The mantle source of REE-rich alkaline silicate magmas can be enriched by continent-derived sediment subduction

Domain: earth_science
Published: 2025-12-17
Editorial outcome: accept

Abstract
Alkaline silicate intrusions host large rare-earth-element (REE) resources, yet the precise origin of the mantle enrichment that generates these magmas remains unresolved. Here, we use a combination of Li-Ba-Sr-Nd isotopic data to clarify the processes responsible for the metasomatic enrichment of the mantle source. We focus on alkaline complexes in the North China Craton that host giant REE deposits, which exhibit broadly similar Li-Ba-Sr-Nd isotopic signatures, despite forming in different tectonic settings. Neither recycling of altered oceanic crust nor serpentinite-related metasomatism can reproduce these coupled geochemical signatures; instead, they require the addition of subducted continent-derived sediment derived from weathered continental crust. Storage of these REE-rich sediments within the lithospheric mantle lowers its solidus and creates chemically fertile domains. Lithospheric extension subsequently triggers selective melting of these domains, generating REE-rich alkaline magmas. Our findings suggest that continent-derived sediment recycling plays an important role in shaping the critical-metal budget of the Earth.

Introduction
Alkaline igneous rocks, which commonly occur along craton margins (Fig. 1A), host ~30% of global rare earth element (REE) resources. The REE-rich alkaline complexes are generally considered to form from low-degrees of partial melting of the sub-continental lithospheric mantle (SCLM) that is highly enriched in REEs, alkali elements, and volatiles1–3. This enrichment is commonly attributed to metasomatism by slab-derived fluids/melts that introduce volatiles and REEs4–7, with plate subduction thought to play a critical role1,2. Despite broad agreement that the source enrichment of alkaline rocks is subduction‑related, there is no consensus on the nature of the subducted material1,2; whether it is derived primarily from marine sediment or altered oceanic crust (AOC)2,6,8–12. Moreover, the “marine sediment” end-member is intrinsically heterogeneous, comprising mixtures of oceanic authigenic phases with continent-derived detrital minerals. Furthermore, recycled materials can be stored at subsolidus conditions in the SCLM for hundreds of million of years13–17 before being remobilized by tectonic events that trigger mantle remelting and generate alkaline magmas18. This implies that recycled material may have been introduced into the SCLM through multiple subduction events.

Element partitioning during slab subduction controls which elements are transferred to the mantle. Aqueous fluids released by slab dehydration do not mobilize high-field-strength elements (HFSE) and REEs in significant amounts19, whereas partial melting of the subducting slab can transport REEs and even some HFSEs20. These slab-derived agents percolate into and react with the overlying mantle, crystallizing distinctive metasomatic mineral assemblages that sequester the introduced elements into enriched veins or pockets21–23. Typical products include phlogopite and amphibole, which host Li, Ba, K, and REEs, together with clinopyroxene enriched in light REEs21–23. Subsequent partial melting of this metasomatized mantle generates alkaline magmas that inherit the chemical fingerprints of these metasomes22,24. Incompatible elements, Li, Ba, and REEs partition preferentially into the melt during mantle melting. Thus, the enrichment of Li and Ba is directly linked to that of the REEs from mantle source enrichment to magma generation.

In this context, Li and Ba and their isotopes (δ7Li and δ138/134Ba) serve as sensitive tracers for identifying the sources of REE-rich alkaline rocks and distinguishing contributions from different recycled reservoirs. The depleted mantle has relatively restricted δ7Li and δ138/134Ba values, as inferred from fresh MORB (e.g., δ7Li=+3.5±1.0 ‰; δ138/134Ba=+0.03 to +0.05‰)25,26. In contrast, sediments and altered oceanic crust display wider ranges in Li and Ba isotopic compositions, generally overlapping but collectively spanning δ7Li=−5 to +20‰25,27,28 and δ138/134Ba=−0.21 to +0.40‰29–31. Addition of Li and Ba from slab‑derived materials can partially modify the isotopic composition of the lithospheric mantle, with the shift depending on the nature of the subducted source24,31. Altered oceanic crust and oceanic authigenic sediments typically impart heavier Li and Ba isotopic compositions, whereas continent-derived sediments are generally lighter (see Discussion). Consequently, the δ7Li and δ138/134Ba values of the lithospheric mantle will tend to increase where enrichment reflects subducted oceanic crust and authigenic sediments, and decrease where continent‑derived sediments are incorporated24,32–34.

There are numerous alkaline intrusions within the North China Craton (NCC), several of which host notable REE mineralization. Particularly important are the alkaline igneous complexes in the Luxi REE belt and the Saima REE-Nb-Ta deposit at the eastern margin of the NCC (Supplementary Fig. S1). Previous radiogenic and oxygen isotope studies have provided evidence of significant crustal contributions to the mantle sources of these alkaline rocks35–37. Despite geochemical similarities, the tectonic settings that triggered SCLM melting differ. The NCC has experienced four tectono-thermal events since the Paleozoic. These are: 1) the southward subduction of the Paleo-Asian oceanic plate beneath the NCC during the late Carboniferous to Late Permian; 2) the closure of the Proto-Tethys along the Qinling-Dabie belt in the Devonian; 3) the thrusting of Yangtze crust beneath the NCC during the Late Permian to Late Triassic along the Dabie-Sulu orogenic belt; and 4) the subduction of the Izanagi (Paleo-Pacific) plate beneath the eastern NCC during the Jurassic and Early Cretaceous38. At Luxi, the magmas (130–120 Ma) were generated during the rollback and steepening of the Izanagi slab, whereas magmatism at Saima (231–224 Ma) is associated with post-collisional crustal extension35–37. Detailed descriptions of the Luxi and Saima deposits and the analyzed samples are provided in the Supplementary Material.

In this work, we report Li-Ba-Sr-Nd isotope data for the Luxi and Saima alkaline complexes (sample information in Supplementary Table S1) to show that the mantle became metasomatized before sourcing the giant REE-bearing alkaline rocks. Our results provide compelling evidence that recycling of continent-derived sediments exerts a primary control on the mantle enrichment that ultimately generates the REE-bearing alkaline magmas, implying a similar petrogenesis for regions elsewhere that host large REE deposits.

Results
Analytical results
The alkaline rocks from the Luxi REE belt and the Saima complex have contrasting major element compositions; most importantly, they differ in their contents of SiO2, Al2O3 and total alkali elements (Supplementary Fig. S2). Those from Luxi vary considerably in composition, with SiO2 contents ranging from 58.9 to 70 wt.%, but for one sample that contains 75.3 wt.% SiO2, Na2O+K2O from 7.36 to 12.1 wt.%, and Al2O3 from 11.9 to 16.2 wt.%. Although most of the rocks are classified as syenites based on their mineralogy (Supplementary Table S2), a few samples fall in the granite, granodiorite, and monzonite fields on a TAS diagram (Supplementary Fig. S2). The Luxi rocks are transitional between alkaline and subalkaline compositions (Supplementary Fig. S2). Despite the variation in major element composition, the subalkaline and alkaline rocks from the Luxi belt have similar primitive mantle-normalized trace-element signatures, characterized by enrichments in LREEs, LILEs (e.g., Sr, Ba, and Pb) and depletions in Ti, Nb, Ta, and P, as well as in the HREEs relativeto LREEs (Fig. 2A). The Li contents of these rocks range from 1.8 to 108 ppm and the δ7Li values from −4.6‰ to +4.8‰ (Fig. 3A). The Luxi alkaline rocks generally have high Ba contents (from 1436 to 4573 ppm) and variable δ138/134Ba values, ranging from −0.26‰ to +0.13‰ (Fig. 3B). The alkaline rocks are characterized by relatively high initial Sr isotope ratios (87Sr/86Sr(i) = 0.7068–0.7112) and low Nd isotope ratios (εNd(t) = −15.65 to −7.63) (Fig. 4).

Alkaline rocks from the Saima complex have lower SiO2 contents (57.4–63.4 wt.%), higher Na2O+K2O contents (10.9–15.4 wt.%) than those of the Luxi belt, and similar Al2O3 contents (13.4–17.0 wt.%). They plot in the syenite and foid syenite fields in the TAS diagram (Supplementary Fig. S2). The Saima alkaline rocks commonly have higher trace element contents and more variable trace-element signatures than the Luxi rocks. The primitive mantle-normalized trace-element profile of the Saima samples shows distinct positive Pb anomalies and negative P and Ti anomalies (Fig. 2B). Some samples also display distinctly negative Nb and Ta anomalies, which is typical for subduction-modified mantle rocks, whereas other samples exhibit no Nb and Ta anomalies. Similarly, some samples show positive U and Th anomalies and enrichments of Zr and Hf relative to LREE, whereas other samples have negative U and Th anomalies and are depleted in Zr and Hf relative to LREE. The alkaline rocks from Saima have Ba and Li contents ranging from 1431 to 4440 ppm and from 20 to 88 ppm, respectively (Fig. 3). The δ138/134Ba values range from −0.22‰ to +0.01‰ and the δ7Li values range from −2.2‰ to +4.1‰ (Fig. 3). The alkaline rocks have relatively radiogenic initial Sr isotope ratios (87Sr/86Sr(i) = 0.7072–0.7086) and relatively low Nd isotope ratios (εNd(t) = −14.1 to −11.7) (Fig. 4). The two groups of alkaline rocks from Saima, i.e., the foid-syenites and the syenites, have similar ranges of Ba, Sr, and Nd isotope compositions.

Some of the Luxi samples display highly negative δ7Li values, whereas others display MORB-like positive δ7Li values. Most of the Saima rocks have positive δ7Li values, but one sample has a highly negative δ7Li value (Fig. 3A). As is the case for some arc magmas, the δ7Li values (−4.64 to +4.85‰) are mostly lighter than those of depleted mantle; the δ138/134Ba values (−0.26 to +0.13‰) are also lighter (Fig. 3B). The contents of Li and Ba do not correlate with the isotopic compositions of the corresponding elements. These samples, collectively, have high 87Sr/86Sr ratios (0.7068–0.7129) and low εNd(t) values (−15.6 to −7.6) (t=130 Ma for Luxi samples and 230 Ma for Saima samples) (Fig. 4), deviating from the mantle array. For comparison, calc-alkaline lamprophyres from Luxi have δ7Li values in the range of –1.3 to –0.1‰, i.e., lower than those of MORB and OIB but within the middle of the range for the alkaline rocks (Fig. 3A).

The REE contents of the samples, although elevated (the color ramp in Fig. 3), do not reach ore grade (typically 0.4–1.4 wt% TREO39; Total Rare Earth Oxide). In alkaline complexes, economic REE mineralization commonly results from late-stage processes (advanced differentiation and hydrothermal alteration) that can concentrate REEs by orders of magnitude relative to the parental magmas40,41. Because these processes can also perturb Li and Ba isotope ratios (see Supplementary Material), we targeted less-evolved, fresh rocks from the Luxi and Saima suites to characterize their primary geochemical signatures.

Recycled continent-derived material dominates the source of REE-rich syenite magmas
The Li-Ba-Sr-Nd isotopic compositions of the investigated alkaline complexes reflect their mantle source, since these compositions cannot be explained by partial melting, magmatic differentiation, crustal assimilation, or surface alteration and weathering (see discussion in the Supplementary Material). Alkaline rocks from Luxi and Saima have low δ7Li, δ138/134Ba, and εNd(t) values, as well as high 87Sr/86Sr(i) values, clearly deviating from depleted mantle compositions but closely resembling the signature of slab lithologies introduced into the lithospheric mantle (Figs. 3 and 4).

