Formation of giant carbonatite rare earth deposits controlled by deep-seated magma chambers

Domain: neuroscience
Published: 2026-02-02
Editorial outcome: minor_revision

Abstract
Carbonatite-associated rare earth element (REE) deposits are currently the primary source of REE resources. Their formation requires REE enrichment during prolonged magma evolution, achieved by suppressing REE-rich mineral crystallization and promoting REE-enriched brine melt formation. Our experiments on the fractional crystallization of carbonatitic magmas indicate that pressure (emplacement depth) is the primary factor controlling REE enrichment. High-pressure (>0.3 GPa) promotes early olivine crystallization, depleting silica and suppressing REE-rich apatite formation. Deep emplacement also delays aqueous fluid exsolution, thereby stabilizing brine melts that enhance phosphate dissolution and prevent REE dispersion into apatite. In contrast, low-pressure conditions (<0.3 GPa) lead to exsolution of REE-poor hydrothermal fluids, dispersing REE into magmatic apatite and preventing the deposition of economically significant REE-carbonates in subsequent hydrothermal stages. Our pressure-dependent model highlights deep emplacement as crucial for passive REE enrichment in residual brine melts, driving large-scale mineralization through precipitation of burbankite and/or bastnäsite.

Introduction
Rare earth elements (REE) are indispensable for the global transition to clean energy and digital technologies, serving as vital components in permanent magnets, catalysts, and optoelectronic devices1–3. Carbonatites, the predominant source of these critical resources, account for more than half of the world’s REE reserves4,5. Global compilations document over 600 carbonatite occurrences, yet fewer than 5% demonstrate economic-grade REE mineralization6. The rare occurrence of REE mineralization in carbonatites reflects the extraordinary combination of some essential physical and chemical conditions, including an enriched mantle source7,8, passive REE enrichment through extreme differentiation of REE-poor minerals9,10, preferential partitioning of REE into carbonatitic melts during liquid immiscibility11,12, and a transition from carbonatitic melt to highly alkaline brine melts13,14. A key question is how the evolution of carbonatitic melts can both suppress the fractionation of REE-rich minerals to prevent REE dissipation and facilitate the formation of highly REE-enriched brine melts9,13,14.

Emplacement depth (i.e., pressure) is likely the key factor governing both the crystallization sequence and the stability of brine melt versus hydrothermal fluid, thereby controlling the enrichment of REE in carbonatites. First, pressure could determine whether a carbonatitic melt evolves into a REE-enriched brine melt or exsolves a REE-depleted hydrothermal fluid15. Brine melts form through extensive fractional crystallization of carbonatitic magmas, concentrating volatiles (H2O, F-, SO42-) and alkalis (Na, K) in a transitional phase between melt and hydrothermal fluid16,17. Notably, Na+ and CO32- serve as crucial transport agents in the brine melt, facilitating the dissolution and migration of REE14,15. Compared to hydrothermal fluids, brine melts serve as the predominant medium for REE transport and precipitation and are essential for forming large-scale REE mineralization14,15,18–20. Previous experiments revealed that pressure acts as a key control factor in the differentiation of carbonatitic melts, governing whether they evolve into brine melts or exsolved into low-salinity hydrothermal fluids15. Furthermore, pressure could also influence the capacity of REE to incorporate into REE-rich minerals (e.g., apatite) by controlling the composition of carbonatitic melts. Apatite is a common early-crystallizing phase in carbonatitic magmas and a major host mineral for REE21–23. Given the compatible behavior of REE in apatite (D=1‒10), its crystallization can deplete the melt in these elements13,24–27. Consequently, effective late-stage REE enrichment requires suppressing the early formation of REE-rich apatite. Previous studies reveal that the REE uptake capacity of apatite is strongly influenced by its composition, particularly its SiO2 and Na2O content25,27,28. Pressure can play an important role in determining the sequence of fractional crystallization29, which governs the compositional evolution of the carbonatitic melts. Therefore, it is plausible that pressure can indirectly modulate the incorporation of REE into apatite through its control on melt compositions.

In fact, global major carbonatite intrusions display variable emplacement depths, spanning from shallow subvolcanic environments (<3 km) to deep-seated plutonic regimes (>10 km)30–34. While emplacement depth theoretically controls key processes like crystallization sequences and fluid-melt stability, its impact on carbonatite-hosted REE mineralization remains poorly constrained. To examine the hypothesis that magma chamber emplacement depth controls REE mineralization in carbonatites, high temperature-pressure experiments were conducted in this study to simulate the crystallization differentiation of carbonatite melts under low- and high-pressure conditions. The experiments employed synthetic carbonatite compositions representative compositions under five distinct pressure regimes (0.2, 0.3, 0.4, 0.5 and 0.6 GPa), corresponding to mid- to upper-crustal depths. The starting material was first homogenized at 1000°C and then cooled from 1000 to 200°C, facilitating progressive differentiation of the carbonatitic melt. Detailed experimental protocols, including starting material compositions and precise run conditions, are fully documented in the Methods section and presented in Tables 1, 2.

Results
Mineral phase assemblages in experimental products
During progressive cooling, mineral crystallization propagates inward from the chamber margins, with phase assemblages recording a well-defined fractional crystallization sequence that correlates systematically with distance from the edge (Fig. 1a–e). Experiments conducted at 0.2 and 0.3 GPa using both piston-cylinder and gas-medium high-pressure apparatuses produced identical products (Supplementary Fig. S1). The experimental results reveal distinct mineral crystallization sequences between high-pressure (0.4–0.6 GPa) and low-pressure (0.2–0.3 GPa) conditions (Figs. 1 and 2). Under high pressure, the paragenetic sequence progresses from calcite (CaCO3) → olivine ((Mg,Fe)2SiO4) → apatite (Ca5(PO4)3(F,Cl,OH)) → fergusonite ((Dy,La)(Nb,Ta)O4) and/or pyrochlore ((Dy,Ca,La)2Nb2O6(OH,F)) → dolomite (MgCaCO3) and fuyuanite (MgNb2O6(OH)2) → hematite (Fe2O3) → sellaite (MgF2) → eitelite (Na2Mg(CO3)2) → bonshtedtite (Na3MgPO4CO3) → burbankite (Na3(La,Ca)3(CO3)5). In contrast, the crystallization sequence under low-pressure conditions follows calcite → apatite → periclase → olivine → fergusonite and/or pyrochlore → dolomite → hematite → nahcolite (NaHCO3). High-pressure experiments yield products that differ systematically from their low-pressure counterparts in three key aspects: (1) olivine crystallizes earlier than apatite, (2) lower abundance of apatite, (3) late-stage crystallization yielded abundant carbonate minerals enriched in REE and phosphorus, such as bonshtedtite, eitelite and burbankite. Electron microprobe mapping of experimental products further reveals differential enrichment of LREE (Supplementary Figs. S2–S6).

Notably, the late-stage assemblage of Na- and LREE-enriched carbonates is consistent with alkali- and LREE-enriched brine melts observed in natural carbonatite inclusions and experimental studies13,17,35–38. In contrast, only trace amounts of fine-grained nahcolite were observed in the sample capsules of the low-pressure experiments (<0.3 GPa), potentially representing a low-alkali, REE-depleted hydrothermal fluid (Fig. 2). Burbankite is a crucial LREE-carbonate, regarded as a precursor to bastnäsite13,14,18. Extensive evidence demonstrates that hydrothermal processes typically redistribute REE from primary igneous minerals (e.g., burbankite) to secondary alteration phases (e.g., bastnäsite)39–41. The occurrence of burbankite signifies extreme LREE enrichment during the late-stage evolution of carbonatitic melts. We observed that large volumes of burbankite grains form only in higher-pressure systems (0.4, 0.5 and 0.6 GPa), whereas in lower-pressure experiments (0.2–0.3 GPa), LREE are predominantly hosted in REE-rich apatite. The formation of REE-rich apatite leads to the dispersion of REE during melt crystallization differentiation. Our experiments demonstrated that 0.3–0.4 GPa represents a critical transition pressure for LREE mineralization in carbonatite.

