6). The Ni and Co concentrations of the Fe-Ni metal vary from 0.36–12.24 wt% and 0.82–2.38 wt% (Supplementary Data 4), respectively, which fall within the field of endogenous Fe-Ni metals21. Minor quantities of troilite (~0.37 wt%) have been observed in the CE6 basalts (Supplementary Table 1). They occur sporadically as inclusions within pyroxene or plagioclase, or as individual grains in the late-stage mesostasis associated with fayalitic olivine and apatite (Supplementary Fig. 7). The δ34S of the troilite varies between –2.6‰ and 1.7‰, with the inclusions having higher δ34S values than the grains in the mesostasis (Supplementary Data 5). This trend was also observed in the CE5 basalts and has been considered to be the result of magma degassing22,23.

The fO2 of the CE6 basalts
The fO2 of the CE6 basalts was determined using the clinopyroxene-melt Eu partition coefficient (DEu) and the spinel-melt V partition coefficient (DV). These coefficients can be calculated using the Eu concentrations in clinopyroxene and the V concentrations in spinel measured in this study, as well as previously reported whole-rock Eu and V concentrations18. The whole-rock Mg# of the CE6 basalts is ~30, with average Eu and V concentrations of 1.99 μg g−1 and 80.53 μg g−1, respectively18. According to the equilibrium Fe-Mg partition coefficient between pyroxene and melt (KD(Fe-Mg)cpx-liq = 0.28 ± 0.08; ref. 24), the clinopyroxene in equilibrium with the CE6 basalts is expected to have Mg# values ranging from 54.3–60.5. The fO2 of the CE6 basalts, as determined by the clinopyroxene Eu oxybarometer25, varies from ΔIW –2.62 ± 0.22 to ΔIW –1.25 ± 0.54, with an average of ΔIW –1.97 ± 0.60 (2σ) (Fig. 1 and Supplementary Data 6). The fO2 of the CE6 basalts derived from the spinel V oxybarometer26 ranges from ΔIW –1.82 ± 0.08 to ΔIW –1.63 ± 0.08, with an average of ΔIW –1.73 ± 0.04 (2σ) (Fig. 1 and Supplementary Data 7). The average fO2 of the CE6 basalts is then estimated to be ΔIW –1.93 ± 0.58 (2σ). Given that the Mg-rich clinopyroxene and Cr-spinel are the earliest crystallizing phases of the basalts20, this fO2 estimation can be considered to represent the redox state of the original melt of the CE6 basalts.

Methods
Sample preparation and properties
This study utilized 23 basaltic fragments sourced from several Chang’e-6 (CE6) lunar soil samples (including CE6C0200YJFM001EGP01, CE6C0200YJFM001EGP02, CE6C0200YJFM001EGP03, CE6C0200YJFM001EGP04, CE6C0200YJFM001EGP05, CE6C0200YJFM001EGP06, CE6C0200YJFM001EGP07, CE6C0200YJFM001EGP08, CE6C0200YJFM001EGP09 and CE6C0200YJFM001EGP10) provided by the China National Space Administration. They were embedded in one-inch epoxy mounts and polished prior to examination. These basaltic fragments exhibit different textures, such as typical sub-ophitic, poikilitic, and coarse-grained textures are also found in these basaltic fragments. Coarse-grained texture denotes to the basaltic fragments >100 μm, characterized by big and few mineral grains (2–5), which do not distinctly align with the aforementioned textures. The sizes of these basaltic fragments range from a minimum of 0.3 mm to a maximum of 2.7 mm. All basaltic fragments comprise principal minerals such as clinopyroxene (20.22–79.89 vol%), plagioclase (13.75–59.31 vol%), and ilmenite (0.16–23.34 vol%), along with auxiliary minerals including olivine (0–27.01 vol%), silica (0–7.77 vol%), spinel (0–4.47 vol%), sulfide (0.04–3.46 vol%), phosphate (0–2.09 vol%) (Supplementary Table 1) and other minor minerals (Fe-Ni metal, whitlockite, baddeleyite, zirconolite, tranquillityite, etc.).

Scanning electron microscopy
Petrographic analyses, encompassing back-scattered electron (BSE) imaging, textural assessments, and mineralogical abundance modeling of the CE6 lunar basalts, were conducted utilizing a Thermofisher Apreo field emission scanning electron microscope (FE-SEM) equipped with a Bruker XFlash 60 energy dispersive spectrometer (EDS) detector in the Electron Microscopy Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS)51. The EDS data were obtained using the Maps-Nanomin automated mineralogy software. This study utilized electron beam currents ranging from 6.4 nA–13 nA and acceleration voltages between 15 kV and 20 kV. An X-ray for mineral analysis was conducted with a step size of 2 μm and a dwell period of 8 ms.

Electron probe micro-analyzer (EPMA) major and trace elemental analysis
EPMA can effectively identify the principal constituents of diverse minerals with excellent spatial resolution (1–5 μm), and high detection limits of up to several hundred μg g−1. The major elements in olivine, clinopyroxene, plagioclase, and ilmenite in the CE6 lunar basalts were conducted using the JEOL JXA8100 electron probe at IGGCAS. The experiment utilized an acceleration voltage of 15 kV, a beam current of 20 nA, a beam diameter of 3 μm, and a counting duration of 10–30 s. The calibration of data using various natural minerals and synthetic oxides aimed to enhance the accuracy and precision of these major elements in the primary minerals of the CE6 basalts. The analytical errors for major elements are below 1.5%.

Recent advancements in EPMA have facilitated the detection of trace elements, effectively addressing the limitation of laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) in detecting trace concentrations due to the diminutive size of minerals. Recent research introduced a high-precision EPMA technique capable of concurrently identifying major and trace elements in spinel, demonstrating accuracy in the spinel of CE5 basalts52. On this basis, we also employed the JEOL JXA8100 electron probe micro-analyzer at IGGCAS to estimate the concentrations of major and trace elements in the spinel of the CE6 basalts. Spinel was examined using an acceleration voltage of 25 kV and a beam diameter of 1 μm. The beam currents for trace elements and major elements were 200 nA and 60 nA, respectively. The major and trace elements were identified utilizing five analytical crystals as follows: two TAP for Mg (Kα), Al (Kα), and Si (Kα), one LIF for Cr (Kα), Ni (Kα), and Mn (Kα), one LPET for Ti (Kα) and V (Kα), and one LLIF for Fe (Kα), Co (Kα), and Zn (Kα). Corrections for peak overlap of Cr (Kß) to Mn (Kα) and Ti (Kß) to V (Kα) were implemented. Natural minerals and synthetic oxides employed for high-precision elemental analysis included rhodonite (Si and Mn), rutile (Ti), FeS2 (Fe), ZnS (Zn), Cr2O3 (Cr), CoO (Co), NiO (Ni), MgO (Mg), and V2O5 (V). The peak counting durations were as follows: 10 s for Al, Mg, and Fe; 20 s for Cr; 80 s for Ti, V, Co, and Zn; 120 s for Mn and Ni; and 240 s for Si. The homogeneous spinel standard LBS13-04 was examined to assess machine drift, hence enhancing the accuracy and precision of these trace elements of spinel derived from the CE6 basalts. The mean detection limits (3σ) for trace elements ranged from 16 μg g−1 to 55 μg g−1, with a specific value of 18 μg g−1 for vanadium (V). The estimated accuracies (1σ) for the major elements and trace elements were superior to ±2 and ±6% (~2% for V), respectively.