Evidence of recycled materials in Luxi and Saima alkaline rocks. Additions from subducted slab materials are also apparent from the arc-like trace-element patterns (Fig. 2) and elevated Th/Yb and Nb/Yb ratios compared to the MORB array (Supplementary Fig. S6). An excess of Th relative to Nb and Yb is often interpreted to indicate a sediment or continental-crust contribution, since Th is enriched in the continental crust and sediment42. Thus, the geochemical characteristics suggest recycled materials in the mantle source. In subduction zones, forearc serpentinites typically have low concentrations of Li and Ba25,31. As a result, any fluid released from serpentinites would have only a minor effect on the Li and Ba isotopic composition of the overlying mantle. In addition, although a substantial fraction of slab-hosted Ba and Li is released at sub-arc depths, a non-negligible portion can be retained and recycled into the convecting mantle, as indicated by the near-mantle δ138/134Ba in OIB (HIMU only up to +0.11‰; EM only down to –0.07‰)43 and the slightly elevated Li isotopic compositions of OIBs (Fig. 3). Nevertheless, the small magnitude of these deviations implies that such recycled inputs have a limited ability to modify the deep mantle and are unlikely to account for the isotopic features of our samples. Importantly, large peralkaline complexes that formed in plume settings (e.g., in the Kola Peninsula, Russia) typically have depleted Sr and Nd isotope ratios (low initial 87Sr/86Sr and positive εNd(t) values) consistent with their derivation from a mantle that has been long depleted44, whereas the NCC alkaline complexes have enriched lithospheric signatures (negative εNd(t); elevated initial 87Sr/86Sr; Fig. 4), indicating a lithospheric source. Thus, the possibility of a significant contribution from the deep mantle can be ruled out. In fact, most of the subducted Li and Ba are sequestered in the SCLM (creating an enriched slab-influenced domain)45. The Luxi and Saima magmas likely originated from this enriched SCLM beneath the craton.

Minor influence of altered oceanic crust. Subducted altered oceanic crust has often been considered a potential source of trace element enrichment in mantle-derived magmas9,12,46,47. Altered oceanic basalts and oceanic authigenic sediments typically acquire heavy Li and Ba isotope signatures due to interaction with seawater48,49. Modern seawater is highly enriched in heavy Li (δ7Li up to +31‰) and heavy Ba (δ138/134Ba up to +0.6‰)25,31. Consequently, oceanic crust altered at low temperature can record very elevated δ7Li (often reaching +10 to +20‰)25 and moderately high δ138/134Ba31 values (Fig. 3). If melts or fluids derived from such AOC were added to the mantle source beneath Luxi and Saima, it is expected that the metasomatized mantle would have shifted toward heavier Li and Ba isotopic compositions25,32,50. This effect would even be enhanced if equilibrium isotope fractionation during fluid formation occurred, since for both isotope systems, the heavy isotope is preferentially incorporated into the fluid51,52. Additionally, low δ138/134Ba values (−0.22 to −0.06‰) have been reported for several lower-oceanic-crust samples53. These samples, however, contain very little Ba (6.5–8.0 ppm), substantially less than MORB, and therefore, such a Ba-poor source cannot explain the positive Ba anomalies (i.e., high Ba contents) that are characteristic of alkaline rocks (Fig. 2).

Several lines of evidence indicate that AOC alone cannot account for the isotopic compositions observed in the alkaline rocks. Firstly, dehydration reactions in the oceanic crust drive the heavier Li and Ba isotopes into the fluid phase, leaving the residual slab depleted in heavy isotopes51,54,55. Thus, when the AOC reaches sub-arc depths, it may no longer carry a heavy isotopic signature as much of the heavy Li and Ba may have been lost to fluids that escaped into the forearc. Slab dehydration can cause at most a 2–4‰ decrease in the δ7Li of the residual slab55, which would not be sufficient to produce the much lower δ7Li values (down to –4.6‰) recorded in some samples. Moreover, if slabs are stripped of heavy Li and Ba by fluid release, they would retain only minor amounts of these elements, which in turn are unlikely to significantly affect the source magmas of the alkaline rocks. Secondly, the AOC typically has positive εNd(t) values because its Nd isotopic composition remains similar to that of MORB even after alteration. In contrast, the Luxi and Saima rocks have strongly negative εNd(t) values (Fig. 4). This signature is more consistent with old continental crust than with young oceanic crust. Finally, if pelagic (deep-sea) sediments or AOC were the dominant contributors of REEs and other metasomatic components, one might expect heavier Li and Ba isotopic compositions in the resulting magmas31,56. Instead, the data show the opposite (lighter), ruling out a simple AOC-dominated source.

Dominant influence of subducted continent-derived sediments. Our data suggest that continent-derived sediments are the principal source of the unusually light Li and Ba isotope signatures in the Luxi and Saima mantle. Unlike pelagic sediments or AOC, continent-derived sediments have intrinsically low δ7Li and δ138/134Ba28, which can be imparted to the mantle when these sediments are subducted and release fluids or melts24,31. Siliciclastic sediments from continental crust typically carry the isotopic imprint of chemical weathering and erosion. During weathering on the continents, secondary minerals (like clays) preferentially take up the lighter isotope 6Li, while 7Li is leached into fluids57. This leaves the weathered rock and soils depleted in 7Li and thus characterized by low δ7Li values (e.g., down to −6.8‰ in weathered granites)58. Moreover, diagenesis and prograde metamorphism of sediments in the subduction environment can further lower their δ7Li values by driving off 7Li-rich fluids59, yielding residual solid phases with lower δ7Li values. Barium isotopes behave similarly. Thus, continental sediments are a potentially light δ138/134Ba reservoir, because weathering and metamorphic dehydration both remove 138Ba from the rocks51,60.

In general, metasomatic agents derived from these sediments will have lower δ7Li and δ138/134Ba values, especially if the sediment has undergone prior slab dehydration. This creates a heterogeneous SCLM for later alkaline magmatism. The data cluster toward the light end of δ7Li-δ138Ba space and overlap the isotopic signatures of continent-derived sediments, and together with the negative εNd(t) and high 87Sr/86Sr(i) values, indicate that continent-derived sediment was the dominant metasomatic agent.

Isotopic mixing models of crustal recycling. The variable ranges of δ7Li and δ138/134Ba values in the samples reflect the heterogeneous nature of the source. Subducted slabs are compositionally diverse, ranging from relatively heavy (marine chemical sediments or AOC-influenced) to very light (weathered continental detritus-dominated)27–30. To quantify the proportion of recycled material, mixing models have been developed using representative end-member compositions. One end-member is the mantle (SCLM) and the other end-members correspond to two distinct continental crustal materials: (i) an average upper continental crust (UCC) with δ7Li=+0.6±0.6‰ and δ138/134Ba=0.00±0.04‰57,61 (this “normal” UCC reflects limited weathering/metamorphism); and (ii) a highly weathered or dehydrated crustal component with much lighter isotope ratios (set as δ7Li=–6‰ and δ138/134Ba=–0.3‰). These values, for the light end-member, represent the materials from continental shales, weathered granites, and metamorphosed sediments. Details of the modeling are provided in the Supplementary Material.

The modeling results indicate that an addition of 10–20% of sediment or crustal material is required in the mantle source to account for the isotopic composition observed in the Luxi and Saima alkaline magmas (Fig. 4). In Li-Ba isotope space, the data for the alkaline rocks plot between the mixing trajectories of mantle-UCC and mantle-weathered UCC mixtures, rather than along the mantle-AOC line. This suggests that both “normal” upper crust and “light” weathered crust components contributed to the source. Most samples fall in a field that can be explained by a two-component mixing between the mantle and sediment. A few samples have slightly higher δ7Li values (approaching 0‰) and lie above the mantle-UCC mixing curve; these samples could reflect a modest contribution from an AOC-derived fluid or a less weathered sedimentary component (Fig. 4). In essence, the REE-rich alkaline melts appear to sample a hybrid source that was originally depleted SCLM but has been overprinted by recycled continental material.

Mantle heterogeneity and multiple metasomatic events
The mantle source of REE-fertile alkaline magmas is highly heterogeneous. We consider two possible scenarios to explain this heterogeneity, in light of the isotopic evidence, namely a single multi-component metasomatic event and multiple metasomatic events.

In the first scenario, a single metasomatic event introduced a diversity of materials into the mantle (e.g., mélange)4,62. If released over a short geological interval, such diverse materials in the downgoing slab, e.g., continental and pelagic sediments as well as AOC could metasomatize the surrounding mantle during the same event. As a result, some mantle domains could be overprinted by sediment-derived material, whereas others could receive a contribution from AOC-derived material. Such a metasomatic event would create a heterogeneous mantle source.

In the second scenario, the lithospheric mantle was enriched through repeated episodes of subduction over a protracted interval of time. The eastern NCC margin experienced successive subduction events during the Phanerozoic (as discussed in the Geological background)38. Earlier events might have introduced ancient sediment-derived metasomatism into the lithospheric mantle (e.g., hydrous melts from subducted continent-derived sediments). Isotopic studies of the Luxi and Saima alkaline complexes have revealed elevated initial 87Sr/86Sr and very low εNd(t) values corresponding to two-stage model ages mostly around 2 Ga (Supplementary Table S5), consistent with the incorporation of Proterozoic continental material in their source. Note, the model age of the sediments is not related to the time of subduction. Subsequent subduction events (such as the Paleo-Pacific subduction) could then have introduced fresh inputs of slab fluids or melts, producing greater source heterogeneity in the Luxi complexes, particularly in εNd(t) values (Fig. 4). Over time, these episodic enrichments would have accumulated in the mantle as multiple metasomatic domains.

Despite contrasting tectonic settings (back-arc rifting at Luxi and post-collisional intrusion at Saima), both complexes display isotope signatures dominated by ancient continent-derived components. The ~110 Ma offset between their melting events indicates that mantle enrichment and melt generation were unrelated. Prolonged sediment influx and metasomatic modification preconditioned the SCLM, with later mobilization of continental materials stored in the SCLM. Because these metasomes also contain volatiles, they lower the peridotite solidus strongly and are far more fusible than the surrounding dry depleted mantle. During subsequent tectonic events, these enriched domains are the first to melt, even at very low degrees of partial melting63. This selective melting of enriched domains explains both the preference of REE-rich alkaline silicate systems for craton margins and their link to extensional tectonics (Fig. 1A). Thus, alkaline magmatism reflects delayed melting of a metasomatized SCLM. Subduction-related recycling produced the enrichment, whereas crustal extension triggered the melting and transferred REEs into the magmas.

Continent-derived sediments enrich the mantle sources for REE-rich alkaline magmas
The lithospheric mantle beneath Archean continental crust can act as a REE source only where subduction‑related metasomatism has replenished its REE budget1,2. Calc-alkaline lamprophyres represent low-degree melts of the SCLM rather than the deeper asthenosphere64. Globally, the high REE contents and light Li isotopic compositions of orogenic lamprophyres (Fig. 5) demonstrate that melts from subducted crustal rocks can introduce significant amounts of REE and Li to metasomatized domains of the SCLM22,65,66. At Luxi, calc‑alkaline lamprophyres accompany the syenites and have low δ7Li and δ138/134Ba values comparable to those of REE-rich syenites (Fig. 3). Taken together, the elevated Ba/La (Fig. 5A) and low Zr/Nb values (Fig. 5B), and the correlation of their isotopic signatures (low δ7Li and δ138/134Ba in both lamprophyres and syenites; Fig. 3) strongly support continent-derived sediment metasomatism as a major source of LILE, LREE, and HFSE enrichment in the SCLM (Fig. 2). Most importantly, alkaline complexes hosting giant REE deposits such as at Luxi and Saima (NCC), Mianning-Dechang (eastern Tibetan Plateau), and Ilímaussaq (southwestern Greenland) consistently have low δ7Li values (Fig. 6), regardless of their tectonic environments (subduction-related at Luxi, post-collisional at Saima and Mianning-Dechang, and rift-related at Ilímaussaq)10,36,67. Furthermore, our modeling confirms that a SCLM metasomatized by recycled continental sediments adequately reproduces the observed compositional range (Fig. 6).

Across the global arc compilation, bulk REE abundances increase systematically with Th/Yb (Fig. 5C), a proxy for addition of sediment-derived melts42,68, defining a positive trend. By contrast, REE abundances show no coherent relationship with Sr/Y (Fig. 5D), a parameter commonly used to track the evolution of magmas derived from AOC69,70. This decoupling implies that REE enrichment in arc magmas is governed more plausibly by the involvement of sediment-derived melts rather than by AOC-derived melts. A complementary view is that, even without extensive melting, subducted sediment can become less dense than the surrounding mantle and rise as a solid-state diapir, transporting its REE inventory71. These mechanisms are not mutually exclusive; both introduce REE-enriched material into the mantle and may operate in tandem.