Major and trace element characteristics of mineral phases
We conducted electron microprobe analyses (EPMA) to determine the major and trace element compositions of minerals, with detailed results presented in Supplementary Data 1‒7.

Apatite from different experiment runs exhibits significant variations in La2O3 and Dy2O3 contents (Supplementary Fig. S7). In high-pressure runs, the concentration ranges of La2O3 and Dy2O3 in apatite are 0.53‒1.73 wt.% (mean=0.88 wt.%) and 0.41‒1.16 wt.% (mean=0.72 wt.%), respectively; while in low-pressure runs, these ranges shift to 0.62‒2.83 wt.% (mean=1.20 wt.%) for La2O3 and 0.42‒3.26 wt.% (mean=1.38 wt.%) for Dy2O3. Overall, apatite formed under low-pressure conditions contains higher REE concentrations (Fig. 3). With progressive fractional crystallization and decreasing temperature, apatite shows a gradual increase in Na2O content accompanied by a continuous decrease in SiO2 content. Under lower pressure, the Na2O and SiO2 contents in apatite vary from 0.23‒3.09 wt.% and 0.01‒4.39 wt.%, respectively. Under higher pressure, they vary from 0.47‒1.90 wt.% and 0.09‒1.34 wt.%, respectively. The La2O3 and Dy2O3 concentrations in apatite exhibit a concave-upward trend (decreasing then increasing) with rising Na2O and SiO2 content (Fig. 3).

The carbonate minerals in the experimental system primarily include calcite, dolomite, eitelite and burbankite. It is noteworthy that burbankite occurs exclusively in the high-pressure experimental system, with its La2O3 content varying between 25.86‒32.15 wt.%. As magma undergoes fractional crystallization, the carbonate mineral assemblage evolves progressively from calcite to dolomite and finally to eitelite, which is consistent with the earlier findings of Anenburg and colleagues13. Notably, high-pressure experiments yield abundant eitelite grains, whereas these phases are absent in low-pressure experiments. The La2O3 and Dy2O3 contents in calcite, dolomite and eitelite range from 0‒0.47 wt.% and 0‒0.43 wt.%, respectively, while Nb concentrations remain extremely low. The study reveals a systematic correlation between REE content and the Mg/Ca ratio in carbonate minerals: REE content decreases with increasing Mg but increases with higher Ca (Supplementary Fig. S8). This trend aligns with previously reported partition coefficients between carbonate minerals and melts42,43.

The compositional ranges of olivine are 52.50‒56.97 wt.% for MgO, 41.28‒43.60 wt.% for SiO2, and 0.40‒4.26 wt.% for FeO. The MgO and SiO2 contents reflect the variations of Mg and Si in the equilibrated melt, with low-Mg, low-Si olivine crystallizing during the early stages of melt evolution, whereas high-Mg, high-Si olivine forms from carbonatitic melts at lower temperatures with higher Mg and Si contents (Supplementary Fig. S9). High-pressure experiments demonstrate that olivine crystallization initiates at early evolutionary stages, whereas under low-pressure conditions, olivine appears significantly later compared to high-pressure scenarios.

Niobium (Nb) and heavy rare earth elements (HREE) are typically associated in our experiments, forming minerals such as fergusonite and pyrochlore. Besides, niobium may also occur in the form of aeschynite. In fergusonite, Nb2O5 content ranges from 40.77 to 43.80 wt.%, while Dy2O3 is significantly enriched (46.97–50.74 wt.%). Pyrochlore exhibits higher Nb2O5 (48.59–55.26 wt.%) but lower Dy2O3 (19.77–25.50 wt.%) comparing to the fergusonite. In contrast, fuyuanite contains the highest Nb2O5 levels (58.58–59.63 wt.%) with negligible Dy2O3 content.

Fractional crystallization of carbonate, oxide, and silicate minerals results in REE enrichment
Carbonate minerals dominate the crystalline phases in natural carbonatites, exhibiting diverse phases such as calcite, dolomite, ankerite, siderite and alkali-rich carbonates10,44. Consistent with natural carbonatites, carbonate minerals (calcite, dolomite, bonshtedtite, eitelite and nahcolite) constitute the dominant crystalline phases in our experimental products. As the temperature in the experimental capsule decreases below the liquidus, calcite begins to crystallize from the carbonatitic melt. Existing data indicate that the partition coefficients of REE between calcite and carbonatitic melt are generally <1, with a decreasing trend as temperature declines45,46. In all our experiments, calcite exhibits low REE concentrations (La2O3: 0.1–0.5 wt.%; Dy2O3: 0.1–0.5 wt.%), lower than the initial REE content of the starting material (La2O3 and Dy2O3 =~0.5 wt.%). This confirms that calcite crystallization enriches REE in residual melts. Other carbonates such as dolomite and eitelite crystallize, exhibiting very low REE contents (0.002‒0.2 wt.%). Their crystallization significantly increases REE concentrations in the residual melt, creating optimal conditions for late-stage mineralization.

Olivine crystallizes early due to the limited silica solubility (<0.5 wt.%) in carbonatitic melts under crustal P-T conditions47. REE partition coefficients between olivine and carbonatitic melt are far less than 18. However, due to its low modal abundance, this effect for REE enrichment is geochemically negligible. Hematite can incorporate only trace amounts of REE; thus, its removal further enriches REE in the residual melt. In our experiments, we observed that Nb and HREE can form minerals such as fergusonite and aeschynite. The formation of these minerals may cause Nb and HREE to precipitate during the early stages of fractional crystallization. Consequently, the fractional crystallization of these minerals likely represents a critical mechanism governing both LREE-HREE fractionation (increase LREE/HREE ratios) and Nb-LREE decoupling.

Pressure controls the REE concentrations of apatite
During the crystallization differentiation of carbonatite melts, apatite serves as a crucial host mineral for REE23,24,26. Experimental studies on element partitioning show that REE exhibit compatible behavior in apatite, with partition coefficients between apatite and carbonatitic melt ranging from 1 to 1024,26. Thus, the extent of apatite crystallization and its capacity to incorporate REE govern whether REE can be efficiently enriched in late-stage melts. The incorporation of REE into apatite is governed by two principal substitution mechanisms24,25,27, both of which are strongly influenced by the SiO2 and Na2O contents of the apatite. The first mechanism involves coupled substitution: Ca2+ + P5+ → REE3+ + Si4+. The second mechanism operates through charge-balanced substitution: 2Ca2+ → REE3+ + Na+28. In both cases, higher concentrations of SiO2 and Na2O in apatite enhance the incorporation of REE into the crystal structure.

As shown in Fig. 3, apatite formed under high pressure (0.4‒0.6 GPa) contains markedly lower REE concentrations (mean = 0.88 wt.% for La2O3 and 0.72 wt.% for Dy2O3) than those formed at low pressures (0.2‒0.3 GPa; mean = 1.24 wt.% for La2O3 and 1.16 wt.% for Dy2O3). Consistent with the substitution mechanisms above9,25,27,28, high-pressure apatite also has a narrow range of SiO2 and Na2O contents. This compositional difference stems from a pressure-controlled shift in crystallization sequence. Under high-pressure conditions (>0.3 GPa), a significant portion of olivine crystallizes prior to apatite (as shown in the Fig. 1c–e). The early and fractional crystallization of olivine systematically depletes the melt in SiO28. Consequently, this results in the formation of apatite with low SiO2 content and lower REE concentrations (mean = 0.88 wt.% La2O3), as the primary Si-coupled substitution mechanism is suppressed9,25,27. In addition, the formation of brine melts under high pressure further limits the crystallization of Na- and REE-rich apatite, thereby reducing the overall proportion of apatite in the experimental products. This mechanism will be thoroughly examined in the following discussion section.