The compositions of troilite in the CE6 basalts were examined with a JEOL JXA8100 electron probe micro-analyzer at IGGCAS. The experiment employed a 40 nA beam current, a 20 kV accelerating voltage, and a beam diameter of 2 μm. The analytical standards included FeS2 (S and Fe), apatite (P), rutile (Ti), pure Ni, Co and Cu metals (Ni, Co and Cu), Ca-Al silicate glass (Si), PbCrO4 (Cr), ZnS (Zn). The peak counting times were 10 s for Fe, S and Ni; 20 s for Ti, Si, P, Cu and Zn; and 40 s for Cr and Co. The detection limits (μg g−1) are as follows: 150 for Fe, 70 for S, 180 for Ni, 50 for Ti, 45 for Si, 40 for P, 60 for Cu and 100 for Zn. The analytical and testing protocols were derived from a previously published study54. The laser diameter is roughly 44 um with a repetition rate of 5 Hz. The laser energy density is approximately 5 J/cm2. Helium was used as a carrier gas to enhance the signal. An acoustics smoothing instrument (ASI) was employed to enhance signal accuracy. Each spot analysis started with a 20 s background collection (gas blank), succeeded by a cleaning process including two laser pulses, prior to the 35 s ablation data capture. The NIST SRM 610 and ARM-1 glass were utilized for external calibration. The GOR128-G and GOR132-G standards were employed for quality control assessment. The NIST SRM 610, ARM-1, GOR128-G, and GOR132-G standards were evaluated for accuracy, with each set comprising ten measurements. Data reduction was implemented by bulk normalization as a 100 wt% strategy utilizing the Iolite 3.4 software package with a self-developed data reduction scheme code. The mean detection limits (2σ) for most trace elements are better than 0.05 μg g−1. The analytical errors for the majority of trace elements (>0.05 μg g−1) are superior to 10% (σ). Additionally, the LA-ICP-MS approach may concurrently measure the concentrations of major elements at identical locations, with an accuracy above 5% (σ).

NanoSIMS S isotopic analysis
The Sulfur isotopic ratios of troilite were conducted using a CAMECA nano-scaled secondary ion mass spectroscopy (NanoSIMS 50L) at IGGCAS. The samples were coated with carbon and pre-loaded in the sample chamber two days ahead in advance to enhance vacuum quality and reduce interference from 32S1H− and 33S1H−. The vacuum in the analysis chamber was approximately ~5 × 10−10 mbar. The samples were pre-sputtered using a high primary beam of around 500 pA across a region of 10 × 10 μm2 area for 2–3 min to eliminate the carbon coating and to stabilize the secondary ions yield. For sample analysis, the raster area was diminished to 5 × 5 μm2 to 7 × 7 μm2, contingent upon the size of troilite, while maintaining the same beam current. In the static multicollection mode with nuclear magnetic resonance-based magnet regulation, the secondary ion species of 32S, 34S were detected with FC1 and FC2, respectively. The FC preamplifier boards were set with 1011 Ω resistors for 34S, 1010 Ω resistors for 32S, and temperature regulation was implemented to stabilize the FC preamplifiers. The equipment was calibrated to attain a mass resolving power of around 5000. Under these operational conditions, the counting rate of troilite ranged from the counting rate of troilite was 5 × 108 to 6 × 108 cps for 32S−. Each analysis has 200 cycles with a counting duration of 1 s per cycle. And the background of FCs was measured 20 s before and after each analysis to correct the baseline. Two troilites Canyon Diablo and Nandan were examined to assess the precision of our measurements. The external reproducibility (2SD) was better than 1.0‰ for the measurement of 34S/32S. The measured 34S/32S ratios are expressed as δ34S in the standard per mil notation (‰): δ34S=[(34S/32S sample)/(34S/32S standard –1)] × 1000, where the standard adopted is VCDT (Vienna Canyon Diablo Troilite) with a 34S/32S value of 0.044162 (ref. 55).

Spinel V oxybarometer
Experimental research shows that the partitioning behavior of V between minerals and melt is sensitive to redox conditions, shown by the increasing Dspinel/meltV with decreasing oxygen fugacity56,57. Previous results show that V oxybarometers have enormous potential because these methods allow for applicability over a range of redox conditions from the most reduced materials in the solar system, to the most oxidized terrestrial magmas13,56,57. The oxygen fugacity of volcanic pyroclastic glasses and lunar basalts has been determined by V oxybarometers14,15. This not only validates the efficacy of V oxybarometers but also aids in comprehending the oxygen fugacity of the lunar mantle. Given the similarity in whole rock MgO content18,58 and pyroxene composition15,20 (Supplementary Fig. 2) exhibited by the CE6 and CE5 basalts, the temperature of both basalts was estimated to be ~1200 °C. The experimental data of Dspinel/meltV and fO2 at approximately 1200 °C (ref. 26) were compiled, and the following equation was obtained by means of fitting: fO2 (ΔFMQ) = 3.193 (±0.228) – 1.930 (±0.115) × ln(Dspinel/meltV) (Supplementary Fig. 11a). In this equation, ΔFMQ signifies the fO2 disparity between the experimental fO2 and FMQ buffer. The V concentration in this study was ascertained from three CE6 basaltic fragments, with an average value of 80.53 μg g−1 (ref. 18). ΔIW is the fO2 disparity between experimental fO2 and IW buffer, with ΔIW is roughly 3.5 log units lower than ΔFMQ (ΔIW = ΔFMQ – 3.5) at a temperature of 1200 °C (ref. 15).

The calculation of the fO2 uncertainties was conducted by propagation law, utilizing the uncertainties of the equation and the V analysis by EPMA. As the SE values for V analysis are almost identical (~2%), the uncertainties of fO2 for each point are also identical (±0.08). The applicable lower limit of the above equation is approximately ΔIW –0.5, whereas the oxygen fugacity of CE6 basalts is estimated at approximately ΔIW –1.73, a value for which a model extrapolation is required. However, the magnitude of this model extrapolation is minimal (from ΔIW –0.5 to ΔIW –1.73). In principle, Dspinel/meltV and fO2 should remain negatively correlated (Supplementary Fig. 11a). Therefore, the uncertainty engendered by this model extrapolation is deemed to be minimal.

Pyroxene Eu oxybarometer
The Eu mineral/melt partition coefficient (DEu) depending on the Eu3+/Eu2+ ratio in the magma is predominantly a function of the prevailing redox conditions during crystallization25,59–61. This reliance of the Eu2+/Eu3+ ratio on fO2 is exemplified by the reaction: Eu3+ + 1/2O2− → Eu2+ + 1/4O2 (ref. 62). The partitioning of Eu between pyroxene and melt as a function of fO2 has proven to be an effective oxybarometer, especially for basaltic samples from the Mars and Moon as their pyroxenes display zonation in REE concentrations that aligns with fractional crystallization from their parent magma20,25,61,62. This implies that they have preserved their initial magmatic DEu ratio between pyroxenes and melt, being unaltered by subsequent processes. Moreover, pyroxenes are the first REE-bearing minerals to crystallize from the parent melt of lunar basalts28,63. Therefore, their DEu ratios remain unaffected by the previous crystallization of plagioclase (which scavenges Eu) and are primarily a function of magmatic redox conditions. Eu2+ exhibits lower compatibility in pyroxene than Eu3+. The greater the negative anomaly in pyroxene, the lower the oxygen fugacity during its crystallization. Research indicates that the distribution coefficient (DEu) between pyroxene and melt constantly increases with elevated fO2 (ref. 25). We compiled four partition coefficients (DEu = 0.086, fO2 = ΔIW–1; DEu = 0.157, fO2 = ΔIW; DEu = 0.182, fO2 = ΔIW+1; DEu = 0.274, fO2 = ΔIW+3.5) to construct a function relating fO2 and DEu, expressed as fO2 (ΔIW) = 24.503 (±2.990) × DEu – 3.407 (±0.560) (Supplementary Fig. 11b). The R² of this function is 0.971, aligning with ref. 25. The calculation of the fO2 uncertainties was conducted by propagation law, utilizing the uncertainties of the equation and the Eu analysis by LA-ICP-MS. The Eu content of pyroxene in the CE6 basalts ranges from 0.055 μg g−1 to 0.247 μg g−1 and the 2SE values range from 0.018 μg g−1 to 0.082 μg g−1. The calculated fO2 uncertainties range from 0.22 to 0.78. The lower limit of the equation that is applicable is approximately ΔIW –1, while the oxygen fugacity of CE6 samples is estimated at approximately ΔIW –1.97. This also requires a modal extrapolation. However, the magnitude of this model extrapolation is relatively small (from ΔIW –1 to ΔIW –1.97). In principle, the positive correlation between DEu and fO2 should not change (Supplementary Fig. 11b). Therefore, the uncertainty engendered by this model extrapolation is also deemed to be minimal.

