Carbon-rich lunar core evidenced by isostructural phase transitions in iron

Carbon-rich lunar core evidenced by isostructural phase transitions in iron | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article Carbon-rich lunar core evidenced by isostructural phase transitions in iron Mingqiang Hou, Hong Liu, Yoshio Kono, Xiaoming Cui, Xiaodong Song, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7903808/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The successful Chang'e mission has reignited scientific interests in the Moon, with analyses of returned samples advancing our understanding of lunar composition and evolution 1-4 . Pioneering models proposed that the Moon harbors a liquid iron-sulfur outer core and a pure face centered cubic (fcc) structured iron (Fe) inner core 5-8 . However, the Apollo-era data described a lunar inner core with seismic velocity of 27% lower than that of fcc-Fe 5 , 6 , posting a long-standing challenge to understand the structure and chemistry of the lunar core. Here, we identify new phases of both bcc- and fcc-Fe that could resolve the discrepancy. In-situ high-pressure and high-temperature X-ray diffraction (XRD) and acoustic velocity measurements show that the Fe alloyed with 0.2-1.7 wt.% carbon (C) undergoes an isostructural phase transition around 4-5 GPa accompanied by a notable velocity drop of ~1 km/s. The seismic velocity profile of the lunar inner core can be directly matched by this new phase in Fe-C solid solution (hereafter denoted as fcc-II; conventional fcc-Fe is referred as fcc-I). These findings provide compelling evidence that the lunar inner core is composed of fcc-II Fe-C with a temperature potentially below 1500 K. Earth and environmental sciences/Planetary science/Mineralogy Earth and environmental sciences/Solid Earth sciences/Seismology Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The Chang’e missions stimulate a new upsurge after the Apollo missions in the exploration of the Moon. Far-side lunar regolith samples returned by the Chang’e-6 mission have advanced insights into surface volcanic processes 1 , the origin of the ancient lunar magnetic field 2 , and mantle water distribution 3 , 4 . These new findings extend our knowledge of the Moon from its surface to deep mantle. Although there is no overt data from Chang’e missions about the lunar core, Moon has inferred to share similar core structure to the Earth with an liquid outer core and a solid inner core, dominated by iron (Fe) with trace light elements (e.g. carbon (C) and sulfur (S)) 5-8 . Recent tidal deformation studies 9 constrain a lunar inner core with radius of 258±40 km and a density of 7822±1615 kg/m 3 crystallized from a larger outer core (250-430 km in radius) 5 , 8-11 . Although vastly smaller than the Earth’s core, the lunar core sustained a dynamo for 1.0-3.4 Ga, as confirmed by the paleomagnetic records 12-18 . In contrast with the Earth’s core, the lunar core is unique and mysterious for its ultra-low seismic velocities. Apollo seismograms yielded a compressional wave velocity ( V P ) of ~4.1 km/s for the outer core, and V P and shear wave velocity ( V S ) of 4.3 km/s and ~2.3 km/s for the inner core 5 , respectively. The seismic velocities are ~7% and ~27% lower compared with liquid Fe and solid fcc-Fe 5-7 , 19 at lunar core conditions, respectively. Nickel (Ni) and S could reduce the V P of liquid Fe solely or jointly at lunar core conditions 7 , 20-22 . Similarly, 3.5 wt.% carbon is reported to lower down the V P of liquid Fe by ~2%, but its effect is reversed in the presence of Ni 23-25 . To date, the ~7% V P deficit of lunar outer core can be well explained by the incorporation of 4-10.5 wt.% S 7 , 20 , 22 . Yet if S is the solo light element in the lunar liquid outer core, the inner core would be pure Fe because the partitioning coefficient of S between solid and liquid Fe is nearly zero at lunar core conditions 26-28 . This leaves the ~27% velocity large discrepancy between the lunar inner core and pure fcc-Fe unresolved, hindering the understanding of the core structure, dynamics, chemical evolution, and the enigmatic history of the lunar dynamo. Light elements like silicon (Si), hydrogen (H), and oxygen (O) rarely partition into lunar core due to the high oxygen fugacity and relatively low pressure 11 . In contrast, phase equilibria experiments 15 and mass balance calculations 29 indicate that C could reside in the lunar core with a content ranging from 0.3-4.8 wt.%. Fe is known to present a liquid phase and four solid phases, which are body-centered cubic (bcc, including and ), fcc (or ), and hexagonal close-packed (hcp or ) phases.Recently, the incorporation of C into Fe is reported to induce a liquid-liquid (L-L) phase transition in molten Fe around 5 GPa 30-35 . The L-L transition in molten Fe-C has been attributed to solid phase transition in Fe 31 , but such L-L phase transition does not occur in liquid Fe 36 , invalidating it as the mechanism for the L-L phase transition in Fe-C. The expected transition pressure of ~5 GPa is exactly the pressure in the lunar inner core, prompting us to test whether similar structural changes occur in solid bcc-Fe-C (hereafter bcc means only α phase) and fcc-Fe-C. If such structure changes exist in solid Fe-C, the ultra-low seismic velocity of lunar inner core might be explained. To address these questions, we conducted in-situ energy-dispersive X-ray diffraction (XRD) and acoustic velocity measurements on Fe alloyed 0.2 wt.% C (Fe-0.2C) and 1.7 wt.% C (Fe-1.7C) solid solutions up to 7 GPa and 1300 K in a Paris-Edinburgh press coupled with Pulse-echo overlap technique. Room-temperature behaviors of Fe-C alloys The acoustic velocities of Fe-C alloys at room temperature are illustrated in Fig. 1. The V P of bcc-Fe is in the range of 6.0-6.5 km/s and linearly increases as a function of pressure (0-7.0 GPa), while its V S varies little and remains nearly constant at 3.5 km/s. Our results of bcc-Fe are consistent with prior work 37 . C doping fundamentally alters these properties of bcc-Fe. At pressures below 4.0 GPa, 0.2 wt.% C could increase the V P of bcc-Fe by ~0.4 km/s, while it slightly decreases the V S by 0.1-0.3 km/s. The effect of C on V P of bcc-Fe is the same as that of Si 38 . Light elements are generally supposed to increase the V S of Fe 39-43 , however, we find that the incorporation of C could decrease the V S of Fe. This effect is same as our recent results on the bcc-Fe-Si alloys 19 . Above 4.0 GPa, we observe a gradual decrease in the V P of Fe-0.2C and the V P drops to ~5.7 km/s above 5.5 GPa. Its V S , meanwhile, increases linearly with pressure below 4.0 GPa, but plateaus at ~3.4 km/s above 4.0 GPa. The acoustic velocity discontinuities in both V P and V S indicate a phase transition in Fe-0.2C around 5 GPa. The XRD characterization (Fig. 1b) shows that the bcc structure phase persists up to 7.3 GPa. The XRD and acoustic velocity results evidence that it is an isostructural phase transition (ISPT). We denote the low-pressure phase structure as bcc-I (5.5 GPa), respectively. At pressures between 4.0 GPa and 5.5 GPa, the V P lies between those of bcc-I and bcc-II, indicating a mixed-phase region of Fe-0.2C. A ~1.6%volume collapse is also observed at ~4.5 GPa, further confirming the ISPT in Fe-0.2C (Fig. 1c). Further, the starting Fe-1.7C sample was employed to clarify how varying carbon concertation affects the aforementioned ISPT. Fe-1.7C starts as a bcc-fcc mixture at room temperature. Below 4.5 GPa, the V P stays constant at 6.2 km/s and the V S linearly increases from 2.5 km/s to 3.3 km/s (Fig. 1a). 