In contrast, at low pressures (0.2–0.3 GPa), a significant portion of apatite crystallizes prior to olivine (Fig. 1a, b). The compositional signatures of olivine under varying pressure conditions effectively record these contrasting crystallization sequences (Supplementary Fig. S9). Olivine formed under low pressure exhibit significantly limited MgO compositional variability (54.5‒56 wt.%) than their high-pressure counterparts (52.5‒57 wt.%). Consequently, these early, Si-rich and Na-rich apatites efficiently scavenge REE from the melt, incorporating high concentrations (mean=1.24 wt.% La2O3 and 1.16 wt.% Dy2O3). This premature removal locks a substantial fraction of the REE budget into a disseminated mineral phase, effectively depleting the melt and preventing significant late-stage enrichment.

Pressure controls phase separation: brine melt or hydrothermal fluid
Our experiments reveal that pressure governs the final physical state of the late-stage carbonatitic melt, including REE-depleted hydrothermal fluids and REE-enriched brine melts. Under high-pressure conditions (>0.3 GPa), water solubility in the carbonatitic melt increases significantly, preventing the exsolution of a separate aqueous fluid48. Instead, the carbonatitic melt evolves towards a F-P-REE-rich Na2CO3-dominated brine melt, as confirmed by our late-stage mineral assemblage (Figs. 1 and 2). The composition of residual liquid is consistent with previous experimental studies and melt inclusions from natural carbonatites13,17,35–38.

Notably, brine melts can effectively dissolve high phosphorus, fluorine and chlorine concentrations13,14, as demonstrated by our experimental products containing bonshtedtite and sellaite. Current experimental research also reveals that brine melts exhibit significantly higher REE solubility (up to 8 wt.%) compared to hydrothermal fluids18,49,50. This enables co-transport of fluorine-REE-phosphorus complexes within the melt18. This melt environment, rich in halogens and carbonates, is highly effective at complexing phosphate groups, thereby significantly increasing phosphorus solubility in the melt. The enhanced phosphorus solubility in this stage effectively limits Na- and REE-rich apatite crystallization (e.g., due to enrichment in complexing ligands like F⁻, Cl⁻, CO32-), preserving REE for late-stage mineralization.

In contrast, under low-pressure experimental conditions, the carbonatitic melt undergoes exsolution of a low-salinity hydrothermal fluid15. Such fluids have extremely low REE solubilities (<1 ppm at near-neutral pH)20, rendering them ineffective agents for REE transportation. Instead, a significant proportion of the REE are sequestered into abundant REE-rich apatite, which precipitates and becomes dispersed within the carbonatites, thereby preventing effective ore formation. Through hydrothermal diamond-anvil cell experiments with in situ observation, Yuan and coworkers15 established a critical pressure threshold of 0.3 GPa controlling phase separation in the Na2CO3 + H2O system: below this pressure, hydrothermal fluids coexist with solid Na2CO3, whereas above 0.3 GPa only brine melts form without the exsolution of hydrothermal fluids. Both their in situ observations and our experimental results demonstrate remarkable consistency, jointly revealing that emplacement depth plays a critical role in promoting brine melt formation and subsequent extreme REE enrichment during this stage.

Implications for giant carbonatite-host REE deposits formation
Our study demonstrates that emplacement depth is the primary controlling factor in the mineralization of carbonatite-hosted REE deposits. In deep crustal environments (>0.3 GPa), the suppression of REE-rich apatite crystallization and enhanced brine-melt formation collectively promote extreme REE enrichment in residual liquids. This mineralization style typically exhibits mineralogical transitions from burbankite to bastnäsite-group REE minerals, accompanied by fluid inclusion signatures of high salinity, homogenization temperatures and pressures32. Representative examples include the Palabora51, Maoniuping,52 Tova,53 Fen, Ambar Dongar, Oka32. Recent geophysical data revealing a massive magma chamber at depths >15 km beneath the Bayan Obo deposit furthers support this model31. In contrast, shallow emplacement (<0.3 GPa) leads to significant REE sequestration in apatite and monazite. In these scenarios, the enrichment of REE is terminated by the solidification of the carbonatitic melts, leading to REE dispersing in the carbonatitic rocks and generating only low-grade mineralization (e.g., Songwe Hill54 carbonatites) or barren carbonatites (e.g., Alnö33, Ol Doinyo Lengai55 and Laacher See ref. 56 carbonatites) (Fig. 4). It is noteworthy that estimating the emplacement depth of carbonatites remains a challenging task. The depths presented here are primarily derived from fluid inclusion analysis and correlation with geophysical data. A compilation of published depth estimates for various carbonatites is provided in Supplementary Data 8.

The proposed genetic model predicts that high-grade carbonatite-hosted REE deposits are more likely associated with complexes that exhibit geophysical signatures of deep-seated magma chambers. These signatures include high-gravity anomalies, high magnetic anomalies, deep low-velocity zones (LVZs) and low resistivity zones are interpreted as solidified fossil magma chambers31,33,44. Conversely, carbonatite systems lacking these deep-seated features might represent shallow-level intrusions that are more likely to form barren apatite-rich carbonatite rocks. This approach enables more effective discrimination of high‑grade exploration targets.

Methods
Piston-cylinder experiments
We conducted high P‒T experiments at the Guangzhou Institute of Geochemistry (GIGCAS) to investigate the fractional crystallization of carbonatitic melts under varying pressure conditions (0.2‒0.6 GPa). This study in particular focuses on the geochemical behavior of LREE (La), HREE (Dy) and Nb during carbonatite magma evolution. The experiments employed synthetic carbonatite compositions calibrated against natural systems under five distinct pressure regimes (0.2, 0.3, 0.4, 0.5 and 0.6 GPa, corresponding to mid- to upper-crustal depths). The chemical composition of the starting carbonatite powder is presented in Table 1, while the detailed experimental conditions and parameters are summarized in Table 2. The experimental procedure consisted of three sequential stages: (1) initial pressurization to target conditions, (2) isothermal holding at 1000°C for 24 h to achieve complete melt homogenization, and (3) controlled cooling at a rate of 5°C/hour (total duration: ~7 days) to 200°C to accurately simulate natural fractional crystallization kinetics. This multi-stage protocol ensured proper simulation of magmatic processes from initial melt formation through progressive crystallization.

All experiments (0.2‒0.6 GPa) were conducted using an end-loaded piston-cylinder apparatus with a pressure capacity of 0.2‒3 GPa. The pressure calibration was carefully performed for low-pressure conditions by employing both the NaCl melting curve57–59, ensuring high precision across the entire experimental pressure range. The ½-inch PC assembly included NaCl pressure media, crushable MgO spacers, and straight-walled graphite heaters, all encased in Pb foil. This setup served to protect the fragile NaCl assembly and minimize friction in the experimental assembly. For monitoring and controlling the temperature of the PC experiments, a type-S thermocouple was positioned axially with respect to the heater and placed adjacent to the capsule. Pressure and temperature uncertainties were estimated to be ±0.1 GPa and ±5°C, respectively.