Continent-derived sediments are heterogeneous in respect to the REE. Fine-grained shales (e.g., NASC; North American shale composite) cluster near average UCC background (~1.6 × 102 ppm ΣREE)72, whereas placer-type heavy-mineral sands, in which monazite/allanite are abundant, can reach >103 ppm ΣREE73,74. The subduction of continent-derived sediments derived from weathered continental rocks is necessary to refertilize an otherwise REE-depleted mantle. During continental chemical weathering, REEs undergo strong mineralogical partitioning. In acidic, highly leached soil environments (e.g., tropical laterites), the middle to heavy REEs are relatively immobile and tend to be retained in the regolith by sorption onto secondary clay minerals like kaolinite and illite75,76. This forms ion-adsorption type REE deposits that are strongly enriched in MREEs and HREEs. By contrast, the LREEs are largely sequestered in some refractory detrital minerals. Accessory minerals like monazite, apatite, and bastnäsite serve as hosts for LREEs in weathered profiles75. These LREE-rich heavy mineral particles and clay complexes are readily eroded and transported by rivers, eventually accumulating as continent-derived sediment in subduction zones77,78.

When REE-enriched sediments are subducted, prograde dehydration of the slab generates aqueous fluids that efficiently scavenge the most labile constituents. The REEs absorbed on clays or hosted by soluble carbonate phases (e.g., calcite/dolomite) are therefore liberated at relatively shallow levels during early devolatilization. In contrast, apatite and bastnäsite both typically release significant REE at hotter, sub-arc conditions or upon partial melting. Monazite is more refractory and requires high temperatures or partial melting to destabilize and liberate its REEs. Consequently, early slab-derived fluids are expected to carry minor REEs, consistent with experimental results19. At higher temperatures, silicic partial melts can dissolve LREE-bearing phases (apatite and, at higher temperatures, monazite), thereby enhancing LREE mobility20.

Compilations show that most deep-sea REE-rich sediments exhibit pronounced negative Ce anomalies [Ce/Ce*=2×CeN/(LaN+PrN)]79. For example, REE-rich bio-apatites in pelagic sediments have Ce/Ce* values ranging from 0.05 to 0.2080. However, ore-related alkaline magmas do not consistently display such negative Ce anomalies79 (e.g., ~0.95 for Saima, and ~0.97 for Luxi; Supplementary Table S4). In fact, the nature of the sedimentary REE source affects Ce concentrations in the resulting magmas. Many deep-sea REE-rich sediments (biogenic apatite, early authigenic phosphates, and carbonates) acquire their REEs from seawater or pore fluids and inherit the characteristic negative Ce anomaly of seawater79,81. By contrast, purely detrital sediments dominated by aluminosilicate clays and oxides typically show little or no evidence of the Ce anomaly characteristic of continental crust (e.g., ~0.99 for average UCC). Furthermore, if these sediments incorporate seawater-derived carbonates or have undergone pore-water exchange, a slightly negative Ce anomaly may appear81. Mixing variable proportions of such continent-derived components into mantle sources therefore reproduces the observed range of Ce/Ce* in alkaline magmas.

In summary, weathering concentrates REEs into sediments, and subduction then conveys these REEs into the mantle, ultimately yielding REE-enriched magmas. The influx of these materials from the continent-derived sediments produces a metasomatized mantle source that is characteristically enriched in LILE and LREE, explaining the geochemical signatures of REE-rich alkaline rocks (Fig. 2) and some arc volcanic rocks. Our model links the weathering of the Earth’s surface and subduction in the Earth’s interior.

The role of plate tectonics in the critical-metal budget of the mantle
Giant REE deposits formed primarily during the Proterozoic and Phanerozoic (Fig. 1B), with a major peak at ~1.3–1.2 Ga, coeval with the assembly of Rodinia (and/or Columbia), and a smaller Paleozoic peak at ~450–400 Ma2. These peaks align with supercontinent cycles alternating between collisional orogenesis and extensional breakup. Geological records from the Rodinia assembly also document abundant non-arc magmas (rift- and plume-related) that were enriched in HFSEs and REEs82. By the Mesoproterozoic, plate-tectonic processes (e.g., prolonged subduction during the amalgamation of Rodinia) had evolved to permit extreme enrichment of these incompatible elements in the SCLM83. Subsequent tectonic events (whether continental rifting or collisional reactivation) could then tap into these pre-enriched mantle domains, generating the large melt volumes needed to form giant REE deposits1,2. In contrast, evidence of such REE-rich alkaline magmatism is scarce in the Archean, consistent with the idea that early Earth tectonics did not efficiently concentrate these elements in the mantle.

A key geodynamic inflection point was the onset of “modern” cold, deep subduction around the Neoproterozoic (after ~800 Ma)84. Unlike the hotter subduction of the Archean-Paleoproterozoic (which likely caused most sediments to be released at shallow depths), late Proterozoic subduction zones became cool enough to carry substantial quantities of sediments deep into the mantle85. Modern-style subduction thus continuously transports incompatible-element-rich sedimentary and crustal materials into the mantle wedge. Some of this recycled material is also conveyed into the deeper mantle, seeding mantle plumes and intraplate sources with enriched signatures. Crucially, silicate and carbonate melts of subducted sediments, characterized by high LREE abundances and volatile fluxing agents (CO2, H2O), provide prime sources for exceptionally enriched magmas tapped during rifting and orogenesis. Thus, sediment subduction is a prerequisite for the source enrichments typical of Proterozoic-Phanerozoic alkaline complexes.

A unified model
Alkaline silicate magmas associated with many REE deposits typically are the products of low-degree partial melting of a metasomatized mantle source1,2. Because REEs are highly incompatible, very small melt fractions can concentrate them to levels orders of magnitude higher than those produced by high-degree melting; even when the source is only modestly enriched, magmas generated by <1% partial melting may have exceptional REE contents3. Thus, exceptionally REE-enriched sources are not required. Equally critical is the coincidence of alkalis and volatiles in the source, which sustains REE mobility and promotes extreme differentiation (e.g., protracted fractional crystallization, liquid immiscibility), thereby amplifying REE concentrations in residual melts and/or exsolved fluids40,41,86,87. Consequently, an intensely metasomatized lithospheric mantle is a prerequisite for generating alkaline magmas with the potential to attain ore-forming REE concentrations during subsequent differentiation. In the case of a heterogeneously metasomatized mantle and/or a difference in the nature of the subducted material, this implies that some alkaline silicate magmas will be REE-fertile and others that are temporally and spatially associated with them will be REE-barren.

A unified model for REE-rich alkaline magmatism begins with recycled crustal material (either continental crust or continent-derived sediment) that must infiltrate and enrich the mantle tens to hundreds of millions of years prior to magmatism. This early metasomatism imparts distinct geochemical signatures (e.g., elevated Sr-Nd isotope ratios, high LREE/HREE ratios, and anomalous δ7Li and δ138/134Ba values) to the source. A second requirement is a suitable tectonic event to trigger partial melting of the enriched SCLM. In the NCC, this involved Mesozoic extension and possible lithospheric destruction38; in Tibet (Mianning-Dechang), Cenozoic collision and orogenic collapse67; and in Greenland (Ilímaussaq), anorogenic rifting10. Despite their differences, these settings share deep-seated fractures or thermal anomalies, enabling the metasomatized mantle to cross its solidus. Magma differentiation and emplacement govern the final REE enrichment, with prolonged melt residence, fluid exsolution, and crystal fractionation concentrating the REE to ore-forming levels3. To conclude, our study shows that only a pre-enriched mantle source can yield alkaline magmas capable of producing giant REE deposits, underscoring the universal significance of prior SCLM fertilization by continent-derived sediment recycling.

Methods
Major and trace element whole-rock analyses were conducted at the Lunan Geo-engineering Exploration Institute, Shandong, China. The samples were crushed to a diameter of a few centimeters and fresh pieces were selected, washed with deionized water, dried, and ground to <200 mesh for analysis. The resulting powders were then mixed with Li2B4O7 flux (in a ratio of 1:8) and fused at a temperature of 1250°C for major element analysis using a V8C automatic fusion machine manufactured by the Analymate Company, China. A X-ray fluorescence spectrometer (Zetium, PANalytical, XRF-1800, Shimadzu Corporation, Japan) was used for the major element analyses. Calibration was performed using both international and internal reference materials. The analytical errors for the major elements were better than 1%. Trace and rare earth element analyses were performed using ~25 mg of powder, which was dissolved in a mixture of 1 mL HF and 0.5 mL HNO3 in screw-top Teflon beakers and left to dissolve for ~48 h at a temperature of 120°C. A second dissolution used a mixture of 2 mL of 1:1 HCl:HNO3 for 8 hours at 120°C. The trace and rare earth elements were analyzed using high-resolution inductively coupled plasma mass spectrometry (ICP-MS) with an analytical precision that was typically better than 2%. The reproducibility of the concentration was determined to be better than 5%. The results of major, trace, and rare earth element analysis are reported in Supplementary Tables S3, S4.

Whole-rock analyses were performed by Nanjing FocuMS Technology Co. Ltd., Nanjing, using a Nu Plasma II multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) to determine the Sr-Nd isotope ratios. Raw Sr and Nd isotope ratios were corrected internally for mass fractionation by normalizing to 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219, respectively, using the exponential law. The Sr and Nd isotope ratios of the reference materials BCR-2 and AGV-2 obtained in this study (Supplementary Table S5) are consistent with published values88.

Barium isotope ratios were determined at the University of Science and Technology of China (USTC), Hefei, China. A detailed description of the method, including sample dissolution, chemical purification, and mass spectrometric analysis, is provided in ref. 89. Isotope ratios were measured using the double-spike method in a low-resolution mode under “dry” plasma conditions. We used the international reference material NIST SRM 3104a as the Ba isotope standard. The total procedural blank was 0.12 ng (n=4). The Ba isotope ratios of the reference materials GSP-2 and G-2 determined in this study (Supplementary Table S5) are consistent with the published values89. The long-term external precision of δ138/134Ba is better than ±0.05‰ (2SD). Barium isotope fractionation is expressed relative to reference material SRM 3104a using the δ138/134Ba (‰) notation: δ138/134Ba (‰) = [(138Ba/134Ba sample / 138Ba/134Ba standard) − 1] × 1000. The results of the Ba isotope analyses are presented in Supplementary Table S5.