1.7 wt.% C is supposed to significantly increase the V P of bcc-Fe, but the V P of bcc/fcc-Fe-1.7C is smaller than that of Fe-0.2C and comparable to that of pure bcc-Fe. Notably, 1.7 wt.% C reduces the V S of Fe by 6-29%, which is much larger than the effect of 0.2 wt.% C (3-8%) likely due to both carbon doping and the fcc phase coexistence. Above 4.5 GPa, the V P suddenly drops to ~5.6 km/s and the V S keeps constant at ~3.2 km/s. These results illustrate that the Fe-1.7C sample also undergoes an ISPT around 4.5 GPa at room temperature. We also obtained the V P and V s of Fe-1.7C during decompression. The V P and V s are ~5.6 km/s and ~3.2 km/s above 4.0 GPa which are same with their compression values. Below 4.0 GPa, V P suddenly increases to ~6.2 km/s and V S drops to 2.5-3.3 km/s, which means both V P and V s recovers to the values in compression process below 4.5 GPa. The variation trend of V P and V s indicates that the ISPT is reversible. Mechanism of the ISPT in iron-carbon alloys For decades, Fe is believed to present a liquid phase and four solid phases which are bcc (including and ), fcc, and hcp.A L-L phase transition in molten Fe-C was reported around 5 GPa 30-35 . Here one more isostructural phase, bcc-II, is observed in solid Fe with the presence of C. In order to investigate the mechanism of ISPT in Fe-C alloy, synchrotron Mӧssbauer spectra (SMS) and x-ray emission spectra (XES) were conducted. Fe-0.2C persists in ferromagnetic state up to 12.3 GPa and then transform to nonmagnetic above 15.4 GPa (Supplementary Fig. S4). The magnetic change can be attributed to the bcc-hcp phase transition. No macroscopic magnetic change is observed around 5 GPa, indicating macroscopic magnetic would not be the reason for ISTP in Fe-C alloys. In contrast, XES provides a way to probe local magnetic momentum. The K β emission is due to a radiation decay of the 1 s core hole from a 3 p level. Due to a net magnetic moment ( m ) in the 3 d valence shell, the K β emission spectrum splits into a main line K β1,3 and a satellite line K β ’ . The intensity of K β ’ is proportional to the net spin of the 3 d shell of the transition metal. No satellite line K β ’ presents at ~7045 eV in Fig. 2. The K β1,3 peak shifts to the lower energy as pressure increases, which can be attributed to the responses of Coulomb repulsion (U) to the pressure. We calculated the integrals of the absolute values of the difference spectra (IAD) 44 to analyze the shift of K β1,3 of Fe-0.2C by using the data at 11.5 GPa as the reference spectrum. The inset in Fig. 2 shows the IAD values of Fe-0.2C under high pressure conditions. The IAD value increases as a function of pressure below 4.6 GPa, while a sudden drop in the IAD is observed at 5-6 GPa. Above 5-6 GPa, the IAD value monotonously decreases as a function of pressure. The XES results show that Coulomb repulsion between Fe-Fe atoms suddenly decreases, which might be the cause of the observed ISTP in Fe with the presence of light element C in Fig. 1. High-temperature behaviors of Fe-C alloys The observed ISPT in Fe-1.7C with mixed bcc and fcc phases at room temperature indicates that the ISTP may also occur in fcc-Fe-C. We conducted high-temperature acoustic velocity measurements of fcc-Fe-C alloys up to 1300 K (Fig. 3). Above 1000 K, both Fe-0.2C and Fe-1.7C are in fcc phase (Supplementary Fig. S5 and S6). The V P of fcc-Fe-0.2C is 5.4 km/s below 4.5 GPa. It is about 1.0 km/s lower than that of its bcc-I counterpart. Temperature has negligible effect on V P in these P-T conditions. However, significant temperature effect can be observed in the V S (Supplementary Fig. S7). At 1100 K, the V S is ~2.5 km/s, while it is ~2.2 km/s at 1300 K. We were not be able to obtain V P datum for fcc-Fe-0.2C above 4.5 GPa, but the sudden V S drop around 5.3 GPa indeed confirm the ISPT in fcc phase. The V S is ~2.0 km/s at 1100 K, which is ~0.5 km/s lower than its lower pressure (<4.5 GPa) counterpart. We denote the ISPT in fcc phase as fcc-I and fcc-II. The fcc ISTP can be more clearly observed in Fe-1.7C. Below 4.0 GPa, the V P and V S of fcc-I Fe-1.7C are ~5.0 km/s and ~2.6 km/s at 1100 K, respectively. While above 4.0 GPa, the V P and V S drop to ~4 km/s and ~2.1 km/s, respectively. Implications for the lunar core Lunar core conditions (5-6 GPa and 1300-1900 K) 6 favor fcc-Fe in its inner core 45-48 . The V P of pure fcc-Fe has been measured up to 19 GPa and 1100 K 6 and extrapolated data to 1-1.5 GPa is in good agreement with our measurements. At lunar inner core conditions, the V P of fcc-Fe is 6.0 km/s with estimated V S of 3.0 km/s. In contrast, the Apollo seismic detection of the Moon’s inner core found that the V P and V S are about 4.3 km/s and 2.3 km/s, respectively, which are significantly smaller than those of fcc-Fe. The contradiction leads to an outstanding issue in determining the composition of lunar core, especially considering the conventional wisdom that the incorporation of light elements would boost the acoustic velocities of solid iron 38-43 , 49 . S was suggested the main light element in the lunar outer core 7 , 20-22 , 50 , 51 . However, the content of sulfur in the lunar inner core would be negligible, because the partitioning coefficient of sulfur between solid and liquid iron increases from 0 at ~20 GPa to 0.1 at ~40 GPa 26-28 . The ISPT in Fe-0.2C and Fe-1.7C could induce a V P collapse from 5.0-5.5 km/s to ~4-4.5 km/s and the V S to ~2.1 km/s. These data are in good agreement with the seismic velocity profile of the lunar core when the data scattering is considered, which means the incorporation of 0.2-1.7 wt.% carbon into fcc-Fe could reproduce the Apollo seismic velocities of the lunar core above 5 GPa (Fig. 3). The carbon-rich lunar core deduced from the acoustic velocity of fcc-II Fe-C is also supported by phase equilibria experiments in Fe-S-C system 15 , 52 and mass balance calculations combined with metal-silicate partitioning and bulk silicate Moon abundance of carbon 29 . The melting experiments of low carbon content Fe-C-S ternary system 53 showed that the carbon partitioning coefficient in liquid and solid iron is around 0.5. If there is 0.2-1.7 wt.% carbon in Moon’s inner core, the carbon content in its liquid outer core would be 0.1-0.8 wt.% (Fig. 4). It is reported that ~4 wt.% sulfur is required to reproduce the V P of the Moon’s outer core 7 . Then 0.1-0.8% carbon would co-exist with ~4 wt.