Long-term reduced lunar mantle revealed by Chang’e-5 basalt

Domain: earth_science
Published: 2024-09-27
Editorial outcome: minor_revision

Abstract
The redox state of a planetary mantle affects its thermal evolution. The redox evolution of lunar mantle, however, remains unclear due to limited oxygen fugacity (fO2) constraints from young lunar samples. Here, we report vanadium (V) oxybarometers on olivine and spinel conducted on 27 Chang’e-5 basalt fragments from 2.0 billion years ago. These fragments yield an average fO2 of ΔIW-0.84 ± 0.65 (2σ), which closely aligns with the Apollo samples from 3.6–3.0 billion years ago. This temporal uniformity indicates the lunar mantle remained reduced. This observation reveals that the processes responsible for oxidizing mantles of Earth and Mars either did not occur or had negligible oxidizing effects on the Moon. The long-term reduced mantle may lead to a distinctive volatile degassing pathway for the Moon. It could also make the lunar mantle more difficult to melt, preventing internal heat dissipation and consequently resulting in a slow cooling rate.

Introduction
Oxygen fugacity (fO2), a measure of the redox conditions of rocks, is a leading factor controlling several planetary properties and the speciation of volatiles during volcanic outgassing1–4. The mantles of large differentiated terrestrial bodies, such as Earth and Mars, have undergone oxidation over time from an originally reduced state. Such increases in oxygen fugacity have profoundly impacted both the composition of their mantles as well as the two planets’ surface environments1–6. However, it is still unclear if such an oxidation evolution is widespread among differentiated terrestrial bodies. Based on lunar samples from ca. 3.6–3.0 billion years ago (Ga), an fO2 of ~ΔIW-1 was estimated for the lunar mantle6. However, the redox evolution of the lunar mantle remains unknown owing to the absence of younger samples. China’s Chang’e-5 (CE-5) mission, delivering the first samples from the Moon in over four decades after the Apollo and Luna missions, successfully retrieved 1731 g of lunar soil samples from northern Oceanus Procellarum7. Petrographic and geochronological analyses of the basalt fragments in the CE-5 soil reveal that they formed during volcanic eruptions at ca. 2.0 Ga (in refs. 8,9). Based on their homogeneous 87Sr/86Sr ratios, εNd(t) values, and comparable REE patterns, the CE-5 basaltic fragments were likely derived from a series of lava flows10,11. These relatively young basalt samples thus provide a unique opportunity to investigate the redox evolution of the lunar mantle over time.

Various approaches have been applied for estimating the oxygen fugacity of lunar basalts and pyroclastic glasses, such as solid electrolytes12,13, mineral assemblages14, melt and mineral compositions15, and the oxidation state of multiple valence cations16. Although each approach has its limitations, most results suggest that the lunar basalts and pyroclastic glasses crystallized at relatively reduced conditions ranging from the ilmenite breakdown reaction (ilmenite = rutile+iron) to the iron-wüstite buffer (IW)12–16. To determine the fO2 at reduced conditions, vanadium (V) oxybarometers have been developed17–20. Varying in different valence states (V2+, V3+, V4+, and V5+), the behavior of V differs considerably between the Earth, Moon, and Mars: in terrestrial basalts, V4+ is dominant; in lunar basalts, V3+ is dominant; and in Martian basalts, V3+ and V4+ are both abundant19–21. Empirical studies have demonstrated that the partitioning behavior of V (Dv) between olivine/spinel and a melt is redox-sensitive22,23. The D olivine/melt and D spinel/melt values tend to increase as the oxygen fugacity decreases24,25. Applying both of these oxybarometers require that the olivine/spinel and melt are in equilibrium. In addition, the partitioning of V between spinel and olivine can also be employed20. The advantage of the D spinel/olivine method is that it does not require samples representing melts or samples with remnant melt present. It only requires that the spinel-olivine pairs co-crystallized or were in equilibrium20. Based on these V oxybarometers, the fO2 of the Apollo lunar mare basalts and pyroclastic glasses has been determined from ΔIW-1.9 to −0.9 (refs. 18,19).

Here, we perform systematic analyses of V oxybarometers for olivine and spinel from 27 CE-5 basalt fragments using their major and trace element compositions to constrain the redox conditions of the lunar mantle source, which can then be compared to other terrestrial bodies in the Solar System.

Results
Petrographic and mineralogical characteristics of CE-5 basalt fragments
The CE-5 basalt fragments analyzed here show variable textures, including subophitic, poikilitic, porphyritic, and equigranular (Supplementary Table 1), which are consistent with previous observations10. The basalt fragments consist primarily of pyroxene, plagioclase, olivine, and ilmenite, with trace amounts of silica, spinel, apatite, whitlockite, baddeleyite, zirconolite, and tranquillityite (Supplementary Table 1). Olivine and spinel co-crystallized during the cooling of the CE-5 basaltic magma. In the early stage of crystallization, olivine became enriched in magnesium (Mg) with high Mg# [atomic Mg/(Mg+Fe)] and spinel enriched in chromium (Cr) with low Ti# [atomic Ti/(Ti + Cr)]. As the crystallization process continues, Mg# in olivine decreases while Ti# in spinel increases (Fig. 1). The compositions and evolutionary trends of olivine, spinel, pyroxene, and plagioclase are consistent with previous studies8,10,26,27 (Supplementary Figs. 1–4; Supplementary Data 1), supporting the interpretation that these CE-5 basalt fragments probably originated from a similar mantle source10,11,28. The V contents in olivine range from 2.8±0.4 to 47.0±1.7 ppm (Supplementary Data 2) and are positively correlated with Mg# (Ol) (Supplementary Fig. 1f). The concentration of V in spinel varies from 681.0±4.0 ppm to 4,826.0±30.0 ppm (Supplementary Data 3) and exhibits a negative correlation with Ti# (Supplementary Fig. 2b).

