Hydration of the lithospheric mantle above big mantle wedges indicated by sapphire deposits

Hydration of the lithospheric mantle above big mantle wedges indicated by sapphire deposits | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Hydration of the lithospheric mantle above big mantle wedges indicated by sapphire deposits Wei-Dong Sun This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4794218/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The deep water cycle is pivotal in shaping Earth’s habitable environments. A fundamental process of this cycle is upward migration of water from Earth’s deep interior. A significant inquiry concerns how water released from the deep mantle hydrates the lithosphere. Here we report hydrothermal experiments of the “phlogopite + H 2 O” system, showing that the incongruent dissolution of phlogopite in water produces sapphire (Al 2 O 3 ) under lithospheric mantle P-T conditions. Our results suggest that sapphire can be leached from phlogopite in the lithospheric mantle by excess water, and subsequently transported to the surface by basaltic magmas. We propose that the magmatic sapphire deposits in eastern China, Southeastern Asia, and eastern Australia provide evidence of lithospheric mantle hydration. The water that leached the lithospheric mantle mainly originated from the mantle transition zone or subducted slabs, while in eastern Australia and Hainan Island, mantle plumes may also contribute. The occurrence of sapphire deposits indicates extensive hydration of lithospheric mantle in big mantle wedges. Earth and environmental sciences/Solid Earth sciences/Geochemistry Earth and environmental sciences/Solid Earth sciences/Petrology Main text Water is requisite to form a habitable Earth 1 , 2 . Plate subduction transports surface water into the upper mantle or even the mantle transition zone (MTZ) 3 . The subducted slabs or MTZ then release water to hydrate the mantle wedge above 4 , a process known as the deep water cycle. This cycle directly induces magmatic activities and promotes ore formation and, consequently, impacts the stability of the ancient cratonic lithosphere 5 . The big mantle wedge beneath eastern China serves as a typical example, which has been influenced by the subduction of the western Pacific plate since the Jurassic 6 , 7 , 8 , 9 . However, there is a disagreement between geophysical and geochemical evidence regarding water migration beneath eastern China. Geophysical researchers reported stagnant slabs beneath eastern China, which currently span a length of 1200−1600 km from the trenches to the west horizontally 10 , 11 . Seismic data shows that the subducted slab can disturb the hydrous MTZ 12 , 13 or cause the breakdown of hydrous phases in the slab 4 , 14 , releasing water to hydrate mantle atop 410 km 9 . However, geochemists argue that the Cenozoic lithosphere beneath eastern China is relatively dry as suggested by low water contents (0−90 ppm on average) of nominally anhydrous minerals (NAMs) in mantle xenoliths. In contrast, the lithospheric mantle source contained >1000 ppm of water during the Mesozoic era 15 , 16 . The lithospheric hydration beneath eastern China lacks geochemical evidence. Here we propose that the magmatic sapphire (Al 2 O 3 ) deposits in eastern China indicate major Cenozoic lithospheric hydration (Fig. 1), based on our hydrothermal experiments. We synthesized sapphires using phlogopite and water under lithospheric mantle P-T conditions, linking sapphire formation with hydration events. This model can also be applied to the Cenozoic sapphire deposits distributed in eastern Australia and Southeast Asia, indicating lithospheric mantle hydration beneath those regions. METHODS 2.1 Hydrothermal diamond anvil cell experiments and Raman spectroscopy The experiments were conducted using a hydrothermal diamond-anvil cell (HDAC-V 17 ) at the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry, China. The HDAC was equipped with two Ia-type diamonds (750 µm culet diameter) and rhenium gaskets with a 500-µm-diameter hole. Microscopic observations and in-situ laser Raman spectroscopic investigations were performed using a Confocal WITec Raman Microscope alpha300 R., experiments. Two diamonds sandwich the rhenium gaskets, forming a reaction cell. The cell can be considered as isometric while heated. Heating was achieved with molybdenum wires, controlled by the Hydrothermal Diamond Anvil Cell Controller 1300, with temperature errors of < 1°C. The cell’s pressure was calculated using an improved method 18 based on the water’s equation of state 19 , with errors of < 0.1 GPa. Pure helium gas protected the diamond from oxidation. A piece of natural phlogopite from Lixian-Dangchang of Gansu Province, in the western Qinling area, was used as the initial material (Extended Data Fig. S1 ). The phlogopite piece measured approximately 1×2 cm with a uniform thickness