% sulfur in Moon’s liquid outer core according to the phase equilibria in Fe-S-C system 15 . Compared with V P , the V S of Fe-C is more sensitive to the temperature (Supplementary Fig. S7). The temperature-dependence of V S can be employed to constrain the temperature of the lunar core if carbon is the sole light element. A reduction of ~0.7×10 -3 km/K for V S can be deduced from the results in Supplementary Fig. S7. The V S of fcc-II Fe-C is around 2.1 km/s at 1300 K and lunar core pressure. Both V P and V S of fcc-II Fe-C at 1300 K could reproduce the seismic velocities of the lunar core. As temperature increases to 1500 K, although the V P for fcc-II Fe-C is comparable to the Apollo seismic observation, the upper bound of V S for fcc-II Fe-C is below the seismic velocity of the lunar core, indicating the temperature profile of the lunar core is likely smaller than 1500 K. We calculated the density of fcc-Fe-0.2C at 5 GPa and 1300-1500 K according to the thermal equation of state 54 , and constrain the density of lunar inner core at 7.71–7.82 g/cm³ , which is in good agreement with the constraint from tidal deformation (7822±1615 kg/m 3 ) 9 . Methods Sample preparation: The starting Fe, Fe-0.2C and Fe-1.7C are synthesized by piston cylinder apparatus at 1 GPa and 1273 K with pure iron powder and graphite (purity 99.9%, purchased from Alfa Aesar Company). All sample are 2 mm in diameter and 5 mm in height. The chemical composition of the synthesized sample was analyzed using EPMA-8050G Electron Probe Microanalyzer at Shanghai Institute of Measurement and Testing Technology. The electron beam conditions are 15 kV and 200 nA with 10 s peak counting time and 10 s background counting time. Five well-characterized Fe-C alloys with carbon contents of 0.0067, 0.160, 0.382, 0.620, and to 0.870 wt.% were used as standards to establish a calibration curve for carbon measurement. Neither the sample nor the standards were coated by conductive layer in order to prevent carbon contamination, while charging was prevented by carbon tape. LiF/CH4 and PbST/CH2 were used for carbon and iron analysis, respectively. The measured carbon contents of the two synthesized samples range from 0.15-0.26 wt.% with an average of 0.2±0.06 wt.% (denoted as “Fe-0.2C”) and 1.65-1.81wt.% with an average of 1.7±0.11wt.% (denoted as “Fe-1.7C”) (Fig. S1). Acoustic velocity measurements: Synthesized samples were carefully cut to disks with thickness of ~0.7 mm. All samples were finely double-polished with 6 μm diamond lapping film. The final thicknesses of all samples were about 0.65 mm. The experiments were conducted using the Paris-Edinburgh press at 16-BM-B, High Pressure Collaborative Access Team (HPCAT) at the Advanced Photo Source, Argonne National Laboratory. Two types of PE cells are employed (Supplementary Fig. S2). Sintered diamond buffering rod and SiO 2 backing were used for room temperature measurements, while alumina buffering rod and backing were used in the high temperature cell. The buffer rod and the backing were placed next to the sample, and ultrasonic signals were reflected between the interfaces. An MgO ring surrounded the sample and it was used to calibrate the pressure 55 . The temperature was read by pre-calibrated power-temperature relations 56 . Pulse-echo overlap technique was employed to obtain the ultrasonic V P and V S signals. An electrical sine wave was generated by waveform generator. Ultrasonic signals from 12 MHz to 40 MHz were collected with a digital oscilloscope (Tektronix DP05104). The wave was reflected in the buffer-rod/sample and sample/backing, which were R 1 and R 2 for both P-wave and S-wave (Supplementary Fig. S3). The travel time ( t ) of ultrasonic signal in the sample could be determined by R 1 and R 2 . Sample length ( l ) was measured by white X-ray radiography image. The acoustic velocity can be calculated by Where l is the length of the sample, and t is the travel time of ultrasonic signals determined by R 1 and R 2 , respectively. The uncertainty of acoustic velocity is ~0.3%. The detailed method can be found in ref. 55 . The energy-dispersed x-ray diffraction (EDXRD) patterns were collected by a Ge solid state detector at . The collection time ranged from 30 s to 200 s. X-ray emission spectra (XES): The XES were performed at 16-IDD, HPCAT. A symmetric diamond anvil cell with culet size of 300 μm was used to generate high pressure for XES measurement. A rhenium gasket was pre-indented at 20 GPa and a 150 μm chamber hole was drilled by laser ablation. A ~25 um thick Fe-0.2C pellet with diameter of ~70 um was loaded into the chamber. Silicon oil served as pressure medium. The pressure was determined by ruby fluorescence with an uncertainty of 0.1 GPa. All XES experiments were conducted at room temperature. An incident X-ray beam with an energy of 11.3 keV and a bandwidth of ~1eV was employed for the experiments. The collection time was ~40 minute for each spectrum. Declarations ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China with grant no. 42394114 and 42394111. AUTHOR CONTRIBUTIONS Conceptualization, B.C., J. L., and M.H.; investigation, M.H., H.L., Y.K., J. L., and X.C.; visualization, M.H., H.L., and J. L.; formal analysis, M.H., H.L., Y.K., X.C., and J. L.; validation, Y.K., B.C. and J. 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Journal of Geophysical Research: Planets 130 , e2024JE008612 (2025). Kono, Y., Park, C., Kenney-Benson, C., Shen, G. & Wang, Y. Toward comprehensive studies of liquids at high pressures and high temperatures: Combined structure, elastic wave velocity, and viscosity measurements in the Paris–Edinburgh cell. Phys. Earth Planet. Inter. 228 , 269-280 (2014). Kono, Y., Irifune, T., Higo, Y., Inoue, T. & Barnhoorn, A. P-V-T relation of MgO derived by simultaneous elastic wave velocity and in situ X-ray measurements: A new pressure scale for the mantle transition region. Phys. Earth Planet. Inter. 183 , 196-211 (2010). Additional Declarations There is NO Competing Interest. Supplementary Files SI.docx Supporting materials Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7903808","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Physical Sciences - Article","associatedPublications":[],"authors":[{"id":544075235,"identity":"4f6e290d-7715-468c-b83d-53bdc406e433","order_by":0,"name":"Mingqiang Hou","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABDUlEQVRIiWNgGAWjYDACZhBhACJ4gLjiAMlazgC1sBFvH1ALYxsRWnTbeQ+/eFNglyfvwHvwMe+8O3ny85uffWCosWPgn92AVYvZYb40yzkGycWGB/iSjXm3PSs2OMZmPIPhWDKDxJ0DOLTwmBnzGDAnbmzgMZPm3XY4cQMbgzHQdQcYDCQS8Gmph2qZczhxfhv7ZwaGf3i1GD/mMQCqZABpaTic2HCMxxgcDvhsYZxjcDxxAzOPseGcY4eBfskpZkjsS+aRuIFDy/kzxh/e/KlOnN/eY/jgTc3hPPnm45sZPnyzk+OfgV0LELBJgCLR4DCElwAjeXCpBwLmDyBZ+QZkLaNgFIyCUTAKkAAAjAtZYVGBn0UAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0001-7003-3025","institution":"Innovation Academy for Precision Measurement Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Mingqiang","middleName":"","lastName":"Hou","suffix":""},{"id":544075236,"identity":"aebb0814-cb98-4543-8915-1d939092b76f","order_by":1,"name":"Hong Liu","email":"","orcid":"","institution":"Institute of Earthquake