The fO2 of CE-5 basalts
The V contents in olivine and spinel, combined with the whole-rock V contents previously reported29, allow us to estimate the fO2 of CE-5 basalt. The whole-rock Mg# of CE-5 basalt is estimated to be 28.9 with a V content of 88.9 ppm (refs. 26,29). The Fo of olivine equilibrated with the CE-5 basalt can be predicted from the forsterite of liquidus phases by the equation (Mg# = 1/([1/Ol Fo - 1]/KD + 1)) (ref. 30). The melt Fo in equilibrium with the CE-5 basalt ranges roughly from 51.9–58.5 when KD = 0.333±0.044 (ref. 31). According to the olivine V oxybarometer20, the fO2 of CE-5 basalt is calculated to be from ΔIW −1.37±0.10 to ΔIW +0.16±0.32, with an average of ΔIW-0.95±0.82 (2σ) (Fig. 2a and Supplementary Data 4). Similarly, the spinel V oxybarometer23, based on Dv in Cr-spinel and melt, yields fO2 ranging from ΔIW -0.94±0.07 to ΔIW -0.55±0.07 with an average of ΔIW −0.76±0.24 (2σ) (Fig. 2a and Supplementary Data 5). Overall, the fO2 of the CE-5 basalt is estimated to be ΔIW-0.84±0.65 (2σ). This result represents the fO2 of the original melt of the CE-5 basalt because the olivine phenocrysts and Cr-spinel are the earliest crystallizing phases of the CE-5 basalt10,11,29. The fO2 of CE-5 basalt closely resembles the average fO2 (ΔIW-1.0) of the lunar mantle derived from the 3.6–3.0 Ga Apollo basalts and pyroclastic glasses6,12–19, suggesting that the lunar mantle maintained its reduced state from 3.6 Ga to 2.0 Ga.

Furthermore, the variation of fO2 throughout the entire crystallization of the CE-5 basalts was measured using the spinel-olivine V oxybarometer20. The fO2 increases from ΔIW -1.86±0.15 to ΔIW+ 2.18±0.54 during late-stage crystallization when the Mg# of the equilibrium melt drops to <20 (Fig. 2a; Supplementary Data 6). This rise in fO2 during the late-stage magma evolution has also been observed in Apollo basalts, which is suggested to result from hydrogen degassing21. This process also occurred during the late-stage crystallization of the CE-5 basalt, as indicated by hydrogen isotopic variations in melt inclusions in ilmenite32. Nevertheless, the average fO2 during the early-stage crystallization (Mg# of the equilibrium melt ≥29) is ΔIW-0.92±0.15, which is consistent with those inferred from olivine and spinel V oxybarometry (Fig. 2a; Supplementary Data 6). Therefore, the rise in fO2 during the late-stage magma evolution does not affect the fO2 estimation of the original melt of the CE-5 basalt.

Methods
Sample preparation and characteristics
Twenty-seven basalt fragments were obtained from a series of Chang’e-5 (CE-5) lunar soil samples (CE5CO100YJFMO0103, CE5CO100YJFMO0104, CE5CO100YJFMO0105, CE5CO100YJFMO0106, CE5CO100YJFMO0107, CE5CO100YJFMO0108, CE5CO100YJFMO0109, CE5CO100YJFMO0110, CE5CO100YJFMO0111, CE5CO100YJFMO0112, CE5CO100YJFMO0113, CE5CO100YJFMO0114, CE5CO100YJFMO0115, CE5CO100YJFMO0116, CE5CO100YJFMO0117, CE5CO100YJFMO0118, CE5CO100YJFMO0119, CE5CO100YJFMO0120, CE5CO100YJFMO0121, CE5CO100YJFMO0122, CE5CO100YJFMO0123, CE5CO100YJFMO0124, CE5CO100YJFMO0125, CE5CO100YJFMO0126, CE5CO100YJFMO0127, CE5CO100YJFMO0128, CE5CO100YJFMO0129). These samples were embedded in one-inch epoxy mounts and polished before analyses. These basalt fragments are composed of primary clinopyroxene (6.13%~81.53%), plagioclase (0.24%~45.96%), olivine (0%~81.16%), and ilmenite (0.54%~13.98%) (Supplementary Table 1), as well as minor amounts of silica, spinel, apatite, whitlockite, baddeleyite, zirconolite, and tranquillityite. Despite exhibiting various petrographic textures and modal abundances of minerals, their mineral chemistry remain highly similar and follows the same evolutionary path (Supplementary Figs. 1–4), consistent with those previously reported8,10,11. The differences in textures and mineralogical abundances of these basalts are likely due to the different cooling rates within different parts of a single lava flow and small sample sizes (<3 mm) of the basalt fragments, respectively10. Recent reports have investigated the petrology, geochemistry, and volatile contents of the CE-5 basalt8,10,11,26,32,50,51 and the bulk compositions of lunar soils7,28,29,52 in detail. The results suggest that the CE-5 basalts are more evolved than the Apollo and Luna samples. The olivine in these basalts with grain sizes ranging from 35–300 μm occurs as phenocrysts with Fo from 60 to 16. These olivine phenocrysts are one of the earliest crystallizing phases in the CE-5 basalt and thus may have been in equilibrium with the original basaltic melt based on their Fo and the whole-rock Mg# (ref. 11), which provides the basis for using olivine V oxybarometers20. The spinel of these basalts, with small grain sizes from 10–50 μm, exists as Cr- and Ti-spinel. The Cr-spinel crystallized at the early stage of the “low-Ti” lunar mare basalts10. The equilibrium between Cr-spinel and basaltic fragments provides the basis for using spinel V oxybarometers23. Olivine and spinel co-crystallized during the cooling of the CE-5 basaltic magma. In the early stage of crystallization, olivine and spinel are enriched in Mg and Cr, respectively, and they gradually enriched in Fe and Ti in the late stage of crystallization, respectively (Fig. 1). The equilibrium between olivine and spinel provides the basis for using olivine-spinel V oxybarometers20.

