Halogen abundance evidence for the formation and metasomatism of the primary lunar crust

Domain: earth_science
Published: 2025-06-20
Editorial outcome: major_revision

Abstract
Halogen abundances in lunar minerals provide unique insight into the Moon’s volatile budget, but incomplete knowledge of halogen incorporation in minerals and melts limits their application. Here we present experimentally derived Cl mineral-melt partition coefficients and couple models of the evolution of the lunar interior to measured Cl and F abundances in lunar crust samples. Most nearside lunar crust materials turn out to be anomalously enriched in Cl compared to expectations from primary crust formation models, likely due to widespread (metal chloride) metasomatism. Mg-suite rocks formed outside the nearside Procellarum KREEP Terrane do not show this Cl enrichment. Plagioclases from these KREEP-poor rocks equilibrated with magma with a low F/Cl ratio of approximately 1.5. This low F/Cl ratio indicates that KREEP-poor Mg-suite parental magma originated from a source containing at least 2% liquid trapped during lunar magma ocean solidification. The hypothesis that Cl metasomatism is limited to the Procellarum KREEP terrane can be further tested by combining our experimental data with halogen measurements of farside Chang’e-6 samples.

Introduction
Since the Moon was found not to be completely devoid of volatile elements1,2, numerous studies over the past decade have revealed that the volatile (e.g., water and halogen) distribution on the lunar crust and in the lunar interior is heterogeneous. Most of our knowledge on the halogen distribution in the Moon is based on studies of lunar apatite from different crustal lithologies, including mare basalts, magnesian-suite (Mg-suite), alkali-suite rocks, and ferroan anorthosite (FAN)3–9. These apatite studies identified two volatile reservoirs with distinct F/Cl ratios in the Moon. The parental magmas of primary crustal lithologies such as the Mg-suite rocks generally have much lower calculated F/Cl ratios (~1 to 5) compared to those of the secondary mare basalts (~20–50)6. This observation has been further verified by halogen abundance measurements of lunar pyroclastic glass beads and olivine-hosted melt inclusions (see a detailed review by McCubbin et al.5).

The origins of these two distinct source regions have been explained within the context of the lunar magma ocean paradigm. McCubbin et al.10 and Potts et al.8 proposed that the source region of mare basalts consists of cumulates from the lunar magma ocean with F » Cl, largely influenced by the much lower mineral-melt partition coefficients of cumulate minerals for Cl compared to F8. In contrast, parts of the primary crust, represented by various Apollo highland lithologies, are generally KREEP-rich (i.e., enriched in potassium, rare earth elements, and phosphorus). Parental magma of most Mg-suite rocks are thought to be associated with the urKREEP reservoir referred as the last dreg of the lunar magma ocean (LMO)11–14. The F/Cl ratios of urKREEP are estimated to be low, between ~0.5 and 4.4, based on halogen abundances in lunar soil10 and apatite15, respectively. This range is close to the inferred range of F/Cl in the source of Mg-suite rocks, and the involvement of urKREEP is also consistent with the KREEP signature in many of these samples.

However, the identification of KREEP-poor Mg-suite rocks suggests that the origin of some primary crustal rocks may not involve the KREEP component16–18, implying that the KREEP component may not be the “petrologic driver” of the formation of Mg-suite magmas19. This agrees well with the observation by remote sensing of the widespread distribution of Mg-suite lithologies on the lunar farside, which is thought to be poor in KREEP20–22. As the recent Chang’e-6 sample-returned mission to the lunar farside likely returned new Mg-suite samples in an area not affected by KREEP23,24, it is important to reassess the role of KREEP in Mg-suite formation from a halogen perspective.

Here, we aim to reassess the formation and evolution of the halogen reservoirs in the Moon by studying the partitioning of Cl between nominally anhydrous minerals (NAMs) that are key to the formation of the lunar crust and co-existing lunar melt. Unlike apatite, which forms only during the final stages of magma crystallization, fractionating F/Cl ratios of final dregs of melt6, continuously crystallizing NAMs can faithfully record nearly constant F/Cl ratios without modifying F/Cl ratios in melt because F and Cl are generally highly incompatible. Several recent studies have reported the F and Cl contents in lunar NAMs, including alkali feldspar, plagioclase and pyroxenes from highland rocks25–29. These results show large variations in F/Cl ratios ranging from less than one to hundreds, suggesting the existence of heterogeneous halogen reservoirs or lunar crust alteration by late processes. The F/Cl ratios of parental magma of non-KREEP Mg-suite rocks also remain enigmatic, given the paucity of halogen partition coefficients between NAMs and silicate melt under lunar-relevant conditions8,30,31.

In this study, we report new Cl partitioning data between plagioclase, orthopyroxene and melt at lunar conditions derived from high-temperature, high-pressure experiments, and derive a set of partition coefficients that can be used at pressures, temperatures, and water contents that are relevant to the Moon. Combined with our previously reported F partition coefficients31, we are able to reconstruct the F/Cl ratios of melts in equilibrium with lunar crustal minerals, providing new insight into the evolution of the halogen budgets of the lunar crust and the origin of the Mg-suite.

Results
The composition of the starting material for the experiments was based on a residual melt composition formed at around 80% solidification of a nominally dry lunar magma ocean (experiment LBS6 from ref. 32, Supplementary Data 1). High-pressure and high-temperature experiments were conducted at pressures of 0.5-1 GPa and temperatures of 1120–1240 °C. Full details of the starting material synthesis, high-pressure, high-temperature experimental techniques, and analytical methods are provided in the Methods. Representative back-scattered electron microscope images of run products are shown in Fig. 1. Experiments run at 0.5-1 GPa and 1120–1240 °C all produce orthopyroxene and plagioclase in equilibrium with melt. The experiments with low melt fractions (e.g., at 1120 oC), in addition, contain quartz and rutile. Most experiments show sharp contacts between liquid and crystals (Fig. 1) consistent with mineral-melt equilibrium. In case of suspected bulk disequilibrium in terms of phase relations or core-to-rim orthopyroxene zonation in experiments at low temperatures (e.g., Fig. 1), the major and chlorine element abundances of mineral grains were obtained from crystal rims at a distance of ~10 μm from the crystal-melt interface. Such locations are closest in chemical equilibrium with the surrounding melt pool, while avoiding the analytical problem of crystal analyses being contaminated by secondary fluorescence from adjacent melt. For orthopyroxene grains with rims thinner than 10 μm, some analysis points fall within 5 μm of the crystal-melt interface. Quantitative secondary fluorescence (SF) modeling on the extent of the Cl contribution from Cl-rich melt shows that the Cl contribution from the melt is insignificant at a distance of 10 μm from the crystal-melt interface (Supplementary Fig. S1, Supplementary Information). The analysis positions in inhomogeneous orthopyroxene grains (e.g., in JQCl9 and JQCl10) and all plagioclase grains were obtained from positions in the cores of several randomly selected crystals.

Chlorine is detected in all orthopyroxene (37–210 μg/g) and plagioclase (34–824 μg/g), well above the electron microprobe detection limit of about 28 μg/g, while the silicate melt contains 3.06–8.90 wt.% Cl contents. As the orthopyroxene grains with zonations show both Mg-rich rims and Fe-rich rims, orthopyroxene-liquid Fe-Mg exchange coefficients, Kd have been calculated for individual analyses for further equilibrium state evaluation. Only individual orthopyroxene rim compositions with Kd ranging from ~0.2–0.433–35 were used for each experiment and for partition coefficient calculation (Supplementary Data 2). Although the orthopyroxene Mg-rich rims generally have disequilibrium Kd values (e.g., 0.16 in JQCl1), their Cl abundances show no systematic difference with Fe-rich rims. The calculated Cl mineral/melt partition coefficients (Dmin=melt) are presented in Table 1. The Cl contents of individual phases and partition coefficients for quartz and rutile are not discussed in the main text. Full detailed descriptions on individual mineral phases, and Cl contents of all phases are provided in the Supplementary Information.

In order to evaluate the quality and reproducibility of the derived Cl partition coefficients, we have conducted duplicate experiments with varying experimental durations. The Cl partition coefficients show no correlation with Cl contents in melt (Supplementary Fig. S2), suggesting it obeys Henry’s law. At 1120 oC and 0.5 GPa, three experiments with durations of 24–48 h have different mineral phases composition between one 24h experiment and two 48-h experiments (Supplementary Data 2). The 48-h experiments have Cl contents of 8.21±0.19 wt% and 8.90±0.57 wt% in the silicate melt, but the 24-h experiment has lower 5.94±0.14 wt% Cl abundance. This indicates that bulk equilibrium was not reached in this 24-h experiment at a temperature of 1120 oC. However, all three experiments yield consistent DOpx=melt and DPlag=melt (Table 1 and Fig. 2), indicating 24 h is sufficient to reach equilibrium Cl partitioning at temperatures of 1120 oC and above. This conclusion is further supported by the consistent DOpx=melt and DPlag=melt values in duplicate experiments at 1160 oC and 0.5 GPa (Table 1 and Fig. 2).