Lithium isotope compositions were analyzed at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China, using a Nu Plasma II MC-ICP-MS instrument. Sample preparation and analytical procedures are described in refs. 90, 91. Isotope ratios were obtained using the standard-sample-standard bracketing method and L-SVEC as Li isotope reference material. The international rock reference materials GSP-2 and BCR-2 were used to evaluate the accuracy of the determined δ7Li values. The values obtained agree well with the published values for these reference materials (Supplementary Table S5). The 2σ long-term external precision, as determined by repeated measurements of pure Li standard solutions and sample solutions, is better than ±0.47‰90. Lithium isotope fractionation is expressed relative to the L-SVEC reference material using the δ7Li (‰) notation: δ7Li (‰) = [(7Li/6Li sample / 7Li/6Li standard) − 1] × 1000. The results of Li isotope analyses are presented in Supplementary Table S5.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper presents Li-Ba-Sr-Nd isotopic analyses of the Mesozoic alkaline rocks in the North China Craton, to constrain the origin of these alkaline magma. They made a conclusion of that subducted terrigenous sediment or continental crust plays a critical roles on mantle metasomatism and formation of REE-bearing alkaline magma. The topic of this study is interesting, the interpretation is sound, and the conclusions are conventional and prosaic. My main concerns are as following. (1) The samples of alkaline rocks in this study from both Luxi and Saima have not enough REE contents to form large REE deposits, with total REE less than 1000 ppm (as showing Figures 3). Actually, there are some REE-rich syenite in Luxi, for example the Weishan. (2) The distinct positive Pb anomalies in both Luxi and Saima alkaline rocks is not a diagnostic evidence. It is likely induced by contamination in analysis. (3) The low δ7Li and δ138/134Ba signatures of the alkaline rocks have been attributed to the subducted low δ7Li and δ138/134Ba continental source. It seems very speculative. Why such continental crust materials recycling can lead to REE-fertilized metasomatized mantle source for alkaline magma? (4) In the model presented in this paper, the metasomatized mantle is suggested to be a result of continental crust recycling. However, it is unclear whether it formed through partial melting or buoyant diapirs, as these processes are significantly different. Additionally, the mantle εNd(t) value of -5 used in geochemical modeling, as shown in Figure 4, appears to be unusual.", "reviewer_2": "The manuscript presents a comprehensive Li-Ba-Sr-Nd isotopic study of REE‑bearing alkaline complexes (Luxi, 120–130 Ma; Saima, 224–231 Ma) in the North China Craton. By combining new high‑precision δ7Li, δ138/134Ba, 87Sr/86Sr, and εNd data with trace‑element modelling, the authors argue convincingly that (1) the metasomatic agent responsible for mantle enrichment was dominantly subducted terrigenous sediment/continental crust, not altered oceanic crust, and (2) metasomatism and partial melting were temporally decoupled, with later extensional events triggering low‑degree melting of a fertile lithospheric mantle. The work is well organised and addresses a question of broad interest. I find the study suitable for publication after the minor issues below are addressed.\nMajor (but minor‑revision level) Comments:\nA more applied angle, but still debated, is how important crustal recycling is for the global REE budget of the mantle and crust. The manuscript suggests that recycling of continental material “plays a significant role in shaping Earth’s critical-metal budget”. This implies that without sediment subduction, the mantle might be poorer in REEs and other critical metals, potentially limiting the formation of large deposits. Some researchers question if we can quantify this: for instance, is there a secular increase in REE content of the lithospheric mantle over time as more sediments have been subducted? Hulett ++ (2016, NatGeo) looked at B isotopes in carbonatites over time and inferred progressive sediment involvement. Others might point to the peak in REE deposit formation around ~1.2 Ga (when supercontinents were assembling) as non-coincidental. The authors could strengthen the manuscript by expanding on whether modern-style plate tectonics (active since ~800 Ma or so) was necessary for generating the giant REE deposits, as these deposits are mostly Proterozoic or Phanerozoic in age. If so, sediment subduction (a hallmark of modern subduction zones) could be the key differentiator that allowed extreme REE enrichment in certain mantle regions after a certain time. This remains an open question, tying mantle geochemistry to tectonic evolution.\nThere is also the question of how metasomes survive in the mantle over geologic time without being homogenized. The fact that we find localized veins with extreme isotopic signatures (e.g., δ7Li ~ –10 permil in some mantle domains) implies that the mantle is not fully homogenized by convection. Some debate revolves around whether the convecting asthenosphere itself holds recycled signatures (as suggested by enriched MORB and OIB), or if essentially all the enriched components reside in the stationary lithosphere or deep transition zone until tapped. The MORB Ba and Li isotope evidence for pervasive sediment suggests even the upper convecting mantle isn’t pristine, albeit with very low concentration. But certainly the largest anomalies (and richest metasomes) seem to reside in lithospheric “reservoirs.” Some workers have suggested that metasomatic veins can undergo multiple melting events over time, each time producing a batch of alkaline magmas, until they are exhausted. The “recurrent magmatism over 2.5 Gy” in Greenland hints that either metasomatism was episodic or that new metasomes kept forming over that time. The debate here is more about how many times can the same metasome be partially melted. Incorporating these discussions would significantly strengthen the manuscript.\nMinor Comments:\n1. Line 16: Change “significant” into “large”.\n2. Line 33-34: “Alkaline igneous rocks account for ~30 % of global rare earth element (REE) resources.”\n3. Lines 154-156: The conclusions presented here are crucial for in furhter discussions, especially regarding Li and Ba isotopes. I recommend incorporating these discussions from the supplementary material into the main manuscript.\n4. Line 145-146: Several Luxi samples show δ7Li as low as –4.6 ‰. Discuss whether these values could reflect diffusive Li loss during late‑stage alteration.\n5. Line 192-194: Are these ratios (used here to represent the index of subduction input) elevated or decreased in your samples?\n6. Line 214: “generates sediments with low δ7Li values”\n7. Line 252-262: The discussion emphasises terrigenous sediments yet briefly mentions delamination. Given emerging evidence for recycled lower crust in post‑collisional K‑rich magmas worldwide, please add 2–3 sentences assessing whether partial melts/diapirs of foundered Archean lower crust could mimic the observed Li–Ba–Sr–Nd systematics.\n8. Line 292-293: “At typical sub-arc depths, slab-derived fluids carry abundant LILE and lesser LREE”.\n9. Line 314: Be specific about what simulation work is cited here.\n10. Figure 1 and the concluding model nicely generalise to other REE provinces. However, Mianning-Dechang (eastern Tibet) and Gardar (SW Greenland) are only treated in Fig. 7. Given their similar Li-Ba signatures, a short paragraph in the main text comparing NCC results with these belts would strengthen the global relevance.\n11. Fig. 3: Add a grey MORB field for visual reference", "reviewer_3": "Comments for the manuscript submitted to Nature Communications, by Qiu et al.\nFirst, I want to apologize for my overdue review.\nThis manuscript reports a novel combination of Li-Ba-Sr-Nd isotopic data of alkaline complexes in the North China Craton to clarify the processes responsible for the metasomatic enrichment of their mantle source. They observed that the alkaline complexes exhibit broadly similar Li-Ba-Sr-Nd isotope signature despite forming in different tectonic settings. They explained that neither altered oceanic crust recycling nor serpentinite fluid metasomatism can reproduce their geochemistry, instead, they require the addition of subducted terrigenous sediment or continental crust. They further suggest that recycling of continental crustal material plays a significant role in shaping Earth’s critical-metal budget. I recommend rejection for the following reasons:\n1. The biggest question of this manuscript is how to link the Ba-Li isotopes of alkaline complexes with the genesis of REE deposits? For now, we tentatively attribute the light Ba and Li isotopes in the alkaline complexes of the North China Craton to terrigenous sediments or continental crustal sources. However, the REE content in the continental crust is not high, thus, the lithospheric mantle metasomatized by continental crust does not exhibit particularly high REE concentrations. Low-degree melting of such sources cannot form REE deposits. Moreover, the properties of Ba and Li are entirely different from those of REEs—Ba and Li are fluid-mobile elements, whereas rare earth elements are fluid-immobile.\n2. The Ba and Li isotopic compositions of altered oceanic crust and terrigenous sediments exhibit an extremely wide range, spanning from values lower than to those higher than the mantle values (Zhang et al., 2024: Deciphering Contribution of Recycled Altered Oceanic Crust to Arc Magmas Using Ba‐Sr‐Nd Isotopes). Therefore, the authors should provide a detailed discussion on the potential contributions of these two surface materials to the alkaline complexes in the North China Craton. Additionally, the Ba isotopic compositions of OIB (Ocean Island Basalt) also show a broad variation, ranging from values higher than the mantle (e.g., HIMU endmember) to those lower than the mantle (e.g., EM endmembers) (Bai et al., 2022: Barium isotopes in ocean island basalts as tracers of mantle processes). Thus, the authors cannot simply conclude that Li and Ba cannot be subducted into the convecting mantle.\n3. Although most researchers believe that alkaline complexes originate from the partial melting of metasomatically enriched lithospheric mantle, a growing number of recent studies suggest that the mantle sources of alkaline rocks—including alkaline complexes, kimberlites, lamproites, and ultrapotassic lamprophyres—may instead derive from the asthenosphere (Giuliani et al., 2020, Science Advances; Sarkar et al., 2022, Geology……). Therefore, the authors should appropriately consider the possibility that REE deposits associated with alkaline complexes could also be sourced from the asthenosphere."}, "author_response": "Response to Reviewers’ Comments\nReviewer #1 (Remarks to the Author):\nThis paper presents Li-Ba-Sr-Nd isotopic analyses of the Mesozoic alkaline rocks in the North\nChina Craton, to constrain the origin of these alkaline magma. They made a conclusion of that\nsubducted terrigenous sediment or continental crust plays a critical roles on mantle\nmetasomatism and formation of REE-bearing alkaline magma. The topic of this study is\ninteresting, the interpretation is sound, and the conclusions are conventional and prosaic.\nMy main concerns are as following.\nResponse: Thank you for the feedback. Our revised manuscript has included expanded\ndiscussions where necessary to strengthen our interpretations.\n(1) The samples of alkaline rocks in this study from both Luxi and Saima have not enough REE\ncontents to form large REE deposits, with total REE less than 1000 ppm (as showing Figures\n3). Actually, there are some REE-rich syenite in Luxi, for example the Weishan.\nResponse: We agree that the Luxi and Saima samples analyzed here have modest whole-rock\nREE contents (113 to 1385 ppm; avg. 591 ppm) and are not, themselves, ore-grade. It is\nnecessary to distinguish (i) ore-grade REE concentrations from (ii) magmatic and mantle-\nsource signatures of REE fertility. In alkaline provinces, economic REE mineralization\ntypically arises from late-stage processes (advanced differentiation, volatile exsolution, and\nhydrothermal alteration) that can concentrate REE by orders of magnitude relative to the\nparental magmas.\nOur sampling targeted relatively less-evolved rocks from the Luxi (including units within the\nWeishan complex) and Saima suites to minimize overprinting by these late processes and to\nisolate mantle-source signals. In such rocks, whole-rock ΣREE may remain modest even when\nthe magmas are REE-fertile. By contrast, REE-mineralized syenites represent more evolved\nparts of the same alkaline system, where original Li and Ba isotope compositions are usually\nmodified.\nThe isotope data we present constrain the metasomatized agents in sources that make the\nparental magmas fertile. They do not require the specific hand samples to be ore-grade. To\naddress this explicitly, we have added the sampling strategy:\n“Notably, in alkaline complexes, economic REE mineralization commonly results from late-\nstage processes (advanced differentiation and hydrothermal alteration) that can concentrate\nREEs by orders of magnitude relative to the parental magmas36,37. Because these processes can\nalso perturb Li and Ba isotopic systematics (see Supplementary Material), we targeted\nrelatively less-evolved, fresh rocks from the Luxi and Saima suites to isolate mantle-source\nsignals. As expected, the REE contents of the samples, while elevated, do not reach ore grade\n(Fig. 3). Thus, the isotopic data are used to constrain the source processes that govern\nparental-magma fertility.”\n(2) The distinct positive Pb anomalies in both Luxi and Saima alkaline rocks is not a diagnostic\nevidence. It is likely induced by contamination in analysis.