Gas-medium high-pressure apparatus
The low-pressure experiments (0.2, 0.3 GPa) were also conducted in the High-Temperature and High-Pressure Hydrothermal Laboratory at the GIGCAS. Rapid-quench TZM (titanium zirconium molybdenum, Mo-based alloy) cold-seal pressure vessel assemblies (outer diameter: 30 mm, inner diameter: 6.5 mm, length: 250 mm) served as the reaction container, capable of stably achieving temperature and pressure conditions up to 1100°C and 300 MPa. The TZM vessel is mostly encased into a Hastelloy C276 sheath to protect it from oxidation at high temperatures. The other end of the vessel is joined with a stainless-steel (304) extension through a water-cooled double cone coupling, with 350 mm long and 30 and 6.5 mm outer and inner diameters, respectively. Ar gas was employed as the pressure medium, with system pressure continuously monitored in real time using a high-precision pressure gauge (±10 MPa accuracy). Temperature control was implemented via an electric furnace, while real-time temperature measurement was precisely executed using a NiCr-Ni (K-type) thermocouple connected to a high-precision electronic temperature controller (±5°C temperature control accuracy). The same three-stage experimental procedure—comprising pressurization, melt homogenization at 1000°C for 24 h, and slow cooling at 5°C/h—was applied to accurately replicate the fractional crystallization conditions of the piston-cylinder experiments.

Electron microprobe analysis
We used a JEOL JXA-8230 EPMA, and wavelength-dispersive spectrometry (WDS) to determine the concentrations of major elements in both minerals and quenched melts at the GIGCAS. Backscattered electron (BSE) imaging was utilized to observe the textural relationships among phases. The major element data of minerals are presented in Supplementary Data 1‒7, respectively. For the analyses, we used an accelerating voltage of 15 kV and a beam current of 10 nA, with a spot size of 10 μm for melt pools, and a focused spot size on mineral grains. All elements were counted for 10 s on peak and 5 s on each background. For the analyses of minerals and silicate melts, Si was calibrated on quartz, Na on albite, K on orthoclase, Mg on olivine, Fe on magnetite, P and Ca on apatite, F on BaF2, Cl on tugtupite, Nb on LiNbO3, La on LaP5O14, Dy on DyP5O14.

Electron probe microanalysis (EPMA) elemental mapping
High-resolution X-ray elemental intensity mapping was analyzed with a JEOL JXA-iSP100 Electron Probe Microanalyzer equipped with five wavelength-dispersive spectrometers (WDS) at the GIGCAS. Operating conditions for the X-ray mapping involved an accelerating voltage of 15 kV, a beam current of 100 nA, a step size of 2 μm and dwell time of 30 ms. Na, Mg, and Si were analyzed using a TAP crystal. F was analyzed using a LDE1 crystal. La were analyzed with a LIFL crystal. P, Ca, Nb were analyzed with a PETJ crystal. Cl were analyzed with a PETH crystal. Dy were analyzed with a LIFH crystal. Elemental Kα line was chosen for F, Na, Mg, Si, P, Ca, and Elemental Lα line was chosen for Nb and Dy.

TIMA analysis
The phase proportions of the experimental products were determined using TIMA (TESCAN Integrated Mineral Analyzer). The analysis was conducted with a MIRA3 scanning electron microscope at Tuoyan Testing Technology Co., Ltd. in Guangzhou. Prior to the experiment, the resin target samples were coated with carbon. During the analysis, the acceleration voltage was set to 25 kV, and the current was 8.24 nA. Both the current and BSE signal intensity were calibrated using an automated program with a platinum Faraday cup, while the EDS signal was calibrated using a manganese standard. The dissociation mode was employed during the test, simultaneously capturing both BSE images and EDS data. The pixel size was 3 μm, and the EDS step size was 9 μm.