Scanning electron microscope imaging
Back-scattered electron (BSE) images were obtained with a Thermo scientific Apreo scanning electron microscopy (SEM) using an FEI Apreo equipped with an energy dispersive x-ray spectrometer (EDS) at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) in Beijing. The accelerating voltage was 15.0 kV, and the probe current was 6.4 nA. The EDS data were used to identify the textures and mineral characteristics of the CE-5 basalts.

Electron microprobe analysis
The major element concentrations of primary minerals in the samples were measured using a JEOL JXA 8100 electron probe at IGGCAS. The experiment utilized an accelerating voltage of 15 kV, a beam current of 12 nA, a beam diameter smaller than 5 μm, and a counting period ranging from 10–30 s. Elemental data was calibrated using a series of natural minerals and synthetic oxides. The precision for major elements is better than 1.5%, as shown by the study of internal laboratory standards.

High-precision electron probe microanalysis
Spinel in the CE-5 basalts was identified and measured using the high-precision electron probe microanalysis method (HP-EPMA) on the CAMECA SXFive electron microscope at IGGCAS. Analytical procedures referred to ref. 53. The acceleration voltage was set to 25 kV. The beam currents for trace elements (Ti, V, Mn, Co, Ni, and Zn) and major elements (Cr, Al, Mg, and Fe) were 200 nA and 60 nA, respectively, with a beam diameter of 1 μm. The natural minerals and synthetic oxides were used for high-precision (elemental) analyses: rhodonite (Si and Mn), apatite (Ca), rutile (Ti), FeS2 (Fe), ZnS (Zn), Cr2O3 (Cr), CoO (Co), NiO (Ni), MgO (Mg), and V2O5 (V). Peak count times were as follows: 10 s for Al, Mg, and Fe; 20 s for Cr; 80 s for Ca, Ti, V, Co, and Zn; 120 s for Mn and Ni; and 240 s for P and Si. The LBS13-04 spinel standard was analyzed two times per 10 measurements to monitor machine drift. Average detection limits (3σ) for Ti, V, Mn, Co, Ni, Zn, Ca, and Si are 17, 18, 33, 20, 55, 48, 16, and 17 µg/g, respectively.

In situ trace-element analysis
The trace element analyses for olivine in the CE-5 basalts were carried out using a Geolas HD 193 nm ArF excimer LA system (Coherent; Gottingen, Germany) coupled to an Element XR sector field (SF)-ICP-MS (Thermo Fisher Scientific; Bremen, Germany) at the State Key Laboratory of Lithospheric Evolution, IGGCAS. The laser diameter is 24 μm with a repetition rate of 5 Hz. The laser energy density is ca. 3 J/cm2. Helium was employed as the ablation gas to improve the transporting efficiency of ablated aerosols. All trace element determinations were performed using time-resolved analysis in the fast, peak jumping mode. The signal intensities of 7Li, 23Na, 25Mg, 27Al, 29Si, 31P, 43Ca, 45Sc, 49Ti, 51V, 53Cr, 55Mn, 57Fe, 59Co, 60Ni, 63Cu, 67Zn, 69Ga, and 89Y were monitored during each analysis. The GOR132-G olivine standard54,55 was used to correct for the instrument- and time-dependent fractionations of Fe/Mg ratios and all trace elements. The MongOLSh11-2 and XEN olivine standards56,57 were employed for quantity control. The GOR132-G, MongOLSh11-2, and XEN olivine standards were analyzed each time per 10 measurements. Average detection limits (3σ) for most elements are better than 0.1 µg/g. The accuracies of the major elements (Mg, Fe, and Si) and trace elements (2 RSD) are better than 1.5% and 10%, respectively. The analytical uncertainty of Fo (2 RSD) is ±0.17% for the long-term analysis. More details of the analytical procedures and data acquisition are provided in ref. 58.

V oxybarometer
In this study, we used the vanadium (V) in olivine oxybarometer (D olivine/melt = 0.316 × e-0.356 × ΔIW; ref. 20) and spinel V oxybarometer (log (D spinel/melt) = −1.09 − 0.186 × ΔFMQ + 2477/T + 0.004 × Cr#; ref. 23) to constrain the fO2 of the CE-5 basalt. The olivine V oxybarometer was developed by studying the distribution of V between synthetic olivine and melt at a constant temperature (1,200 °C) under varying oxygen fugacity conditions. The spinel V oxybarometer was developed by systematically assessing the impact of various factors (including fO2, T, P, and mineral or melt composition) on the D spinel/melt ratio. In the above equations, T represents the temperature in Kelvin; Cr# = Cr/(Al+Cr) (%); ΔIW and ΔFMQ are the fO2 difference between experimental fO2 and IW and FMQ buffer, respectively. The compositions in our investigation were obtained from two CE-5 basalt fragments29. The eruption temperature estimated for CE-5 basalts is around 1150–1230 °C (ref. 11), so the average temperature of 1200 °C was adopted in this study. According to the IW and FMQ buffer formulations59,60 at a given temperature of 1200 °C, the ΔIW is approximately 3.5 log units below ΔFMQ (ref. 61).