of ~ 2 mm. For each experiment, a small slice of phlogopite and deionized pure water were placed into the anvil. The volume of the phlogopite was estimated by measuring its area and thickness, and the diameter of the air bubble under the microscope. This allowed for estimating the volumes of water, the air bubble, and the phlogopite slice. In all experiments, the mass ratios of water/mineral ranged from 2 to 10, with errors of about ± 10%. The unpolarized Raman spectra were obtained during and after the experiments using a WITec confocal Raman microscope. The microscope utilized a 532 nm (frequency doubled Nd:YAG) laser excitation source operating at 20 mW, with a 100 mm confocal aperture and an 1800 gr/mm grating. Measurements were performed in backscattering geometry under an Olympus microscope, employing a Mitutoyo MPlan Apo SL 20× long-working distance objective. The lateral spatial resolution of the microscope was less than 1 µm, and the spectrometer’s accuracy was ± 1.5 cm ‒ 1 . Each spectrum was collected for 180 seconds with three accumulations. The collected data from different spectral windows, each with a width of 500 cm ‒ 1 , was processed using Project Five software. The HDAC device was operated from room temperature up to 950°C. Each run was maintained at the target temperature (500 − 950°C) for at least 30 minutes to allow the system to achieve equilibrium, after which heating was abruptly stopped to induce a rapid decrease in temperature. Despite the low solubility of phlogopite and the fact that the working temperature was kept below its melting point 20 , only a few parts of the phlogopite piece were dissolved in our experiments. The products of phlogopite dissolution were investigated in situ under the microprobe (Extended Data Fig. S1 ). Following the experiments, these products on the surface of the diamond anvil were carefully investigated using a 100× objective lens to verify their mineralogy. 2.2 Electron microprobe analyses The chemical compositions of the phlogopite used in our experiments were determined using an Electron Probe Micro-Analyzer (EPMA) (JEOL JXA-8230) at the Institute of Oceanology, Chinese Academy of Sciences. The analyses were conducted with a 15 kV acceleration voltage, a 20 nA beam current, and a defocused beam spot of 1 − 5 mm. Routine analyses for Na and K were obtained by counting for 10s at the peak and 5s on the background, while for other elements, they were obtained by counting for 20s at the peak and 5s on the background. Analytical standards such as jadeite (Na, Si, and Al), wollastonite (Ca), K-feldspar (K), magnetite (Fe), rutile (Ti), apatite (P), periclase (Mg), and manganese oxide (Mn) were used. The determination of ferric iron in minerals followed a previous method 21 . Three different parts from the phlogopite piece were extracted for target fabrication and EPMA analysis. RESULTS The EMPA analysis of the phlogopite indicates a homogeneous chemical composition (see Supplementary Information). The ideal chemical composition of phlogopite is KMg 3 AlSi 3 O 10 (OH) 2 with a water content of 4.3%. Our initial sample inhibited a high loss on ignition (~ 7.1 wt%), suggesting the presence of halogen elements. To estimate the relative proportions of each element, we performed a simple calculation: elements with low contents (< 0.1 wt%) were excluded, and the total electrovalence of all cations was set at 22. The chemical formula of our sample used in experiments can be expressed as K 0.887 Na 0.065 Mg 2.632 Fe 0.139 Al 1.296 Si 2.850 Ti 0.054 O 10 (OH, F, Cl) 2 . The detailed results of HDAC experiments on the “phlogopite + H 2 O” system at 500 − 950°C and 10 − 18 kbar are shown in Supplementary Information. Phlogopite remains stable below 500°C but dissolves at higher temperatures. At 950°C and 1.8 GPa, the sharp edges of the primary mineral become less defined, and acicular phlogopites grow around it (Fig. 2 A; Extended Data Fig. S1 ). This indicates the dissolution and recrystallization of phlogopite in water, with the fluids becoming over-saturated in major components like K, Mg, Al, and Si. Corundum precipitation occurs when phlogopite interacts with H 2 O at 1.5 GPa and 800°C. The quenched products after the experiments preserve corundum, with crystals exhibiting triangular or hexagonal cross-sections and sharp edges (Fig. 2 B, C), consistent with sapphire’s crystallographic properties 22 . Raman bands at 400 cm ‒ 1 , 580 cm − 1 , and 760 cm ‒ 1 indicate the vibration modes of corundum, with slight differences in characteristic Raman peaks between natural and synthetic corundum due to the small crystal size (~ 4 µm diameter) and strong fluorescence effects 23 . The shapes of corundum crystal, combined with the Raman spectrum, suggest ongoing growth before quenching. Amorphous substances observed in the quenched products have a similar composition to natural pyroxene based on Raman