Forecasting","correspondingAuthor":false,"prefix":"","firstName":"Hong","middleName":"","lastName":"Liu","suffix":""},{"id":544075237,"identity":"1e133614-21ea-4a94-9d42-c620ed8b49ad","order_by":2,"name":"Yoshio Kono","email":"","orcid":"https://orcid.org/0000-0001-5916-7524","institution":"Kwansei Gakuin University","correspondingAuthor":false,"prefix":"","firstName":"Yoshio","middleName":"","lastName":"Kono","suffix":""},{"id":544075238,"identity":"b01ab532-4049-43e2-9b33-739326c55497","order_by":3,"name":"Xiaoming Cui","email":"","orcid":"","institution":"Innovation Academy for Precision Measurement Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Xiaoming","middleName":"","lastName":"Cui","suffix":""},{"id":544075239,"identity":"46021fd3-0ee0-4d31-94e1-b1ff5e08b0f7","order_by":4,"name":"Xiaodong Song","email":"","orcid":"","institution":"School of Earth and Space Sciences, Peking University","correspondingAuthor":false,"prefix":"","firstName":"Xiaodong","middleName":"","lastName":"Song","suffix":""},{"id":544075240,"identity":"e81c2a0f-fc57-46d7-9a1e-0f38b4787830","order_by":5,"name":"Heping Sun","email":"","orcid":"https://orcid.org/0000-0002-2243-6353","institution":"Innovation Academy for Precision Measurement Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Heping","middleName":"","lastName":"Sun","suffix":""},{"id":544075241,"identity":"9f6eced0-0e21-4cea-9a3d-ce4c13a33bec","order_by":6,"name":"Sidao Ni","email":"","orcid":"","institution":"Innovation Academy for Precision Measurement Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Sidao","middleName":"","lastName":"Ni","suffix":""},{"id":544075242,"identity":"bd46522e-ca4b-4d84-aaf6-314aaa8a1937","order_by":7,"name":"Yongjun Tian","email":"","orcid":"https://orcid.org/0000-0002-4594-4879","institution":"Yanshan University","correspondingAuthor":false,"prefix":"","firstName":"Yongjun","middleName":"","lastName":"Tian","suffix":""},{"id":544075243,"identity":"4d36c6a5-2139-4a96-991c-e906855ac7ab","order_by":8,"name":"Jin Liu","email":"","orcid":"https://orcid.org/0000-0002-1670-8199","institution":"Yanshan University","correspondingAuthor":false,"prefix":"","firstName":"Jin","middleName":"","lastName":"Liu","suffix":""},{"id":544075244,"identity":"5bdbb92b-2f9d-4df0-9dc1-c6b39e373c65","order_by":9,"name":"Bin Chen","email":"","orcid":"","institution":"Center for High Pressure Science and Technology Advanced Research","correspondingAuthor":false,"prefix":"","firstName":"Bin","middleName":"","lastName":"Chen","suffix":""}],"badges":[],"createdAt":"2025-10-20 08:41:11","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7903808/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7903808/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":95821653,"identity":"cb0c66d8-dd5f-47cd-b2d5-8cb00dc91ac2","added_by":"auto","created_at":"2025-11-13 10:48:33","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":163895,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe acoustic velocity and X-ray diffraction characterizations of Fe and Fe-C alloys at room temperature.\u003c/strong\u003e (A) Acoustic velocities of bcc-Fe, bcc-Fe-0.2C, and bcc-Fe-1.7C. De. is short for decompression. The pressure uncertainty is 0.3 GPa. The black, red, and blue curves are hand-drawn relations of acoustic velocities and pressure for bcc-Fe, bcc-Fe-0.2C, and bcc\u0026amp;fcc-Fe-1.7C, respectively. (B) X-ray diffraction of Fe-0.2C; (C) \u003cem\u003eP\u003c/em\u003e-\u003cem\u003eV\u003c/em\u003e relations of bcc-Fe and bcc-Fe-0.2C. The volumes of bcc-Fe and bcc-Fe-0.2C are calculated according to peaks of (211), (220), (310), and (222).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7903808/v1/70b82b3b8644c4579fe342b5.png"},{"id":95821612,"identity":"384473a2-eb44-4d2f-bbd5-d41066223e74","added_by":"auto","created_at":"2025-11-13 10:48:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":89239,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eX-ray emission spectra of Fe-0.2C at 1.3-11.5 GPa and room temperature.\u003c/strong\u003eThe bottom curves are the differences of spectra compared with the reference spectrum at 11.5 GPa. The data points in the inset are the integrals of the absolute values of the difference spectra. The two solid lines are linear fits.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7903808/v1/a725a4697010d5c85e7b840e.png"},{"id":95821684,"identity":"39987cbd-185f-4aa1-8ba9-d264cdde4c5c","added_by":"auto","created_at":"2025-11-13 10:48:45","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":131597,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAcoustic velocities of Fe-0.2C (A) and Fe-1.7C (B) at elevated temperatures.\u003c/strong\u003e The black, blue, bold gray, bold blue curves (solid and dash) are hand-drawn variations of bcc-I, bcc-II, fcc-I, and fcc-II Fe-C alloys, respectively. The red star represents the \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eP\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e of the lunar core \u003ca href=\"#_ENREF_5\" title=\"Weber, 2011 #138\"\u003e\u003csup\u003e5\u003c/sup\u003e\u003c/a\u003e.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7903808/v1/f4acf67387f68105625840f2.png"},{"id":95821651,"identity":"273957ca-fd8e-477d-8990-3b6ffacfb106","added_by":"auto","created_at":"2025-11-13 10:48:29","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":336689,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe revised phase diagram of Fe (A), comparison of sound velocities of fcc-Fe, fcc-II Fe-C, and the lunar core (B), and the structure and chemistry of the lunar core (C).\u003c/strong\u003e The phase diagram of iron is from ref. \u003ca href=\"#_ENREF_45\" title=\"Swartzendruber, 1982 #56\"\u003e\u003csup\u003e45\u003c/sup\u003e\u003c/a\u003e. The circles and rectangles with crosses are phases of the sample at corresponding \u003cem\u003eP-T\u003c/em\u003e conditions. Blue vertical lines are phase boundaries of bcc-I/bcc-II and fcc-I/fcc-II. Lunar core \u003cem\u003eP-T\u003c/em\u003e conditions are represented by the pink shaded area in (A). The shaded areas in (B) are uncertainties of sound velocity from fitting error caused by data scattering. The phase diagram for Fe-1.7 C can be found in the supplementary materials. No visible temperature-dependence for \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eP\u003c/em\u003e\u003c/sub\u003e of fcc-II Fe-C is observed in supplementary Fig. S7. We assume \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eP\u003c/em\u003e\u003c/sub\u003e is irrelevant with temperature in (\u003cstrong\u003eB\u003c/strong\u003e).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7903808/v1/3e9f7c9f15fb5b8f964a65ff.png"},{"id":100369480,"identity":"415f7ead-dc90-4642-b6b6-0e744aad5b60","added_by":"auto","created_at":"2026-01-16 07:59:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1507441,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7903808/v1/e248d239-3208-4f5c-a23b-d216a0e22eb0.pdf"},{"id":95821613,"identity":"d15a179d-a3f9-4feb-80ac-d36d2fd274e1","added_by":"auto","created_at":"2025-11-13 10:48:24","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1768360,"visible":true,"origin":"","legend":"Supporting materials","description":"","filename":"SI.docx","url":"https://assets-eu.researchsquare.com/files/rs-7903808/v1/64f6e106307aafe7f64cbe84.