We employed the spinel-olivine V oxybarometer20 (D olivine/spinel = 29.42 × ΔIW + 89.41) to quantify the changes in oxygen fugacity in the CE-5 basalts during crystallization. First, a relationship (Fo = -0.7581 × Ti# + 82.466; R2 = 0.99) between the Ti# of spinel and Fo of olivine was established using representative samples in which spinel and olivine coexist (Fig. 1). Then, the V contents in olivine co-crystallized with spinel were calculated based on the positive correlation between V contents and Fo in olivine (V content = 0.8853 × Fo - 10.459; R2 = 0.84). It should be noted that the average Dv spinel/olivine value of our samples (~122) is slightly higher than the calibration formulas previously reported (~60-120; ref. 20). The main reason should be that the spinel-olivine V oxybarometer was established by homogeneous synthesized Mg and Al-spinel (MgAl2O4) and Mg-olivine20. However, the late-stage crystallization minerals in the CE-5 basalt are Fe-olivine and Ti-spinel10,11. Therefore, the accuracy of the estimated fO2 during late-stage crystallization still needs to be verified by future experimental studies. Nevertheless, these results of the late-stage crystallization do not affect the fO2 estimation of the original melt of the CE-5 basalt.

Mantle potential temperature calculation
The mantle potential temperature (Tp) of the CE-5 basaltic fragments was calculated using a spreadsheet provided by ref. 31. The average bulk compositions of low-Ti lunar basalts, meteorites, and CE-5 basalts were collected from the literature26,62,63. The pressures of ca. 4.4–3.0 Ga lunar samples and ca. 2.0 Ga CE-5 basalts were assumed to be 3.5 GPa and 2.0 GPa, respectively, as suggested by ref. 64. Detailed results are listed in Supplementary Data 7.