spectrum analysis (Fig. 2 C). These amorphous pyroxenes were not observed under the in situ microscope, suggesting its precipitation from the fluid during quenching. Elemental solubilities in the fluid decrease during rapid cooling, leading to precipitation of some components (like Mg, Fe, Si) as pyroxene, while soluble components (Na and K) remain in fluids, suggesting a silicate-rich component in the fluid at pressures > 1.5 GPa. The phenomenon described above can be attributed to the incongruent dissolution of Al-rich minerals. Due to the lower solubility of Al compared to other elements like K, Mg, and Si, most elements dissolve and are leached out completely except for Al, which forms corundum as a residue. Similar phenomena have been reported in other “Al-rich mineral + water” systems (Extended Data Fig. S2). For example, in the “K-feldspar + water” system, higher temperatures lead to the conversion of K-feldspar into muscovite and eventually into corundum 24 , as in the “albite + water” system 25 . However, K-feldspar and albite break down in natural mantle P-T conditions 3 . This study demonstrates the incongruent dissolution of phlogopite, which can be stable in deep mantle up to 6 GPa. In summary, our experiments demonstrated that phlogopite incongruently dissolves at pressures > 1.5 GPa, while Al remains partially dissolved due to its low solubility, resulting in the formation of corundum. Under higher pressure and higher temperatures, phlogopite is expected to dissolve further in water or even melt, leading to increased corundum precipitation. However, conducting hydrothermal experiments beyond 1000°C is challenging with the current HDAC setup, as the diamond cells would be oxidized to CO 2 at such high temperatures even when protected by helium gas. Therefore, corundum precipitation is predicted to favor even higher P-T conditions than those revealed in this study (Fig. 3 ). DISCUSSION 4.1 Sapphire formation in the lithospheric mantle The corundum gem deposits (a brief introduction see Supplementary information) consist of ruby and sapphire deposits, and this study focuses on six Cenozoic magmatic sapphire deposits in eastern China: Muling, Kuandian, Changle, Luhe, Mingxi, and Penglai (details see Supplementary Dataset 1). In these deposits, sapphires are found in mantle xenoliths or as xenocrysts, not in the basaltic matrix. Previous works showed that sapphires did not crystallize in the basaltic magma but were trapped during magma upwelling 26 . They initially formed in the lithospheric P-T conditions of 790 − 1200°C and 0.85 − 2.5 GPa. In these sapphire deposits, the sapphire gems have been widely discovered in mantle xenoliths of alkaline basalts 26 , which typically originate from the lithosphere-asthenosphere boundary (LAB) 27 , and are rarely found in the Mesozoic calc-alkaline basalts (lithospheric origin) 8 . This suggests that the sapphire primarily formed in the deep lithosphere. Additionally, the MORB-like 3 He/ 4 He and high 40 Ar/ 36 Ar isotopic ratios 28 and the presence of multiple fluid inclusions 26 in these sapphires indicate the involvement of asthenosphere-derived melts or fluids in their formation. Previous models did not explain why sapphires form at such deep lithosphere and how water influences their production 28 , 29 . Our experiments show that sapphire forms when phlogopite interacts with water beyond 1.5 GPa, which equals > 50 km depth in the lithospheric mantle (Fig. 3 ). This reaction answers two key questions about the sapphire deposits: how do sapphires form in the lithospheric mantle, and why do these deposits only form in the Cenozoic? The depth of sapphire formation, as indicated by this study, approximates the melting depth of Mesozoic calc-alkaline basalts 8 and can only be brought to the surface by Cenozoic alkaline basaltic magma from LAB 27 . These sapphires are initially produced in the lithosphere, trapped by deep-origin magmas, and then transported to the surface. From a traditional viewpoint, an Al-saturated environment rarely occurs in the mantle. This study provides a relatively universal mechanism for sapphire formation in the lithospheric mantle (> 50 km depth) (Fig. 4 ). The timescale of sapphire formation is controlled by water migration in the mantle. Before the Cenozoic era, LAB beneath eastern China was generally more than 90 km 8 , and the upward-moving water was first captured by pargasite 30 , which hindered the direct hydration of the lithospheric mantle. However, during the Cenozoic era, with the destruction of the cratonic lithosphere, LAB become less than 90 km, and thus pargasite breaks down. Abundant water can directly react with phlogopite in the lithospheric mantle to form sapphire. Phlogopite is frequently found in Paleozoic mantle xenoliths, even as phlogopitites (~ 100% phlogopite), while Cenozoic xenoliths lack phlogopite-bearing rocks 31 , 32 , possibly due to mantle metasomatism consumption. Our hydrothermal diamond-anvil cell experiments of the “phlogopite + H 2 O” system at 500 − 950°C and 1.0 − 1.8 GPa simulate a typical mantle metasomatism process, essential to understanding sapphire formation. On the other hand, this finding highlights the role of P-T conditions in phlogopite dissolution and sapphire formation (Fig. 3 ). Thus, the presence of sapphire deposits provides unique information about hydration events. 