\nResponse: We appreciate the reviewer’s attention to analytical contamination, though the\nreason for raising this issue is not entirely clear to us. Independent replication in the\nliterature supports that the Pb anomalies we report are real and geologically coherent,\nnot artifacts. Positive Pb anomalies of comparable magnitude have been documented in the\nLuxi and Saima alkaline rocks by independent groups using different laboratories and protocols\n(please see Wu et al., 2016; Wei et al., 2019; Ju et al., 2019; Lan et al., 2024).\n(3) The low δ7Li and δ138/134Ba signatures of the alkaline rocks have been attributed to the\nsubducted low δ7Li and δ138/134Ba continental source. It seems very speculative. Why such\ncontinental crust materials recycling can lead to REE-fertilized metasomatized mantle source\nfor alkaline magma?\nResponse: In fact, we emphasize the key role of subducted terrestrial sediments, rather\nthan the continental crust itself. In the revision, we discussed these points in detail:\n“The subduction of terrigenous sediments derived from weathered continental rocks is\nnecessary to refertilize an otherwise REE-depleted mantle. During continental chemical\nweathering, REEs undergo strong mineralogical partitioning. In acidic, highly leached soil\nenvironments (e.g., tropical laterites), the middle to heavy REEs are relatively immobile and\ntend to be retained in the regolith by sorption onto secondary clay minerals like kaolinite and\nillite65,66. This forms ion-adsorption type REE deposits that are notably enriched in MREEs\nand HREEs. By contrast, the LREEs are largely sequestered in refractory detrital minerals.\nAccessory minerals like monazite, apatite, and bastnäsite serve as hosts for LREEs in\nweathered profiles65. These LREE-rich heavy mineral particles and colloidal clay complexes\nare readily eroded and transported by rivers, eventually accumulating as terrigenous sediment\nin subduction zones67,68.\nWhen REE-enriched sediments are subducted, prograde dehydration of the slab generates\naqueous fluids that efficiently scavenge the most labile constituents. REEs absorbed on clays\nor hosted by soluble carbonate phases (e.g., calcite/dolomite) are therefore liberated at\nrelatively shallow levels during early devolatilization. In contrast, apatite and bastnäsite\ntypically both release significant REE at hotter, sub-arc conditions or upon partial melting.\nMonazite is more refractory and requires high temperatures or partial melting to destabilize\nand liberate its REEs. Consequently, early slab-derived fluids are expected to carry minor\nREEs, consistent with experimental results15. At higher temperatures, silicic partial melts can\ndissolve LREE-bearing phases (apatite and, at higher temperatures, monazite), thereby\nenhancing LREE mobility16.\nNotably, compilations show that most deep-sea REE-rich sediments exhibit pronounced\nnegative Ce anomalies [Ce/Ce* = 2×Ce /(La +Pr )]69. For instance, REE-rich bioapatites in\nN N N\npelagic sediments exhibit Ce/Ce* values ranging from 0.05 to 0.2070. However, ore-related\nalkaline magmas worldwide do not consistently display such negative Ce anomalies69 (e.g.,\n~0.94 for Manning-Dechang69, ~0.95 for Saima, and ~0.97 for Luxi; Table S4). In fact, the\nnature of the sedimentary REE source affects Ce anomalies in resulting magmas. Many deep-\nsea REE-rich sediments (biogenic apatite, early authigenic phosphates, and carbonates)\nacquire their REEs from seawater or pore fluids and inherit seawater’s characteristic negative\nCe anomaly69,71. By contrast, purely terrigenous detrital sediments dominated by\naluminosilicate clays and oxides typically show little or no Ce anomaly characteristic of\ncontinental crust (e.g., ~0.99 for average UCC). Furthermore, if these sediments incorporate\nseawater-derived carbonates or have undergone pore-water exchange, a slightly negative Ce\nanomaly might appear71. Mixing variable proportions of such terrigenous components into\nmantle sources therefore reproduces the observed range of Ce/Ce* in alkaline magmas.\nIn summary, weathering concentrates REEs into sediments, and subduction processes then\nconvey those REEs into the mantle, ultimately yielding the REE-enriched magmas. The influx\nof LREE-rich material from the terrestrial sediments produces a metasomatized mantle source\nthat is characteristically enriched in LILE and LREE, explaining the geochemical signatures\nof REE-rich alkaline rocks (Fig. 2) and some arc volcanic rocks. Our model links the\nweathering on the Earth’s surface and subduction in the Earth’s interior.”\n(4) In the model presented in this paper, the metasomatized mantle is suggested to be a result\nof continental crust recycling. However, it is unclear whether it formed through partial melting\nor buoyant diapirs, as these processes are significantly different. Additionally, the mantle εNd(t)\nvalue of -5 used in geochemical modeling, as shown in Figure 4, appears to be unusual.\nResponse: i) In our model, we intentionally allowed both partial melting and buoyant diapirism\nas possible mechanisms. Below, we clarify why we do not distinguish and why both\nmechanisms are plausible given the available evidence.\nFirst, they have a common effect. Both partial sediment melts and sediment diapirs introduce\na silica-rich, REE-enriched component into the mantle. Geochemically, the mantle source was\nenriched by a continental crustal contribution (high LILE, Th, LREE, continental Li-Ba-Sr-Nd\nisotope signatures). Whether that contribution arrived as an infiltrating melt or as a rising solid\nmélange, the end result is a mantle domain that carries an imprint of continental material. In\nessence, the data tell us what was added, but not explicitly how it was delivered.\nSecond, they are not mutually exclusive but potentially interlinked processes. For example,\na subducted sediment layer might begin to partially melt; the presence of melt reduces the\ndensity and strength of that layer, which promotes diapiric rise (Currie et al., 2007). As the\ndiapir ascends, additional melting can occur within it. The ascending diapir thus carries melt\nand solid phases upward. This scenario combines both mechanisms, i.e., a melt-bearing diapir.\nThird, unlike arc magmas, alkaline magmas sourced from the subcontinental lithospheric\nmantle typically postdate subduction and form during later extensional phases; thus, the slab-\nderived component may have been introduced long before melting and not in a single event.\nAs mentioned by Reviewer 2, recycled components can persist under subsolidus conditions in\nthe SCLM for up to ~1 Gyr; many SCLM sections document multiple, time-separated\nmetasomatic additions associated with successive subduction cycles. In our context, isolating\na single transport mechanism is neither feasible nor especially relevant.\nIn the revision, we stress: “A complementary view is that, even without extensive melting,\nsubducted sediment can become less dense than the surrounding mantle and rise as a solid-\nstate diapir, transporting its REE inventory64. These mechanisms are not mutually exclusive;\nboth introduce REE-enriched material into the mantle and may operate in tandem.”\nii) A mantle ε (t) of −5 would be unusual if the “mantle” end-member was global depleted\nNd\nMORB mantle (DMM), which has positive ε (e.g., Workman and Hart, 2005). However, in\nNd\nour case, the “mantle” end-member refers to the lithospheric mantle beneath the North China\nCraton, not DMM. Cratonic SCLM typically shows low to negative ε (t) because of early\nNd\nstrong melt extraction and long-term enrichment (resulting in low Sm/Nd). Thus, cratonic\nlithospheric mantle has negative ε , whereas younger mantle may be slightly positive. The ε\nNd Nd\nvalue of -5 for NCC SCLM is documented by Zhang and Yang (2007). For clarity, we revised\nthe caption of Fig. 4 to state that the mantle end‑member represents NCC SCLM explicitly.\nReviewer #2 (Remarks to the Author):\nThe manuscript presents a comprehensive Li-Ba-Sr-Nd isotopic study of REE‑bearing alkaline\ncomplexes (Luxi, 120–130 Ma; Saima, 224–231 Ma) in the North China Craton. By combining\nnew high‑precision δ7Li, δ138/134Ba, 87Sr/86Sr, and εNd data with trace‑element modelling,\nthe authors argue convincingly that (1) the metasomatic agent responsible for mantle\nenrichment was dominantly subducted terrigenous sediment/continental crust, not altered\noceanic crust, and (2) metasomatism and partial melting were temporally decoupled, with later\nextensional events triggering low‑degree melting of a fertile lithospheric mantle. The work is\nwell organised and addresses a question of broad interest. I find the study suitable for\npublication after the minor issues below are addressed.\nResponse: We are pleased that you find our interpretations solid and insightful. We have\ncarefully addressed each of your comments in our revision.\nMajor (but minor‑revision level) Comments:\nA more applied angle, but still debated, is how important crustal recycling is for the global REE\nbudget of the mantle and crust. The manuscript suggests that recycling of continental material\n“plays a significant role in shaping Earth’s critical-metal budget”. This implies that without\nsediment subduction, the mantle might be poorer in REEs and other critical metals, potentially\nlimiting the formation of large deposits. Some researchers question if we can quantify this: for\ninstance, is there a secular increase in REE content of the lithospheric mantle over time as more\nsediments have been subducted? Hulett ++ (2016, NatGeo) looked at B isotopes in carbonatites\nover time and inferred progressive sediment involvement. Others might point to the peak in\nREE deposit formation around ~1.2 Ga (when supercontinents were assembling) as non-\ncoincidental. The authors could strengthen the manuscript by expanding on whether modern-\nstyle plate tectonics (active since ~800 Ma or so) was necessary for generating the giant REE\ndeposits, as these deposits are mostly Proterozoic or Phanerozoic in age. If so, sediment\nsubduction (a hallmark of modern subduction zones) could be the key differentiator that\nallowed extreme REE enrichment in certain mantle regions after a certain time. This remains\nan open question, tying mantle geochemistry to tectonic evolution.\nResponse: We appreciate this suggestion. We have supplemented a new section:\n“The role of plate tectonics in the mantle’s critical-metal budget\nGiant REE deposits occur primarily during the Proterozoic and Phanerozoic (Fig. 1B), with a\nmajor peak at ~1.2–1.3 Ga, coeval with the assembly of Rodinia (and/or Columbia), and a\nsmaller Paleozoic peak at ~450–400 Ma2. Notably, these peaks align with supercontinent\ncycles alternating between collisional orogenesis and extensional breakup. Geological records\nfrom Rodinia assembly also document abundant non-arc magmatism (rift- and plume-related)\nthat was enriched in HFSEs and REEs56. By the Mesoproterozoic, plate-tectonic processes (e.g.,\nprolonged two-sided subduction during Rodinia’s amalgamation) had evolved to permit\nextreme enrichment of these incompatible elements in the SCLM57. Subsequent tectonic events\n(whether continental rifting or collisional reactivation) could then tap into these pre-enriched\nmantle domains, generating the large sustained melt volumes needed to form giant REE\ndeposits1,2. In contrast, such REE-rich alkaline magmatism is scarce in the Archean, consistent\nwith the idea that early Earth tectonics did not efficiently concentrate these elements in the\nmantle.\nA key geodynamic inflection point was the onset of “modern” cold, deep subduction around\nthe Neoproterozoic (after ~800 Ma)58. Unlike the hotter subduction of the Archean-\nPaleoproterozoic (which likely caused most sediments to be released at shallow depths), late\nProterozoic subduction zones became cool enough to carry substantial quantities of sediments\ndeep into the mantle59. Modern-style subduction thus continuously transports incompatible-\nelement-rich sedimentary and crustal materials into the mantle wedge. Some of this recycled\nmaterial is also conveyed into the deeper mantle, seeding mantle plumes and intraplate sources\nwith enriched signatures. Crucially, silicate and carbonate melts of subducted sediments,\ncharacterized by high LREE abundances and volatile fluxing agents (CO , H O), provide prime\n2 2\nsources for exceptionally enriched magmas tapped during rifting and orogenesis. Thus,\nsediment subduction is a prerequisite for the source enrichments typical of Proterozoic-\nPhanerozoic alkaline complexes.”\nThere is also the question of how metasomes survive in the mantle over geologic time without\nbeing homogenized. The fact that we find localized veins with extreme isotopic signatures (e.g.,\nδ7Li ~ –10 permil in some mantle domains) implies that the mantle is not fully homogenized\nby convection. Some debate revolves around whether the convecting asthenosphere itself holds\nrecycled signatures (as suggested by enriched MORB and OIB), or if essentially all the\nenriched components reside in the stationary lithosphere or deep transition zone until tapped.\nThe MORB Ba and Li isotope evidence for pervasive sediment suggests even the upper\nconvecting mantle isn’t pristine, albeit with very low concentration. But certainly the largest\nanomalies (and richest metasomes) seem to reside in lithospheric “reservoirs.” Some workers\nhave suggested that metasomatic veins can undergo multiple melting events over time, each\ntime producing a batch of alkaline magmas, until they are exhausted. The “recurrent\nmagmatism over 2.5 Gy” in Greenland hints that either metasomatism was episodic or that new\nmetasomes kept forming over that time. The debate here is more about how many times can\nthe same metasome be partially melted. Incorporating these discussions would significantly\nstrengthen the manuscript.