Mantle fO2 data for Earth, Mars, and the Moon
The Earth mantle fO2 constraints from two available studies show a similar fO2 trend1,2, but each dataset was determined using a different oxybarometer. The fO2 derived from different oxybarometers shows a systematic offset. Ref. 3 corrected these two datasets using the current canonical modern MORB value of ΔFMQ+0.2 ± 0.3. Upon correction, the two mantle oxidation trends overlap within error, thus supporting the mantle oxidation concept. The emendated data suggest a notably elevated fO2 of Earth’s mantle compared to the values (~ΔIW-2) during the final stages of core formation33. The oxygen fugacity of Mars may be directly estimated by analyzing Martian meteorites such as shergottites, nakhlites, and chassignites. The fO2 of shergottites based on the V olivine oxybarometry yields an average fO2 of ΔIW+1.4 ± 1.8 (ref. 5). In contrast, chassignites and nakhlites are relatively oxidized (ΔIW+5.6 ± 0.4; ΔIW+2.7 to ΔIW+3.4; refs. 5,65). The elevated fO2 observed in chassignites and nakhlites is thought to result from metasomatism5,6,66. Hence, the average fO2 of shergottite is considered the most accurate approximation for bulk silicate Mars5. According to models of early Mars differentiation, it is concluded that the Martian mantle reached equilibrium with core-forming metal under reduced circumstances (~ΔIW-1.5; ref. 34). This result suggests that the fO2 of the Martian mantle must have oxidized since the formation of its core. The fO2 estimations for Apollo materials, including basalts and pyroclastic glasses of various crystallization ages (3.6–3.0 Ga) were obtained from two earlier studies18,19. The fO2 values used in this study were derived only using the V oxybarometer, thus avoiding any potential systematic bias among different oxybarometers.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The result that the lunar mantle has remained reduced thru geologic time is noteworthy, and I think the\nhypothesis that body size as a first-order control of oxidation thru time is noteworthy also.\nI think the research builds on and summarizes the fo2 work done on Earth, Moon and Mars and makes\nrelevant conclusions based on the work. The work is done on a new set of \"young\" lunar basalts and this\nis a sound interpretation of the results and is significant to planetary petrologists.\nI think the methodology is sound and the analyses of the fragments looks good.\nI have a few suggestions that I believe would improve the ms.\nFrom Line 74 - 77 and Figure 1 caption: I think you are trying to say that olivine is normally zoned from\nhigh- Mg in cores to low- Mg in rims and spinels zone from low Ti to high Ti in same areas. I would say\nthat \"Fe and Ti increase with increasing crystallization\" instead of \"accumulated increasing amounts of Fe\nand Ti....\" I would try state this a little more clearly. For Fig. 1 and caption- Show way less labels on the\nBSE images and just give a couple examples of olivine-spinel pairs that are high Mg#, low Ti# and a\ncouple that are low Mg#, low Ti#. That is the point of the BSE images, correct? Unless you want to show\nthe different textures...\nLine 139. Maybe put the formula in for Bridgmanite, so one can get an idea of the proposed Fe-\ndisproportionation reaction.\nFigure 2, 3. Increase the font size of the axes labels for easy reading.\nLine 159. Should this be \"magma <ocean> cooling instead of just \"magma cooling?\" Maybe spend a\ncouple of sentences explaining H2 loss and the oxidation of mantles, how and when it occurs.\nLine 168. Rewrite this sentence as it doesn't quite make sense as is.\nFigure 3. Explain what the red star is and where that point is from. Also, maybe spend a sentence in the\ntext or figure caption on what basalts are measured for Earth and Mars.\nFigure 4. Why not put in an image of Mercury instead of the symbol? Even though it lies off the trend,\nit would be better (imo) for the figure.\nLine 193. I think it should read, \"Under low fO2 conditions <multivalent> elements usually.....\"\nLine 195-196. This sentence which begins \"The long-term reduced.....\" seems out of place or disjointed\nfrom the previous and following sentence?\nLine 196-208. This section is a little convoluted- try rewording to emphasize the higher melting temp in\nthe reduced lunar mantle which leads to a slow cooling rate.", "reviewer_2": "This paper uses a vanadium oxybarometer on olivine and spinel to determine the oxygen fugacity of\nlunar mantle-derived melts with ages of ~2 billion years. These oxygen fugacity measurements are then\ncompared to values derived from Apollo samples aged 3.6-3 billion years to show that there has been no\nevolution of the lunar mantle redox state, likely since its formation. This is a very interesting result, and\nworth publication. However, the paper then reviews proposed oxidation mechanisms that may have\noperated on other planets and argues that all of the proposed mechanisms have a size-dependency,\nwhich is why the Moon has remained reduced whereas Mars and Earth have gradually oxidized over\ntime. I disagree with the direction of the size-dependency for a few of the proposed oxidation\nmechanisms, so I cannot support this final claim. Please note that I am a theorist, so I will leave review of\nthe analytical results on the oxybarometers to others.\nThe three proposed planetary oxidation mechanisms include : 1) Fe-disproportionation at high\npressures, 2) late accretion of oxidizing materials, and 3) hydrogen degassing/escape. I have no issue\nwith the discussion of Fe-disproportionation. The paper suggests that both late accretion and hydrogen\ndegassing/escape will produce more oxidation on larger planets. I disagree that this is necessarily the\ncase for late accretion, and suggest that this is simply incorrect for hydrogen degassing/escape, as\nexplained below.\nLate accretion: the proposal that late accretion is more oxidizing for larger planets rests on the\nassumptions that 1) larger planets experience more late accretion (which may be accurate) and 2) late\naccretion is oxidizing. Assumption #2 is in disagreement with recent isotopic studies that have suggested\nthat Earth’s late accreting material was dominated largely by inner disk material (see e.g. Dauphas 2017\nNature, Fisher-Godde & Kleine 2017 Nature, Hopp et al. 2020 EPSL, Zhu et al. 2021 Nature Astronomy,\nWorsham & Kleine 2021 Sci.Adv.) which is likely more metal-rich and volatile-depleted than\ncarbonaceous chondrites. Hirschmann (2023) EPSL models the redox history of the Earth, in which he\nassumes late accretion of dominantly metal-rich materials could have neutralized the early oxidation of\nthe surface and near-surface. There is no a priori reason to assume that all late accretion would be\noxidizing. This argument must be better supported to be included.