Although we attempted to do experiments that were nominally anhydrous, in practice, this is impossible to achieve in terrestrial laboratories. Nominally anhydrous experiments of the type performed in this study contain on the order of 500 μg/g water36,37. In the absence of hydrous minerals, this water would all concentrate in the melt phase. To assess if water concentrations affect the behavior of Cl, we measured the water contents in the silicate glasses from our experiments. These range from 0.20–1.00 wt.% H2O (Supplementary Data 3). DOpx=melt and DPlag=melt values show no dependence on pressure, temperature, or water content within the experiment temperature and pressure range and in the measured range of water contents. Recent studies have shown a significant increase in DPlag=melt for water itself as the amount of water in silicate melt is decreased below 1 wt.%38,39. This is not observed in the case of Cl partitioning. The mean DOpx=melt and DPlag=melt values are calculated as 0.0019±0.0006 and 0.0024±0.0006 (Supplementary Data 4), respectively, excluding some high values that are accompanied by much larger errors (e.g., DPlag=melt = 0.0077±0.0051, in experiment JQCl4). Our data are in good agreement with existing data (where available) obtained at lunar conditions8 and most of the data focused on terrestrial conditions40 (see detailed discussion in the Supplementary Information about the exception related to their DOpx=melt).

Methods
Starting material synthesis and experimental techniques
Synthesis of starting materials followed the methods described in our previous work on fluorine mineral-melt partitioning31. A residual melt composition formed at around 80% solidification of a nominally dry lunar magma ocean (experiment LBS6)32 was chosen as the starting material. This starting material is in the simple system CaO-Al2O3-MgO-SiO2-FeO-TiO2. Three weight percent Cl was added to this composition in the form of CaCl2 without changing the major element ratios in LBS6 (Supplementary Data 1). Starting materials were prepared by mixing appropriate amounts of high-purity (99.5–100.05%, Alfa Aesar) oxide powders (MgO, Fe2O3, Al2O3, TiO2, SiO2) and carbonates (CaCO3). As described in ref. 31, the oxides MgO, Al2O3, TiO2 and SiO2 were fired overnight at 1000 oC and then stored at 110 oC, and the other chemicals were stored at 110 oC prior to use. After mixing the components under ethanol in an agate mortar for 1 h, the mixture was dried in air and decarbonated in an iron-saturated Pt crucible in a box furnace by gradually raising the temperature from 650 to 1000 oC in 7 h31. The resulting mixture was melted in an iron-saturated Pt crucible for 30 mins at 1550 oC to promote homogeneity and quenched to a glass by immersing the bottom of the Pt crucible in water. The glass was subsequently crushed, dried, and reground under ethanol with appropriate amounts of CaCl2 powder in an agate mortar for 1 h and then kept at 110 oC until use.

High-pressure and high-temperature experiments at pressures of 0.5–1 GPa and temperatures of 1120–1240 oC were conducted in a QUICKpress piston cylinder press using a half-inch (12.7 mm) diameter talc-pyrex cell at the Vrije Universiteit Amsterdam, using methods described in detail in our previous work31. Temperatures were measured using a PtRh30-PtRh6 (type B) thermocouple. Graphite-lined Pt capsules are used in all experiments. As in ref. 31, a hand-machined graphite bucket, with an ID of 0.7 mm, OD of 1.6 mm and a length of 3-4 mm, was filled with starting material, closed with a graphite lid and inserted in a Pt capsule, with an ID of 1.7 mm, OD of 2 mm, and a length of ~6 mm. Capsules loaded with starting material were kept at 300 oC in a furnace overnight to dry before welding. The graphite-lined Pt capsules impose an oxygen fugacity close to that of the carbon-carbon monoxide (CCO) buffer36, ensuring that iron in our experiments is present predominantly as Fe2+. This is close to the current best estimates of oxygen fugacity conditions in the LMO61,62. Experiments were kept at 800 oC and target pressure for 30 mins for sintering31. Then they were heated to an near-liquidus temperature of 1300 oC for 30 min and cooled to the target temperature at a rate of 5 oC per 30 mins. Subsequently, the experiments were kept at their final target temperatures for 22 to 48 h. At the end of experiments, samples were quenched by turning off the power, after which temperatures typically dropped to values below 50 oC in approximately 20 s. Experimental conditions are summarized in Supplementary Data 2.

Electron microprobe analysis
Experimental run products were mounted in epoxy and polished, and carbon-coated for electron microprobe analysis. The chemical compositions of the run products (minerals and glasses) were determined using a JEOL JXA-8530F field emission electron probe microanalyzer (EPMA) at the Universität Münster. As previously described in Zhang et al.63 and Flemetakis et al.64, the electron beam size was set to a diameter of 2 and 10 μm for the mineral phases and glasses, respectively. Major element abundances were determined using a beam current of 15 nA and operated at 15 kV accelerating voltage. Counting times were 15 s for peak and 5 s for background. Chlorine abundances in the minerals and glasses were determined in a second step using counting times of 120 s on peak and 60 s on background at beam currents were 60 nA. The electron microprobe gave a detection limit of ~28 μg/g for Cl analysis. Both analyses, major elements and Cl, were merged using the Offline-matrix correction EPMA software. The experiment results of mineral major and Cl compositions are provided in the Supplementary Data 2.

Water content measurement
The water contents of the glasses from our experiments were measured by a combination of a Fourier-transform infrared spectrometer (FTIR, JASCO IRT-5200) and a Raman spectrometer (JASCO NRS-5100gr) at the Geodynamics Research Center (GRC), Ehime University. Most samples were analyzed by Raman spectrometer only, because most of the recovered samples with quench cracks were difficult to doubly polish, and the area of quenched glass pockets in the polished run products was too small to enable FTIR measurements (with the sole exception of sample No. JQCl9). In order to assess the validity of the Raman measurements, three water-bearing standard glasses were synthesized using high-pressure apparatuses and measured by FTIR as well. Two water-bearing standard glasses (JB-1b_#1 and JB-1b_2#) were synthesized from a mixture of water-bearing natural alkaline basalt (JB-1b)65, and different amounts of dry oxides after decarbonation (Si-Mg-Ca-Al-Fe) using a 2000-ton multi-anvil apparatus at GRC, Ehime University. One standard glass (PLW1-18N10) was synthesized from dry oxides after decarbonation by a piston-cylinder press at the Vrije Universiteit Amsterdam.

The water content of quenched glass (CH2O in weight percent) in these doubly polished standard glasses and sample JQCl9 was determined by FTIR using the following equation:
CH2O = (M * A) / (rho * d * epsilon) * 100 (1)
where M is the molar weight of H2O (18.02 g/mol), A is absorbance at the OH peak, rho is glass density, d is glass thickness, and epsilon is an excitation coefficient. Here we assumed rho of 2800 g/L66 and epsilon of 78 L/mol/cm67. Transmitted IR spectra of doubly polished samples below a 100 μm thickness were collected at ~20 Pa with an aperture size of 50×50 μm2, a spectral resolution of 4 cm−1, and 128 scans (Supplementary Fig. S3). The water content of the glass was determined using Eq. (1), d of the glass, and A (the peak height at 3540 cm−1 after the linear background subtraction between 2500 and 3800 cm−1).

The water contents of most quench glasses were measured by Raman spectrometer following method developed in previous studies68–70. Raman spectra at 100–4000 cm−1 were collected using a laser having a 532.15-nm excitation wavelength with a ×100 objective lens, 1800 l/mm grating, counting times of 60 sec, and 5–10 times accumulation (Supplementary Fig. S4). The spatial resolution and the spectral resolution are 1–2 μm2 and 0.42 cm−1, respectively. The power input of the laser was reduced to 5.4 mW to minimize possible damage to the samples. A silicon standard having 520 cm−1 peak was used for the daily calibration of the spectrometer.

For the data treatment of Raman spectra, to apply the internal calibration of the previous study70 to our Raman spectra, the effects of elastic scattering of the excitation beam and temperature were corrected by the following equation:
I = Iobs * (v0^3 / v^3) * (1 - exp(-hcv / kT)) * (v0 - v)^4 (2)
where I is the corrected intensity, Iobs is the measured intensity of the raw spectrum, v0 is the wavenumber of incident laser light (532.15 nm: 18792 cm−1), h is the Plank constant (6.62606957×10−34 Js), c is the speed of light (2.9979×1010 ms−1), v is the measured wavenumber in cm−1, k is the Boltzmann constant (1.3806488×10−23 JK−1), and T is temperature in K. A spline cubic baseline was fitted using the corrected Raman spectrum in the regions of 100–400 cm−1, an intermediate minimum of 620–680 cm−1, and 1200–1500 cm−1 for Si-O vibrations and 2800–3400 cm−1 and 3700–4000 cm−1 for O-H stretching vibration. Typical spectra after the baseline subtraction are shown in Supplementary Fig. S5. The water content (CH2O) in weight percent of the quenched glass was estimated by the following equation:
CH2O = a * As / Aw (3)
a = 0.096 * CFeO + 0.663 (4)
where As and Aw are area of the O-H stretching vibration region and the Si-O vibration region, respectively. Slope a is determined using the Eq. (4) with FeO content in weight percent of the glass from electron microprobe analyses. The resulting water contents in quenched glasses are summarized in Supplementary Data 3. In comparison with FTIR measurements, the water contents of synthetic standard glasses and JQCl9 determined by Raman measurements are reproduced with a root-mean-square error (RMSE) of 0.24 wt.% H2O (Supplementary Fig. S6 and Supplementary Data 3).