Sulfur degassing and primitive S content of CE6 basalts
The δ34S of the CE6 primitive magma prior to degassing and the bulk CE6 magma was determined to be approximately 0.2‰ ± 0.5‰ and –1.6‰ ± 0.5‰ derived from the maximum δ34S (1.7‰ ± 0.5‰) and the average δ34S (–0.1‰ ± 0.5‰) of the troilite (Supplementary Data 5), respectively, utilizing the equilibrium fractionation factor of 1.0015 between troilite and melt64. The degassing loss of sulfur was determined to be ~45% according to the equilibrium Rayleigh fractionation model. Equations and variables employed: δ34Sm = (δ34Si + 1000)f(ɑ–1) – 1000; C = Ci(1–F)D–1; α = 34S/32Sgas / 34S/32Smelt; D = Sgas/Smelt; f = the fraction of S remained in the melts; F = fraction of degassing removed; m = residual melt; i = initial melt. Here the average αgas–melt value (1.003) was chosen under reduced conditions based on ref. 65. The estimated average modal abundance of sulfides in the CE6 basalts is approximately 0.37 wt% based on EDS analysis (Supplementary Table 1). The sulfur abundance of the bulk CE6 basalts was estimated to be approximately 1343 ± 672 ppm derived from the average sulfur content of troilite (36.3 wt%; Supplementary Data 8) and its modal abundance (~0.37 wt%), accounting for a 50% error in the statistical modal abundance. The primordial sulfur abundance of the CE6 magma is estimated to be approximately 2442 ± 1221 ppm predicated on a degassing rate of about 45%. Data of CE5 and Apollo samples are from refs. 22,66, respectively.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_2": "Summary: The manuscript reports measurements of the oxygen fugacity of lunar far side magmas. The samples were collected within the South Pole Aitken (SPA) basin by the Chang’e 6 mission. These are the only samples that have been collected from the far side or SPA, and so any measurement on the samples is new and scientifically highly valuable. The results show that the far side magmas have an oxygen fugacity that is about 1 log unit lower than magmas from the near side. This is a very interesting and important result. It adds to the existing list of differences between the near and far side, most notably in crustal thickness and the abundance of basaltic magmas, and maybe provides new clues to the origin of those differences. The manuscript suggests two mechanisms by which the SPA impact caused the oxygen fugacity of the far side mantle to be lower than the near side. The manuscript further suggests that the paucity of basaltic lavas on the far side may be due to the low oxygen fugacity, as decreases in oxygen fugacity have been shown to increase mantle melting temperature.\n\nGeneral comments: The new oxygen fugacity measurements presented in this manuscript are highly significant to our understanding of the evolution of the lunar magma ocean and provide additional context to the lunar nearside-farside dichotomy. The chemical analyses seem to be of high quality and the oxybarometers selected for this study are generally appropriate for application to lunar samples. However, it should be noted that the models were initially applied to terrestrial and Martian compositions with more oxidizing oxygen fugacities than found in the CE6 samples. Additionally, there are some small discrepancies in the fit and uncertainties of these models, although any adjustments result in even more reducing oxygen fugacities for the CE6 samples, and therefore ultimately do not impact the major finding of this study, that the farside is more reduced than the nearside.\n\nThe two mechanisms by which the SPA impact produced the low oxygen fugacity seem plausible, but both have significant problems associated with them. These issues are actually quite well described in the text, so are not repeated here. But overall, one is left feeling like neither of the proposed mechanisms seems viable, even to the authors. This manuscript would be improved by exploring other models for the difference in oxygen fugacity. The other main conclusion of the manuscript is that the paucity of basaltic volcanism on the far side is due to the lower oxygen fugacity increasing the melting temperature of the mantle. This seems implausible as a 1 log unit decrease in oxygen fugacity should only increase mantle melting temperature by ~10°C, which does not seem enough to explain the stark difference in volcanism between the near and far sides of the Moon. On the other hand, the paper provides data indicating that the mantle that produced the CE6 basalts was highly melt-depleted. This would provide a more plausible explanation for the paucity of volcanism. So overall, the finding of low oxygen fugacity on the far side is significant, but the explanations given for its origin are unconvincing, and the causal connection of the low fO2 to the low amount of volcanism on the far side seems unlikely.\n\nSpecific comments:\nEffect of mantle depletion on mantle solidus: The pyroxenes in the CE6 lavas have ~10 times lower trace element concentrations than the pyroxenes in the CE5 lavas from the near side (Supplementary figure 3), suggesting that the melt was also low in trace elements and was sourced from a strongly melt-depleted source. This idea is supported by the title of reference 18 in the paper ‘Ultra-depleted mantle source of Chang’e-6 basalts from the South Pole – Aitken Basin’. We did not have access to this paper, so can only guess that it provides geochemical analyses of the CE6 basalts, showing them to be as depleted as the pyroxenes in the current manuscript suggest they are.\nIf the mantle on the far side is melt-depleted, it would provide a straightforward explanation for the paucity of volcanism on the far side. Looking at papers on the effect of mantle depletion on the melting temperature of the mantle (Robinson et al, 1998, EPSL 155: 97-111; Maaloe, 2004, Min and Petr 81: 1-17) shows that differences between fertile and depleted lherzolite (10-20% melt depletion) melting temps can be 50-70°C, and between lherzolite and harzburgite (>20% melt depletion) can be 150 to 250°C. This is much larger than the effect of fO2 on the solidus, and seems like a much more plausible explanation for the lack of melts on the far side. So then the question is whether the cause of the mantle melt depletion on the far side mantle could also be linked to the lower fO2. If so, then the low fO2 and paucity of volcanism are both effects of the same underlying cause (depleted mantle), rather than low fO2 being the cause of the low volcanism.\nLine 98: Figure 1 should include a description of what the error bars represent.\nLine 154: Add “and” after “in the melt”.\nLine 162: The manuscript states that the process of CO degassing cannot be verified by current analytical techniques. It would be helpful to include what is limiting in analytical techniques to confirm this model, and what types of measurements would need to be completed?\nLine 176: Effect of oxygen fugacity on the solidus of the mantle – The manuscript does not give a quantitative estimate of how much 1 log unit oxygen fugacity should change the mantle solidus. Looking at the Lin et al (2021, cited in the manuscript), their experiments show about a 40°C increase in melting temperature over 6 log units of fO2 at 1 bar total pressure. So less than 10°C per log unit. A 10°C difference seems like it is too small of an effect to explain how few basaltic lavas there are on the far side compared to the near side.\nLine 304: Change “researches” to “research”. Change “show” to “shows”.\nLine 313: The cited reference for the spinel V oxybarometer (Wang et al., 2019- reference 24) is calibrated to FMQ-4 (IW-0.5). It would be beneficial to include a brief justification for the extrapolation of this calibration to the lower oxygen fugacities of the CE6 samples, including a discussion of resulting uncertainties.\nLine 313: The Wang et al. (2019) oxybarometer equation includes uncertainties. Our brief set of calculations suggest that accounting for these uncertainties would not change the ultimate conclusions that CE6 was more reducing, but it would be beneficial to include a note considering these uncertainties in the supplement.\nLine 337: The cited reference for the Eu oxybarometer (Karner et al., 2010- reference 23) does not provide an equation for the model. The regression used in this manuscript to find the cited equation should be described in the supplement for replication by future readers. We explored fitting the data and the best-fit line we find varies slightly from that described in this manuscript (25.2525 × DEu – 3.5378). However, this only produces more reducing fO2.\nLine 337: Karner et al. (2010) (reference 23) oxybarometer is calibrated to IW-1. The range of calculated oxygen fugacities for the CE6 samples is as low as IW-2.6. It would be beneficial to include a brief justification for the extrapolation of this calibration, including a discussion of resulting uncertainties.\nSupplementary Data 6: How is the error calculated for each fO2? The 2SE values are the same for each sample in Supplementary Data 7 (spinel V oxybarometer) but are different for each sample in Supplementary Data 6. This should be clarified in the supplement.