\nHydrogen outgassing/escape: This argument confounds “volatile-rich” with “larger”. The part of this\nprocess that yields a net planetary oxidation is the complete loss of H2(g) from the planet by\natmospheric escape. Atmospheric escape is most strongly dependent on planetary size, with smaller\nobjects experiencing more escape (the reverse of the trend claimed here). The other factor not\nmentioned here, which effects both escape and initial volatile-budget, is the formation distance from the\nSun. Escape will be stronger at closer distances, whereas volatile accretion will be stronger at further\ndistances. Some have argued that Mars was initially more volatile-rich than the Earth. Extended data Fig.\n5 plots present-day H content versus object size with a nice trend. However, especially for the smaller\nobjects, if H-escape has been significant, these H-abundances cannot be representative of the initial\nbudget and hence would not correlate with the amount of mantle oxidation produced.\nAn additional oxidation mechanism not mentioned in this paper is the possible loss of hydrogen from a\nwater vapor-dominated atmosphere. While Sharp et al. (2013) are correct that the magnitude of\noxidation produced by this mechanism may be smaller than H2-escape, it is not a negligible effect.\nModels comparing Venus and Earth (e.g. Hamano et al. 2013) suggest that residual oxygen, which cannot\nescape at the same rate as H, could significantly oxidize Venus, much more than the larger and cooler\nEarth would experience.\nIn all cases, these arguments have ignored Mercury, which does not fit into these proposed size-based\ntrends. However, bulk redox trends (as opposed to the mantle redox trends proposed here) (e.g. Tronnes\net al. 2019) have proposed that there is a radial redox gradient in the Solar System, with the Moon\n(instead of Mercury) appearing to be an outlier due to its formation from the remnants of a giant impact\nwhich resulted in metal sequestration in the primary object (Earth). Many papers have also argued that\nthe Moon formed volatile-depleted, as a result of the giant impact, which would have limited possible\noxidation by hydrogen escape. Therefore, it is unclear to me whether the Moon can realistically be\nconsidered in the same category as other planetary objects that formed through similar processes\n(whether those are planetesimal or pebble-based accretion is likely unimportant to this distinction)."}, "author_response": "Point-by-point response to the issues raised by the referees\nNote: Comments and suggestions are in black, Responses in blue.\nReviewer #1\nThe result that the lunar mantle has remained reduced thru geologic time is noteworthy,\nand I think the hypothesis that body size as a first-order control of oxidation thru time is\nnoteworthy also.\nI think the research builds on and summarizes the fO work done on Earth, Moon and\nof \"young\" lunar basalts and this is a sound interpretation of the results and is\nsignificant to planetary petrologists.\nI think the methodology is sound and the analyses of the fragments looks good.\nI have a few suggestions that I believe would improve the ms.\nResponse: Thank you for the positive and detailed review. We have carefully\nconsidered all the suggestions and comments and have revised the manuscript\naccordingly.\n(1) From Line 74 - 77 and Figure 1 caption: I think you are trying to say that olivine is\nnormally zoned from high- Mg in cores to low- Mg in rims and spinels zone from low\nTi to high Ti in same areas. I would say that \"Fe and Ti increase with increasing\ncrystallization\" instead of \"accumulated increasing amounts of Fe and Ti....\" I would try\nstate this a little more clearly. For Fig. 1 and caption- Show way less labels on the BSE\nimages and just give a couple examples of olivine-spinel pairs that are high Mg#, low\nTi# and a couple that are low Mg#, low Ti#. That is the point of the BSE images,\ncorrect? Unless you want to show the different textures...\nResponse: Thank you for your suggestions. Yes, what we are trying to say is that as the\ncrystallization process continues, Mg# in olivine decreases while the Ti# in spinel\nincreases. We have revised this sentence (see lines 74-76). Figure 1 has also been\ndesigned to illustrate the same observation, and we have modified it by simply labeling\n1\na few examples of olivine-spinel pairs with various Mg# and Ti# (see Figure 1 and lines\n85-86).\n(2) Line 139. Maybe put the formula in for Bridgmanite, so one can get an idea of the\nproposed Fe-disproportionation reaction.\nResponse: Thank you for your suggestion. We have added the formula for Bridgmanite\n[(Mg,Fe)(Al,Si)O )] into the revised manuscript (see line 142).\n3\n(3) Figure 2, 3. Increase the font size of the axes labels for easy reading.\nResponse: Fixed.\n(4) Line 159. Should this be \"magma <ocean> cooling instead of just \"magma cooling?\"\nMaybe spend a couple of sentences explaining H loss and the oxidation of mantles,\n2\nhow and when it occurs.\nResponse: Thanks for the suggestion. This sentence has been removed from the revised\nmanuscript. After careful consideration of reviewer #2’s comments, we have decided to\nfocus the discussion on the mechanisms maintaining the long-term reduced lunar\nmantle.\n(5) Line 168. Rewrite this sentence as it doesn't quite make sense as is.\nResponse: We have rewritten this sentence. The insufficient pressure for\nFe-disproportionation, the low mass ratio of late-accreted materials, and the low initial\nwater content of the Moon may collectively contribute to the maintenance of its\nlong-term reduced state. (see lines 181-183).\n(6) Figure 3. Explain what the red star is and where that point is from. Also, maybe\nspend a sentence in the text or figure caption on what basalts are measured for Earth and\nMars.\nResponse: Thanks for the suggestion. The red star represents the original average fO\n2\nvalues of Earth, Mars, and the Moon derived from Frost et al. (2008) and Righter et al.\n(2008). The fO data for the Earth’s mantle are from mid-ocean ridge basalts and\n2\n2\nultramafic lavas (Kadoya et al., 2020). The fO data for the Martian mantle are from\n2\nshergottites (Nicklas et al., 2021). See lines 174-177.\nFrost, D.J., Mann, U., Asahara, Y. & Rubie, D.C. The redox state of the mantle during and just\nafter core formation. Philosophical Transactions of The Royal Society A-mathematical\nPhysical and Engineering Sciences 366, 4315–4337 (2008).\nRighter, K., Yang, H., Costin, G. & Downs, R.T. Oxygen fugacity in the Martian mantle\ncontrolled by carbon: new constraints from the nakhlite mil 03346. Meteoritics & Planetary\nScience 43, 1709–1723 (2008).