Modeling F/Cl evolution
The partition coefficients used in our modeling are summarized in Supplementary Data 4. The Cl partition coefficients for orthopyroxene and plagioclase are the mean values with 1σ standard deviation from this study after exclusion of measurements with abnormally high uncertainty, while F partition coefficients for orthopyroxene and plagioclase are the mean values reported in ref. 31. The number of available olivine-melt partition coefficients is limited and we chose a pair of partition coefficient for F and Cl from Experiment Fe_5 conducted by Potts et al.8. All olivine partition coefficients are from nominally anhydrous samples.

The F/Cl ratio evolution modeling includes two processes. First, we have modeled the F/Cl ratio of minerals (plagioclase and orthopyroxene) crystallized from their equilibrated magmas with different F/Cl ratios. As the plagioclase and orthopyroxene have an identical DF=DCl ratio, the F/Cl ratios of both minerals formed from the same magma are indistinguishable. The modeling results are plotted in Fig. 3b. Second, we have calculated the halogen contents and F/Cl ratios of different mantle sources, including (1) olivine and orthopyroxene; (2) olivine and plagioclase; (3) orthopyroxene and plagioclase.

The halogen concentrations of the mantle sources (composed of LMO cumulates) are controlled by their concentrations in the LMO magmas with which the cumulates equilibrated. McCubbin et al.5 concluded that the bulk silicate Moon (BSM) has 3.1–4.9 μg/g F and 1.0–4.1 μg/g Cl halogen contents with F/Cl ratios ranging from approximately 1 to 5. In our modeling, the F/Cl ratio of the initial LMO is set within this range (Supplementary Table S4). Since halogens are highly incompatible, we can simply model the halogen contents in LMO residual magma after any degree of solidification by assuming all halogens remain in the liquid during crystallization, regardless of whether it is equilibrium or fractional crystallization. This also indicates that the F/Cl ratio of the partial melts formed from a mantle source would be nearly constant even the melt underwent certain degrees of evolution.