\nSupplementary Data 7: The spinel Cr# seem to be high (~79-80). When we calculated the Cr# of samples using the geochemical analyses in Supplementary Data 3, we were not able to reproduce those values and instead find Cr# values between ~67-68. These calculations should be double-checked. Additionally, the highest spinel Cr# of the literature compiled by Prissel et al. (2016, American Mineralogist, Appendix Table 1) is 68 (Dymek et al., 1975). If there are higher Cr# lunar spinel in line with the measurements from this manuscript, it would be beneficial to include those.", "reviewer_3": "Review of Zhang et al., Nat Comm\nBy Keith Putirka\n2/19/25\nThis paper present some highly intriguing results from the farside lunar surface that deserve widespread attention in a publication like Nat. Comm. I am not sure that I accept that observed contrasts in fO2 could affect melt productivity to the extent needed here, but I do think that Figure 1 is important and compelling and will add to our thoughts about Moon’s formation. I’ll touch on the fO2 and melt productivity below, but I would also suggest to the authors that they may be too quickly dismissing Stevenson’s core-formation scenario for explaining contrasts in fO2. I am not expert on Moon formation, and perhaps that earlier model is not valid, but Sossi et al. (2024) and others make a good case that the Mars-size impact model for Moon is not entirely certain. The main problem, as I understand it, is that Moon and Earth are nearly identical in their isotopic compositions but any nominal “Theia” impactor would likely have different isotopic ratios – and most models suggest that Theia, not Earth would contribute the larger fraction to Moon’s total mass. Fischer et al. argue that Theia could have lost its silicate mantle prior to impact. That’s an interesting idea. But even Mercury preserves a large fraction of its silicate material, so this is not a compelling solution to the problem.\nSo I can’t help but wonder if the contrasts in fO2 might play some larger role in our tests of Moon-forming hypotheses. But again, I am too little educated on such issues to say much about this possibility. In any case, this observed difference in fO2 appears to be real and should impact how we think about the evolution of the moon\nAs to the minor but still somewhat emphasized issue of partial melting, I doubt that the observed fO2 contrast is large enough to greatly affect melt productivity and so explain basalt thickness contrasts between the nearside and farside. For example, Lin et al. (2021) require nearly 11 orders of magnitude to obtain a 105 degree contrast in the mantle solidus; that means a ca. 1 order of magnitude should then translate to only about a 10 degree difference in the mantle solidus. That’s at the limit of what we can detect in experiments. Another concern is that the more recent study by Lin et al. that is also cited here involves experiments at very high pressures – much higher than in the lunar interior, and so the relevant reactions might not apply in the lunar interior.\nIf the authors want to push the issue, they could quantify the analysis a little better: they could calibrate a delta-T in solidus temperature relative to changes in fO2 using all the Lin et al. experiments (as I’ve just done in the above paragraph), and estimate the possible shift in solidus based on Figure 1. And then they could look at melt productivity models by, say, Charlie Langmuir, or Paul Asimow, or Claude Herzberg, etc. and try entering such a contrast in solidus temperature translates to a predicted thickness of basalts. And then tell us what the contrasts are in mare basin basalt thickness and put this altogether to see if fO2 has any chance of explaining the observed mare basin basalt thicknesses.\nOn another issue, the authors state on line 113 that there are differences in volatile contents and degrees of differentiation between nearside and farside compositions. But their cited reference (#27) doesn’t appear to make such a comparison. It would be very helpful (even if I have overlooked something in cit. no. 27) to have a plot that compares these compositions such as a plot of MgO contents vs. volatile contents, or maybe alkalis if they are a proxy for volatiles.\nI hope these comments are helpful.\nKeith Putirka\nLine-by-line comments\nLine 30: here and perhaps in the abstract as well it’s probably best to refer to the asymmetry as being relative to the nearside and farside mare basalt as readers who are not aware of the issues regarding lunar volcanism may think of Earth’s ocean basins as anologies, where lots of volcanism = thick crust.\nLines 34-35: which site has thicker crust? Cite the actual estimates of crust thickness at both localities.\nLines 36-39: And if the proposed explanation is an excess of heat-producing elements, would that not only provide more volcanism, but more crust as well? Unless one believes that the heat-producing elements on the near side mantle were introduced only later, after the crust had formed. But then how does that work?\nThe more passive, transport-restricted model of Head et al., seems to make more sense although the discovery of a contrast in fO2 may be important. Let’s read further…\nLine 43: I think it’s OK to cite other papers besides Lin et al. (2024) but only Lin et al. (2024) (citation # 12) and Lin et al. (2021) (citation #49) truly make the point that the authors are attempting to emphasize here. Until those two experimental studies, the effects of fO2 were not expected to be important, being overwhelmed by effects such as MgO, total alkalis, water, and pressure; the earlier studies cited here do not affect that conventional view. What is impressive about the Line et al. (2021, 2024) studies is that they show that solidus temperatures can be affected by100s of degrees C or more with changes in fO2.\nI know of no other study that shows the same effect, though no one else has precisely looked for it. In think in their 2021 study, they needed 11 orders of magnitude change in fO2 to get ca. 100 degrees of change in solidus T. So it might still be a stretch to apply it here.\nIn any case, it’s the Lin et al. papers that should be cited here and not the others as the earlier papers do no indicate a change in solidus T with changes in fO2.\nLines 65-66: Mg# can only range form 0 – 1 (so the range to 2.1 must be a typo).\nLine 113: the authors write “melting and fractional crystallisation processes they have experienced” I understand that they cite a reference but a plot of MgO vs. K2O, or even better, MgO vs. some estimate of volatile concentrations would be helpful here. The cited paper (ref. 27) does not compare nearside to farside compositions. A simple chart, using the color coding in the authors Fig. 1, is needed.\nLines 136-139: Or, maybe the large impactor hypothesis is wrong.\nLines 182-183: The authors write “basalts have similar Mg# values (~30), they could have experienced equivalent degrees of magma evolution, thus potential Tp deviations resulting from different degrees of fractional crystallisation can be eliminated50.” Since they calculate Tp, this is not a big deal, but I’d just like to note that Mg# is not the defining quality. Two rocks with very different FeO contents can have the same Mg# but the rock with higher FeO would also have a higher MgO content and so could have a very different Tp."}, "author_response": "Point-by-point response tothe issues raised by the referees\nNote: Comments and suggestions are in black, Responses in blue, and the revisions\nin red.\nReviewer #1\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse: Thank you for your contribution to the review of our manuscript.\nReviewer #2\nSummary: The manuscript reports measurements of the oxygen fugacity of lunar far\nside magmas. The samples were collected within the South Pole Aitken (SPA) basin by\nthe Chang’e 6 mission. These are the only samples that have been collected from the far\nside or SPA, and so any measurement on the samples is new and scientifically highly\nvaluable. The results show that the far side magmas have an oxygen fugacity that is\nabout 1 log unit lower than magmas from the near side. This is a very interesting and\nimportant result. It adds to the existing list of differences between the near and far side,\nmost notably in crustal thickness and the abundance of basaltic magmas, and maybe\nprovides new clues to the origin of those differences. The manuscript suggests two\nmechanisms by which the SPA impact caused the oxygen fugacity of the far side mantle\nto be lower than the near side. The manuscript further suggests that the paucity of\nbasaltic lavas on the far side may be due to the low oxygen fugacity, as decreases in\noxygen fugacity have been shown to increase mantle melting temperature.\nResponse: Thank you for your comprehensive and positive evaluation of our\nmanuscript.\nGeneral comments: The new oxygen fugacity measurements presented in this\nmanuscript are highly significant to our understanding of the evolution of the lunar\nmagma ocean and provide additional context to the lunar nearside-farside dichotomy.