\nResponse: This comment is helpful. We now clarify that the Luxi and Saima signatures are\nbest explained by sampling long‑lived metasomes in the lithospheric mantle that were\nreactivated and partially melted during a late-stage thermal event, as follows:\n“Mantle heterogeneity and multiple metasomatism events\nThe mantle source of REE-fertile alkaline magmas is highly heterogeneous. We consider two\npossible scenarios to explain this heterogeneity in light of the isotopic evidence.\nSingle-event, multi-component metasomatism: one hypothesis is that a single metasomatic\nevent introduced different components into the mantle (e.g., mélange)3,54. If released over a\nshort geological interval, diverse materials, such as terrigenous and pelagic sediments as well\nas AOC, in the downgoing slab could metasomatize the surrounding mantle almost\nsimultaneously. As a result, some mantle domains could be overprinted by more terrigenous\nsediment-derived materials, while others receive a contribution from AOC-derived materials.\nSuch a single-event metasomatism would create a heterogeneous mantle source. However, the\nLuxi alkaline magmas were emplaced in the Early Cretaceous (~120 Ma)31, whereas the Saima\ncomplex formed in the Late Triassic (~230 Ma)33. The subduction enrichment beneath Luxi and\nSaima occurred during two significantly discrete events. A single metasomatic event is\ntherefore hard to account for the similar geochemical signatures in both complexes across such\na vast time gap.\nMultiple episodic enrichments: metasomes can survive in the mantle over geologic time without\nbeing fully homogenized by convection. A more plausible scenario is that the lithospheric\nmantle was enriched through repeated subduction episodes over a protracted period. The\neastern NCC margin experienced successive subduction regimes during the Phanerozoic (as\ndiscussed in the Geological background)35. Earlier events might have introduced ancient\nsediment-derived metasomatism in the lithospheric mantle (e.g., hydrous melts from subducted\nterrigenous sediments. Isotopic studies of the Luxi and Saima alkaline complex reveal elevated\ninitial 87Sr/86Sr and very low ε (t) values corresponding to two-stage model ages mostly\nNd\naround 2 Ga (Table S5), consistent with the incorporation of Proterozoic continental material\nin their source. Later subduction events (such as the Paleo-Pacific subduction) could then have\nadded fresh inputs of slab fluids or melts, producing greater source heterogeneity in the Luxi\ncomplexes, particularly in ε (t) values (Fig. 4). Over time, these episodic enrichments would\nNd\naccumulate in the mantle as multiple metasomatic layers or domains.”\nMinor Comments:\n1. Line 16: Change “significant” into “large”.\nResponse: Change made as suggested.\n2. Line 33-34: “Alkaline igneous rocks account for ~30 % of global rare earth element (REE)\nresources.”\nResponse: Change made as suggested.\n3. Lines 154-156: The conclusions presented here are crucial for in furhter discussions,\nespecially regarding Li and Ba isotopes. I recommend incorporating these discussions from the\nsupplementary material into the main manuscript.\nResponse: We appreciate the suggestion. However, moving the full discussion from the\nSupplementary Material into the main text would disrupt the manuscript’s narrative flow. We\ntherefore hope to retain the details in the Supplementary Material.\n4. Line 145-146: Several Luxi samples show δ7Li as low as –4.6 ‰. Discuss whether these\nvalues could reflect diffusive Li loss during late‑stage alteration.\nResponse: 6Li diffuses faster than 7Li (D /D = ~0.998). During diffusion, the escaping flux is\n7 6\n6Li‑rich and the residual rock becomes isotopically heavier, not lighter. If diffusion controlled\nthe signal, we would expect a positive covariation between δ7Li and Li depletion, which is\nopposite to our observations. Accordingly, the very low δ7Li values are more plausibly record\nsource processes other than simple diffusion, which we discuss in the Supplementary Materials.\n5. Line 192-194: Are these ratios (used here to represent the index of subduction input)\nelevated or decreased in your samples?\nResponse: In the revision, we no longer use those ratios as proxies for subduction input and\nhave reorganized the paragraph accordingly.\n6. Line 214: “generates sediments with low δ7Li values”\nResponse: Change made as suggested.\n7. Line 252-262: The discussion emphasises terrigenous sediments yet briefly mentions\ndelamination. Given emerging evidence for recycled lower crust in post‑collisional K‑rich\nmagmas worldwide, please add 2–3 sentences assessing whether partial melts/diapirs of\nfoundered Archean lower crust could mimic the observed Li–Ba–Sr–Nd systematics.\nResponse: Although partial melts or solid-state diapirs of foundered Archean lower crust could\nreproduce some aspects of the isotopic signal (e.g., Sr and Nd), two observations argue against\na dominant lower-crustal control: (i) unweathered lower crust does not supply the bulk REE\ninventory needed to generate source enrichment; and (ii) bulk REE increase systematically with\nTh/Yb but show no coherent relation with Sr/Y, a pattern characteristic of sediment-melt\naddition in arc settings. We therefore infer that terrigenous sediments were the principal agents\nof metasomatism, while acknowledging that multiple enrichment episodes and delivery\npathways may have also operated.\n8. Line 292-293: “At typical sub-arc depths, slab-derived fluids carry abundant LILE and lesser\nLREE”.\nResponse: This section has been re-organized.\n9. Line 314: Be specific about what simulation work is cited here.\nResponse: Change made as suggested.\n10. Figure 1 and the concluding model nicely generalise to other REE provinces. However,\nMianning-Dechang (eastern Tibet) and Gardar (SW Greenland) are only treated in Fig. 7.\nGiven their similar Li-Ba signatures, a short paragraph in the main text comparing NCC results\nwith these belts would strengthen the global relevance.\nResponse: We have supplemented in the main text.\n11. Fig. 3: Add a grey MORB field for visual reference\nResponse: Change made as suggested.\nReviewer #3 (Remarks to the Author):\nComments for the manuscript submitted to Nature Communications, by Qiu et al.\nFirst, I want to apologize for my overdue review. This manuscript reports a novel combination\nof Li-Ba-Sr-Nd isotopic data of alkaline complexes in the North China Craton to clarify the\nprocesses responsible for the metasomatic enrichment of their mantle source. They observed\nthat the alkaline complexes exhibit broadly similar Li-Ba-Sr-Nd isotope signature despite\nforming in different tectonic settings. They explained that neither altered oceanic crust\nrecycling nor serpentinite fluid metasomatism can reproduce their geochemistry, instead, they\nrequire the addition of subducted terrigenous sediment or continental crust. They further\nsuggest that recycling of continental crustal material plays a significant role in shaping Earth’s\ncritical-metal budget. I recommend rejection for the following reasons:\nResponse: Thank you for your review. We have carefully examined each concern you outlined\nand revised the manuscript accordingly.\n1. The biggest question of this manuscript is how to link the Ba-Li isotopes of alkaline\ncomplexes with the genesis of REE deposits? For now, we tentatively attribute the light Ba and\nLi isotopes in the alkaline complexes of the North China Craton to terrigenous sediments or\ncontinental crustal sources. However, the REE content in the continental crust is not high, thus,\nthe lithospheric mantle metasomatized by continental crust does not exhibit particularly high\nREE concentrations. Low-degree melting of such sources cannot form REE deposits. Moreover,\nthe properties of Ba and Li are entirely different from those of REEs—Ba and Li are fluid-\nmobile elements, whereas rare earth elements are fluid-immobile.\nResponse: We addressed these comments as follows:\ni) It is important to note that not all slab-derived agents are simple aqueous fluids. As\nsubduction proceeds deeper (or in continental collision scenarios like the Saima case), the slab\ncan begin to melt. Silicic (hydrous) melts derived from subducted sediments or crust are\ncapable of mobilizing REEs and other LILE like Li and Ba. This concept is directly relevant to\nour case: if the metasomatizing agent in the lithospheric mantle was not just a slab fluid, but\nrather a low-degree melt of terrigenous sediment, it could introduce both fluid-mobile elements\n(Ba, Li, etc.) and some amount of LREEs into the mantle wedge. In the revised introduction,\nwe relate the use of Li and Ba to the REE source enrichment of alkaline rocks, as follows:\n“Element partitioning during slab subduction controls which elements are transferred to the\nmantle. Aqueous fluids released by slab dehydration only weakly mobilize high-field-strength\nelements (HFSE) and REEs15, whereas partial melting of the subducting slab can transport\nREEs and even some HFSEs16. These slab-derived agents percolate into and react with the\noverlying mantle, crystallizing distinctive metasomatic mineral assemblages that sequester the\nintroduced elements into enriched veins or pockets17–19. Typical products include phlogopite\nand amphibole, which host Li, Ba, K, and REEs, along with clinopyroxene enriched in Ca and\nlight REEs17–19. Subsequent partial melting of this metasomatized mantle generates alkaline\nmagmas that inherit the chemical fingerprints of these metasomes18,20. As incompatible\nelements, Li, Ba, and REEs preferentially partition into the melt during mantle melting. Thus,\nthe enrichment of Li and Ba is directly linked to that of REEs from mantle source enrichment\nto magma generation.\nIn this context, Li and Ba and their isotopes (δ7Li and δ138/134Ba) serve as sensitive tracers for\nidentifying the sources of REE-rich alkaline rocks and distinguishing contributions from\ndifferent recycled reservoirs.”\nii) We highlight the key role of terrstrial sediments in source enrichment rather than the\ncontinental crust itself, as follows:\n“The subduction of terrigenous sediments derived from weathered continental rocks is\nnecessary to refertilize an otherwise REE-depleted mantle. During continental chemical\nweathering, REEs undergo strong mineralogical partitioning. In acidic, highly leached soil\nenvironments (e.g., tropical laterites), the middle to heavy REEs are relatively immobile and\ntend to be retained in the regolith by sorption onto secondary clay minerals like kaolinite and\nillite65,66. This forms ion-adsorption type REE deposits that are notably enriched in MREEs\nand HREEs. By contrast, the LREEs are largely sequestered in refractory detrital minerals.\nAccessory minerals like monazite, apatite, and bastnäsite serve as hosts for LREEs in\nweathered profiles65. These LREE-rich heavy mineral particles and colloidal clay complexes\nare readily eroded and transported by rivers, eventually accumulating as terrigenous sediment\nin subduction zones67,68.\nWhen REE-enriched sediments are subducted, prograde dehydration of the slab generates\naqueous fluids that efficiently scavenge the most labile constituents. REEs absorbed on clays\nor hosted by soluble carbonate phases (e.g., calcite/dolomite) are therefore liberated at\nrelatively shallow levels during early devolatilization. In contrast, apatite and bastnäsite\ntypically both release significant REE at hotter, sub-arc conditions or upon partial melting.\nMonazite is more refractory and requires high temperatures or partial melting to destabilize\nand liberate its REEs. Consequently, early slab-derived fluids are expected to carry minor\nREEs, consistent with experimental results15. At higher temperatures, silicic partial melts can\ndissolve LREE-bearing phases (apatite and, at higher temperatures, monazite), thereby\nenhancing LREE mobility16.\nNotably, compilations show that most deep-sea REE-rich sediments exhibit pronounced\nnegative Ce anomalies [Ce/Ce* = 2×Ce /(La +Pr )]69. For instance, REE-rich bioapatites in\nN N N\npelagic sediments exhibit Ce/Ce* values ranging from 0.05 to 0.2070. However, ore-related\nalkaline magmas worldwide do not consistently display such negative Ce anomalies69 (e.g.,\n~0.94 for Manning-Dechang69, ~0.95 for Saima, and ~0.97 for Luxi; Table S4). In fact, the\nnature of the sedimentary REE source affects Ce anomalies in resulting magmas. Many deep-\nsea REE-rich sediments (biogenic apatite, early authigenic phosphates, and carbonates)\nacquire their REEs from seawater or pore fluids and inherit seawater’s characteristic negative\nCe anomaly69,71. By contrast, purely terrigenous detrital sediments dominated by\naluminosilicate clays and oxides typically show little or no Ce anomaly characteristic of\ncontinental crust (e.g., ~0.99 for average UCC). Furthermore, if these sediments incorporate\nseawater-derived carbonates or have undergone pore-water exchange, a slightly negative Ce\nanomaly might appear71. Mixing variable proportions of such terrigenous components into\nmantle sources therefore reproduces the observed range of Ce/Ce* in alkaline magmas.”\n2. The Ba and Li isotopic compositions of altered oceanic crust and terrigenous sediments\nexhibit an extremely wide range, spanning from values lower than to those higher than the\nmantle values (Zhang et al., 2024: Deciphering Contribution of Recycled Altered Oceanic\nCrust to Arc Magmas Using Ba‐Sr‐Nd Isotopes). Therefore, the authors should provide a\ndetailed discussion on the potential contributions of these two surface materials to the alkaline\ncomplexes in the North China Craton. Additionally, the Ba isotopic compositions of OIB\n(Ocean Island Basalt) also show a broad variation, ranging from values higher than the mantle\n(e.g., HIMU endmember) to those lower than the mantle (e.g., EM endmembers) (Bai et al.,\n2022: Barium isotopes in ocean island basalts as tracers of mantle processes). Thus, the authors\ncannot simply conclude that Li and Ba cannot be subducted into the convecting mantle.