For modeling partial melts from an olivine-dominated mantle, we adopted a 1.9% melting model based on ref. 55, with the accumulated fractional melting equation expressed as:
C melt = C0 * (1/F) * [1 - (1 - F)^Dbulk] (5)
To model the F/Cl ratio of partial melts derived from an orthopyroxene-dominated mantle, we applied a 10% batch melting degree following Sheng et al.56, using the batch melting equation:
C melt = C0 / (Dbulk + F(1 - Dbulk)) (6)
where C melt is the halogen content of partial melts, C0 is the halogen content of the mantle source, F is the melt fraction, Dbulk is the bulk partition coefficient calculated based on modal abundances of mineral phases. The olivine and orthopyroxene in the mantle source are considered to be in equilibrium with the LMO after 50% solidification and expected to be Mg-rich, and the plagioclase is considered to have formed in equilibrium with the LMO after 80% solidification when plagioclase is expected to occur38,71–73. In this modeling, the LMO residual magma after 50% solidification has been considered as TIRL. All the modeling results and associated uncertainties were generated by a Monte Carlo approach on the basis of 1,000,000 simulations using Eqs. (5) and (6). The details and code of the Monte Carlo simulation are provided in the Supplementary Information and Supplementary Software 1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of manuscript entitled “Halogen evidence for the formation and vapor-phase metasomatism of the primary lunar” by Jie-Jun Jing et al. submitted to Nature Communications. This manuscript reported newly-obtained Cl partition coefficients between orthopyroxene/plagioclase and melt, and applied their data to the discussion of the origin of lunar crustal samples. There has been limited work on halogen partitioning between nominally anhydrous minerals and silicate melt under relevant lunar conditions. This work is a welcome addition. Moreover, application of these data to the discussion of the origin of lunar near-side crustal samples and non-KREEP Mg-suite is quite interesting. However, I do have several concerns: 1) The experimental methods adopted in this study produced zoned minerals, which raises the question if these partition coefficients represent equilibrium conditions. 2) The experimental data are not investigated adequately in terms of the effects of pressure, temperature on Cl contents in minerals and melt, and consequently the partition coefficients. 3) Discussion in regards to the vapor-phase metasomatism on lunar nearside is heavy on literature review, weak in argument. 4) The method for “modeling F/Cl evolution” needs to be better explained and justified. Hence this manuscript requires major revision before publication. Detailed comments in relation to these concerns are listed below. My main concerns lie with the experimental methods adopted in this study: 1) My first concern is in relation to the preparation of the starting material. Using Fe2O3 in the starting mix and melting the starting mixture in air ensure that all Fe was present as Fe3+ in the starting composition. Performing the experiments in graphite-lined Pt capsules with talc-pyrex cell leads to reduction of the starting material, which means that hydrogen diffusion inwards and the formation of H2O in the starting composition. Therefore, I highly recommend Raman analysis of the experimental glass to identify the present of H2O. The analytical total for glass from electron microprobe analyses vary between 97.83-99 wt%, supporting the presence of H2O in glass. The subsequent question is what is the effect of H2O on Cl partitioning? 2) The temperature path for experiments involves a cooling stage from 1300°C to target temperature at a rate of 5°C per 30 mins, which explains the mineral zonation observed in all experiments (Fig.2). Unfortunately, this is not the ideal method for investigating partition coefficients under equilibrium conditions. The authors stated that they used the rim compositions of zoned mineral phases for calculating partition coefficients, a more detailed discussion is required to demonstrate the validity of this method. For example, the homogeneity of the rim compositions from the same experiment, the reproducibility of the rim compositions in duplicate experiments, systematic variation of rim compositions with pressure and temperature, etc. Line 118-120: In regards to the three experiments at 0.5 GPa, 1120°C, the mineral assemblages are different between 24-hour run and 48-hour runs. The Cl content in glass also varied significantly, e.g., 5.94±0.14 wt% for the 24-hour run, and 8.21±0.19 wt% and 8.90±0.57 wt% for the two 48-hour runs. I think it is safe to say that 24-hour duration is not sufficient for reaching steady state. The authors made the same observation as stated in the supplementary material. It is important to say the same here. Line 123-124: In regards to the statement “Chlorine partition coefficients do not show clear dependence on pressure and temperature”, I have the following comments: 1) could this be a result of the large uncertainties associated with the Cl partition coefficients? 2) It is important to distinguish data points representing different pressure and temperature conditions in Fig. 2, so the reader can judge for themselves. 3) The mineral assemblages are different for different pressure and temperature conditions, and Cl contents in minerals and melt also vary between experiments of different pressure and temperature conditions. It is an important aspect to discuss before reaching such a conclusion regarding partition coefficients. Section “Widespread Cl enrichment on lunar nearside”: In my opinion, discussion of vapor-phase metasomatism is better done with the knowledge of Cl isotope compositions. Moreover, the text here is heavy on literature review, in contrast, the argument seems vague and weak. Section “Modeling F/Cl evolution”: 1) F and Cl partition coefficients reported in the literature show orders of magnitude of variation. The choice of a single value for F and Cl partition coefficients listed in Table S3 should be explained. 2) Moreover, these partition coefficients are associated with large uncertainties. Is this taken into account in the modeling? 3) In regards to the statement “the F/Cl ratio of the mantle source will be nearly consistent with its partial melt, regardless of the partial melting degree”, what is the basis for such an assumption? Considering that DF/DCl >1, low degree partial melts should have higher Cl/F ratios than the mantle source. Fig.2 The authors mentioned duplicate experiments with different run durations, however, this distinction is not shown in Fig.2. Moreover, in the plot against temperature, data for different pressure conditions are not distinguished; similarly, in the plot against pressure, different temperature conditions are not distinguished.", "reviewer_2": "Overall comment: This manuscript seeks to constrain the Cl abundances of primary crustal sources in the Moon, and they provide new and much needed experimental data to better constrain mineral-melt partition coefficients for Cl. The topic of this work is of broad interest and has wide-reaching implications for the thermochemical evolution of the Moon, and it is also timely given the implications for testing the hypotheses of this work through analyses that could be conducted on the recently returned Chang’e-6 samples. Before I could recommend this work for publication in Nature Communications, I have some concerns with the Cl data collected on mineral rims and hence the new D values that are derived from those analyses. In particular, it is not discussed in the manuscript what efforts have been done to correct or mitigate for contributions from secondary fluorescence of the glass phase to the Cl abundances in the mineral rims. Furthermore, Cl abundance data are not reported for mineral cores, and these data are needed to evaluate the data quality. I also raise several other points as outlined in my comments below. I suggest major revisions at this point so that the authors are given adequate time to address the concerns, however, I do think that the work is important and timely, and I look forward to seeing their responses. Lines 22-23: Is the F/Cl ratio meant to be a mass ratio or a molar ratio? Please indicate either way. If the former, I am not seeing that from the ranges provided for F (19-32 ppm) and Cl (13-25 ppm). Lines 37 and 38: Please carefully check that all cited references in the manuscript are listed in the reference section at the end of the manuscript. Of the citations listed here, all but McCubbin et al., 2023 and Hayden et al., 2024 are missing from the list (i.e., 5 of the listed references are missing in the bibliography). Lines 347-354: Why was the target temperature approached at such a slow cooling rate? Lines 355-369: You mentioned measuring the rims of the grains because they are most likely to be in equilibrium with the melt. Given the slow cooling rates that were chosen, I can understand the concern. However, secondary fluorescence of Cl from the nearby glass phase could affect the Cl concentrations measured in the rims of the mineral grains. There are models for computing and or correcting for the expected contribution from secondary fluorescence in the literature (e.g., Gavrilenko et al., 2023; Borisova et al., 2018), but this is not mentioned in the manuscript. Has this been taken into consideration? If not, I think it is necessary to do such computations prior to publication because it could have a large effect on the D values. In fact, I think it would be very informative to provide Cl abundances across the grains rather than just reporting the rim values. Line 372: I think it is worth pointing out that the usage of nominally anhydrous here is a bit misleading because it is another way of saying that you did not intentionally add water but you also did not measure for water. Based on the description of your experimental technique, particularly the use of talc-pyrex cells, I would presume that the glasses could have as much as wt.% levels of water. Lines 101-102: On what basis was the equilibrium between mineral and melt determined? Please elaborate. Also, if the minerals seem to be in equilibrium with the melt, why is it only the Cl abundances of mineral rims that are thought to be in equilibrium with the melt? Lines 105-108: Just looking at mineral rims for Cl seems like an odd precaution to take based on the worry that mineral cores may not be in equilibrium with the melt. Were the Cl abundances of mineral cores measured or were there only rim measurements? As detailed in my earlier comment, I have substantial concerns about secondary fluorescence from the Cl-rich glass that is adjacent to the mineral rims contributing to the measured Cl abundances, which would affect the computed D values. I think that the Cl concentrations of the mineral cores need to be reported, and I think that, if not done already, specific efforts need to be taken to account for potential contributions to Cl from secondary fluorescence. Both of these points need to be adequately addressed before I could recommend this work for publication. Line 112: ~9 wt.% Cl in a silicate melt is very high. Have other studies reported such a high solubility of Cl in silicate melts? The studies I am aware of have not been much higher than ~5 wt.% Cl. Though beyond the scope of the present study, it may be worth following up on this observation further because it may imply that there is additional effort needed to constrain the limits of Cl solubility in silicate melts. Lines 137-138: I believe the text should say “fall below detection limits” Line 173: Both of the Ustunisik references are missing from the bibliography at the end. I did not do a comprehensive check of all references, but the two places I spot checked seemed to have missing references, so please check throughout the manuscript. Lines 200-208: I have seen in different places that the degassed vapor would be rich in 35Cl and the residue enriched in 37Cl (e.g., Boyce et al., 2015), but it seems the opposite is being argued here. Could you please elaborate on the process a bit more that is thought to drive the enrichment in 37Cl? Lines 209-223: This section seems a bit too speculative (i.e., broad implications about the lunar crust being drawn from previously published data from a single felsite sample) and is not adequately linked to the new experimental results that are the main new contribution within this manuscript. I suggest either making that link a bit stronger in this section or maybe consider removing this section from the manuscript. References: There seems to be a number of cited references that do not appear to be cited in the manuscript text, including the 3 Barnes references and the Boyce 2015 reference. All four of those manuscript are relevant to this work, and two of them (Boyce et al., 2015 and Barnes et al., 2016a) are the primary contrasting models for understanding the origin Cl isotopic fractionation in lunar samples. I have not done an exhaustive check of the references here, I just checked the first few and noted several discrepancies. Please do a more exhaustive check during revision. Supplement: Line 63: should be anomalous instead of anonymous."