\nThe chemical analyses seem to be of high quality and the oxybarometers selected for\nthis study are generally appropriate for application to lunar samples.However, it should\nbe noted that the models were initially applied to terrestrial and Martian compositions\nwith more oxidizing oxygen fugacities than found in the CE6 samples. Additionally, there\nare some small discrepancies in the fit and uncertainties of these models, although\nany adjustments result in even more reducing oxygen fugacities for the CE6 samples,\nand therefore ultimately do not impact the major finding of this study, that the farside is\nmore reduced than the nearside.\nResponse: Thank you for pointing out this issue. We agree with you that the V and Eu\noxybarometers used in this study were established for more oxidizing oxygen fugacities\n1\nthan those observed in the CE6 samples. When applying these models to the CE6\nsamples, there is a problem of model extrapolation. To fully solve this problem,\nexperiments must be conducted to ascertain the partition coefficients of V and Eu under\nmore reduced oxygen fugacities. The design of this experiment is complex and cannot\nyet be completed in this study. However, the estimated oxygen fugacity of the CE6\nbasalt (ΔIW −2) is only slightly lower than the lower limit of both oxybarometers (ΔIW\n−0.5 and ΔIW −1). In principle, the trend of the partition coefficients of V and Eu with\noxygen fugacity is not expected to change. Therefore, the impact of extrapolation\ncalculations on the results should be minimal. At least, it will not affect the main\nconclusion of the study that the CE6 basalt is more reduced.\nRevision: To ensure that readers can easily identify the issue with the oxybarometers\nused in this paper, we have included a statement in the Methods section that addresses\nthe problem of modal extrapolation. Please see Lines 315-320 and Lines 341-346 in the\nrevised manuscript.\n“The applicable lower limit of the above equation is approximately ΔIW –0.5,\nwhereas the oxygen fugacity of CE6 basalts is estimated at approximately ΔIW –1.73, a\nvalue for which a model extrapolation is required. However, the magnitude of this\nmodel extrapolation is minimal (from ΔIW –0.5 to ΔIW –1.73). In principle, D spinel/melt\nv\nand fO should remain negatively correlated (Supplementary Fig. 11a). Therefore, the\n2\nuncertainty engendered by this model extrapolation is deemed to be minimal.”\n“The lower limit of the equation that is applicable is approximately ΔIW –1, while\nthe oxygen fugacity of CE6 samples is estimated at approximately ΔIW –1.97. This also\nrequires a modal extrapolation. However, the magnitude of this model extrapolation is\nrelatively small (from ΔIW –1 to ΔIW –1.97). In principle, the positive correlation\nbetween D and fO should not change (Supplementary Fig. 11b). Therefore, the\nEu 2\nuncertainty engendered by this model extrapolation is also deemed to be minimal.”\nThe two mechanisms by which the SPA impact produced the low oxygen fugacity seem\nplausible, but both have significant problems associated with them. These issues are\nactually quite well described in the text, so are not repeated here. But overall, one is left\nfeeling like neither of the proposed mechanisms seems viable, even to the authors. This\nmanuscript would be improved by exploring other models for the difference in oxygen\nfugacity. The other main conclusion of the manuscript is that the paucity of basaltic\nvolcanism on the far side is due to the lower oxygen fugacity increasing the melting\ntemperature of the mantle. This seems implausible as a 1 log unit decrease in oxygen\nfugacity should only increase mantle melting temperature by ~10°C, which does not\nseem enough to explain the stark difference in volcanism between the near and far sides\nof the Moon. On the other hand, the paper provides data indicating that the mantle that\nproduced the CE6 basalts was highly melt-depleted. This would provide a more\nplausible explanation for the paucity of volcanism. So overall, the finding of low\noxygen fugacity on the far side is significant, but the explanations given for its origin\nare unconvincing, and the causal connection of the low fO to the low amount of\n2\nvolcanism on the far side seems unlikely.\nResponse: Thank you for your valuable comments. It is indeed challenging to identify a\n2\nmechanism that can elucidate the observed differences in oxygen fugacity between the\nnear and far sides. Recent studies have revealed that the CE6 basalt source is dryer (He\net al., 2025 Nature) and more depleted (Zhou et al., 2025 Nature). Together with the\nmore reduced mantle observed in this study, it is evident that the lunar asymmetry is\nmanifested not only in crustal thickness, KREEP distribution and volcanic activity, but\nalso in the composition of the deep mantle. However, further analysis is still required to\ndetermine the processes contributing to lunar asymmetry. We can only offer two\npotential explanations for these observations: the asymmetric crystallization of the lunar\nmagma ocean and/or the asymmetry induced by the SPA massive impact. It is\nacknowledged that both models have their limitations, and further work is required to\ndistinguish them.\nIn addition, we are in agreement with your comment that the effect of oxygen\nfugacity on the melting temperature is likely to be negligible. An increase of one unit in\noxygen fugacity results in a decrease of 14°C in melting temperature. It appears that this\nwill not be the cause of the low amount of volcanism on the farside.\nRevision: We have made two revisions. (1) We have added a paragraph about the\nsignificance of our findings for future lunar research and exploration. Please refer to\nLines 169-176.\n“While the verification of the above two mechanisms remains challenging, the\nasymmetric crystallisation of the lunar magma ocean and/or the SPA-forming massive\nimpact, our observation opens new directions for future lunar exploration and research.\nFor instance, the analysis of the CE6 basalt for volatile isotopes, such as K, Cl, Zn,\ncould elucidate the influence of degassing processes resulting from the SPA massive\nimpact. Alternatively, the sampling of lunar farside outside the SPA basin could\ndetermine whether this reduced mantle is prevalent on the lunar farside or confined\nsolely in the SPA basin. Nevertheless, the reduced nature of the underlying mantle\nbeneath the SPA basin provides another aspect of lunar asymmetry, which should be\nconsidered by the models for the lunar asymmetry origin.”\n(2) We have removed the discussion on the role of fO in regulating lunar\n2\nvolcanism. The revised manuscript focuses only on the formation mechanism of the\nreduced mantle beneath the SPA basin.\nZhou, Q., et al. Ultra-depleted mantle source of basalts from South Pole–Aitken Basin.\nNature, accepted (2025).\nHe, H.C., et al. Water abundance in the lunar farside mantle. Nature (2025). DOI:\nhttps://doi.org/10.1038/s41586-025-08870-x.\nSpecific comments: Effect of mantle depletion on mantle solidus: The pyroxenes in the\nCE6 lavas have ~10 times lower trace element concentrations than the pyroxenes in the\nCE5 lavas from the near side (Supplementary figure 3), suggesting that the melt was\nalso low in trace elements and was sourced from a strongly melt-depleted source. This\nidea is supported by the title of reference 18 in the paper ‘Ultra-depleted mantle source\nof Chang’e-6 basalts from the South Pole-Aitken Basin’. We did not have access to this\npaper, so can only guess that it provides geochemical analyses of the CE6 basalts,\nshowing them to be as depleted as the pyroxenes in the current manuscript suggest they\n3\nare. If the mantle on the far side is melt-depleted, it would provide a straightforward\nexplanation for the paucity of volcanism on the far side. Looking at papers on the effect\nof mantle depletion on the melting temperature of the mantle (Robinson et al, 1998,\nEPSL 155: 97-111; Maaloe, 2004, Min and Petr 81: 1-17) shows that differences\nbetween fertile and depleted lherzolite (10-20% melt depletion) melting temps can be\n50-70°C, and between lherzolite and harzburgite (>20% melt depletion) can be 150 to\n250°C. This is much larger than the effect of fO on the solidus, and seems like a much\n2\nmore plausible explanation for the lack of melts on the far side. So then the question is\nwhether the cause of the mantle melt depletion on the far side mantle could also be\nlinked to the lower fO . If so, then the low fO and paucity of volcanism are both effects\n2 2\nof the same underlying cause (depleted mantle), rather than low fO being the cause of\n2\nthe low volcanism.\nResponse: Thank you for your comments. We agree with your perspective. Reference\n18 has been accepted by Nature. The CE6 basalt is distinguished by its ultra-depleted,\nlow water content, and more reduced characteristics. There is a possibility that the low\nvolcanism observed on the farside may be attributed to the presence of these features.