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eCarbon-rich lunar core evidenced by isostructural phase transitions in iron\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Chang\u0026rsquo;e missions stimulate a new upsurge after the Apollo missions in the exploration of the Moon. Far-side lunar regolith samples returned by the Chang\u0026rsquo;e-6 mission have advanced insights into surface volcanic processes\u003csup\u003e1\u003c/sup\u003e, the origin of the ancient lunar magnetic field\u003csup\u003e2\u003c/sup\u003e, and mantle water distribution\u003csup\u003e3\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e4\u003c/sup\u003e. These new findings extend our knowledge of the Moon from its surface to deep mantle. Although there is no overt data from Chang\u0026rsquo;e missions about the lunar core, Moon has inferred to share similar core structure to the Earth with an liquid outer core and a solid inner core, dominated by iron (Fe) with trace light elements (e.g. carbon (C) and sulfur (S))\u003csup\u003e5-8\u003c/sup\u003e. Recent tidal deformation studies\u003csup\u003e9\u003c/sup\u003e constrain a lunar inner core with radius of 258\u0026plusmn;40 km and a density of 7822\u0026plusmn;1615 kg/m\u003csup\u003e3\u003c/sup\u003e crystallized from a larger outer core (250-430 km in radius)\u003csup\u003e5\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e8-11\u003c/sup\u003e. Although vastly smaller than the Earth\u0026rsquo;s core, the lunar core sustained a dynamo for 1.0-3.4 Ga, as confirmed by the paleomagnetic records\u003csup\u003e12-18\u003c/sup\u003e. In contrast with the Earth\u0026rsquo;s core, the lunar core is unique and mysterious for its ultra-low seismic velocities. Apollo seismograms yielded a compressional wave velocity (\u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e) of ~4.1 km/s for the outer core, and \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and shear wave velocity (\u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e) of 4.3 km/s and ~2.3 km/s for the inner core\u003csup\u003e5\u003c/sup\u003e, respectively. The seismic velocities are ~7% and ~27% lower compared with liquid Fe and solid fcc-Fe\u003csup\u003e5-7\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e19\u003c/sup\u003e at lunar core conditions, respectively. Nickel (Ni) and S could reduce the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of liquid Fe solely or jointly at lunar core conditions\u003csup\u003e7\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e20-22\u003c/sup\u003e. Similarly, 3.5 wt.% carbon is reported to lower down the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of liquid Fe by ~2%, but its effect is reversed in the presence of Ni\u003csup\u003e23-25\u003c/sup\u003e. To date, the ~7% \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e deficit of lunar outer core can be well explained by the incorporation of 4-10.5 wt.% S\u003csup\u003e7\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e20\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e22\u003c/sup\u003e. Yet if S is the solo light element in the lunar liquid outer core, the inner core would be pure Fe because the partitioning coefficient of S between solid and liquid Fe is nearly zero at lunar core conditions\u003csup\u003e26-28\u003c/sup\u003e. This leaves the ~27% velocity large discrepancy between the lunar inner core and pure fcc-Fe unresolved, hindering the understanding of the core structure, dynamics, chemical evolution, and the enigmatic history of the lunar dynamo. \u003c/p\u003e\n\u003cp\u003eLight elements like silicon (Si), hydrogen (H), and oxygen (O) rarely partition into lunar core due to the high oxygen fugacity and relatively low pressure\u003csup\u003e11\u003c/sup\u003e. In contrast, phase equilibria experiments\u003csup\u003e15\u003c/sup\u003e and mass balance calculations\u003csup\u003e29\u003c/sup\u003e indicate that C could reside in the lunar core with a content ranging from 0.3-4.8 wt.%. Fe is known to present a liquid phase and four solid phases, which are body-centered cubic (bcc, including and ), fcc (or ), and hexagonal close-packed (hcp or ) phases.Recently, the incorporation of C into Fe is reported to induce a liquid-liquid (L-L) phase transition in molten Fe around 5 GPa\u003csup\u003e30-35\u003c/sup\u003e. The L-L transition in molten Fe-C has been attributed to solid phase transition in Fe\u003csup\u003e31\u003c/sup\u003e, but such L-L phase transition does not occur in liquid Fe\u003csup\u003e36\u003c/sup\u003e, invalidating it as the mechanism for the L-L phase transition in Fe-C. The expected transition pressure of ~5 GPa is exactly the pressure in the lunar inner core, prompting us to test whether similar structural changes occur in solid bcc-Fe-C (hereafter bcc means only \u0026alpha; phase) and fcc-Fe-C. If such structure changes exist in solid Fe-C, the ultra-low seismic velocity of lunar inner core might be explained. To address these questions, we conducted in-situ energy-dispersive X-ray diffraction (XRD) and acoustic velocity measurements on Fe alloyed 0.2 wt.% C (Fe-0.2C) and 1.7 wt.% C (Fe-1.7C) solid solutions up to 7 GPa and 1300 K in a Paris-Edinburgh press coupled with Pulse-echo overlap technique.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eRoom-temperature behaviors of Fe-C alloys\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe acoustic velocities of Fe-C alloys at room temperature are illustrated in Fig. 1. The \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of bcc-Fe is in the range of 6.0-6.5 km/s and linearly increases as a function of pressure (0-7.0 GPa), while its \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e varies little and remains nearly constant at 3.5 km/s. Our results of bcc-Fe are consistent with prior work\u003csup\u003e37\u003c/sup\u003e. C doping fundamentally alters these properties of bcc-Fe. At pressures below 4.0 GPa, 0.2 wt.% C could increase the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of bcc-Fe by ~0.4 km/s, while it slightly decreases the \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e by 0.1-0.3 km/s. The effect of C on \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of bcc-Fe is the same as that of Si\u003csup\u003e38\u003c/sup\u003e. Light elements are generally supposed to increase the \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e of Fe\u003csup\u003e39-43\u003c/sup\u003e, however, we find that the incorporation of C could decrease the \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e of Fe. This effect is same as our recent results on the bcc-Fe-Si alloys\u003csup\u003e19\u003c/sup\u003e. Above 4.0 GPa, we observe a gradual decrease in the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of Fe-0.2C and the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e drops to ~5.7 km/s above 5.5 GPa. Its\u003cem\u003e V\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e, meanwhile, increases linearly with pressure below 4.0 GPa, but plateaus at ~3.4 km/s above 4.0 GPa. The acoustic velocity discontinuities in both \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e indicate a phase transition in Fe-0.2C around 5 GPa. The XRD characterization (Fig. 1b) shows that the bcc structure phase persists up to 7.3 GPa. The XRD and acoustic velocity results evidence that it is an isostructural phase transition (ISPT). We denote the low-pressure phase structure as bcc-I (\u0026lt;4.0 GPa) and the high-pressure phase as bcc-II (\u0026gt;5.5 GPa), respectively. At pressures between 4.0 GPa and 5.5 GPa, the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e lies between those of bcc-I and bcc-II, indicating a mixed-phase region of Fe-0.2C. A ~1.6%volume collapse is also observed at ~4.5 GPa, further confirming the ISPT in Fe-0.2C (Fig. 1c).