}, "author_response": "Reply: Thank you for your interest in our manuscript and very useful comments. The manuscript has been modified according to the comments and suggestions. Detailed responses and modifications are listed below. My main concerns lie with the experimental methods adopted in this study: 1) My first concern is in relation to the preparation of the starting material. Using Fe2O3 in the starting mix and melting the starting mixture in air ensure that all Fe was present as Fe3+ in the starting composition. Performing the experiments in graphite-lined Pt capsules with talc-pyrex cell leads to reduction of the starting material, which means that hydrogen diffusion inwards and the formation of H2O in the starting composition. Therefore, I highly recommend Raman analysis of the experimental glass to identify the present of H2O. The analytical total for glass from electron microprobe analyses vary between 97.83-99 wt%, supporting the presence of H2O in glass. The subsequent question is what is the effect of H2O on Cl partitioning? Reply: Thanks for the constructive suggestions. We understand the concern and agree that the glass in our experiment should contain a non-negligible amount of water, and agree it is important to evaluate the water content effect. Firstly, we tried to use Fourier-transform infrared spectroscopy (FTIR). However, most of the recovered samples with quench cracks were difficult to doubly polish and the size of quenched glass pools was too small to be analyzed by FTIR measurements with the exception of sample No. JQCl9. Thus, most samples were analyzed by Raman spectroscopy following the method established in literature (Behrens et al., 2006; Le Losq et al., 2012; Di Genova et al., 2017). In order to assess the validity of the Raman measurement results, we also synthesized three extra water-bearing glasses with different water contents (ranging from 0.33 to 1.33 wt.% water content, measured by FTIR) using high-pressure apparatuses. The comparison of results between FTIR and Raman is now shown in Supplementary Fig. S4 (as below). The water contents in glasses from experiments are presented in Supplementary Table S3 and revised Fig. 2 (as reproduced below). The method description with associated figures and tables has been added in the revised Method section and Supplementary materials. Our results show that the recovered glasses in our experiments contain ranging from 0.20 wt.% to 1.00 wt.% (2000-10000 μg/g) water. This qualitatively consistent with exceptions : previous work has shown that nominally dry experiments typically contain around 500 μg/g water, which in the case of our experiments, which do not contain any hydrous minerals, would all concentrate in the melt phase. This information has been added to the revised main text. As described below, we have modified the reported partition coefficients for orthopyroxene after equilibrium assessment, which has been updated in the revised Fig. 2 (as below). The blue area in the revised Fig. 2 represents the mean value with standard deviation of partition coefficient data from our experiments, which are used for modeling. See more details in the response letter. The numbers of experiments that assessed if there is an effect of water on Cl partitioning is very limited at lunar conditions, and in the available dataset we cannot see dependence of Cl partitioning on melt water content. Within the associated uncertainties, the experiment from Potts et al. (2021) with the highest water content also shows a partition coefficient consistent with our mean values. Recent studies have shown that hydrogen partitioning between plagioclase and melt changes significantly at melt water contents below 1 wt.%. This is clearly not the case for Cl partitioning. We have added all this information to the revised section on water contents in the main text. Overall, we conclude that Cl partition coefficients for orthopyroxene and plagioclase do not show a significant effect by water content at relatively dry conditions (< 1 wt.%). Although the water content of the mantle source for Mg-suite magma is not constrained well, the lunar primitive mantle is thought to contain at least 110 μg/g water (Chen et al., 2015). Thus, we can assume any magmas generated by the lunar mantle are relatively dry (<1 wt.%). In this case, the mean values should be a highly appropriate choice for F/Cl modeling in this study. 2) The temperature path for experiments involves a cooling stage from 1300°C to target temperature at a rate of 5°C per 30 mins, which explains the mineral zonation observed in all experiments (Fig.2). Unfortunately, this is not the ideal method for investigating partition coefficients under equilibrium conditions. The authors stated that they used the rim compositions of zoned mineral phases for calculating partition coefficients, a more detailed discussion is required to demonstrate the validity of this method. For example, the homogeneity of the rim compositions from the same experiment, the reproducibility of the rim compositions in duplicate experiments, systematic variation of rim compositions with pressure and temperature, etc. Reply: Because the cooling rate of our experiments was also mentioned by the Reviewer #2, here we firstly explain the rationale of the experimental strategy. The cooling rate was chosen as it can promote large plagioclase crystallization, which is critical as plagioclase was the main target mineral in this project. Our lab has conducted numerous projects aiming for low-temperature lunar magma ocean crystallization and found that plagioclase grains are generally tiny and elongated like needles, which precludes accurate trace element abundance measurements. Two undergraduate students in our lab further investigated the factors controlling the plagioclase crystal growth. A slow cooling rate from temperatures above the liquidus helps reduce the nucleation rate by minimizing cooling-driven magma supersaturation. In other words, it promotes crystal growth by minimizing the plagioclase crystal numbers. The work in our lab on this topic was not published yet, but a similar study on ‘crystal growth’ can be seen in literature (e.g., Cashman, 2020). As a result of this cooling strategy, Fe-Mg zonation developed in the orthopyroxene grains. The early-crystallized orthopyroxene (i.e., the grain core) is dark and Mg-rich, while the rims are Fe-rich. We apologize for the incomplete description on the zonation in the original manuscript. In fact, this zonation pattern (Fe-rich rim with an Mg-rich core) was observed exclusively in orthopyroxene, whereas plagioclase grains remained compositionally homogeneous (see their low standard deviations in major elements in all runs). Considering this observation and the 2 μm diameter beam size used for mineral analysis, we generally positioned the beam 10 μm away from orthopyroxene rims, whereas for plagioclase we randomly selected different euhedral grains and measured their cores. However, for orthopyroxene grains with Fe-rich rims thinner than 10 μm, some analysis points inevitably fall within the Mg-rich regions (e.g., orthopyroxene grains in Fig. 2b). Consequently, the measured orthopyroxene rims exhibited major element compositional heterogeneity, leading to large standard deviations in FeO and MgO contents (as seen in the original Table S2, e.g., JQCl1). However, the Cl abundance in the Mg-rich rims still falls within the range of the Fe-rich rims from the same experiment (e.g., JQCl1 and JQCl8). In addition, we also measured orthopyroxene core Cl abundance in JQCL6 (84 μg/g) which is consistent with the Cl range of 77-130 μg/g in its rims. In experiments JQCl9 and JQCl10, all orthopyroxene grains were measured in their core, which show relatively homogeneous Cl abundances. According to these observations and comparison, in the original manuscript, we reported the average compositions for orthopyroxene and used the mean Cl abundance for calculating the partition coefficients, although the standard deviation of FeO and MgO in some orthopyroxene are obviously larger than other major elements. This information which was missing from the previous version of the manuscript has now been added in the Supplementary Material. In agreement with the suggestions by both reviewers, we now report the individual data for orthopyroxene including both Fe/Mg-rich rims and core (Table S2B). More importantly, we introduce an additional parameter, the Fe-Mg partition coefficients (K , D /D ), which represents the ratio of partition coefficients between orthopyroxene and melt), to evaluate equilibrium conditions. K values range from 0.20-0.40 in equilibrium high-pressure, high-temperature experiments (e.g., Grove and Bryan, 1983; Kinzler and Grove, 1992; Sisson and Grove, 1993). The results indicate that Fe-rich rims generally exhibit equilibrium K values, whereas Mg-rich regions do not (Table S2B). Therefore, only the orthopyroxene with equilibrium K were taken into consideration for the determination of average major and Cl compositions. Data have all been updated in Table 1 and S2A. The above information has been added in the Result section in revised manuscript in lines 111-132. Line 118-120: In regards to the three experiments at 0.5 GPa, 1120°C, the mineral assemblages are different between 24-hour run and 48-hour runs. The Cl content in glass also varied significantly, e.g., 5.94±0.14 wt% for the 24-hour run, and 8.21±0.19 wt% and 8.90±0.57 wt% for the two 48-hour runs. I think it is safe to say that 24-hour duration is not sufficient for reaching steady state. The authors made the same observation as stated in the supplementary material. It is important to say the same here. Reply: Thanks for the suggestions. We agree that the 24-hour duration at 1120 oC is not sufficient for reaching steady state. We also found, following up from the previous point in this rebuttal that most orthopyroxene individual points from this run do not show equilibrium K . ‘This indicates the bulk equilibrium was not reached in this 24 hour experiment at a temperature of 1120 oC.’ has been added in lines 144-145 in the revised manuscript. Line 123-124: In regards to the statement “Chlorine partition coefficients do not show clear dependence on pressure and temperature”, I have the following comments: 1) could this be a result of the large uncertainties associated with the Cl partition coefficients? 2) It is important to distinguish data points representing different pressure and temperature conditions in Fig. 2, so the reader can judge for themselves. 3) The mineral assemblages are different for different pressure and temperature conditions, and Cl contents in minerals and melt also vary between experiments of different pressure and temperature conditions. It is an important aspect to discuss before reaching such a conclusion regarding partition coefficients. Reply: As suggested, we have revised Fig. 2 to show plots against temperature, in which we have clearly differentiated data points corresponding to different pressure conditions. Besides, we have now distinguished the duplicate experiments with different run durations. This has helped improve the clarity of our figure. We agree with the comment. The primary reason for the absence of an obvious correlation is the extremely low Cl partition coefficients associated with large uncertainties. Under such conditions, any potential relationship between the partition coefficients and controlling factors (e.g., temperature, pressure, or water content in the melt) cannot be reliably established across the entire dataset, as the uncertainties may obscure the underlying trends. In addition, the current dataset of Cl partition coefficients remains very limited, especially for plagioclase. We found that at 1240 oC, the experiment at 1 GPa has higher D than the run at 0.5 GPa (Fig. b). However, a robust conclusion cannot be drawn by comparing between only two experiments. Furthermore, the mineral assemblages are different for different pressure and temperature conditions, which caused the Cl content variation in the residual melt after mineral crystallization. For example, the experiment that crystallized quartz and rutile indicates high-degree crystallization and would have higher Cl content in the residual melt. In this case, whether the Cl partitioning obeys Henry’s law would be important before discussing the dependence of partition coefficients on temperature and pressure. In the revised Supplementary material, we added an additional plot of the partition coefficients v.s. melt Cl contents (Supplementary Fig. S2). It shows that Cl partitioning still follows Henry’s law. Overall, we believe that the existing data do not provide sufficient evidence to establish any clear relationship between the partition coefficient and temperature or pressure or water content. Any such effect appears to be too weak to be distinguished within the uncertainty range