\nKadoya, S., Catling, D.C., Nicklas, R.W., Puchtel, I.S. & Anbar, A.D. Mantle data imply a\ndecline of oxidizable volcanic gases could have triggered the great oxidation. Nature\nCommunications 11, 2774 (2020).\nNicklas, R.W., et al. Uniform oxygen fugacity of shergottite mantle sources and an oxidized\nMartian lithosphere. Earth and Planetary Science Letters 564, 116876 (2021).\n(7) Figure 4. Why not put in an image of Mercury instead of the symbol? Even though it\nlies off the trend, it would be better (imo) for the figure.\nResponse: Thank you for your suggestion. This figure has been removed from the\nrevised manuscript. After careful consideration of reviewer #2’s comments, we have\nremoved the discussion on body size as a first-order control of oxidation with time.\n(8) Line 193. I think it should read, \"Under low fO2 conditions <multivalent> elements\nusually.....\"\nResponse: Fixed.\n(9) Line 195-196. This sentence which begins \"The long-term reduced.....\" seems out of\nplace or disjointed from the previous and following sentence?\nResponse: This sentence has been deleted and we have rewritten this paragraph (see\nlines 185-189).\n(10) Line 196-208. This section is a little convoluted- try rewording to emphasize the\nhigher melting temp in the reduced lunar mantle which leads to a slow cooling rate.\nemphasizing that the elevated melting temperature in the reduced lunar mantle may lead\nto its slow cooling rate (see lines 193-205).\nReviewer #2\nThis paper uses a vanadium oxybarometer on olivine and spinel to determine the\noxygen fugacity of lunar mantle-derived melts with ages of ~2 billion years. These\noxygen fugacity measurements are then compared to values derived from Apollo\nsamples aged 3.6-3 billion years to show that there has been no evolution of the lunar\nmantle redox state, likely since its formation. This is a very interesting result, and worth\npublication. However, the paper then reviews proposed oxidation mechanisms that may\nhave operated on other planets and argues that all of the proposed mechanisms have a\nsize-dependency, which is why the Moon has remained reduced whereas Mars and\nEarth have gradually oxidized over time. I disagree with the direction of the\nsize-dependency for a few of the proposed oxidation mechanisms, so I cannot support\nthis final claim. Please note that I am a theorist, so I will leave review of the analytical\nresults on the oxybarometers to others.\nThe three proposed planetary oxidation mechanisms include: 1) Fe-disproportionation at\nhigh pressures, 2) late accretion of oxidizing materials, and 3) hydrogen\ndegassing/escape. I have no issue with the discussion of Fe-disproportionation. The\npaper suggests that both late accretion and hydrogen degassing/escape will produce\nmore oxidation on larger planets. I disagree that this is necessarily the case for late\naccretion, and suggest that this is simply incorrect for hydrogen degassing/escape, as\nexplained below.\nResponse: Thank you for your detailed review. After careful consideration of your\ncomments and suggestions, we have decided to remove the discussion (including related\nfigures) of body size controlling planetary redox evolution and focus only on the\nmechanisms maintaining the long-term reduced lunar mantle in the revised manuscript.\n4\n(1) Late accretion: the proposal that late accretion is more oxidizing for larger planets\nrests on the assumptions that 1) larger planets experience more late accretion (which\nmay be accurate) and 2) late accretion is oxidizing. Assumption #2 is in disagreement\nwith recent isotopic studies that have suggested that Earth’s late accreting material was\ndominated largely by inner disk material (see e.g. Dauphas 2017 Nature, Fisher-Godde\n& Kleine 2017 Nature, Hopp et al. 2020 EPSL, Zhu et al. 2021 Nature Astronomy,\nWorsham & Kleine 2021 Sci.Adv.) which is likely more metal-rich and\nvolatile-depleted than carbonaceous chondrites. Hirschmann (2023) EPSL models the\nredox history of the Earth, in which he assumes late accretion of dominantly metal-rich\nmaterials could have neutralized the early oxidation of the surface and near-surface.\nThere is no a priori reason to assume that all late accretion would be oxidizing. This\nargument must be better supported to be included.\nResponse: Thank you for pointing out this problem. We had originally considered that\nthe late accreted materials might be oxidized. However, it seems that this assumption\nmight not be correct. Thus, we have rewritten paragraph [8] to remove the discussion\nthat late accreted material may oxidize the mantles of Earth and Mars. We have simply\nemphasized that the Moon accreted a significantly smaller volume of materials. Even if\nwe assume that oxidized materials dominated the late accretion, this process may still be\ninsufficient to oxidize the lunar mantle (see lines 153-159).\n(2) Hydrogen outgassing/escape: This argument confounds “volatile-rich” with “larger”.\nThe part of this process that yields a net planetary oxidation is the complete loss of\nH2(g) from the planet by atmospheric escape. Atmospheric escape is most strongly\ndependent on planetary size, with smaller objects experiencing more escape (the reverse\nof the trend claimed here). The other factor not mentioned here, which effects both\nescape and initial volatile-budget, is the formation distance from the Sun. Escape will be\nstronger at closer distances, whereas volatile accretion will be stronger at further\ndistances. Some have argued that Mars was initially more volatile-rich than the Earth.\nExtended data Fig. 5 plots present-day H content versus object size with a nice trend.\nHowever, especially for the smaller\nobjects, if H-escape has been significant, these\ncorrelate with the amount of mantle oxidation produced.\nResponse: We are grateful for your constructive comments. We agree with you that the\npresent-day H abundance of a planet may not represent its initial budget, and that the\nformation distance from the Sun can affect the initial H budget, and smaller objects\nwould experience more extensive hydrogen escape. All these points do not support our\noriginal idea that larger planets would lose more hydrogen and are more easily oxidized.\nThus, we have rewritten paragraph [9], and now only focus on the long-term reduced\nlunar mantle. It indicates that hydrogen degassing of the lunar mantle was not sufficient\nto exhaust Fe0 and increase oxygen fugacity (see lines 161-171).\n(3) An additional oxidation mechanism not mentioned in this paper is the possible loss\nof hydrogen from a water vapor-dominated atmosphere. While Sharp et al. (2013) are\ncorrect that the magnitude of oxidation produced by this mechanism may be smaller\nthan H2-escape, it is not a negligible effect. Models comparing Venus and Earth (e.g.