Data availability
All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Overall Comments\nREE mineralization process is a hot topic recently. This manuscript presents a study on high-temperature and pressure experiments with natural observations, unequivocally identifying pressure (emplacement depth) as a primary controlling factor for REE mineralization in carbonatites. Lots of factors controlling small fraction of global carbonatites hosting giant REE deposits but were not clearly understood. The proposed \"deep-seated enrichment vs. shallow-level dispersion\" model provides more illumination. Meanwhile, still some comments and questions need to be fixed.\n1. At first, I do not think the Abstract and Introduction have been well written as the novelty is not presented in good pattern and nice logic. For examples, why pressure is a first-order control REE mineralization among many factors governing the formation of giant deposits.\n2. In the Abstract, more explicitly state that this model revises current models that overemphasize hydrothermal fluid immiscibility or apatite fractionation, establishing the primacy of pressure/depth.\n3. In the final paragraph of the Introduction (line ~67), specify \"other, potentially more critical factors\" as \"emplacement depth (pressure) and its control on the crystallization sequence and fluid/melt stability\" to more clearly introduce the aim of this study.\n4. This is the key problem of the ms. Fluent logic is lacking throughout the whole manuscript. The logical chain is largely complete, but the connectivity of a few key links could be tighter. For examples, high-pressure apatite suppression mechanism: The chain \"early olivine crystallization-silica depletion-suppression of REE-rich apatite\" relies on a critical implicit assumption that silica activity is the primary limiting factor for REE-rich apatite crystallization. Is there some literature or your data to support the view that \"apatite crystallization in carbonatitic systems is strongly controlled by silica activity\"\n5. Lines 273-274 state that the saline melt can \"enhance phosphate dissolution... effectively limits Na- and REE-rich apatite crystallization.\" A brief mechanistic explanation of why the saline melt enhances phosphate solubility (e.g., due to enrichment in complexing ligands like F⁻, Cl⁻, CO₃²⁻) would make this logical step smoother.\n6. lines 29-30 or in the relevant discussion, references or statement explicitly supporting the control of silica activity on apatite saturation are necessary.\n7. After line 273, a sentence is needed such as: \"This melt environment, rich in halogens and carbonates, is highly effective at complexing phosphate groups, thereby significantly increasing phosphorus solubility in the melt.\"\n8. Integration and Focus of Results and Discussion. The current \"Results\" section (e.g., \"Mineral Phase Assemblages,\" \"Major and Trace Element Characteristics\") contains extensive descriptive data, some of which includes interpretive statements that could be moved to the Discussion. Move interpretive statements like \"We propose...\" to the Discussion.\n9. Discussion Section: Use the facts from the Results to build a concentrated, in-depth discussion of the two core mechanisms: \"Pressure-Controlled Crystallization Sequence\" and \"Pressure-Controlled Phase Separation (Saline Melt vs. Hydrothermal Fluid).\" Integrate the mechanistic analysis currently scattered in the Results here to create a more forceful argument.\n10. Figure 3: The label \"Pressure (Gpa)\" should be corrected to \"GPa\". The \"black arrow\" indicating the \"direction of magmatic crystallization differentiation\" in panels c-f deserves a more detailed explanation in the figure caption (e.g., what petrological process does this trend represent?).\n11. Figure 4: The compilation of global cases is very convincing. Consider using different symbols/colors in the plot or caption to explicitly distinguish deep-seated (>10 km) from shallow-level (<10 km) deposits, creating a direct visual link to the model in the text.\n12. The conclusion effectively summarizes the findings. The exploration implications could be made more concrete to enhance the paper's applied value.\n13. At the end of the Abstract (lines 38-39) and in the main conclusions, specify what \"integrated geophysical exploration\" entails under this model. For instance, state that this model predicts high-grade deposits are more likely associated with carbonatite complexes showing geophysical signatures of deep-seated magma chambers (e.g., high gravity anomalies, deep low-velocity zones), whereas shallow systems might only form barren apatite-rich rocks. This provides a direct theoretical basis for interpreting geophysical data.\n________________________________________\nMinor comments:\n1. Line 27: the primary factor controlling\n2. Line 175: resolving or exsolving?\n3. Line 282: exposition or exsolution?\nAnyway, please strengthening the logic and optimizing figure presentation.", "reviewer_2": "Very interesting experiments modelled on what we did but at varied pressure. This is a very good idea, and I applaud them for doing these and regret we did not.\n.\nThe experiments are great and worthy of publication but the weakness here is pressure.\nAll of us working on carbonatites and REE would LOVE to be able to accurately estimate pressure but, alas, we can’t. How then was Fig 4 derived? Simply based on guesses by previous authors? I am VEY sceptical of the reported depths.\nAlso, barren carbonatites may just be carbonatites that have not been explored. How do we know what hides under Lengai?\nRegarding pressure control, when I moved to ANU 31 years ago to learn how to do piston cylinder experiments, I was told not to do them below 5kbar. Since then, we have improved things considerably but at 2kbar I do not feel terribly certain of the pressure. Given this, I’d like the authors to explain how certain they are of 2kbars being considerably different than 3 kbar. To make things even less clear they state, in the experimental section, uncertainties of 1 kbar! Which means 2 kbar experiments are somewhere between 1 and 3……. Why not do an experiment at 20kbar just to be sure?\nAnother thing: These experiments contain hematite without magnetite, ankerite or siderite. This tells me fO2 wasn’t buffered. How much does this affect the results? In our experiments we put in a piece of graphite to keep us buffered at CCO.\nI made specific suggestions on the manuscript.\nSo, although I love the concept, this needs major improvements before I could recommend publication.\nJohn Mavrogenes"}, "author_response": "We sincerely thank you for your positive and constructive feedback. We are especially gratified by your recognition of our study’s clear demonstration of pressure as a key control on REE mineralization, as well as the significance of the proposed genetic model. Your insightful comments have significantly enhanced the logical flow, readability, and overall impact of our manuscript. All raised points have been addressed point-by-point in our responses below, with corresponding revisions integrated into the manuscript.\nAt first, I do not think the Abstract and Introduction have been well written as the novelty is not presented in good pattern and nice logic. For examples, why pressure is a first-order control REE mineralization among many factors governing the formation of giant deposits. In the Abstract, more explicitly state that this model revises current models that overemphasize hydrothermal fluid immiscibility or apatite fractionation, establishing the primacy of pressure/depth.\nResponse: Thanks for your suggestion! The Abstract and Introduction sections have now been mostly reorganized and rewritten. The revised Abstract and Introduction now better emphasize the novelty and logical flow of our research.\nWe have revised the Abstract to explicitly state that “pressure primarily controls REE enrichment through two key mechanisms: the sequence of mineral crystallization and the nature of phase separation into either a hydrothermal fluid or a brine melt.” In lines 41‒43, we also highlighted the key role of deep emplacement, concluding that “Our pressure-dependent model highlights that deep emplacement is crucial for suppressing REE-scavenging apatite crystallization and promoting the formation of REE-enriched brine melts.”\nIn the opening paragraph of the Introduction, we summarized the key question for carbonatite-related REE mineralization as follows: “A key question is that the formation of these economically significant REE deposits critically requires that the evolution of carbonatitic melts both suppresses the fractionation of REE-rich minerals to prevent REE dissipation and facilitates the formation of highly REE-enriched brine melts [1, 2, 3].” Meanwhile, we have added content regarding the critical role of suppressing apatite crystallization differentiation in the late-stage enrichment of REEs. As detailed below:\n“Apatite is a common early-crystallizing phase in carbonatitic magmas and a significant host mineral of REE [4, 5, 6]. Given the compatible behavior of REE in apatite (D = 1‒10), its crystallization can deplete the melt in these elements [3, 7, 8, 9, 10]. Consequently, effective late-stage REE enrichment requires suppressing the early formation of REE-rich apatite. Previous studies reveal that the REE uptake capacity of apatite is strongly influenced by its composition, particularly its SiO and Na O content [8, 10, 11]. Pressure can play an important role in determining the sequence of fractional crystallization [12], which governs the compositional evolution of the carbonatitic melt. Therefore, it is plausible that pressure can indirectly modulate the incorporation of REEs into apatite through its control on melt compositions.”\n1. In the final paragraph of the Introduction (line ~67), specify “other, potentially more critical factors” as “emplacement depth (pressure) and its control on the crystallization sequence and fluid/melt stability” to more clearly introduce the aim of this study.