of our measurements. Therefore, we take a conservative approach, stating that within the experimental temperature and pressure and water content range, there is no clear correlation between partition coefficient and these variables—rather than outright denying their potential influence. This rationale also explains why, in our F/Cl modeling, we use the average partition coefficient derived from our experimental results (excluding anomalously high values with large uncertainties). As shown in the figure, the blue-shaded region represents the mean value and its associated uncertainty, which encompasses most of the data points. Below, we will also adopt this average value and its uncertainty in our simulation calculations. The above information has been added in the Result section in revised manuscript in lines 142-148, and in lines 72-89 in the Supplementary Material. Section “Widespread Cl enrichment on lunar nearside”: In my opinion, discussion of vapor-phase metasomatism is better done with the knowledge of Cl isotope compositions. Moreover, the text here is heavy on literature review, in contrast, the argument seems vague and weak. Reply: Thanks for the suggestion. Reviewer #2 also raised the same comment on this section and has suggested removing the original lines 209-223 ‘which is too speculative and not adequately linked to the new experimental results that are the main new contribution within this manuscript’. Upon reflection, we agree that the knowledge of Cl isotope compositions is necessary for an in depth discussion of the ‘vapor-phase metasomatism’. Thus, we removed the detailed discussion of ‘vapor-phase metasomatism’ in this section but emphasized our observation on Cl enrichment in lunar nearside samples. We still attribute this observed Cl enrichment to possible late metasomatism while only briefly mentioning the ‘vapor phase’ hypothesis. In lines 239-246 in the revised manuscript, we added the sentence of ‘A conclusion on which metasomatic process caused the Cl enrichment cannot be drawn without Cl isotopic data. Whether this Cl enrichment occurs only on the lunar nearside, or if it also affected the lunar farside, and which metasomatic process may be responsible, remain open questions. An analyses of halogen abundances in the recently returned Chang’e-6 samples (e.g., Cui et al., 2024; Zhang et al., 2024), could shed light on the geographical distribution of Cl enrichment, while Cl isotopic analyses could shed light on whether the metasomatic process(es) are comparable between nearside and farside.’ This may provide insights for future studies on Cl isotopes in this context. To accommodate the changes in the manuscript, we have also made appropriate modifications to the title – ‘Halogen evidence for the formation and metasomatism of the primary lunar crust’. Section “Modeling F/Cl evolution”: 1) F and Cl partition coefficients reported in the literature show orders of magnitude of variation. The choice of a single value for F and Cl partition coefficients listed in Table S3 should be explained. 2) Moreover, these partition coefficients are associated with large uncertainties. Is this taken into account in the modeling? 3) In regards to the statement “the F/Cl ratio of the mantle source will be nearly consistent with its partial melt, regardless of the partial melting degree”, what is the basis for such an assumption? Considering that DF/DCl >1, low degree partial melts should have higher Cl/F ratios than the mantle source. Reply: Thanks for the comments. The F and Cl partition coefficients used for modeling in the Table S3 are the mean values of the data produced in experiments at lunar conditions from previous literature (Jing et al., 2024) and this studies for orthopyroxene and plagioclase (except the data with abnormal uncertainties), respectively. The available halogen partitioning data for olivine are limited. For our modeling, we used a single pair of F and Cl values from one experiment conducted by Potts et al. (2021). We agree that the uncertainties associated with the selected values should be taken into account. In the revised manuscript, we provide an explanation of our choice of partition coefficients in lines 461-467 and incorporate Monte Carlo simulations into our F/Cl evolution modeling to propagate these uncertainties. We apologize for insufficient explanations for our modeling which was oversimplified in the original manuscript. It is true when the D /D > 1, low degree partial melts (especially <10%) should have higher Cl/F ratios than the mantle source. As the olivine has D /D being around 1, in the original manuscript we had only considered and tested the case for high degree partial melting (>10%, Sheng et al., 2024) for orthopyroxene-dominated mantle for the modeling. Sheng et al. (2024) has modeled 10-50% partial melting of an orthopyroxene-dominated mantle using the THERMOCALC program and successfully produced the major-element characteristics of the enclosed melt in plagioclase grains from a noritic anorthosite. In contrast, Prissel and Gross (2020) proposed that the primary Mg-suite magma was generated by 1.9% partial melting of an olivine-dominated mantle. While this represents a low-degree partial melt, olivine exhibits D /D being around 1. We have now cited these two different partial melting models and applied them in our modeling for olivine-dominated mantle (1.9% fractional melting) and orthopyroxene-dominant mantle (10% batch melting), respectively. To model the F/Cl ratio of partial melts derived from an orthopyroxene-dominant mantle, we applied a 10% batch melting degree following Sheng et al. (2024), using the batch melting equation: C = C ⁄[D +F(1-D )] (1) melt 0 bulk Bulk For partial melts from an olivine-dominant mantle, we adopted a 1.9% melting model based on Prissel and Gross (2020), with accumulated fractional melting equation expressed as: 1 ( ) C = C (1/F) [1-(1-F) Dbulk , (2) melt 0 where C is the halogen content of partial melts, C is the halogen content of mantle source, F is the melt fraction, D is the bulk partition coefficient calculated based on modal abundances of mineral phases. As same as in the original manuscript, the olivine and orthopyroxene in the mantle source are considered to be in equilibrium with the LMO after 50% solidification as expected to be Mg-rich, and the plagioclase in equilibrium with the LMO after 80% solidification when plagioclase is expected to occur (e.g., Lin et al., 2017; Charlier et al., 2018; Rapp and Draper, 2018; Schmidt and Kraettli, 2022). To account for the propagated uncertainties in our modeling, particularly those associated with partition coefficients, we employed a Monte Carlo algorithm to integrate the modeling results with their standard deviations. The partition coefficients were assumed to follow normal distributions, with means and standard deviations provided in Table S4. To ensure physically meaningful values, only non-negative partition coefficients were considered in the Monte Carlo simulations. As an example, the result shows that starting from an LMO with F/Cl ratio of 1, the partial melt of a mantle source (90% Opx + 10% Plag) after batch melting would yield F/Cl ratio of 11.79 with standard deviation of 3.75. All the figures (Figs. 4 and 5) showing modeling results have been updated to reflect these new models. Using the same Monte Carlo simulation, the F/Cl ratio of the Mg-suite parental magma was calculated using the results of 1,000,000 simulations. The F and Cl contents in plagioclase from non-KREEP Mg-suite rock were considered within the specified ranges of 0.6–1 ppm for F and 0.03–0.06 ppm for Cl. The partition coefficients for F and Cl were assumed to follow normal distributions without negative values. The derived F/Cl ratio of the parental magma is 1.51 ± 0.72, present as orange area in Figs. 4 and 5. The above description has been added in the lines 225-260 revised Method section (lines 485-507) and visualized in revised Figs. 4 and 5. The Python codes for conducting the Monte Carlo simulation have been added as Supplementary Materials. Reviewer #2 (Remarks to the Author): Overall comment: This manuscript seeks to constrain the Cl abundances of primary crustal sources in the Moon, and they provide new and much needed experimental data to better constrain mineral-melt partition coefficients for Cl. The topic of this work is of broad interest and has wide-reaching implications for the thermochemical evolution of the Moon, and it is also timely given the implications for testing the hypotheses of this work through analyses that could be conducted on the recently returned Chang’e-6 samples. Before I could recommend this work for publication in Nature Communications, I have some concerns with the Cl data collected on mineral rims and hence the new D values that are derived from those analyses. In particular, it is not discussed in the manuscript what efforts have been done to correct or mitigate for contributions from secondary fluorescence of the glass phase to the Cl abundances in the mineral rims. Furthermore, Cl abundance data are not reported for mineral cores, and these data are needed to evaluate the data quality. I also raise several other points as outlined in my comments below. I suggest major revisions at this point so that the authors are given adequate time to address the concerns, however, I do think that the work is important and timely, and I look forward to seeing their responses. Reply: Thank you for reviewing our manuscript and for the positive feedback. We have performed secondary fluorescence modeling based on our data and have further discussed this topic in the revised manuscript. The manuscript has been modified according to the comments and a point-by-point response has been provided in the following section. Lines 22-23: Is the F/Cl ratio meant to be a mass ratio or a molar ratio? Please indicate either way. If the former, I am not seeing that from the ranges provided for F (19-32 ppm) and Cl (13-25 ppm). Reply: It is a mass ratio. In the revised manuscript, we have done a Monte Carlo simulation for calculating the F/Cl ratios by considering the uncertainties of the partition coefficients and the range of the halogen contents in the plagioclase from that non-KREEP Mg-suite sample. The modeled F/Cl ratio result is 1.51±0.72. The sentence in the abstract has been modified it as ‘Plagioclases from these KREEP-poor rocks equilibrated with magma with a low ratio F/Cl ~1.5’ Lines 37 and 38: Please carefully check that all cited references in the manuscript are listed in the reference section at the end of the manuscript. Of the citations listed here, all but McCubbin et al., 2023 and Hayden et al., 2024 are missing from the list (i.e., 5 of the listed references are missing in the bibliography). Reply: Thanks for checking. The citation in the bibliography and the manuscript have been checked and modified thoroughly, apologies that this was not correct in the original manuscript. Lines 347-354: Why was the target temperature approached at such a slow cooling rate? Reply: This is strategy was chosen based on our previous lab experiences for low-temperature experiments. Our lab has conducted numerous projects aiming for low-temperature lunar magma ocean crystallization and found that plagioclase grains are generally tiny and elongated like needles, which is insufficient for accurate measurements. Elements partitioning coefficients between plagioclase and silicate melt is our main project in last several years. Thus, as an extension, two students in our lab further investigated the factors controlling the plagioclase crystal growth. A slow cooling rate from temperatures above the liquidus helps reduce the nucleation rate by minimizing cooling-driven magma supersaturation. In other words, it promotes crystal growth. As our work in our lab on this topic was not published yet, a more thoughtful study on ‘crystal growth’ can be seen in literature (e.g., Cashman, 2020). However, as it is shown, the slow cooling rate also promotes the iron-zonation in orthopyroxene from low temperature experiments, but plagioclase grains are homogeneous and their cores were measured. The advantage of this strategy is the plagioclase size can be at up to hundred μm levels and homogeneous (Fig. 1). According to the comments about the orthopyroxene zonation, more details have been provided in the revised manuscript and a further description is given below. Lines 355-369: You mentioned measuring the rims of the grains because they are most likely to be in equilibrium with the melt. Given the slow cooling rates that were chosen, I can understand the concern. However, secondary fluorescence of Cl from the nearby glass phase could affect the Cl concentrations measured in the rims of the mineral grains. There are models for computing and or correcting for the expected contribution from secondary fluorescence in the literature (e.g., Gavrilenko et al., 2023; Borisova et al., 2018), but this is not mentioned in the manuscript. Has this been taken into consideration? If not, I think it is necessary to do such computations prior to publication because it could have a large effect on the D values. In fact, I think it would be very informative to provide Cl abundances across the grains rather than just reporting the rim values. Reply: Thanks for the suggestions. Due to the same experience with measuring low F content in silicate minerals from our previous study (also wt.