\nRevision: We have revised this part of discussion by summarized all these above\nfeatures of the CE6 basalt (Lines 45-47).\n“Recent studies have revealed that the majority of the basalt fragments in the CE6\nsoil sample are low-Ti basalt formed 2.8 billion years ago (Ga) (refs. 9,16,17), which are\ncharacterized by ultra-depleted Sr–Nd isotopic compositions18 and low water content19.”\n(1) Line 98: Figure 1 should include a description of what the error bars represent.\nRevision:We have added a description in the Figure 1 caption (Lines 102-103).\n(2) Line 154: Add “and” after “in the melt”.\nRevision: Fixed (Line 154).\n(3) Line 162: The manuscript states that the process of CO degassing cannot be verified\nby current analytical techniques. It would be helpful to include what is limiting in\nanalytical techniques to confirm this model, and what types of measurements would\nneed to be completed?\nRevision:We have added these contents to the revised manuscripts (Lines 162-166).\n“However, the process remains unverifiable due to technical challenges in\ndetermining the indigenous carbon in lunar samples. This necessitates the development\nof high-precision techniques, the enhancement of better resolution stepwise combustion\nwith minimal sample loss, and the establishment of a relationship between exposure\nages of samples and their carbon content and isotopic composition52.”\n(4) Line 176: Effect of oxygen fugacity on the solidus of the mantle-The manuscript\ndoes not give a quantitative estimate of how much 1 log unit oxygen fugacity should\nchange the mantle solidus. Looking at the Lin et al (2021, cited in the manuscript), their\nexperiments show about a 40°C increase in melting temperature over 6 log units of fO2\nat 1 bar total pressure. So less than 10°C per log unit. A 10°C difference seems like it is\n4\ntoo small of an effect to explain how few basaltic lavas there are on the far side\ncompared to the near side.\nRevision: The discussion on the role of fO in regulating lunar volcanism has been\n2\nremoved. The revised manuscript focuses only on the formation mechanism of the\nreduced mantle beneath the SPA basin.\n(5) Line 304: Change “researches” to “research”. Change “show” to “shows”.\nRevision:Fixed (Line 298).\n(6) Line 313: The cited reference for the spinel V oxybarometer (Wang et al., 2019-\nreference 24) is calibrated to FMQ-4 (IW-0.5). It would be beneficial to include a brief\njustification for the extrapolation of this calibration to the lower oxygen fugacities of the\nCE6 samples, including a discussion of resulting uncertainties.\nRevision: Thanks for your suggestion. A brief explanation for the extrapolation of this\ncalibration to the lower oxygen fugacity of the CE6 samples has been incorporated,\nalong with a discussion of the resultant uncertainties (Lines 315-320).\n“The applicable lower limit of the above equation is approximately ΔIW –0.5,\nwhereas the oxygen fugacity of CE6 basalts is estimated at approximately ΔIW –1.73, a\nvalue for which a model extrapolation is required. However, the magnitude of this\nmodel extrapolation is minimal (from ΔIW –0.5 to ΔIW –1.73). In principle, Dvspinel/melt\nand fO should remain negatively correlated (Supplementary Fig. 11a). Therefore, the\n2\nuncertainty engendered by this model extrapolation is deemed to be minimal.”\n(7) Line 313: The Wang et al. (2019) oxybarometer equation includes uncertainties. Our\nbrief set of calculations suggest that accounting for these uncertainties would not change\nthe ultimate conclusions that CE6 was more reducing, but it would be beneficial to\ninclude a note considering these uncertainties in the supplement.\nRevision: Thanks for your valuable suggestion. Since the lack of clear indication in the\nliterature (Wang et al, 2019) on whether there is bias for oxybarometer equation, we\nhave compiled the data of D spinel/melt and fO at around 1,200℃ and obtained an\nV 2\nequation by means of fitting. Please see Lines 304-308 in the Methods section.\n“Given the similarity in whole rock MgO content15,18,20 and pyroxene composition\n(Supplementary Fig. 2) exhibited by the CE6 and CE5 basalts, the temperature of both\nbasalts was estimated to be 1,200℃. The experimental data of D spinel/melt and fO at\nV 2\napproximately 1,200℃ (ref. 26) were compiled and the following equation was obtained\nby means of fitting: fO (ΔFMQ) = 3.193(±0.228) – 1.930(±0.115) × ln(D spinel/melt)\n2 v\n(Supplementary Fig. 11a).”\nThe fO uncertainties can be calculated by propagation law, utilizing the\n2\nuncertainties of both the equation and the V analysis by EPMA. Please see Lines\n313-315 in the Methods section.\n“The calculation of the fO uncertainties was conducted by propagation law,\n2\nutilizing the uncertainties of the equation and the V analysis by EPMA. As the 2SE\nvalues for V analysis are almost identical (~2%), the uncertainties of fO for each point\n2\nare also identical (±0.08).”\n5\n(8) Line 337: The cited reference for the Eu oxybarometer (Karner et al., 2010-\nreference 23) does not provide an equation for the model. The regression used in this\nmanuscript to find the cited equation should be described in the supplement for\nreplication by future readers. We explored fitting the data and the best-fit line we find\nvaries slightly from that described in this manuscript (25.2525 × DEu – 3.5378).\nHowever, this only produces more reducing fO .\n“Research indicates that the distribution coefficient (D ) between pyroxene and\nEu\nmelt constantly increases with elevated fO (ref. 25). We compiled four partition\n2\ncoefficients (D =0.086, fO =ΔIW−1; D =0.157, fO =ΔIW; D =0.182, fO =ΔIW+1;\nEu 2 Eu 2 Eu 2\nD =0.274, fO =ΔIW+3.5) to construct a function relating fO and D , expressed as\nEu 2 2 Eu\nfO (ΔIW) = 24.503(±2.990) × D – 3.407(±0.560) (Supplementary Fig. 11b). The R²\nof this function is 0.971, aligning with ref. 25.”\n(9) Line 337: Karner et al. (2010) (reference 23) oxybarometer is calibrated to IW-1.\nThe range of calculated oxygen fugacities for the CE6 samples is as low as IW-2.6. It\nwould be beneficial to include a brief justification for the extrapolation of this\ncalibration, including a discussion of resulting uncertainties.\nRevision: We have added a brief justification for the extrapolation of this calibration\nand a discussion of resulting uncertainties (Lines 341-346).\n“The lower limit of the equation that is applicable is approximately ΔIW –1, while\nthe oxygen fugacity of CE6 samples is estimated at approximately ΔIW –1.97. This also\nrequires a modal extrapolation. However, the magnitude of this model extrapolation is\nrelatively small (from ΔIW –1 to ΔIW –1.97). In principle, the positive correlation\nbetween D and fO should not change (Supplementary Fig. 11b). Therefore, the\nEu 2\nuncertainty engendered by this model extrapolation is also deemed to be minimal. ”\n(10) Supplementary Data 6: How is the error calculated for each fO ? The 2SE values\n2\nare the same for each sample in Supplementary Data 7 (spinel V oxybarometer) but are\ndifferent for each sample in Supplementary Data 6. This should be clarified in the\nsupplement.\nRevision: We have added descriptions in the Methods section (Lines 313-315 and Lines\n338-341).\n“The calculation of the fO uncertainties was conducted by propagation law,\n2\nutilizing the uncertainties of the equation and the V analysis by EPMA. As the 2SE\nvalues for V analysis are almost identical (~2%), the uncertainties of fO for each point\n2\nare also identical (±0.08).”\n“The calculation of the fO uncertainties was conducted by propagation law,\n2\nutilizing the uncertainties of the equation and the Eu analysis by LA-ICP-MS. The\neuropium content of pyroxene in the CE6 basalts ranges from 0.055 μg g⁻¹ to 0.247 μg\ng⁻¹and the 2SE values of each analytical spot range from 0.018 μg g⁻¹ to 0.082 μg g⁻¹.\nThe calculated fO uncertainties range from 0.22 to 0.76.”\n2\n(11) Supplementary Data 7: The spinel Cr# seem to be high (~79-80). When we\ncalculated the Cr# of samples using the geochemical analyses in Supplementary Data 3,\n6\nwe were not able to reproduce those values and instead find Cr# values between ~67-68.\nThese calculations should be double-checked. Additionally, the highest spinel Cr# of the\nliterature compiled by Prissel et al. (2016, American Mineralogist, Appendix Table 1)\nis 68 (Dymek et al., 1975). If there are higher Cr# lunar spinel in line with the\nmeasurements from this manuscript, it would be beneficial to include those.\nResponse: We recalibrated the Cr# of spinel according to Supplementary Data 3,\nyielding a range of 67.31 to 68.85, consistent with your calculations.