\nResponse: i) In the revision, we clarify why AOC is unlikely to be the dominant source for the\nalkaline complexes, and instead emphasize the key role of terrigenous sediments:\n“Minor influence of altered oceanic crust\nSubducted altered oceanic crust has often been considered a potential source of trace element\nenrichment in mantle-derived magmas. Altered oceanic basalts and oceanic authigenic\nsediments typically acquire heavy Li and Ba isotope signatures due to interaction with seawater.\nModern seawater is highly enriched in heavy Li (δ7Li up to 31 ‰) and heavy Ba (δ138/134Ba up\nto 0.6 ‰)21,27. Consequently, oceanic crust altered at low temperatures can record very\nelevated δ7Li (often reaching +10 to +20 ‰)21 and moderately high δ138/134Ba27 (Fig. 3). If\nmelts or fluids derived from such AOC were added to the mantle source beneath Luxi and Saima,\nit is expected that the metasomatized mantle would shift toward heavier Li and Ba isotopic\ncompositions21,28,47.\nSeveral lines of evidence indicate that AOC alone cannot account for the isotopic compositions\nobserved in the alkaline rocks. First, dehydration reactions in the oceanic crust preferentially\ndrive the heavier Li and Ba isotopes into the fluid phase, leaving the residual slab depleted in\nheavy isotopes39,44,48. This means that when the AOC reaches sub-arc depths, it may no longer\ncarry an extremely heavy isotopic signature. Thus, much of the heavy Li and Ba may have been\nlost to earlier fluids that escaped into the forearc. However, slab dehydration can cause at most\n2–4 ‰ decrease in δ7Li of the residual slab39, which would not be sufficient to produce the\nmuch lower δ7Li recorded in some samples. Second, AOC typically has positive ε (>0)\nNd\nbecause its Nd isotopic composition remains similar to MORB even after alteration. In contrast,\nthe Luxi and Saima rocks have strongly negative ε (t) (Fig. 4). This is a signature more\nNd\nconsistent with old continental crust than with young oceanic crust. Finally, if pelagic (deep-\nsea) sediments or AOC were the dominant contributors of REEs and other metasomatic agents,\none might expect heavier Li and Ba isotopic compositions in the resulting magmas27,49. Instead,\nthe data show the opposite (lighter), ruling out a simple AOC-dominated source.\nDominant influence of subducted terrigenous sediments\nOur data suggests continent-derived sediments as the key source of the unusual light Li and Ba\nisotope signatures in the Luxi and Saima mantle. Unlike pelagic sediments or AOC, terrigenous\nsediments have intrinsically low δ7Li and δ138/134Ba24, which can be imparted to the mantle\nwhen these sediments are subducted and release fluids or melts20,27. Siliciclastic sediments\nfrom continental crust typically carry the isotopic imprint of chemical weathering and erosion\nprocesses. During weathering on the continents, secondary minerals (like clays) preferentially\ntake up the lighter isotope 6Li, while 7Li is leached into fluids50. This leaves the weathered rock\nand soils depleted in 7Li and thus characterized by low δ7Li values (e.g., down to −6.8 ‰ in\nweathered granites)51. Moreover, diagenesis and prograde metamorphism of sediments in the\nsubduction environment can further lower their δ7Li by driving off 7Li-rich fluids52, yielding\nresidual solid phases with lower δ7Li. Ba isotopes behave analogously. Continental sediments\nare thought to represent a light δ138/134Ba reservoir, because weathering and metamorphic\ndehydration both remove 138Ba from the rocks43. While comprehensive Ba isotope data on\nsediments are still emerging, an extremely low δ138/134Ba value of –0.49 ‰ has been\ndocumented in terrestrial sediment samples26.\nIn general, metasomatic agents derived from these sediments will have lower δ7Li and\nδ138/134Ba, especially if the sediment had undergone prior slab dehydration. This creates a\nheterogeneous SCLM for later alkaline magmatism. The data cluster toward the light end of\nδ7Li-δ138Ba space and overlap terrigenous signatures, along with the extremely negative ε (t),\nNd\nand high 87Sr/86Sr , indicating that continent-derived sediment was the dominant metasomatic\n(t)\nagent.”\nii) The magnitude and variability of δ138/134Ba in OIB are modest compared with arc lavas.\nMost OIBs deviate by only ~0.1‰ from the ambient mantle value (~0.05 ± 0.05 ‰). Extreme\nHIMU lavas are +0.11 ‰ (slightly above mantle), whereas some EM-type basalts reach –0.07 ‰\n(slightly below mantle). These values fall within or just outside the commonly defined\nmantle range. Carbonatites, including plume-related examples, also display mantle-like\nδ138/134Ba, and Li isotopes in both OIB and carbonatites generally lie within the mantle range.\nTaken together, these observations suggest that while recycled Ba and Li can survive into\nthe convecting mantle, their average isotopic impact on the deep mantle is limited. Given\nthis comment, we have removed the statement that Li and Ba “cannot” be subducted into the\ndeep mantle. In this context, an OIB-like recycled component is unlikely to be the dominant\nsource for our samples. We revised the manuscript as follows:\n“In addition, although a substantial fraction of slab-hosted Ba and Li is released at sub-arc\ndepths, a non-negligible portion can be retained and recycled into the convecting mantle, as\nindicated by the near-mantle δ138/134Ba in OIB (HIMU only up to +0.11 ‰; EM only down to\n–0.07 ‰)41 and the slightly elevated Li isotopic compositions of OIBs (Fig. 3). Nevertheless,\nthe small magnitude of these deviations implies that such recycled inputs have a limited ability\nto modify the deep mantle and are unlikely to account for the isotopic features of our samples.\nThus, the possibility of a significant asthenospheric (deep mantle) contribution can be ruled\nout. In fact, most of the subducted Li and Ba are sequestered in the SCLM (creating an enriched\nslab-influenced domain)46. The Luxi and Saima magmas likely originate from this enriched\nSCLM beneath the craton.”\n3. Although most researchers believe that alkaline complexes originate from the partial melting\nof metasomatically enriched lithospheric mantle, a growing number of recent studies suggest\nthat the mantle sources of alkaline rocks—including alkaline complexes, kimberlites,\nlamproites, and ultrapotassic lamprophyres—may instead derive from the asthenosphere\n(Giuliani et al., 2020, Science Advances; Sarkar et al., 2022, Geology……). Therefore, the\nauthors should appropriately consider the possibility that REE deposits associated with alkaline\ncomplexes could also be sourced from the asthenosphere.\nResponse: The reviewer’s comment references “kimberlites, lamproites, and ultrapotassic\nlamprophyres”, rock types that indeed are often linked to deep mantle sources. However, it is\ncrucial to distinguish what type of lamprophyres are present in our study area. Our\ncomplex is accompanied by calc-alkaline lamprophyre dikes (actually, we have stressed this in\nthe discussion), not the intraplate “alkaline” lamprophyres or lamproites. This distinction is\nwell documented in petrogenetic literature (details in Krmíček et al., 2022; Krmíček and\nChalapathi Rao, 2022). Calc-alkaline lamprophyres are characteristically found in convergent\norogenic settings. They typically occur as dikes associated with calc-alkaline and alkaline\nplutons or post-collisional extensional phases in regions that had recent subduction. In contrast,\nalkaline/ultramafic lamprophyres (e.g., camptonite, monchiquite, kimberlitic lamprophyres)\nare usually associated with extensional regimes such as rifts, continental hotspots, or cratonic\ninteriors. As a result, the source of calc-alkaline lamprophyres is generally the lithospheric\nmantle modified by subduction, whereas alkaline lamprophyres originate in deeper\nasthenospheric mantle or plume sources in the absence of recent subduction.\nThe calc-alkaline lamprophyres in our study (and worldwide), as well as syenites, display “arc-\nlike” geochemical signatures: enrichment in LILE (K, Rb, Ba) and LREE, coupled with strong\ndepletions in Nb, Ta, Ti, and sometimes P. They also often have elevated Pb and Sr, reflecting\ncrustal or subduction input. These features are typical of magmas derived from a subduction-\nmodified lithospheric source (either active subduction or remnant metasomatism). By\ncontrast, truly alkaline lamprophyres (intraplate type) tend to lack strong Nb-Ta negative\nanomalies and can show OIB-like patterns (relatively flat rare-earth patterns, etc.), indicating\na more primitive mantle source with minimal subduction imprint. The presence of calc-alkaline\nlamprophyres thus strongly suggests a lithospheric mantle source that was enriched in a\nsubduction environment. In the revision, we have stressed these points.\nAdditionally, these recent studies do not dismiss the role of the lithospheric mantle altogether.\nInstead, they highlight that the lithosphere often remains critical as a metasomatic filter that\nmodifies those deep melts. For instance, Giuliani et al. (2020) emphasize that lithospheric\nmantle assimilation is a “major process” in kimberlite magmatism. The convective mantle-\nderived kimberlite melt must pass through and interact with lithospheric mantle, which can\ndramatically change its composition (e.g., raising Mg/Si, introducing LILE). Similarly, Sarkar\net al. (2022) require phlogopite-rich lithospheric veins to explain the high K and trace-element\npatterns of lamproites. In summary, the emerging view is a hybrid model.\nReference\nCurrie C A, Beaumont C, Huismans R S. The fate of subducted sediments: A case for backarc\nintrusion and underplating[J]. Geology, 2007, 35(12): 1111-1114.\nGiuliani A, Pearson D G, Soltys A, et al. Kimberlite genesis from a common carbonate-rich\nprimary melt modified by lithospheric mantle assimilation[J]. Science Advances, 2020, 6(17):\neaaz0424.\nKrmíček L, Rao N V C. Lamprophyres, lamproites and related rocks as tracers to\nsupercontinent cycles and metallogenesis[J]. 2022.\nKrmíček L, Magna T, Pandey A, et al. Lithium isotopes in kimberlites, lamproites and\nlamprophyres as tracers of source components and processes related to supercontinent cycles[J].\n2022.\nLan J, Li D, Xing N, et al. Petrogenesis of the alkaline complex in the Weishan REE deposit,\nLuxi Block, eastern North China Craton: Implications for REE mineralization[J]. Ore Geology\nReviews, 2024, 167: 106005.\nJu N, Ren Y S, Zhang S, et al. Metallogenic epoch and tectonic setting of Saima niobium\ndeposit in Fengcheng, Liaoning Province, NE China[J]. Minerals, 2019, 9(2): 80.\nSarkar S, Giuliani A, Phillips D, et al. Sublithospheric melt input in cratonic lamproites[J].\nGeology, 2022, 50(11): 1296-1300.\nWei P, Yu X, Li D, et al. Geochemistry, Zircon U–Pb Geochronology, and Lu–Hf Isotopes of\nthe Chishan Alkaline Complex, Western Shandong, China[J]. Minerals, 2019, 9(5): 293.\nWorkman R K, Hart S R. Major and trace element composition of the depleted MORB mantle\n(DMM)[J]. Earth and Planetary Science Letters, 2005, 231(1-2): 53-72.\nWu B, Wang R C, Yang J H, et al. Zr and REE mineralization in sodic lujavrite from the Saima\nalkaline complex, northeastern China: A mineralogical study and comparison with potassic\nrocks[J]. Lithos, 2016, 262: 232-246.\nResponse to Reviewers’ Comments\nReviewer #1 (Remarks to the Author):\nThe authors made a substantial revision, especially they going deeper to explain the data and\nmake its meaning clearer. In the revised version, authors give a clear logic path in which\ncontinental weathering makes REEs concentrate into sediments; then subduction carries these\nsediments into the mantle; finally, REE-enriched magmas are produced. This direct link from\nsurface weathering to deep-mantle REE enrichment is quite fresh and, in my opinion, really\nadd new value. I recommend publishing after minor revisions.\nResponse: We appreciate the reviewer for the evaluation and positive recommendation.\nSome specific comments:\nIs the Saima rare-metal deposit hosted in a complex or a region? The terms 'Saima' and 'Saima\narea' should be used consistently throughout the text. It is recommended to include information\non the resources (e.g., tonnage, grade) of the rare-metal deposits discussed.\nResponse: The ambiguous term has been removed. Resources were added in the SI.\nPlease add explanatory arrows and descriptions for the green and purple lines. It is unclear\nwhat the purple line represents in Fig. 2A and the green line in Fig. 2B.The legends for Figs.\n3A and 3B appear to be mixed or unclear. Consider combining them or presenting them in a\nclearer format.\nResponse: Changes made as suggested. We labeled the purple and green lines in the figure and\nexplained them in the figure caption. Furthermore, we explain the shaded envelopes (full range\nof values) and the central lines (arithmetic means) in the figure caption.\nLine 33-37: Where are these externally derived fluids/melts sourced from? Are they related to\noceanic or continental materials? This should be clarified.\nResponse: We replace “externally derived fluids/melts” with “slab-derived fluids/melts”.\nLine 148: The previous section does not describe the mantle signature. It would be helpful to\ninclude supporting evidence earlier in the text.\nResponse: The sentence was ambiguous. Revised to “ … to characterize their primary\ngeochemical signatures.”, i.e., stressing what we will do in the paper.