\nResponse: Thanks! We have revised the Introduction as recommended (lines 64‒66), explicitly stating that the “Emplacement depth (i.e., pressure) is likely the key factor governing both the crystallization sequence and the stability of the brine melt versus hydrothermal fluid, thereby controlling the enrichment of REE in carbonatites.”\n2. This is the key problem of the ms. Fluent logic is lacking throughout the whole manuscript. The logical chain is largely complete, but the connectivity of a few key links could be tighter. For examples, high-pressure apatite suppression mechanism: The chain “early olivine crystallization-silica depletion-suppression of REE-rich apatite” relies on a critical implicit assumption that silica activity is the primary limiting factor for REE-rich apatite crystallization. Is there some literature or your data to support the view that “apatite crystallization in carbonatitic systems is strongly controlled by silica activity.”\nResponse: Thanks for your constructive suggestions. We have addressed this in both the Introduction and Discussion sections by detailing how silicon activity governs the formation of REE-enriched apatite, supported by citations from the existing literature.\nThe silicon and sodium contents of apatite govern the capacity of apatite to host REEs [3, 7, 8, 9, 10]. In general, higher Si and Na concentrations in apatite lead to increased REE uptake, primarily through two substitution mechanisms: Ca²⁺ + P⁵⁺ → REE³⁺ + Si⁴⁺ and 2Ca²⁺ → REE³⁺ + Na⁺ [11]. Both mechanisms are observed in our fluorapatite experiments and are further supported by the positive correlation between Si content and REE concentration in natural carbonatite-hosted apatite [7].\nIn the revised manuscript, we further emphasize that low-pressure conditions promote the formation of Si- and Na-rich apatite, which incorporates more REEs. Furthermore, the absence of phosphate-rich brine melt stage allows a greater proportion of phosphorus to be incorporated into the apatite, resulting in a higher mass fraction of crystallized apatite compared to high-pressure conditions. However, these processes lead to the dispersal of REEs within the carbonatites, thereby limiting the formation of large-scale, high-grade REE deposits. In contrast, high-pressure conditions offer a dual advantage: first, apatite crystallizing under such conditions contains less Si and Na, reducing its REE uptake capacity; and second, the formation of phosphate-rich brine melts further suppresses apatite crystallization. We have clarified this interpretation in the revised text and added appropriate references to strengthen the logical progression of our argument.\n5. Lines 273-274 state that the saline melt can “enhance phosphate dissolution... effectively limits Na- and REE-rich apatite crystallization.” A brief mechanistic explanation of why the saline melt enhances phosphate solubility (e.g., due to enrichment in complexing ligands like F⁻, Cl⁻, CO₃²⁻) would make this logical step smoother.\nResponse: Thanks! We agree with you that a mechanistic explanation strengthens the argument. As suggested, we have revised the manuscript (originally lines 273–274, now lines 277–281) to include a brief explanation that “This melt environment, rich in halogens and carbonates, is highly effective at complexing phosphate groups, thereby significantly increasing phosphorus solubility in the melt. The enhanced phosphorus solubility in this stage effectively limits Na- and REE-rich apatite crystallization (e.g., due to enrichment in complexing ligands like F⁻, Cl⁻, CO 2⁻), preserving REEs for late-stage mineralization.”\n3\n6. lines 29-30 or in the relevant discussion, references or statement explicitly supporting the control of silica activity on apatite saturation are necessary.\nResponse: Thanks! As addressed in our response to the second question, we have expanded the discussion in both the Introduction and Discussion sections titled “Pressure controls the REE concentrations of apatite” to investigate how the silica activity governs the incorporation of REEs into apatite. Besides, we have added key references that explicitly document the control of silica activity on REE-rich apatite crystallization in carbonatitic magmas. These citations provide direct support for this foundational aspect of our model.\n7. After line 273, a sentence is needed such as: “This melt environment, rich in halogens and carbonates, is highly effective at complexing phosphate groups, thereby significantly increasing phosphorus solubility in the melt.”\nResponse: Thanks! We have inserted the proposed sentence. The sentence, “This melt environment, rich in halogens and carbonates, is highly effective at complexing phosphate groups, thereby significantly increasing phosphorus solubility in the melt”, has been added at lines 277‒279 as recommended.\n8. Integration and Focus of Results and Discussion. The current “Results” section (e.g., “Mineral Phase Assemblages,” “Major and Trace Element Characteristics”) contains extensive descriptive data, some of which includes interpretive statements that could be moved to the Discussion. Move interpretive statements like “We propose...” to the Discussion.\nResponse: Thanks! We have now carefully revised the “Results” section to focus strictly on the data, removing all interpretive statements. These interpretive statements have been incorporated into the “Discussion” section to ensure a clear separation between data presentation and scientific interpretation.\n9. Discussion Section: Use the facts from the Results to build a concentrated, in-depth discussion of the two core mechanisms: “Pressure-Controlled Crystallization Sequence” and “Pressure-Controlled Phase Separation (Saline Melt vs. Hydrothermal Fluid).” Integrate the mechanistic analysis currently scattered in the Results here to create a more forceful argument.\nResponse: Thanks! We have thoroughly reorganized the Discussion section to build a concentrated and in-depth argument around the two core mechanisms: “Pressure-Controlled Crystallization Sequence” and “Pressure-Controlled Phase Separation.” The mechanistic analyses previously scattered in the Results have been integrated here, allowing us to synthesize the facts into a more forceful and coherent narrative.\n10. Figure 3: The label “Pressure (Gpa)” should be corrected to “GPa”. The “black arrow” indicating the “direction of magmatic crystallization differentiation” in panels c-f deserves a more detailed explanation in the figure caption (e.g., what petrological process does this trend represent?).\nResponse: Thanks! As suggested, we have modified the pressure units to GPa in Figure 3 and provided a detailed description of the black arrows in the figure caption. The black arrows in Figure 3 show a clear geochemical trend of fractional crystallization as temperature decreases. The SiO and Na O contents in apatite effectively reflect their concentrations in the coexisting carbonatitic melt, which can serve as a precise fingerprint of carbonatitic melt evolution (as shown in the black arrow). With the progression of crystal fractionation, the SiO content gradually decreases while the Na O progressively increases in both the carbonatitic melt and co-crystallizing apatite. The decrease in SiO attests to the continuous removal of early-forming silicate minerals (e.g., olivine). Simultaneously, the increase in Na O highlights the incompatible nature of Na O during crystallization fractionation, which is progressively enriched in the evolving residual melt.\n11. Figure 4: The compilation of global cases is very convincing. Consider using different symbols/colors in the plot or caption to explicitly distinguish deep-seated (>10 km) from shallow-level (<10 km) deposits, creating a direct visual link to the model in the text.\nResponse: Thanks for your good suggestion. According to your advice, we have revised Figure 4 to use distinct symbols and colors to differentiate deep-seated (>10 km) and shallow-level (<10 km) deposits, as detailed in the revised figure caption. This revision provides immediate visual support for our depth-based model (As shown in Fig. 1 below).\nFig. 1. Emplacement depth controls the formation of carbonatite-related REE deposits.\n12. The conclusion effectively summarizes the findings. The exploration implications could be made more concrete to enhance the paper’s applied value.\nResponse: Thanks! We have enhanced the main conclusion by incorporating the use of geophysical signatures of carbonatite complexes to facilitate the deep exploration for carbonatite-hosted REE deposits. Our findings establish pressure (depth) as a first-order control, providing a genetic basis for a more concrete and effective exploration strategy.\nWe propose that integrated geophysical surveys should prioritize carbonatite complexes exhibiting signatures of deep-seated magmatic systems. Key targets include geophysical anomalies such as high-gravity anomalies, high magnetic anomalies, deep low-velocity zones (LVZs) and low resistivity zones [13, 14, 15]. In contrast, shallow systems lacking these deep signatures are interpreted as lower-priority targets, likely hosting only barren, apatite-rich rocks. This refined approach enables a more effective discrimination of high-grade REE exploration targets.\n13. At the end of the Abstract (lines 38-39) and in the main conclusions, specify what “integrated geophysical exploration” entails under this model. For instance, state that this model predicts high-grade deposits are more likely associated with carbonatite complexes showing geophysical signatures of deep-seated magma chambers (e.g., high gravity anomalies, deep low-velocity zones), whereas shallow systems might only form barren apatite-rich rocks. This provides a direct theoretical basis for interpreting geophysical data.\nResponse: Thanks! We have revised the Abstract and main conclusions as advised, explicitly linking high-grade mineralization to deep-seated carbonatite complexes with specific geophysical signatures (e.g., high gravity anomalies, high magnetic anomalies, deep low-velocity zones and low resistivity zones) and distinguishing them from barren shallow systems.