\nHamano et al. 2013) suggest that residual oxygen, which cannot escape at the same rate\nas H, could significantly oxidize Venus, much more than the larger and cooler\nEarth would experience.\nResponse: Thank you for your suggestion. We have added the suggested mechanism\n“possible loss of hydrogen from a water vapor-dominated atmosphere” in the revised\nmanuscript (see lines 161-163). In the case of the Moon, if an early atmosphere was\npresent, it is possible that it was H dominated (e.g., Renggli et al., 2017). Therefore,\n2\nthis oxidation mechanism may have little effect on the Moon.\nRenggli, C.J., King, P.L., Henley, R.W. & Norman, M.D. Volcanic gas composition, metal\ndispersion and deposition during explosive volcanic eruptions on the Moon. Geochimica et\nCosmochimica Acta 206, 296–311 (2017).\n(4) In all cases, these arguments have ignored Mercury, which does not fit into these\nproposed size-based trends. However, bulk redox trends (as opposed to the mantle\nredox trends proposed here) (e.g. Tronnes et al. 2019) have proposed that there is a\n6\nradial redox gradient in the Solar System, with the Moon (instead of Mercury)\nappearing to be an outlier due to its formation from the remnants of a giant impact\nwhich resulted in metal sequestration in the primary object (Earth). Many papers have\nalso argued that the Moon formed volatile-depleted, as a result of the giant impact,\nwhich would have limited possible oxidation by hydrogen escape. Therefore, it is\nunclear to me whether the Moon can realistically be considered in the same category as\nother planetary objects that formed through similar processes\n(whether those are planetesimal or pebble-based accretion is likely unimportant to this distinction).\nResponse: Thank you for your suggestions and comments. We agree with you that the\nMoon maybe an outlier instead of Mercury. The positive correlation about the average\nfO of the mantles of Earth, Mars, the Moon, and the asteroid 4-Vesta with their\n2\nrespective radius could be a coincidence. Therefore, we have removed the discussion\n(including related figures) of body size controlling planetary redox evolution and\nfocused only on the mechanisms maintaining the long-term reduced lunar mantle in the\nrevised manuscript.\n7"}, {"round": 2, "reviewer_comments": {"reviewer_2": "I appreciate the time the authors took to consider my previous comments. I think the new focus of the\nargument on the relative constancy of the lunar mantle fO2 remains interesting and noteworthy and is\nbetter supported by the arguments. I have a few minor comments that the authors may consider but\notherwise support publication of the paper with very minor revisions.\nLines 156-158: I agree that late accretion of this mass ratio is likely insufficient to oxidize the lunar\nmantle, but I think the argument would be strengthened if the authors presented a small mass-balance\ncalculation to actually show that this is the case. Assuming e.g. all late accretion is delivered as pure\nwater ice, how much oxidation could occur?\nLines 162-163: Renggli et al. (2017) show that at reasonable pressures for lunar degassing at IW-2, the\ndominant gases are CO and S2. Perhaps you meant that H2 was dominant compared to other H-bearing\nspecies?\nFigure 3a: To emphasize that the lunar mantle remained reduced over its lifetime (and that 2 Ga\nvolcanism was effectively the same fO2 as earlier), then plotting a flat line through the lunar data points\nwould be more consistent than a figure with an increasing slope (indicating continued oxidation over\ntime).\nFigure 3b caption: I would specify that this is “late accretion” mass. All mass is accreted at some point,\nbut you are specifically referring to the mass accreted following the last major core-forming event.\nI support the move of the material on mantle potential temperature into the main text, I think this is an\ninteresting consideration for lunar thermal evolution models."}, "author_response": "Point-by-point response to the issues raised by Reviewer #2\nNote: Comments and suggestions are in black, Responses in blue.\nReviewer #2\nI appreciate the time the authors took to consider my previous comments. I think the\nnew focus of the argument on the relative constancy of the lunar mantle fO2 remains\ninteresting and noteworthy and is better supported by the arguments. I have a few minor\ncomments that the authors may consider but otherwise support publication of the paper\nwith very minor revisions. I support the move of the material on mantle potential\ntemperature into the main text, I think this is an interesting consideration for lunar\nthermal evolution models.\nResponse: Thanks so much for your review. We carefully considered all your\nsuggestions and comments and have revised the manuscript accordingly.\nLines 156-158: I agree that late accretion of this mass ratio is likely insufficient to\noxidize the lunar mantle, but I think the argument would be strengthened if the authors\npresented a small mass-balance calculation to actually show that this is the case.\nAssuming e.g. all late accretion is delivered as pure water ice, how much oxidation\ncould occur?\nResponse: Thank you for constructive suggestion. We have presented a simple\nmass-balance calculation (H O + Fe0 = H + FeO) to strengthened our issue. The mass\n2 2\nof lunar mantle accounts for ~86% of the Moon, so the late accretion materials were\n~0.027% of the mass of the lunar mantle. Even if the late accretion were dominated by\noxidized materials (e.g., pure water ice), such a limited amout of accretion can only\noxidize ~0.08 wt.% Fe metal in the lunar mantle. It is plausible that this process may\nhave an insignificant impact on the redox state of the lunar mantle. See lines 140-143.\nLines 162-163: Renggli et al. (2017) show that at reasonable pressures for lunar\ndegassing at IW-2, the dominant gases are CO and S . Perhaps you meant that H2 was\n2\ndominant compared to other H-bearing species?\ndominant H-species should occur as H (see lines 148-149).\nthat 2 Ga volcanism was effectively the same fO as earlier), then plotting a flat line\n2\nthrough the lunar data points would be more consistent than a figure with an increasing\nslope (indicating continued oxidation over time).\nResponse: We have modified figure 3a with a flat line to better display that the lunar\nmantle remained reduced from 3.6 Ga to 2.0 Ga.\nFigure 3b caption: I would specify that this is “late accretion” mass. All mass is accreted\nat some point, but you are specifically referring to the mass accreted following the last\nmajor core-forming event.\nResponse: Fixed (see lines 492-493)."}]

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