\u003c/p\u003e\n\u003cp\u003eFurther, the starting Fe-1.7C sample was employed to clarify how varying carbon concertation affects the aforementioned ISPT. Fe-1.7C starts as a bcc-fcc mixture at room temperature. Below 4.5 GPa, the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e stays constant at 6.2 km/s and the \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e linearly increases from 2.5 km/s to 3.3 km/s (Fig. 1a). 1.7 wt.% C is supposed to significantly increase the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of bcc-Fe, but the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of bcc/fcc-Fe-1.7C is smaller than that of Fe-0.2C and comparable to that of pure bcc-Fe. Notably, 1.7 wt.% C reduces the \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e of Fe by 6-29%, which is much larger than the effect of 0.2 wt.% C (3-8%) likely due to both carbon doping and the fcc phase coexistence. Above 4.5 GPa, the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e suddenly drops to ~5.6 km/s and the \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e keeps constant at ~3.2 km/s. These results illustrate that the Fe-1.7C sample also undergoes an ISPT around 4.5 GPa at room temperature. We also obtained the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003es\u003c/sub\u003e\u003c/em\u003e of Fe-1.7C during decompression. The \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003es\u003c/sub\u003e\u003c/em\u003e are ~5.6 km/s and ~3.2 km/s above 4.0 GPa which are same with their compression values. Below 4.0 GPa, \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e suddenly increases to ~6.2 km/s and \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e drops to 2.5-3.3 km/s, which means both \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003es\u003c/sub\u003e\u003c/em\u003e recovers to the values in compression process below 4.5 GPa. The variation trend of \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003es\u003c/sub\u003e\u003c/em\u003e indicates that the ISPT is reversible.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMechanism of the ISPT in iron-carbon alloys\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFor decades, Fe is believed to present a liquid phase and four solid phases which are bcc (including and ), fcc, and hcp.A L-L phase transition in molten Fe-C was reported around 5 GPa\u003csup\u003e30-35\u003c/sup\u003e. Here one more isostructural phase, bcc-II, is observed in solid Fe with the presence of C. In order to investigate the mechanism of ISPT in Fe-C alloy, synchrotron Mӧssbauer spectra (SMS) and x-ray emission spectra (XES) were conducted. Fe-0.2C persists in ferromagnetic state up to 12.3 GPa and then transform to nonmagnetic above 15.4 GPa (Supplementary Fig. S4). The magnetic change can be attributed to the bcc-hcp phase transition. No macroscopic magnetic change is observed around 5 GPa, indicating macroscopic magnetic would not be the reason for ISTP in Fe-C alloys. In contrast, XES provides a way to probe local magnetic momentum. The \u003cem\u003eK\u003csub\u003e\u0026beta;\u003c/sub\u003e\u003c/em\u003e emission is due to a radiation decay of the 1\u003cem\u003es\u003c/em\u003e core hole from a 3\u003cem\u003ep\u003c/em\u003e level. Due to a net magnetic moment (\u003cem\u003em\u003c/em\u003e) in the 3\u003cem\u003ed\u003c/em\u003e valence shell, the \u003cem\u003eK\u003csub\u003e\u0026beta;\u003c/sub\u003e\u003c/em\u003e emission spectrum splits into a main line \u003cem\u003eK\u003csub\u003e\u0026beta;1,3\u003c/sub\u003e\u003c/em\u003e and a satellite line\u003cem\u003e K\u003csub\u003e\u0026beta;\u003c/sub\u003e\u0026rsquo;\u003c/em\u003e. The intensity of \u003cem\u003eK\u003csub\u003e\u0026beta;\u003c/sub\u003e\u003csup\u003e\u0026rsquo;\u003c/sup\u003e\u003c/em\u003e is proportional to the net spin of the 3\u003cem\u003ed\u003c/em\u003e shell of the transition metal. No satellite line\u003cem\u003e K\u003csub\u003e\u0026beta;\u003c/sub\u003e\u0026rsquo;\u003c/em\u003e presents at ~7045 eV in Fig. 2. The \u003cem\u003eK\u003csub\u003e\u0026beta;1,3\u003c/sub\u003e\u003c/em\u003e peak shifts to the lower energy as pressure increases, which can be attributed to the responses of Coulomb repulsion (U) to the pressure. We calculated the integrals of the absolute values of the difference spectra (IAD)\u003csup\u003e44\u003c/sup\u003e to analyze the shift of K\u003cem\u003e\u003csub\u003e\u0026beta;1,3\u003c/sub\u003e\u003c/em\u003e of Fe-0.2C by using the data at 11.5 GPa as the reference spectrum. The inset in Fig. 2 shows the IAD values of Fe-0.2C under high pressure conditions. The IAD value increases as a function of pressure below 4.6 GPa, while a sudden drop in the IAD is observed at 5-6 GPa. Above 5-6 GPa, the IAD value monotonously decreases as a function of pressure. The XES results show that Coulomb repulsion between Fe-Fe atoms suddenly decreases, which might be the cause of the observed ISTP in Fe with the presence of light element C in Fig. 1.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eHigh-temperature behaviors of Fe-C alloys\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe observed ISPT in Fe-1.7C with mixed bcc and fcc phases at room temperature indicates that the ISTP may also occur in fcc-Fe-C. We conducted high-temperature acoustic velocity measurements of fcc-Fe-C alloys up to 1300 K (Fig. 3). Above 1000 K, both Fe-0.2C and Fe-1.7C are in fcc phase (Supplementary Fig. S5 and S6). The \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of fcc-Fe-0.2C is 5.4 km/s below 4.5 GPa. It is about 1.0 km/s lower than that of its bcc-I counterpart. Temperature has negligible effect on \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e in these P-T conditions. However, significant temperature effect can be observed in the \u003cem\u003eV\u003csub\u003eS \u003c/sub\u003e\u003c/em\u003e(Supplementary Fig. S7). At 1100 K, the \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e is ~2.5 km/s, while it is ~2.2 km/s at 1300 K. We were not be able to obtain \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e datum for fcc-Fe-0.2C above 4.5 GPa, but the sudden \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e drop around 5.3 GPa indeed confirm the ISPT in fcc phase. The \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e is ~2.0 km/s at 1100 K, which is ~0.5 km/s lower than its lower pressure (\u0026lt;4.5 GPa) counterpart. We denote the ISPT in fcc phase as fcc-I and fcc-II. The fcc ISTP can be more clearly observed in Fe-1.7C. Below 4.0 GPa, the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e of fcc-I Fe-1.7C are ~5.0 km/s and ~2.6 km/s at 1100 K, respectively. While above 4.0 GPa, the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e drop to ~4 km/s and ~2.1 km/s, respectively. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eImplications for the lunar core\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eLunar core conditions (5-6 GPa and 1300-1900 K)\u003csup\u003e6\u003c/sup\u003e favor fcc-Fe in its inner core\u003csup\u003e45-48\u003c/sup\u003e. The \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of pure fcc-Fe has been measured up to 19 GPa and 1100 K\u003csup\u003e6\u003c/sup\u003e and extrapolated data to 1-1.5 GPa is in good agreement with our measurements. At lunar inner core conditions, the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of fcc-Fe is 6.0 km/s with estimated \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e of 3.0 km/s. In contrast, the Apollo seismic detection of the Moon\u0026rsquo;s inner core found that the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e are about 4.3 km/s and 2.3 km/s, respectively, which are significantly smaller than those of fcc-Fe. The contradiction leads to an outstanding issue in determining the composition of lunar core, especially considering the conventional wisdom that the incorporation of light elements would boost the acoustic velocities of solid iron\u003csup\u003e38-43\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e49\u003c/sup\u003e. S was suggested the main light element in the lunar outer core\u003csup\u003e7\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e20-22\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e50\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e51\u003c/sup\u003e. However, the content of sulfur in the lunar inner core would be negligible, because the partitioning coefficient of sulfur between solid and liquid iron increases from 0 at ~20 GPa to 0.1 at ~40 GPa\u003csup\u003e26-28\u003c/sup\u003e. The ISPT in Fe-0.2C and Fe-1.7C could induce a \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e collapse from 5.0-5.5 km/s to ~4-4.5 km/s and the \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e to ~2.1 km/s. These data are in good agreement with the seismic velocity profile of the lunar core when the data scattering is considered, which means the incorporation of 0.2-1.7 wt.% carbon into fcc-Fe could reproduce the Apollo seismic velocities of the lunar core above 5 GPa (Fig. 3). The carbon-rich lunar core deduced from the acoustic velocity of fcc-II Fe-C is also supported by phase equilibria experiments in Fe-S-C system\u003csup\u003e15\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e52\u003c/sup\u003e and mass balance calculations combined with metal-silicate partitioning and bulk silicate Moon abundance of carbon\u003csup\u003e29\u003c/sup\u003e. The melting experiments of low carbon content Fe-C-S ternary system\u003csup\u003e53\u003c/sup\u003e showed that the carbon partitioning coefficient in liquid and solid iron is around 0.5. If there is 0.2-1.7 wt.% carbon in Moon\u0026rsquo;s inner core, the carbon content in its liquid outer core would be 0.1-0.8 wt.% (Fig. 4). It is reported that ~4 wt.% sulfur is required to reproduce the \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e of the Moon\u0026rsquo;s outer core\u003csup\u003e7\u003c/sup\u003e. Then 0.1-0.8% carbon would co-exist with ~4 wt.% sulfur in Moon\u0026rsquo;s liquid outer core according to the phase equilibria in Fe-S-C system\u003csup\u003e15\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eCompared with \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e, the \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e of Fe-C is more sensitive to the temperature (Supplementary Fig. S7). The temperature-dependence of \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e can be employed to constrain the temperature of the lunar core if carbon is the sole light element. A reduction of ~0.7\u0026times;10\u003csup\u003e-3\u003c/sup\u003e km/K for \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e can be deduced from the results in Supplementary Fig. S7. The \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e of fcc-II Fe-C is around 2.1 km/s at 1300 K and lunar core pressure. Both \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e of fcc-II Fe-C at 1300 K could reproduce the seismic velocities of the lunar core. As temperature increases to 1500 K, although the\u003cem\u003e V\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e for fcc-II Fe-C is comparable to the Apollo seismic observation, the upper bound of \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e for fcc-II Fe-C is below the seismic velocity of the lunar core, indicating the temperature profile of the lunar core is likely smaller than 1500 K. We calculated the density of fcc-Fe-0.2C at 5 GPa and 1300-1500 K according to the thermal equation of state\u003csup\u003e54\u003c/sup\u003e, and constrain the density of lunar inner core at 7.71\u0026ndash;7.82 g/cm\u0026sup3; , which is in good agreement with the constraint from tidal deformation (7822\u0026plusmn;1615 kg/m\u003csup\u003e3\u003c/sup\u003e)\u003csup\u003e9\u003c/sup\u003e. \u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eSample preparation:\u003c/strong\u003e The starting Fe, Fe-0.2C and Fe-1.7C are synthesized by piston cylinder apparatus at 1 GPa and 1273 K with pure iron powder and graphite (purity 99.9%, purchased from Alfa Aesar Company). All sample are 2 mm in diameter and 5 mm in height. The chemical composition of the synthesized sample was analyzed using EPMA-8050G Electron Probe Microanalyzer at Shanghai Institute of Measurement and Testing Technology. The electron beam conditions are 15 kV and 200 nA with 10 s peak counting time and 10 s background counting time. Five well-characterized Fe-C alloys with carbon contents of 0.0067, 0.160, 0.382, 0.620, and to 0.870 wt.% were used as standards to establish a calibration curve for carbon measurement. Neither the sample nor the standards were coated by conductive layer in order to prevent carbon contamination, while charging was prevented by carbon tape. LiF/CH4 and PbST/CH2 were used for carbon and iron analysis, respectively. The measured carbon contents of the two synthesized samples range from 0.15-0.26 wt.% with an average of 0.2\u0026plusmn;0.06 wt.% (denoted as \u0026ldquo;Fe-0.2C\u0026rdquo;) and 1.65-1.81wt.% with an average of 1.7\u0026plusmn;0.11wt.% (denoted as \u0026ldquo;Fe-1.7C\u0026rdquo;) (Fig. S1).