\nWe have added and clarified in the abstract that “Consequently, geophysical exploration should target carbonatite complexes with signatures of deep-seated magma chambers (e.g., high gravity anomalies and deep low-velocity zones), as shallow systems tend to be barren.” More specifically, in the chapter “Implications for Giant Carbonatite-hosted REE Deposit Formation,” we provide a detailed explanation of what integrated geophysical exploration entails within this model. The proposed genetic model predicts that high-grade carbonatite-hosted REE deposits are more likely associated with complexes that exhibit geophysical signatures of deep-seated magma chambers. These signatures include high-gravity anomalies, high magnetic anomalies, deep low-velocity zones (LVZs) and low resistivity zones are interpreted as solidified fossil magma chambers [13, 14, 15]. Conversely, carbonatite systems lacking these deep-seated features might represent shallow-level intrusions that are more likely to form barren apatite-rich carbonatite rocks. This approach enables more effective discrimination of high‑grade exploration targets.\nMinor comments:\n1. Line 27: the primary factor controlling\nRevised.\n2. Line 275: resolving or exsolving?\nResponse: Thanks! The expression “the carbonatitic melt undergoes exsolution of a hydrothermal fluid” is reasonable.\n3. Line 285: exposition or exsolution?\nResponse: Thanks! We find “exsolution” to be an accurate term for the phenomenon wherein a carbonatitic melt resolves a hydrothermal fluid upon reaching fluid saturation in its late evolutionary stage.\nReviewer #2 (Remarks to the Author): Very interesting experiments modelled on what we did but at varied pressure. This is a very good idea, and I applaud them for doing these and regret we did not. The experiments are great and worthy of publication but the weakness here is pressure. All of us working on carbonatites and REE would LOVE to be able to accurately estimate pressure but, alas, we can’t. How then was Fig 4 derived? Simply based on guesses by previous authors? I am VEY skeptical of the reported depths. Also, barren carbonatites may just be carbonatites that have not been explored. How do we know what hides under Lengai? Regarding pressure control, when I moved to ANU 31 years ago to learn how to do piston cylinder experiments, I was told not to do them below 5kbar. Since then, we have improved things considerably but at 2kbar I do not feel terribly certain of the pressure. Given this, I’d like the authors to explain how certain they are of 2kbars being considerably different than 3 kbar. To make things even less clear they state, in the experimental section, uncertainties of 1 kbar! Which means 2 kbar experiments are somewhere between 1 and 3……. Why not do an experiment at 20 kbar just to be sure?\nAnother thing: These experiments contain hematite without magnetite, ankerite or siderite. This tells me fO was not buffered. How much does this affect the results? In 2\nour experiments we put in a piece of graphite to keep us buffered at CCO. I made specific suggestions on the manuscript. So, although I love the concept, this needs major improvements before I could recommend publication.\nJohn Mavrogenes\nWe sincerely thank Professor John Mavrogenes for the positive assessment of our experimental design and insightful comments, which have been invaluable in strengthening this manuscript. In response to the crucial issue regarding pressure calibration in piston-cylinder experiments, the influence of oxygen fugacity, and the determination of carbonatite emplacement depths, we have undertaken substantial revisions to the manuscript. Below is our point-by-point response to the specific points raised and the improvements implemented in the revised version.\n1. Validation of Pressure’s Dominant Role: Low-Pressure (0.2-0.3 GPa) Experiments in a Gas-Medium Apparatus\nResponse: Thanks, very professional question! We acknowledge that achieving high precision at lower pressures in piston cylinder experiments is challenging. To further validate our findings, we conducted additional experiments at 0.2 and 0.3 GPa using a gas-medium high-pressure apparatus, which is fully capable of maintaining pressure up to 300 MPa (with an accuracy of ±10 MPa) and temperatures up to 1100 °C (±5°C). A detailed description of the gas-medium high-pressure apparatus experiments has been added to the Methods section. The excellent reproducibility of the results between this apparatus and the piston-cylinder device, strongly corroborates the robustness of our experimental data. This gives us high confidence in the reliability of our main conclusions regarding the pressure-dependent trends.\nThis consistency is primarily reflected in the absence of abundant rare earth minerals such as burbankite during the late crystallization stage (Fig. 2 and Fig.1 in the main text). Moreover, composition data of apatite show high REE contents, indicating that a significant portion of REEs remained dispersed in apatite throughout crystallization, thereby suppressing late-stage REE enrichment (Fig. 2 in the main text). These observations further reinforce the validity of our conclusion. While the proposed 2.0 GPa experiment is an excellent suggestion for future work to extend our model to greater depths, it is beyond the scope of our current study. Our work is specifically focused on establishing the pressure trend within the upper-to-mid crustal range most relevant to these deposits.\nFig. 2. TIMA mineral identification maps of the experimental runs (0.2-1000-200-New and 0.3-1000-200-New). The experiments at 0.2 and 0.3 GPa were performed using a gas-medium high-pressure apparatus. The results of the supplementary experiments were largely consistent with those of the piston-cylinder low-pressure series, with a notable absence of extensive burbankite crystallization in the experimental products.\n2. The Potential Effect of Oxygen Fugacity\nResponse: Thanks! In our original experiments, the oxygen fugacity was not externally buffered but was set by the redox state of the starting materials. The reviewer also raised an interesting point, leading us to perform additional experiments at 0.5 GPa and 0.2 GPa under CCO buffer conditions. The 0.5 GPa and 0.2 GPa experiments were conducted using a piston-cylinder and a gas-medium high-pressure apparatus, respectively. Consistent with our earlier experiments, high-pressure conditions (0.5 GPa) led to the crystallization of abundant rare earth minerals such as burbankite in the late evolutionary stage (Fig. 3). In contrast, under low-pressure conditions, a significant portion of the rare earth elements was dispersed in early-crystallizing phases in the form of REE-rich apatite, thereby suppressing late-stage REE enrichment. The results conclusively confirm that while redox conditions affect iron-bearing mineralogy, they do not change the overriding control of pressure on the late-stage enrichment of REEs.\nThe iron-bearing mineral assemblage under reducing conditions (CCO buffer) is characterized by the crystallization of magnetite and ankerite, in contrast to the oxidizing conditions, which produce hematite.\nFig. 3. TIMA mineral identification maps of the experimental runs (0.2-1000-200-C and 0.5-1000-200-C). The primary difference between the 0.5 GPa and 0.2 GPa experimental products lies in the abundant crystallization of minerals such as burbankite and bonshtedtite, indicating the occurrence of a brine melt stage and late-stage enrichment of REEs. Under CCO buffer conditions, iron oxides predominantly exist as magnetite, accompanied by the presence of minerals such as ankerite.\nThe robustness of our conclusion against redox variations can be understood by considering two key points: first, the redox conditions do not alter the capacity of apatite to incorporate REEs; and second, they do not affect the conditions required for the formation of REE-rich brine melts. This finding robustly demonstrates that the dominant control on REE enrichment is pressure, not oxygen fugacity. Given that the specific role of oxygen fugacity falls beyond the primary scope of this study, we have included the corresponding experimental results and a detailed discussion in the “Response to Editor and Reviewers” document for your consideration.\n3. On the uncertainty of pressure estimates in natural systems and the nature of barren carbonatites.\nResponse: We agree that accurately estimating the pressure (depth) of formation for carbonatite complexes is a significant challenge and a source of uncertainty in the field. The depths plotted in Figure 4 are indeed based on the available estimates from the published literature for each specific deposit, which typically rely on a combination of methods such as fluid inclusions and association with crustal-scale geophysical data. The literature on depth estimates of different carbonatites is summarized in the supplementary Table S8. We acknowledge the inherent uncertainty in these estimates. Regarding the comment on barren carbonatites, we agree that a surface expression being barren does not unequivocally preclude mineralization at depth. Our model does not claim that all surface-barren complexes are unmineralized at depth. Instead, it provides a genetic framework and an exploration prioritization tool. It predicts that complexes exhibiting geophysical signatures of a deep-seated root are more likely to host significant REE mineralization at depth, whereas shallow systems that also lack such deep signatures are assigned a lower exploration priority.\nMinor comments:\nLine 2 chambers\nRevised.\nLine 26 investigated the fractional crystallization of …\nRevised.\nLine 27 control of\nRevised.\nLine 30-31 inhibits change to delays, stabilizing the occurrence of brine melts?\nRevised.\nLine 32 REEs change to REE\nRevised.\nLine 33 residual liquids? Are brine melts residual? Or are water-rich fluids residual?\nRevised.\nLine 42 have become change to are\nRevised.\nLine 58 is change to are\nRevised.\nLine 67 critical?\nRevised.\nLine 68-70 how do you determine depth?