\nReviewer #3 (Prof. Putirka)\nThis paper presents some highly intriguing results from the farside lunar surface that\ndeserve widespread attention in a publication like Nat. Comm. I am not sure that I\naccept that observed contrasts in fO could affect melt productivity to the extent needed\n2\nhere, but I do think that Figure 1 is important and compelling and will add to our\nthoughts about Moon’s formation. I’ll touch on the fO and melt productivity below, but\ncore-formation scenario for explaining contrasts in fO . I am not expert on Moon\n2\nformation, and perhaps that earlier model is not valid, but Sossi et al. (2024) and others\nmake a good case that the Mars-size impact model for Moon is not entirely certain. The\nmain problem, as I understand it, is that Moon and Earth are nearly identical in their\nisotopic compositions but any nominal “Theia” impactor would likely have different\nisotopic ratios-and most models suggest that Theia, not Earth would contribute the\nlarger fraction to Moon’s total mass. Fischer et al. argue that Theia could have lost its\nsilicate mantle prior to impact. That’s an interesting idea. But even Mercury preserves a\nlarge fraction of its silicate material, so this is not a compelling solution to the problem.\nSo I can’t help but wonder if the contrasts in fO might play some larger role in our tests\n2\nof Moon-forming hypotheses. But again, I am too little educated on such issues to say\nmuch about this possibility. In any case, this observed difference in fO appears to be\n2\nreal and should impact how we think about the evolution of the moon.\nResponse: Thank you for detailed reviews and helpful suggestions. Prior to submitting\nthis paper, we have discussed this issue with Professor Stevenson via email. He stated\nthat the assumption in his research regarding a “cold primordial core” may be incorrect,\ngiven the prevailing view of lunar origin through a giant impact is now widely accepted.\nHowever, after carefully considering your suggestion, we feel that even though the cold\nlunar core may not be plausible, asymmetric crystallization of LMO is still a possible\nscenario.\nRevision: Therefore, we have modified this part of discussion (Lines 135-139).\n“Although this proposed cold primordial core is not consistent with the giant\nimpact origin of the Moon which would predict that the early Moon was uniformly\nheated37, there remains a possibility for asymmetric crystallisation of lunar magma\nocean (LMO) with a hot core38-40. However, the current models do not consider the\npotential variation in oxygen fugacity during asymmetric crystallization. Further\nresearch is required to explore this possibility.”\nAs to the minor but still somewhat emphasized issue of partial melting, I doubt that the\nobserved fO contrast is large enough to greatly affect melt productivity and so explain\n2\n7\nbasalt thickness contrasts between the nearside and farside. For example,\nLin et al. (2021) require nearly 11 orders of magnitude to obtain a 105-degree contrast in the\nmantle solidus; that means a ca. 1 order of magnitude should then translate to only\nabout a 10-degree difference in the mantle solidus. That’s at the limit of what we can\ndetect in experiments. Another concern is that the more recent study by Lin et al. that is\nalso cited here involves experiments at very high pressures-much higher than in the\nlunar interior, and so the relevant reactions might not apply in the lunar interior.\nIf the authors want to push the issue, they could quantify the analysis a little better: they\ncould calibrate a delta-T in solidus temperature relative to changes in fO using all the\npossible shift in solidus based on Figure 1. And then they could look at melt\nproductivity models by, say, Charlie Langmuir, or Paul Asimow, or Claude Herzberg,\netc. and try entering such a contrast in solidus temperature translates to a predicted\nthickness of basalts. And then tell us what the contrasts are in mare basin basalt\nthickness and put this altogether to see if fO has any chance of explaining the observed\n2\nmare basin basalt thicknesses.\nResponse: Thank you for your suggestion. We assessed the influence of oxygen\nfugacity on the melting temperature with the identical bulk composition of CE6 basalts.\nOur findings indicate that an increase of 1 unit in oxygen fugacity decreases the melting\ntemperature by 14℃.\nRevision: We have removed the discussion of fO in relation to the asymmetry of lunar\n2\nvolcanism.\nOn another issue, the authors state on line 113 that there are differences in volatile\ncontents and degrees of differentiation between nearside and farside compositions. But\ntheir cited reference (#27) doesn’t appear to make such a comparison. It would be very\nhelpful (even if I have overlooked something in cit. no. 27) to have a plot that compares\nthese compositions such as a plot of MgO contents vs. volatile contents, or maybe\nalkalis if they are a proxy for volatiles. I hope these comments are helpful.\nRevision: Thank you for your comments and suggestions. A figure has been included to\nelucidate the differences in volatile contents and levels of differentiation between the\nlunar nearside and farside. Please refer to page 9 in the supplement.\n(1) Line 30: here and perhaps in the abstract as well it’s probably best to refer to the\nasymmetry as being relative to the nearside and farside mare basalt as readers who are\nnot aware of the issues regarding lunar volcanism may think of Earth’s ocean basins as\nanologies, where lots of volcanism = thick crust.\nRevision: Wedeleted the discussion of fO controlling the lunar volcanism asymmetry\n2\nand focused only on the fO measurements in CE6 basalt and the difference from the\n2\nlunar nearside. Therefore, we have rewritten the introduction (Lines 29-41).\n“The redox conditions, determined by oxygen fugacity (fO ), for basaltic magmas\n2\noriginating from the melting of planetary mantles provide significant insights for\ndetermining the compositions of the precursor materials that contributed to planetary\nformation and the subsequent evolution of their interiors1-3. For example, the oxygen\nfugacity is crucial for ascertaining the stability and composition of crystallizing phases,\nthe oxidation state of multivalent elements, and the composition and stability of volatile\n8\nphases in a magma4,5. Therefore, the oxygen fugacity of the planetary mantle is crucial\nfor comprehending its origin and evolution. The Moon displays a global asymmetry\nencompassing basalt distribution6, geomorphology7, chemical composition8,9 andcrustal\nthickness10. However, the question of whether there is an asymmetry of oxygen fugacity\nbetween the lunar nearside and farside remains unanswered. This is attributable to the\nfact that all studied lunar samples were collected from the nearside of the Moon11,12. The\nApollo lunar mare basalts and pyroclastic glasses yielded an oxygen fugacity of ΔIW –1\nusing olivine/spinel V oxybarometers4,13,14. A recent study indicates that the lunar\nmantle fO derived from CE5 basalts is ΔIW –0.84 using the same oxybarometers15.\n2\n(2) Lines 34-35: which site has thicker crust? Cite the actual estimates of crust thickness\nat both localities.\nRevision:We have rewritten the introduction (Lines 29-41).\n(3) Lines 36-39: And if the proposed explanation is an excess of heat-producing\nelements, would that not only provide more volcanism, but more crust as well? Unless\none believes that the heat-producing elements on the near side mantle were introduced\nonly later, after the crust had formed. But then how does that work?\nThe more passive, transport-restricted model of Head et al., seems to make more sense\nalthough the discovery of a contrast in fO may be important. Let’s read further…\n2\nregulating lunar volcanism has been removed. The revised manuscript focuses only on\nthe formation mechanism of the reduced mantle beneath the SPA basin.\n(4) Line 43: I think it’s OK to cite other papers besides Lin et al. (2024) but only Lin et\nal. (2024) (citation # 12) and Lin et al. (2021) (citation #49) truly make the point that\nthe authors are attempting to emphasize here. Until those two experimental studies, the\neffects of fO were not expected to be important, being overwhelmed by effects such as\nconventional view. What is impressive about the Line et al. (2021, 2024) studies is that\nthey show that solidus temperatures can be affected by100s of degrees C or more with\nchanges in fO .\nlooked for it. In think in their 2021 study, they needed 11 orders of magnitude change in\nfO to get ca. 100 degrees of change in solidus T. So it might still be a stretch to apply it\n2\nhere.\nIn any case, it’s the Lin et al. papers that should be cited here and not the others as the\nearlier papers do no indicate a change in solidus T with changes in fO .\nproposed the influence of oxygen fugacity on magma solidus temperatures.\n(5) Lines 65-66: Mg# can only range form 0 – 1 (so the range to 2.1 must be a typo).\n(6) Line 113: the authors write “melting and fractional crystallisation processes they\nhave experienced” I understand that they cite a reference but a plot of MgO vs. K O, or\n2\neven better, MgO vs. some estimate of volatile concentrations would be helpful here.\nThe cited paper (ref. 27) does not compare nearside to farside compositions. A simple\nchart, using the color coding in the author’s Fig. 1, is needed.\nRevision: Thank you for your recommendation. We have added a figure\n(Supplementary Fig. 8: H O content in the lunar mantle vs bulk Mg# of the basalts).