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcoustic velocity measurements:\u0026nbsp;\u003c/strong\u003eSynthesized samples were carefully cut to disks with thickness of ~0.7 mm. All samples were finely double-polished with 6 \u0026mu;m diamond lapping film. The final thicknesses of all samples were about 0.65 mm. The experiments were conducted using the Paris-Edinburgh press at 16-BM-B, High Pressure Collaborative Access Team (HPCAT) at the Advanced Photo Source, Argonne National Laboratory. Two types of PE cells are employed (Supplementary\u0026nbsp;Fig. S2). Sintered diamond buffering rod and SiO\u003csub\u003e2\u003c/sub\u003e backing were used for room temperature measurements, while alumina buffering rod and backing were used in the high temperature cell. The buffer rod and the backing were placed next to the sample, and ultrasonic signals were reflected between the interfaces. An MgO ring surrounded the sample and it was used to calibrate the pressure\u003ca href=\"#_ENREF_55\" title=\"Kono, 2014 #41\"\u003e\u003csup\u003e55\u003c/sup\u003e\u003c/a\u003e. The temperature was read by pre-calibrated power-temperature relations\u003ca href=\"#_ENREF_56\" title=\"Kono, 2010 #42\"\u003e\u003csup\u003e56\u003c/sup\u003e\u003c/a\u003e. Pulse-echo overlap technique was employed to obtain the ultrasonic \u003cem\u003eV\u003csub\u003eP\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eV\u003csub\u003eS\u003c/sub\u003e\u003c/em\u003e signals. An electrical sine wave was generated by waveform generator. Ultrasonic signals from 12 MHz to 40 MHz were collected with a digital oscilloscope (Tektronix DP05104). The wave was reflected in the buffer-rod/sample and sample/backing, which were R\u003csub\u003e1\u003c/sub\u003e and R\u003csub\u003e2\u003c/sub\u003e for both P-wave and S-wave (Supplementary Fig. S3). The travel time (\u003cem\u003et\u003c/em\u003e) of ultrasonic signal in the sample could be determined by R\u003csub\u003e1\u003c/sub\u003e and R\u003csub\u003e2\u003c/sub\u003e. Sample length (\u003cem\u003el\u003c/em\u003e) was measured by white X-ray radiography image. The acoustic velocity can be calculated by\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"609\" height=\"60\"\u003e\u003c/p\u003e\n\u003cp\u003eWhere \u003cem\u003el\u003c/em\u003e is the length of the sample, and \u003cem\u003et\u003c/em\u003e is the travel time of ultrasonic signals determined by R\u003csub\u003e1\u003c/sub\u003e and R\u003csub\u003e2\u003c/sub\u003e, respectively. The uncertainty of acoustic velocity is ~0.3%. The detailed method can be found in ref. \u003ca href=\"#_ENREF_55\" title=\"Kono, 2014 #41\"\u003e\u003csup\u003e55\u003c/sup\u003e\u003c/a\u003e. The energy-dispersed x-ray diffraction (EDXRD) patterns were collected by a Ge solid state detector at \u0026nbsp;. The collection time ranged from 30 s to 200 s.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eX-ray emission spectra (XES):\u003c/strong\u003e The XES were performed at 16-IDD, HPCAT. A symmetric diamond anvil cell with culet size of 300 \u0026mu;m was used to generate high pressure for XES measurement. A rhenium gasket was pre-indented at 20 GPa and a 150 \u0026mu;m chamber hole was drilled by laser ablation. A ~25 um thick Fe-0.2C pellet with diameter of ~70 um was loaded into the chamber. Silicon oil served as pressure medium. The pressure was determined by ruby fluorescence with an uncertainty of 0.1 GPa. All XES experiments were conducted at room temperature. An incident X-ray beam with an energy of 11.3 keV and a bandwidth of ~1eV was employed for the experiments. The collection time was ~40 minute for each spectrum.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China with grant\u003c/p\u003e\n\u003cp\u003eno. 42394114 and 42394111.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAUTHOR CONTRIBUTIONS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization, B.C., J. L., and M.H.; investigation, M.H., H.L., Y.K., J. L., and X.C.; visualization, M.H., H.L., and J. L.; formal analysis, M.H., H.L., Y.K., X.C., and J. L.; validation, Y.K., B.C. and J. L; funding acquisition, M.H., X.S., S.N.,and H.S.; project administration, M.H. and\u003c/p\u003e\n\u003cp\u003eX.S.; writing – original draft, M.H. and H.L.; writing – review \u0026amp; editing, all authors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDECLARATION OF INTERESTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eZhang, Q. 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Inter.\u003c/em\u003e \u003cstrong\u003e183\u003c/strong\u003e, 196-211 (2010).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7903808/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7903808/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe successful Chang'e mission has reignited scientific interests in the Moon, with analyses of returned samples advancing our understanding of lunar composition and evolution\u003ca href=\"#_ENREF_1\" title=\"Zhang, 2024 #437\"\u003e\u003csup\u003e1-4\u003c/sup\u003e\u003c/a\u003e. Pioneering models proposed that the Moon harbors a liquid iron-sulfur outer core and a pure face centered cubic (fcc) structured iron (Fe) inner core\u003ca href=\"#_ENREF_5\" title=\"Weber, 2011 #138\"\u003e\u003csup\u003e5-8\u003c/sup\u003e\u003c/a\u003e. However, the Apollo-era data described a lunar inner core with seismic velocity of 27% lower than that of fcc-Fe\u003ca href=\"#_ENREF_5\" title=\"Weber, 2011 #138\"\u003e\u003csup\u003e5\u003c/sup\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca href=\"#_ENREF_6\" title=\"Antonangeli, 2015 #6\"\u003e\u003csup\u003e6\u003c/sup\u003e\u003c/a\u003e, posting a long-standing challenge to understand the structure and chemistry of the lunar core. Here, we identify new phases of both bcc- and fcc-Fe that could resolve the discrepancy. In-situ high-pressure and high-temperature X-ray diffraction (XRD) and acoustic velocity measurements show that the Fe alloyed with 0.2-1.7 wt.% carbon (C) undergoes an isostructural phase transition around 4-5 GPa accompanied by a notable velocity drop of ~1 km/s. The seismic velocity profile of the lunar inner core can be directly matched by this new phase in Fe-C solid solution (hereafter denoted as fcc-II; conventional fcc-Fe is referred as fcc-I). These findings provide compelling evidence that the lunar inner core is composed of fcc-II Fe-C with a temperature potentially below 1500 K.\u003c/p\u003e","manuscriptTitle":"Carbon-rich lunar core evidenced by isostructural phase transitions in iron","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-13 10:42:37","doi":"10.21203/rs.3.rs-7903808/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"94a88d82-cb5c-452f-b9be-8debf27104f0","owner":[],"postedDate":"November 13th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":57926813,"name":"Earth and environmental sciences/Planetary science/Mineralogy"},{"id":57926814,"name":"Earth and environmental sciences/Solid Earth sciences/Seismology"}],"tags":[],"updatedAt":"2026-01-13T21:55:17+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-13 10:42:37","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7903808","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7903808","identity":"rs-7903808","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}