\nLine 148-149: “As expected, the REE contents of the samples, while elevated, do not reach ore\ngrade.” Please tell what comparison you use for “ore grade”.\nResponse: Information added to the text: “typically 0.4 to 1.4 wt% TREO39; Total Rare Earth\nOxide”\nLines 178-179: Please briefly explain why εNd for the NCC SCLM you set to negative value.\nA short reason will help reader to understand.\nResponse: The metasomatized SCLM has negative values. The depleted mantle has positive\nvalues. To avoid confusion, we have replaced the “NCC SCLM” with the depleted mantle,\nwhich is widely used as the mantle reference; this results in minor changes in the relative\ncontributions (the depleted mantle does not contribute much to the element budget of\nincompatible elements), but does not affect the overall conclusions.\nLines 206-208: Please add references to support the statements.\nResponse: Two references (48,49) have been added.\nLine 320-322 Please unify the order of ages (either from oldest to youngest or vice versa) for\nconsistency.\nResponse: Revised as suggested.\nLine 354: “Moreo importantly” should be “More importantly.”\nResponse: The typo has been corrected.\nReviewer #2 (Remarks to the Author):\nThank you for re-inviting me to review this revised manuscript. The authors have satisfactorily\naddressed my comments and those of Reviewers 1 and 3. As noted in my initial review, I am\nsupportive of publication. The scope of the additional revisions is substantial, and the quality\nof the presentation has improved markedly. Key contributions to the REE community include\n(1) development of a new framework to identify terrestrial sediment recycling as a principal\ncontrol on mantle REE enrichment, and (2) reconciliation of recent Ce/Ce* observations with\nthe sediment-recycling model. The manuscript is timely. I therefore recommend publication in\nNature Communications after the following minor points are considered:\nResponse: We thank the reviewer for the positive and encouraging assessment.\n(1) The authors have clearly justified the exclusion of an asthenospheric contribution\nhighlighted by Reviewer 3. Nevertheless, I suggest adding one or two concise sentences to help\nreaders who may not work on alkaline REE deposits. For example, you could note that several\nlarge peralkaline complexes formed in plume settings (e.g., the Kola Peninsula, Russia)\ntypically show depleted Sr and Nd isotope ratios consistent with derivation from long-term\ndepleted mantle, whereas NCC alkaline complexes display enriched lithospheric signatures\n(negative εNd; elevated 87Sr/86Sr), indicating a lithospheric rather than asthenospheric source.\nIn addition, the two groups actually show different mineralization styles.\nResponse: We followed this truly helpful suggestion and added the following sentence:\n“Importantly, large peralkaline complexes that formed in plume settings (e.g., in the Kola\nPeninsula, Russia) typically have depleted Sr and Nd isotope ratios (low initial 87Sr/86Sr and\npositive ε (t) values) consistent with their derivation from a mantle that has been long\nNd\ndepleted44, whereas the NCC alkaline complexes have enriched lithospheric signatures\n(negative ε (t); elevated initial 87Sr/86Sr; Fig. 4), indicating a lithospheric source.”\nNd\n(2) In Figure 5, the axis tick directions are inconsistent (panels A and B point inward; C and D\npoint outward). Please unify the tick directions across all panels.\nResponse: Modified. Now, all ticks in all figures point inward.\nReviewer #3 (Remarks to the Author):\nI appreciate the authors' comprehensive response to the reviewers' comments. However, the\ncentral issue of the manuscript remains unresolved: namely, the genetic link between the Ba-\nLi isotopes in the alkaline complexes and the formation of the REE deposits. The concentration\nof rare earth elements (REEs) in terrigenous sediments is inherently low, typically on the order\nof several tens of parts per million (Plank and Langmuir, 1998, CG). Consequently, even a\nlithospheric mantle metasomatized by melts from such sediments is unlikely to achieve REE\nconcentrations high enough for ore formation.\nResponse: Continent-derived sediments are heterogeneous in REE. Fine-grained shales (e.g.,\nNASC; North American shale composite) cluster near upper-crustal background (~1.6×102\nppm ΣREE), while placer-type heavy-mineral sands can reach >103 ppm ΣREE where\nmonazite/allanite are abundant. Typically, the REE patterns of such rocks are strongly LREE-\nenriched as accessory heavy minerals like allanite and monazite dominate the REE budget.\nA simple mixture would require an exceptionally enriched crustal REE source. But this is not\nhow REEs are transferred to the mantle and later mobilized from the metasomatized mantle.\nInstead, partial melts of the subducted sediments are strongly enriched in REE (relative to the\nstarting material). This is the first step of enrichment. These sediment-derived melts are\nrelatively Si-rich and, therefore, react with the mantle to form diffuse and veined metasomatic\nzones with pyroxene and hydrous minerals (phlogopite, richterite, et.). This reaction consumes\nthe sediment-derived melts/fluids, which deliver REE, H O, CO , F, and P among a wide range\n2 2\nof lithophile elements into the metasomatized domains of the mantle wedge, creating vein-rich\ndomains. This is the second step of enrichment. Subsequent low-degree partial melting of this\nvein-rich source results in highly mobile melts that are rich in lithophile elements (including\nREE) and fluids and that have a crustal isotope signature. Collection of these melts, followed\nby magmatic differentiation and late fluid focusing, can amplify the REE contents of these\nmelts to ore-grade concentrations.\nThis information has been included in the revision.\nFurthermore, an alternative petrogenetic model involving the lower oceanic crust has not been\nsufficiently ruled out. Recent Ba isotope data from an intact altered oceanic crust (Nan et al.,\n2023, CG) show that the lower gabbroic section possesses distinctly light δ138/134Ba values\n(-0.22‰ to -0.06‰) compared to the depleted MORB mantle (DMM). This presents a plausible\ncontribution from the lower oceanic crust to the magma source, which the authors have not\nadequately addressed.\nResponse: We appreciate the reviewer’s concern about the role of the lower oceanic crust\n(LOC), which indeed may have light Ba isotope compositions. We expanded our discussion to\nexplain why we think that the lower oceanic crust is not an important source of REE for the\nstudied deposits. (i) Several samples have lighter Ba isotope compositions than LOC. For LOC\nto be a possible source, it would have to be altered by late-magmatic fluids that remove\nisotopically heavy Ba (Nan et al., 2023). LOC samples have very low Ba contents (6.5–8.0\nμg/g), significantly lower than MORB (mean 29.2 ppm; Gale et al., 2013). Alteration would\nreduce these contents even more. The recycling of such Ba-poor lithologies cannot generate\nrocks with high Ba contents, which is characteristic of the studied alkaline rocks (Fig. 2). (ii)\nA simple mass balance would demonstrate that a moderate contribution from the LOC would\nnot change the isotopic composition of the alkaline rocks. (iii) The “abnormal” samples do not\nhave the composition of the AOC end-member. (iv) The LOC scenario is inconsistent with the\nLi–Sr–Nd isotopic signatures and the trace-element patterns of Luxi and Saima rock.\nThis information (except the mass balance argument) has been included in the revision.\nFinally, even if the proposed model is valid, the premise that terrigenous sediments contribute\nto alkaline rock sources is not novel, as it has been previously established by zircon Hf-O\nisotope studies (Zhu et al., 2017, Geology). In light of these fundamental concerns and the lack\nof a demonstrably new conceptual advance, I find the innovative contribution of this work\ninsufficient for the high impact criteria of Nature Communications. I therefore must\nrecommend against its publication here and would suggest submission to a more specialized\njournal.\nResponse: We respectfully clarify that our contribution goes beyond identifying continental\ncrustal materials (this is indeed well-known). Specifically, we emphasize (i) the role of\ncontinental weathering in generating REE-bearing terrestrial sediments and their diagnostic\nisotopic characteristics and (ii) delineate an end-to-end pathway that links surface processes to\nmantle metal enrichment. To our knowledge, this integrated perspective has not previously\nbeen articulated within a single framework. This broader contribution is also acknowledged by\nReviewers 1 and 2. Most importantly, however, (iii) we outline a pathway to form major REE\ndeposits that is an alternative to “mantle only” or “deep mantle only” models. This is especially\nimportant for the understanding of such deposits (and the exploration of them).\nComments linked to the text.\nLine 19: The rare earth element (REE) concentration in the alkaline magmatic complex is\ninsufficient for mineralization.\nResponse: The text at Lines 18-20 discusses isotopic constraints on source provenance. It does\nnot make any statement about REE concentrations in alkaline magmatic complexes.\nLines 23-24: Please explain why storage of REE-rich sediments within the lithospheric mantle\ncan lower its solidus.\nResponse: We explain this now explicitly in the main text: “Because these metasomes also\ncontain volatiles, they lower the peridotite solidus strongly and are far more fusible than the\nsurrounding dry depleted mantle. During subsequent tectonic events, these enriched domains\nare the first to melt, even at very low degrees of partial melting63. This selective melting of\nenriched domains explains both the preference of REE-rich alkaline silicate systems for craton\nmargins and their link to extensional tectonics (Fig. 1A).”\nLine 148: The Luxi samples are characterized by high SiO2 contents up to 75 wt%, indicating\na highly evolved nature.\nResponse: Only one evolved sample (SiO = ~75 wt%) was included for comparison. The\n2\nmajority of investigated Luxi samples have SiO < 70 wt% and do not indicate an overall highly\n2\nevolved character. Importantly, the evolved comparison sample shows no anomalous isotopic\ncompositions relative to the suite and does not affect our isotopic interpretations.\nLines 221-222: What about ancient AOC? The ancient lower oceanic crustal component can\nsimultaneously possess the characteristics of enriched Nd isotopes and light Ba isotopes. How\ncan we rule out this possibility?\nResponse: As discussed above, making the oceanic crust an isotopically light Ba source\nrequires additional processes involving fluid modification. However, these rocks are Ba-poor;\ntherefore, recycling such crust cannot generate the high-Ba alkaline rocks.\nLine 256: Please explain why the sub-continental lithospheric mantle has εNd = 0.\nResponse: We replaced the “NCC SCLM” with the widely used depleted mantle (εNd >0).\nThis change does not affect our conclusions. See also reply to reviewer 1 (comment Lines 178-\n179).\nLines 259-260: Please explain the rationale for selecting these end-member values (set as δ7Li\n= –6 ‰ and δ138/134Ba = –0.3 ‰).\nResponse: This information is / has been provided in the Supplementary Materials of the\npresent and earlier versions (see note below Table S6) and we refer to the Supplemenary\nMaterials in the caption of Figure 4 that presents the binary mixing curve. In brief, the\ndehydrated crust is likely to have a δ138/134Ba value below the reference value for the Upper\nContinental Crust (0 ‰) due to effects of Ba isotope fractionation during fluid-rock interaction\n(weathering, dehydration) with fluids preferring the heavy Ba isotopes. The value for δ7Li is\ntaken from reference 36 (Romer & Meixner 2014), as cited in Table S6.\nLines 319-342: The sudden appearance of this section is puzzling, given the apparent lack of\nconnection between rare earth deposit formation and plate tectonics.\nResponse: The paragraph was added in direct response to Reviewer 2’s suggestion to discuss\nwhether modern-style plate tectonics (and, specifically, sediment subduction) could modulate\nthe probability of generating exceptionally REE-enriched mantle domains.\nLines 361-365: Given that both the Sr/Y and Th/Yb ratios are established indicators of residual\ngarnet, could you please clarify their specific use in distinguishing between sediment melt and\nAOC melt?\nResponse: We agree that Sr/Y is widely used as a proxy for melting/fractionation under\nresidual garnet ± amphibole and suppressed plagioclase (i.e., “adakitic” conditions) and thus is\ncommonly invoked for slab melts derived from AOC/eclogite-facies sources (e.g., Defant &\nDrummond, 1990; Martin, 1999; Castillo, 2012). In contrast, Th/Yb is not an established\nindicator of residual garnet per se; instead, it is most commonly used (especially together with\nNb/Yb) to track sediment-derived components because Th is strongly enriched in sediments,\nwhereas Yb behaves compatibly in mantle sources (e.g., Pearce, 2008; Plank, 2005). Although\nthe presence of residual garnet can depress Yb and thereby secondarily elevate Th/Yb, in\nnatural arc systems, Th is typically dominated by sediment variability, making Th/Yb a\nsediment-input rather than a garnet-residue proxy.\nAccordingly, in our dataset, we use Th/Yb to monitor the sediment-melt contribution and Sr/Y\nto evaluate the influence of AOC-like slab melts. The observed increase of bulk REE with\nTh/Yb supports variable sediment-melt addition, whereas the lack of a coherent relation\nbetween bulk REE and Sr/Y argues against residual-garnet-controlled AOC melts being the\nfirst-order control on our samples."}]

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