\n(7) Lines 136-139: Or, maybe the large impactor hypothesis is wrong.\nResponse: Yes, it could be wrong. But in this paper, we are just focusing on the oxygen\nfugacity and are not going to get involved in this controversial issue.\nRevision:We have revised this paragraph (Lines 135-139).\n“Although this proposed cold primordial core is not consistent with the giant\nimpact origin of the Moon which would predict that the early Moon was uniformly\nheated37, there remains a possibility for asymmetric crystallisation of lunar magma\nocean (LMO) with a hot core38-40. However, the current models do not consider the\npotential variation in oxygen fugacity during asymmetric crystallization. Further\nresearch is required to explore this possibility.”\n(8) Lines 182-183: The authors write “basalts have similar Mg# values (~30), they could\nhave experienced equivalent degrees of magma evolution, thus potential Tp deviations\nresulting from different degrees of fractional crystallisation can be eliminated.” Since\nthey calculate Tp, this is not a big deal, but I’d just like to note that Mg# is not the\ndefining quality. Two rocks with very different FeO contents can have the same Mg#\nbut the rock with higher FeO would also have a higher MgO content and so could have\na very different Tp.\nRevision: The revised manuscript focuses only on the formation mechanism of the\nreduced mantle beneath the SPA basin. This paragraph has been removed.\nNote: Comments and suggestions are in black, Responses in blue, and the revisions\nin red.\nReviewer #1\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse: Thanks for your feedback about the review of our manuscript.\nReviewer #2\nSummary of revisions – The main change has been the removal of the discussion about\nhow the lower fO on the farside might explain the lack of volcanism. Both reviewers\n2\npointed out the issue with this, and the paper is stronger for removing it. The\nexplanations of the oxygen fugacity estimates and the associated uncertainties are also\nmuch better.\nGeneral – The observation of the lower fO in the CE6 samples remains both interesting\n2\nand important. The two explanations for how a nearside/farside dichotomy in fO was\n2\nformed both have their issues, but they both present testable hypotheses for future work,\nand the issues are well-discussed in the manuscript.\nResponse: We much appreciate your acknowledgment of our revisions. Your\nrecommendations have provided us with further insights into the uncertainty concerning\noxygen fugacity estimates and the underlying mechanisms of the nearside/farside\ndichotomy in fO , thereby significantly improving our manuscript.\n(1) Figure 2a – I can’t remember if this was in the original version of the paper, but I\nhave some questions about it. First, how do we know the whole Moon started at IW-2? I\nlooked at the Deng et al reference and they seem to just assume that value as a starting\npoint for their models. I don’t think there are good constraints on the initial fO of the\nfigure of measured fO vs age, with the Apollo and CE data. I suggest showing a range\n2\nfor the initial fO as a vertical bar at 4.5 Ga. It would be worth a deep dive in the\n2\nliterature (more than just the Deng reference) to find what constraints there are on the\ninitial fO . My guess is that there are not many, and that most people have just assumed\nResponse: Thank you for pointing out this issue and we agree with your perspective.\nRevision: Following your suggestion, we haverevised Figure 2, setting the initial fO of\n2\nthe lunar mantle to be IW-1 to IW-2 (Lines 116-117).\n(2) Second, why does path 2 of the red dashed line go up and down? Why does it go up\n1\nto the same fO as the nearside? There should be text in the caption, or in the main text,\n2\nthat specifically discusses this. If the farside is getting oxidized, what is getting reduced?\nThere needs to be more text, either in the main text or in the caption, linking the models\nshown if Figure 2a to the 2 models proposed in the main text. And they should not all\njust start at IW-2.\nResponse: Thank you for your suggestion. Following your comment #1, the initial fO\n2\nof the lunar mantle has been set to IW-1 to IW-2. Therefore, path 2 does not require\noxidation followed by reduction.\nRevision: We have revised Figure 2 to show path 2 underwent a reduction process\nduring the SPA impact at 4.25 Ga. We have added descriptions to the figure caption and\nlinked the two models in the main text (Lines 114-127).\n“The purple vertical bar denotes the initial fO of the lunar mantle (from ΔIW –1 to\n2\nΔIW –2; refs. 32,33). The more reduced lunar farside mantle may be attributable to two\nmechanisms: (i) The asymmetric crystallisation of the lunar magma ocean has induced a\nmore reduced lunar farside mantle in comparison to the nearside mantle. This initial fO\n2\n(ΔIW –2) has remained unchanged since then (corresponding to the trend ①);\n(ii) The initial fO of the lunar farside mantle was analogous to that of the nearside mantle (ΔIW\n2\n–1) and was subsequently reduced by the SPA massive impact (4.25 Ga; ref. 34)\n(corresponding to the trend ②).”\n“Two potential candidates are the asymmetric crystallisation of the lunar magma\nocean30 (trend ① in Fig. 2a), and/or the SPA-forming massive impact31 (the trend ②\nin the Fig. 2a). ”\nReference\nPapike, J.J., Karner, J.M & Shearer, C.K. Comparative planetary mineralogy: V/(Cr/Al) systematics\nin chromite as an indicator of relative oxygen fugacity. Am. Mineral. 89, 1557–1560 (2004).\nWadhwa, M. Redox Conditions on Small Bodies, the Moon and Mars. Rev. Mineral. Geochem. 68,\n493–510 (2008).\nsamples. Natl Sci. Rev. 12, nwaf103 (2025).\n(3) Figure 3 – This figure makes me think about whether the models proposed imply a\nnearside vs farside dichotomy, or a SPA vs PKT centered dichotomy. For example, from\nFigure 3 it seems like the northern hemisphere of the farside and the southern\nhemisphere of the nearside should be similar in fO . But the Fe-rich layer at the CMB is\n2\nnot axisymmetric with the SPA and PKT, but seems pointed at the Earth. I suggest there\nshould be more discussion about where the dichotomy would be centered around in the\ntwo different models. Maybe this would be a way to test the different ideas? By\nsampling regions away from SPA and PKT.\nResponse: Thank you for this constructive comment, which helps us to further consider\nthe origin of the lunar asymmetry. If it was formed by the asymmetric crystallisation of\nthe lunar magma ocean, there should be an asymmetry between nearside and farside.\nHowever, if it was formed by the SPA impact, there should be an asymmetry between\nSPA and PKT. Therefore, comparative studies on the lunar northern hemisphere of the\nfarside and southern hemisphere of the nearside could validate these two mechanisms.\nRevision: We have revised Figure 3 and added discussion to both the figure caption and\nthe main text (Lines 173-178 and Lines 181-191).\n“In the mechanism ①,the Fe metal sank to the core from the nearside mantle and\nelevated its Fe3+/∑Fe, resulting in a relatively oxidized nearside mantle than farside\nmantle. This processshould result in an asymmetry between nearside and farside. In the\nmechanism ②, the abundant S and CO degassing induced by the SPA massive impact\n2\n3\noccurred and caused Fe2+  Fe0, and consequently a relatively reduced mantle beneath\nSPA basin. This process should lead to an asymmetry between the SPA basin and PKT.\nFuture comparative studies on the lunar northern hemisphere of the farside and southern\nhemisphere of the nearside are required to validate these two mechanisms.”\n“As illustrated in Fig.3, the asymmetric crystallisation of the lunar magma ocean\nshould induce an asymmetry between nearside and farside.However, the SPA massive\nimpact is likely to result in an asymmetry between the SPA basin and PKT. It is\ntherefore imperative that future comparative studies be conducted on the northern\nhemisphere of farside and the southern hemisphere of nearside, in order to decipher\nthese two mechanisms.”\nReviewer #3 (Prof. Putirka)\nThe authors have certainly done an excellent job addressing the comments I made on\ntheir original draft, and my reading of the authors' comments to other two reviews seem\nto be very thoughtful as well. I must reiterate that I am much less expert on the issues of\nthe Moon's origin and lunar petrogenesis and the other reviewers are also more up to\ndate on the particulars of using trace elements to calculate fO . But I am nonetheless in\n2\nfavor of publication as I think the authors have some very compelling results that are\nimportant to the planetary science community.\nI have no other comments or suggestions. I think I tend to still believe that Dave\nStevenson's model might still have value, but if he has abandoned it, then maybe it's\ntime to move on.\nResponse: Thank you again for taking the time to review our paper. Your feedback has\nenabled us to make significant improvements to it.\n4"}]

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