\nResponse: The emplacement depths of carbonatites are primarily inferred from previous studies, which rely on barometry of fluid inclusions and interpretations of geophysical anomalies (e.g., low seismic velocity and low electrical resistivity). We acknowledge the inherent uncertainties in these depth estimates. Nevertheless, a positive correlation is observed, whereby systems emplaced at greater depths generally exhibit higher potential for REE mineralization.\nLine 74-75 are those the same?\nResponse: Thanks! We have rephrased the previous repetitive expression in the revised version of the manuscript as follows: However, the potential impacts of depth-dependent parameters (e.g., crystallization sequence and fluid/melt stability) on REE fractionation and enrichment in carbonatites remain poorly constrained.\nLine 83 underwent controlled cooling change to cooled\nRevised.\nLine 90 deleted mineral\nRevised.\nLine 101 periclase\nRevised.\nLine 103-105 But there no other P phase?\nResponse: Thanks! We identified phosphorus-rich minerals other than apatite, specifically bonshtedtite (Na MgPO CO ). This mineral exclusively crystallizes during the late stages of magmatic evolution under high-pressure conditions and contains low abundances of REEs.\n3 4 3\nLine 108 Where?\nResponse: Thanks! We have provided Figs S2-S6 as part of the Supplementary materials.\nLine 111 trace amounts of\nRevised.\nLine 112 chamber cavities?\nRevised.\nLine 119 volumes … form\nRevised.\nLine 121 but low p low apatite (line 105)?\nResponse: Thanks! We describe in the text that a higher proportion of apatite crystallizes under low-pressure conditions compared to high-pressure conditions. This is primarily because high-pressure conditions promote the evolution of brine melts, which dissolve a significant amount of phosphorus and thereby suppress the formation of apatite.\nLine 127 analyses?\nRevised.\nLine 136 demonstrates change to contains\nRevised.\nLine 146-147 we had kaugaukende at 2kbar\nResponse: Our experimental results and repeated tests using a gas-medium high-pressure apparatus both indicate that significant amounts of burbankite form only in high-pressure experiments at 0.4–0.6 GPa, while REEs enrichment is not prominent in low-pressure experiments.\nLine 157 delete the\nRevised.\nLine 161-162 2 digits necessary?\nRevised.\nLine 165 with change to at, but change to with.\nRevised.\nLine 170 delete fractional and the\nRevised.\nLine 174 delete with each other\nRevised.\nLine 194 like change to such as\nRevised.\nLine 195 fractionation change to crystallization\nRevised.\nLine 210 apatite controls late-stage REE enrichment\nRevised.\nLine 229 change to crystal fractionation\nRevised.\nLine 230 delete content\nRevised.\nLine 232 deleted its\nRevised.\nLine 238 change to crystallize\nRevised.\nLine 297 pressures\nRevised.\nLine 299 chambers\nRevised.\nLine 313 in particular focuses on\nRevised.\nLine 321 holding at\nRevised.\nLine 330 consisted of change to included, medium change to media\nRevised.\nLine 331 heaters and deleted a\nRevised.\nLine 335 how to estimated 2 vs 3 and 4 vs 5?\nResponse: Thanks! As mentioned earlier, our new low-pressure experiments conducted using the gas-medium high-pressure apparatus yielded results largely consistent with those from piston-cylinder experiments under low-pressure conditions. This confirms the reliability of our current experimental data.\nLine 343 delete sediment\nRevised.\nLine 345 delete the\nRevised.\nLine 346-347 quartz and tugtupite\nRevised.\nLine 361 delete testing\nRevised.\nFigure 3: can you really trust 1 kbar difference at low P?\nResponse: Thanks! We acknowledge that the piston-cylinder apparatus exhibits relatively larger experimental pressure uncertainties, particularly in the low-pressure range. To address this, we have replicated the low-pressure experiments (0.2 and 0.3 GPa) using a gas-medium high-pressure apparatus, which maintains a pressure uncertainty within ±10 MPa. The results show excellent agreement with those from the piston-cylinder experiments, robustly confirming that pressure is a key controlling factor in REE mineralization.\nFigure 4: do we know we have two melts always? Just because we have not seen the REE? How do you get this depth?\nResponse: Thanks! This cartoon schematic serves for illustrative purposes only. In reality, the occurrence of liquid immiscibility in globally outcropping carbonatites varies significantly. At this stage, our assessment of mineralization potential is based solely on observed REE enrichment phenomena and does not account for potentially concealed ore bodies at depth. Estimates of carbonatite emplacement depth primarily rely on previous studies, which utilize barometry from fluid inclusions and interpretations of geophysical anomalies (such as low seismic velocity and low resistivity zones). While we acknowledge the inherent uncertainties in these depth estimation methods, a consistent correlation is observed: systems emplaced at greater depths generally exhibit higher potential for REE mineralization."}, {"round": 2, "reviewer_comments": {"reviewer_1": "This paper presents a revised manuscript on the cutting‑edge topic of formation mechanisms in carbonatite‑type rare earth element (REE) deposits. It marks another amazing contribution by systematically demonstrating the crucial control of carbonatite emplacement depth on the genesis of large‑scale REE mineralization. The work challenges the long‑standing paradigm in the field, which has traditionally emphasized the role of volatiles and hydrothermal fluids in both field and lab only. Instead, the authors clearly establish that deeper emplacement depths (>10 km) govern REE mineralization potential through two key processes: first, elevated pressure suppresses the incorporation of REEs into early‑crystallizing minerals such as apatite; second, such conditions promote the formation of REE‑enriched brine melts rather than hydrothermal fluids.\nIn the revised version, the authors have fully addressed and incorporated the feedback from the two previous reviewers, which addressed all our concern. By supplementing key experimental data, they have further substantiated the core innovative point—that \"pressure (emplacement depth) controls rare earth mineralization in carbonatites\"—thereby significantly enhancing the reliability of the conclusions. Additionally, the authors have systematically revised and optimized the abstract, introduction, and discussion sections, resulting in a more rigorous logical structure, clearer organization, and more coherent argumentation. Overall, the paper exhibits considerable scientific innovation, robust supporting evidence, and clear, logical presentation. It has reached a high academic standard and is recommended for publication after minor revisions.\nMinor comments:\nLine 33: Change “depletes silica in” to “depleting silica from”.\nLine 35: Add “thereby” before “stabilizing”.\nLine 59: Change “A key question is that...” to “A key question is how the evolution of carbonatitic melts can both suppress... and facilitate...”.\nLine 80: Change “a significant host mineral of REE” to “a major host mineral for REE”.\nLine 95: It is recommended to revise the phrase “that magma chamber emplacement depth control on the REE mineralization of carbonatites” to “that magma chamber emplacement depth controls REE mineralization in carbonatites.”\nLine 209: “dramatically elevates REE concentrations…,” be refined to “significantly increases REE concentrations in the residual melt.”\nLine 231-232: Please revise “strong influenced” to “strongly influenced”\nLine 245: Revise “depleting” to “depletes”\nLine 283-284: It is recommended that the expression “Such fluids are poor solvents for REE, with solubilities of <1 ppm at near-neutral pH,” be refined to “Such fluids have extremely low REE solubilities (<1 ppm at near-neutral pH).”\nLine 305-307: Should be revised to “Recent geophysical data revealing a massive magma chamber at depths >15 km beneath the Bayan Obo deposit further support this model.”", "reviewer_2": "I am satisfied with the improvements made by the authors."}, "author_response": "Thank you very much for your positive evaluation of our revised manuscript and for recommending its publication. We are delighted that you find our work on the pressure-controlled model in carbonatite-type REE deposits to be a valuable contribution that challenges the traditional paradigm in the field. We will carefully implement the remaining minor revisions and are prepared to submit the final version promptly.\nMinor comments:\nLine 33: Change “depletes silica in” to “depleting silica from”.\nRevised.\nLine 35: Add “thereby” before “stabilizing”.\nRevised.\nLine 59: Change“A key question is that...”to “A key question is how the evolution of carbonatitic melts can both suppress... and facilitate...”.\nRevised.\nLine 80: Change “a significant host mineral of REE” to “a major host mineral for REE”.\nRevised.\nLine 95: It is recommended to revise the phrase “that magma chamber emplacement depth control on the REE mineralization of carbonatites” to “that magma chamber emplacement depth controls REE mineralization in carbonatites.”\nRevised.\nLine 209: “dramatically elevates REE concentrations…,” be refined to “significantly increases REE concentrations in the residual melt.”\nRevised.\nLine 231-232: Please revise “strong influenced” to “strongly influenced”\nRevised.\nLine 245: Revise “depleting” to “depletes”\nRevised.\nLine 283-284: It is recommended that the expression “Such fluids are poor solvents for REE, with solubilities of <1 ppm at near-neutral pH,” be refined to “Such fluids have extremely low REE solubilities (<1 ppm at near-neutral pH).”\nRevised.\nLine 305-307: Should be revised to “Recent geophysical data revealing a massive magma chamber at depths >15 km beneath the Bayan Obo deposit further support this model.”\nRevised.\nReviewer #2 (Remarks to the Author):\nI am satisfied with the improvements made by the authors.\nWe thank Reviewer #2 for their positive feedback and for being satisfied with the improvements made in our revised manuscript."}]

Decision letter