% level F in silicate melts, Jing et al. 2024), we were worried about the effect of secondary fluorescence and thus generally chose analysis positions at 10 μm far from boundary in most cases. However, modeling was not conducted and we agree it is better to provide this. We now supply it in the revised Supplementary Material (Fig. S1). More details and discussions are provided below. Additionally, the compositions of all individual analyses for orthopyroxene have now been provided in Table S2B. Plagioclase is a low-temperature mineral that readily crystallizes as small, needle-shaped grains. To facilitate the formation of larger crystals, we implemented a slow cooling procedure from temperatures above the liquidus in our experiments. As a result of this cooling strategy, Fe-Mg zonation developed in the orthopyroxene grains. The early-crystallized orthopyroxene (i.e., the grain core) is dark and Mg-rich, while the rims became Fe-rich. However, this zonation pattern (Fe-rich rim with an Mg-rich core) was observed exclusively in orthopyroxene, whereas plagioclase grains remained compositionally homogeneous. Considering this observation and the 2 μm beam size used for mineral analysis, we generally positioned the beam 10 μm away from orthopyroxene rims, whereas for plagioclase, we randomly selected different euhedral grains and measured their cores. However, for orthopyroxene grains with Fe-rich rims thinner than 10 μm, some analysis points inevitably fell within the Mg-rich dark rims (e.g., orthopyroxene grains in Fig. 2b). Consequently, the measured orthopyroxene rims exhibited compositional heterogeneity, leading to large standard deviations in FeO and MgO contents (as seen in the original Table S2). However, the Cl abundance in the Mg-rich rims still falls within the range observed in the Fe-rich rims (e.g., JQCL1 and JQCL8). Besides, we also measured orthopyroxene core Cl abundance in JQCL6 (84 μg/g) which is consistent with 77-130 μg/g in its rim, and all orthopyroxene grain cores in experiments JQCl9 and JQCl10 with relatively homogeneous Cl abundances. Thus, in the original manuscript, we reported the average compositions for orthopyroxene and used the mean Cl abundance for calculating the partition coefficients, although the standard deviation of FeO and MgO in some orthopyroxene are obviously larger than other major elements. In the revised manuscript, we introduced an additional parameter, the Fe-Mg partition coefficients (K , D /D ), which represents the ratio of partition coefficients between orthopyroxene and melt), to evaluate equilibrium conditions. K values range from 0.20-0.40 in equilibrium high-pressure, high-temperature experiments (e.g., Grove and Bryan, 1983; Kinzler and Grove, 1992; Sisson and Grove, 1993). The results indicate that Fe-rich rims generally exhibit equilibrium K values, whereas Mg-rich rims (or Mg-rich core) does not (Table S2B). Therefore, only the orthopyroxene with equilibrium K were taken into consideration for averaging major and Cl compositions, which have been updated in Table 1 and S2. According to the existing literature (e.g., Borisova et al., 2018; Gavrilenko et al., 2023), we conducted the secondary fluorescence (SF) modeling using CalcZAF/Standard software suite and added the results and description in the Supplementary Material (Supplementary Fig. S1). We have used the Cl-rich melt (8.2 wt% Cl) and orthopyroxene from experiment JQCl1 to model an extreme Cl-rich case. The results indicate that the contribution of coexisting melt at a 10 μm distance is 9.4 μg/g Cl. This is within the uncertainties of Opx Cl abundance (162±49 μg/g) in JQCl1 and will not affect the partition coefficients significantly. In another words, the SF effect may have partly contributed to variations in orthopyroxene Cl contents, but these variations are already incorporated within the 1σ standard deviation. By considering the same effect for silicate melt with 3.06 wt.% Cl (JQCl10), the Cl contribution at 10 μm would be 3.5 μg/g which can be neglected considering the orthopyroxene cores have mean Cl content of 58±20 μg/g. We note that in certain experiments (e.g., JQCl4, Fig. R, attached below), many mineral crystals are relatively small. When measuring the core of plagioclase in this experiment, the distance between the core and rim is less than 10 μm. Given this limitation, a 5 μm distance between the analyzed position and melt was generally chosen. However, the Cl contribution by SF modeling in this experiment is 12 μg/g, which is far less than the 1 sigma standard deviation of average Cl contents in plagioclase (262±150 μg/g). In addition, we also measured the coexisting orthopyroxene cores (due to the size limitation) in JQCL4 but it show much smaller 1 sigma standard deviation of 88±30 μg/g Cl. Thus, it is unlikely the SF is the main contribution to the large variation in Cl contents in these plagioclase grains, otherwise the orthopyroxene should have similar standard deviation value. However, we did not find any correlations between the Cl abundance and their major compositions, it is still unclear why the plagioclase shows large Cl content variations in some experiments (Fig. 2) and we recommend not to use these values. The SF modeling has been added as a separate section and discussed in the lines 15-39 in the revised Supplementary Material. The partition coefficients have been updated and the Result section has been revised accordingly. Line 372: I think it is worth pointing out that the usage of nominally anhydrous here is a bit misleading because it is another way of saying that you did not intentionally add water but you also did not measure for water. Based on the description of your experimental technique, particularly the use of talc-pyrex cells, I would presume that the glasses could have as much as wt.% levels of water. Reply: Thanks for this comment which makes a point that is also raised by Reviewer #1. Using FTIR and Raman spectroscopy, we have now measured the water contents in our glasses, which ranges from 0.20 wt.% to 1.00 wt.%. The results, analytical method, and discussion on the absence of an effect of the presence of water, have been added in the revised manuscript. According to the suggestion by Reviewer#1, we have plotted the partition coefficients as a function of water contents in the silicate melt (revised Fig. 2) and no correlation has been observed. Lines 101-102: On what basis was the equilibrium between mineral and melt determined? Please elaborate. Also, if the minerals seem to be in equilibrium with the melt, why is it only the Cl abundances of mineral rims that are thought to be in equilibrium with the melt? Reply: Thanks for the comment. This has already been answered in previous responses and is only briefly described here. When designing this project, we found that the lack of knowledge of halogen diffusion rate data in minerals make it is hard to choose the experimental duration time. In the original manuscript, whether a steady state has been reached was mainly evaluated by the Cl partition coefficients from the time-series experiments conducted at same conditions. Based on this, we can only conclude that whether Cl element was in a steady state. We agree that the zonation in Opx still indicates bulk disequilibrium, which could be attributed to slower diffusion rate of Fe and Mg compared with Cl. As shown in the above responses, we also attached the dataset reporting all individual analyses points on minerals as Supplementary Table (Table S2B) in the revised manuscript. We further introduced another parameter Fe-Mg partition coefficients (K , D /D , ratios of partition coefficients between orthopyroxene and melt) for evaluating the equilibrium states.The reported Cl partition coefficients for orthopyroxene were calculated based on the measured Cl abundance in orthopyroxene with an equilibrium K value. Lines 105-108: Just looking at mineral rims for Cl seems like an odd precaution to take based on the worry that mineral cores may not be in equilibrium with the melt. Were the Cl abundances of mineral cores measured or were there only rim measurements? As detailed in my earlier comment, I have substantial concerns about secondary fluorescence from the Cl-rich glass that is adjacent to the mineral rims contributing to the measured Cl abundances, which would affect the computed D values. I think that the Cl concentrations of the mineral cores need to be reported, and I think that, if not done already, specific efforts need to be taken to account for potential contributions to Cl from secondary fluorescence. Both of these points need to be adequately addressed before I could recommend this work for publication. Reply: Thanks for the comment. This has already been answered in previous responses and is only briefly described here. Full secondary fluorescence modeling has been conducted, and all individual orthopyroxene compositions have been reported. By introducing a new parameter (K ) for evaluating the equilibrium condition, only orthopyroxene compositions with equilibrium K have been used for calculating the mean orthopyroxene compositions and partition coefficients. Line 112: ~9 wt.% Cl in a silicate melt is very high. Have other studies reported such a high solubility of Cl in silicate melts? The studies I am aware of have not been much higher than ~5 wt.% Cl. Though beyond the scope of the present study, it may be worth following up on this observation further because it may imply that there is additional effort needed to constrain the limits of Cl solubility in silicate melts. Reply: To the best of our knowledge, the high Cl content in silicate is not higher than ~5 wt.% as well. However, the Oxford group (Thomas and Wood, 2021, 2023; Rusiecka and Wood, 2025) has put efforts in constraining the Cl solubility in silicate melts using high-pressure high-temperature experiments. The Cl solubility of silicate melt is highly linked to the silicate composition, experimental temperature and pressures. Using their provided excel tool for calculating Cl solubility in anhydrous silicate melt (Thomas and Wood, 2023), we calculated the Cl solubility is 8.88 wt.% for the experimental glass from JQCl6, which is within error of our measured 8.90±0.57 wt.%, indicating their equation may be valid in very Cl-rich silicate system. Lines 137-138: I believe the text should say “fall below detection limits” Reply: Rewritten as ‘fall below detection limits’. Line 173: Both of the Ustunisik references are missing from the bibliography at the end. I did not do a comprehensive check of all references, but the two places I spot checked seemed to have missing references, so please check throughout the manuscript. Reply: Thanks for the comments. We have checked throughout the manuscript and added the missed literatures. Lines 200-208: I have seen in different places that the degassed vapor would be rich in 35Cl and the residue enriched in 37Cl (e.g., Boyce et al., 2015), but it seems the opposite is being argued here. Could you please elaborate on the process a bit more that is thought to drive the enrichment in 37Cl? Reply: Yes, the degassed vapor would be rich in 35Cl as it is lighter in mass. To drive the 37Cl enrichment by vapor-metasomatism, a partial recondensation process of this Cl-bearing vapor would be required as 37Cl prefers condensation. As long as the vapor is Cl-rich, a partial recondensation would contribute to 37Cl enrichment. Lines 209-223: This section seems a bit too speculative (i.e., broad implications about the lunar crust being drawn from previously published data from a single felsite sample) and is not adequately linked to the new experimental results that are the main new contribution within this manuscript. I suggest either making that link a bit stronger in this section or maybe consider removing this section from the manuscript. Reply: Thanks for the suggestion. Reviewer #1 also raised the same comment on this section. We agree that knowledge of Cl isotope compositions is necessary for a full discussion of the ‘vapor-phase metasomatism’. Thus, we removed the detailed discussion of ‘vapor-phase metasomatism’ in this section but emphasized our observation on Cl enrichment in lunar nearside samples. Meanwhile, we still attribute this observed Cl enrichment to possible late metasomatism while only briefly mentioning the ‘vapor phase’ hypothesis. In lines 239-246 in the revised manuscript, we added the sentences of ‘A conclusion on which metasomatic process caused the Cl enrichment cannot be drawn without Cl isotopic data. Whether this Cl enrichment occurs only on the lunar nearside, or if it also affected the lunar farside, and which metasomatic process may be responsible, remain open questions. An analyses of halogen abundances in the recently returned Chang’e-6 samples (e.g., Cui et al., 2024; Zhang et al., 2024), could shed light on the geographical distribution of Cl enrichment, while Cl isotopic analyses could shed light on whether the metasomatic process(es) are comparable between nearside and farside’ This may provide insights for future studies on Cl isotopes in this context. To accommodate the changes in the manuscript, we have also made appropriate modifications to the title – ‘Halogen evidence for the formation and metasomatism of the primary lunar crust’. References: There seems to be a number of cited references that do not appear to be cited in the manuscript text, including the 3 Barnes references and the Boyce 2015 reference. All four of those manuscript are relevant to this work, and two of them (Boyce et al., 2015 and Barnes et al., 2016a) are the primary contrasting models for understanding the origin Cl isotopic fractionation in lunar samples. I have not done an exhaustive check of the references here, I just checked the first few and noted several discrepancies. Please do a more exhaustive check during revision. Reply: Thanks for checking. Indeed, the submitted manuscript was shortened just before submission and we did not go through a final check of the reference list at that point – we apologize for the oversight. In the revised manuscript, we have further reduced the discussion on vapor-phase metasomatism in Cl isotope studies and carefully reviewed the reference list to ensure consistency. Supplement: Line 63: should be anomalous instead of anonymous. Reply: Revised as ‘anomalously high values’."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review of the revised manuscript entitled “Halogen evidence for the formation and metasomatism of the primary lunar crust” by Jing et al. submitted to NC Dear editor and authors: The revised manuscript has been much improved. I appreciate that the authors took the time to analyze the H2O content in experimental melt, reassessing the F and Cl partitioning data and also incorporating error estimation in the modelling work. I am happy to recommend this manuscript for publication in NC upon the completion of a minor revision of the current manuscript. Some suggestions for a minor revision are listed below. Additionally, a final polish of the writing is recommended, in particular for the abstract, which will increase the appeal of the manuscript. On a separate note, if the authors have a chance to analyze the H2O contents in the mineral phases in their experiments, they should be able to obtain H2O partitioning coefficients, which are also very useful for modeling lunar volatile evolution. Line 160-162: Experimental melts from this study have high Cl contents (> 3 wt%). By judging from the data from Potts et al. (2021) with melt Cl contents < 3 wt%, there could be an increase of Cl partition coefficient between orthopyroxene and melt with decreasing melt Cl content (Fig. S2a). Line 201-203: The F/Cl ratio in melt should be 0.08 when calculated from plagioclase, and 1.8 when calculated from orthopyroxene. It is the wrong way around here. Line 218-223: why would the crystallization of apatite cause an increase of the hydrogen concentration in the residual magma? Apatite crystallization does happen quite late in the magmatic evolution. During the late stage of fractional crystallization, e.g., 90-99.9% of mare basalt crystallization, we do see orders of magnitude variation in terms of volatile contents in the residual magma, which can be attributed to the concentration of volatiles in the last dreg of the melt, and also the modification of volatile contents in melt due to apatite crystallization. This is manifested in the varying apatite compositions in a single lunar basalt sample, which do display orders of magnitude variation in terms of volatile contents. Is this the case for apatite from Mg-suite rocks? Is apatite crystallization relevant for the discussion of F and Cl contents in orthopyroxene and plagioclase from Mg-suite rocks? How about the felsite sample 14321,1062, which has been discussed a lot in the text? Line 225: “alkali” is not a mineral. Line 248-250: what is the criterion for identifying data affected by Cl metasomatism? Line 266-273: It is difficult to tell the differences between the first two models described in line 266-268 and line 268-270, simply based on the authors’ descriptions here. Moreover, this whole section seems rather repetitive, considering clearer and more detailed descriptions are found below. Line 326-330: Please elaborate on how the addition of TIRL can lower the melting point of Mg-rich cumulate by increasing the concentration of incompatible elements, but still in keeping with the “non-KREEP” compositional characteristics.", "reviewer_2": "I have read through the responses to the reviewers and the revised manuscript, and I think the authors have adequately addressed all of the concerns I raised in my initial review. Furthermore, I have no additional suggestions for improvement, and I recommend that this work be accepted for publication in Nature Communications."}, "author_response": "Reply: Thank you for the second review and the positive comments. The manuscript and Abstract have been further polished. As we are also interested in mineral-melt water partitioning at lunar conditions, a future attempt at quantifying the (likely very low) concentrations of hydrogen in the minerals in our experiments could lead to a follow-up study. This is outside the scope of the current manuscript. Line 160-162: Experimental melts from this study have high Cl contents (> 3 wt%). By judging from the data from Potts et al. (2021) with melt Cl contents < 3 wt%, there could be an increase of Cl partition coefficient between orthopyroxene and melt with decreasing melt Cl content (Fig. S2a). Reply: Thanks for the comment. The group of Cl partitioning data in Potts et al. (2021) shown in our data compilation figure is limited in number and was published with an associated error of zero (and plotted as such in our Fig. S2). If we would use the same error propagation method used for our own experiments and applied it to their data, the uncertainties for their data would be high, e.g., D = 0.0040±0.0022 for run Fe_4, and D = 0.0040±0.0010 for run Pt_hal1. The Fig. S2 in the Supplementary Information file has been updated. Their Cl partition coefficients would then be identical to our data range within error, and we think it is not correct to suggest an effect of melt Cl content on D values. Line 201-203: The F/Cl ratio in melt should be 0.08 when calculated from plagioclase, and 1.8 when calculated from orthopyroxene. It is the wrong way around here. Reply: Thank you for spotting this mistake – you are correct and we revised this. Line 218-223: why would the crystallization of apatite cause an increase of the hydrogen concentration in the residual magma? Apatite crystallization does happen quite late in the magmatic evolution. During the late stage of fractional crystallization, e.g., 90-99.9% of mare basalt crystallization, we do see orders of magnitude variation in terms of volatile contents in the residual magma, which can be attributed to the concentration of volatiles in the last dreg of the melt, and also the modification of volatile contents in melt due to apatite crystallization. This is manifested in the varying apatite compositions in a single lunar basalt sample, which do display orders of magnitude variation in terms of volatile contents. Is this the case for apatite from Mg-suite rocks? Is apatite crystallization relevant for the discussion of F and Cl contents in orthopyroxene and plagioclase from Mg-suite rocks? How about the felsite sample 14321,1062, which has been discussed a lot in the text? Reply: Thanks for this comment. The answer to the first question is easy. Reported apatite-melt partition coefficients for hydrogen are low (e.g., D of 0.07-0.24 in McCubbin et al., 2025b). Fluorapatite crystallization alone would therefore already lead to an increasing trend of hydrogen in the residual magma. As other nominally dry silicates would co-crystallize with apatite during fractional crystallization, the bulk D for hydrogen would be even lower than the apatite value. Crystallization in these systems would therefore further increase melt H content. We have further elaborated on the ‘apatite issue’ by referring to the experimental apatite-melt partitioning study of McCubbin et al. (2015b) in lines 220-228 in the revised manuscript. McCubbin et al., (2015b) reported a dataset for apatite-melt partition coefficients, being in the order of D (4.4-19) > D (1.1-5) > D (0.07-0.24). In our study, we focus on the cause for very low F/Cl ratios observed in lunar materials, and in explaining such low ratios. It is true that substantial apatite crystallization in mare basalts at very late stage could contribute to volatile variations in the residual magma. Nearly all apatite in lunar samples is fluorapatite (McCubbin et al., 2015a) so that the fluorapatite data of McCubbin et al. (2015b) are appropriate. The experimental data suggest that fluorapatite crystallization could potentially decrease F/Cl ratios in residual melts, as the partition coefficients of F between apatite and melt (4.4-19) are generally higher than those of Cl (1.1-5). But as suggested by the reviewer, apatite is generally a very minor phase observed in Mg-suite rocks (<1% volume modal, Shearer et al., 2015) as they generally have high Mg# not highly evolved. Thus, its influence on early crystallized orthopyroxene and plagioclase would be limited. And, this scenario is more unlikely for the non-KREEP Mg-suite rocks discussed in this study. For example, Mg-suite samples NWA 5744 and 10140 have Mg-minerals with high Mg# numbers and lack observations of apatite (Kent et al., 2017; Gross et al., 2020). As for the highly evolved felsite sample 14321,1062, the variation of Cl abundances (0.02 to 4.67 μg/g) and low F/Cl ratios in this rock cannot be attributed to fluorapatite crystallization. Given that Cl also prefers to partitioning into apatite (D = 1.1-5), fractional crystallization of fluorapatite would be expected to produce a positive correlation between F and Cl in any co-crystallizing feldspar. However, in sample 14321,1062, Cl abundances in feldspar increase while F contents remain nearly constant (Fig. 3b). This discrepancy leads us to propose alternative process(es) responsible for Cl enrichment that do not involve concurrent F addition, as described in our study. Line 225: “alkali” is not a mineral. Reply: We agree with the reviewer and ‘alkali’ has been replaced by ‘alkali-bearing phases’. Line 248-250: what is the criterion for identifying data affected by Cl metasomatism? Reply: Based on our discussion presented in the ‘Widespread Cl enrichment on the lunar nearside’ section of our manuscript, we suggest that low F/Cl values below 0.5 should be attributed to Cl metasomatism. We added this to the revised text line 253. Line 266-273: It is difficult to tell the differences between the first two models described in line 266-268 and line 268-270, simply based on the authors’ descriptions here. Moreover, this whole section seems rather repetitive, considering clearer and more detailed descriptions are found below. Reply: Thank you for the comment. The original sentences in lines 266–270 that the reviewer refers to here have been removed to make the paragraph more concise and to reduce redundancy. Line 326-330: Please elaborate on how the addition of TIRL can lower the melting point of Mg-rich cumulate by increasing the concentration of incompatible elements, but still in keeping with the “non-KREEP” compositional characteristics. Reply: We apologize for the confusion. The lowering of the melting point of TIRL-bearing Mg-rich cumulates is not due to the increase in concentration of incompatible REE elements. The TIRL itself, through its major element composition, has a lower melting point (solidus) than the lunar mantle cumulates. The addition of TIRL would introduced the addition of fusible materials, but it does not lead to a vast increase in trace element concentrations comparable to the addition of urKREEP materials. The urKREEP reservoir is a very late residual liquid (>99% solidification) from lunar magma ocean crystallization, thus containing nearly 100x REE concentrations of the initial LMO. In contrast, TIRL from in the early lunar cumulates has much lower REE concentrations (only ~2x of the initial LMO at PCS50) compared with the urKREEP. Elardo et al. (2020) calculated that the high REE concentrations of KREEP-rich Apollo troctolites were formed by partial melting of lunar cumulates with 15-50% addition of KREEP, which contains several orders of magnitude higher REE concentrations than the early TIRL. Adding only ~2 wt.% TIRL would therefore not result in KREEP-like characteristics observed in these KREEP-rich Apollo troctolites. The above description has been briefly summarized in lines 328-338 in the revised manuscript to clarify our point. Reply: Thank you for the second review and positive comments."}]

Decision letter