4.2 Spatial and temporal constraints on hydration The distribution and age of sapphire deposits provide spatial and temporal constraints on hydration events. The six Cenozoic sapphire deposits are all distributed along the eastern coast of China (Fig. 1 ), roughly parallel to the Pacific subduction zone. The horizontal distance between these deposits and the subduction trench ranges from 1200 to 1600 kilometers, suggesting that plate subduction has triggered the hydration of the entire mantle wedge. This big mantle wedge extends to a maximum depth of 410 km, with a latitudinal and longitudinal distance of 1600 km and 2000 km, respectively. Notably, Cenozoic basalts in Kuandian and Chifeng have similar petrologic features, approximate ages, and close latitude 33 , 34 . However, sapphires-bearing xenoliths are only reported in Kuandian and rarely in Chifeng. This is probably due to the deeper LAB beneath Chifeng 33 . Deeper LAB means less destruction of the lithospheric mantle, less pargasite breakdown, and thus less water release. There are two timelines for these sapphire deposits: the ages of sapphire formation (T s ) represent when sapphires initially appeared at the lithospheric mantle, while the ages of sapphire-bearing host rocks (T h ) record when those sapphires were transported to the surface 28 . Thus, sapphire formation cannot occur later than the eruption of associated alkaline basaltic magma (T h ≤T s ). These basaltic magmas in eastern China erupted from ~ 19.2 Ma to Pleistocene (T h ), which represents the deadlines of hydration events. Recent studies reported the accurate dating of zircon inclusions inside the sapphire crystals or associated zircon megacrysts 26 , 35 , 36 . However, whether these zircons’ ages can represent when sapphire formed remains controversial. The relatively high temperature of alkaline basaltic magmas may lead to Pb diffusion in zircons and reset the U-Pb ages 37 . However, the preservation of ancient zircons (e.g., ~ 400 Ma zircons inclusions) in Cenozoic sapphire deposits suggests the lack of age resetting in zircons 26 . A recent study 36 suggests that the zircon inclusions trapped in Changle sapphires can be used as the age of sapphire formation (~ 15.5 Ma), which is consistent with the eruption age of the host basalts. This indicates that sapphires formed contemporary to the eruption of basaltic magma. This model can also be applied to Cenozoic magmatic sapphire deposits in Southeastern Asia and eastern Australia. 4.3 A globally applicable model? Eastern China and Australia are both typical examples of the destruction of ancient craton lithosphere due to the subduction of the western Pacific Plate (Fig. 1 ; Extended Data Fig. S4, S5). Extensive magmatic sapphire deposits in Southeast Asia are also distributed parallel to the subduction zone. In these regions, sapphire deposits are all related to alkaline basalts and have the same metallogenic features 26 . All the sapphire deposits can be explained by the model presented in this study. These sapphire deposits provide constraints on the hydration of the lithospheric mantle beneath these areas, respectively. Sapphire deposits in these three regions are all distributed along the coastlines. The deposits in Australia are located farther from the subduction trench (~ 3400 km on average) compared to those in China (~ 1400 km on average), while the sapphire deposits in Southeast Asia are closest to the subduction zone trenches (~ 650 km on average). Some of the early sapphires in eastern Australia may have been influenced by fluids from the subducting slab before 80 Ma. However, with the Tasman Sea spreading 38 , the impact of the subducting slab on eastern Australia has diminished. Seismic profiles reveal that stagnant slabs beneath the lithosphere are only present in northeastern Australia 39 . This suggests that the mantle transition zone or mantle plumes, rather than the slab itself, contribute to the water, needed for forming eastern and southeastern sapphire deposits. The shallow LAB beneath the three regions 40 indicates the destruction of the thick pargasite layer, releasing fluids that interacted with phlogopite in the ancient craton lithosphere to form mantle sapphire. The formation age of sapphire deposits (T h ) in eastern Australia varies from 74.5 Ma to 2 Ma, while those in eastern China and Southeast Asia are concentrated in the Late Cenozoic. Moreover, Asian sapphires are limited to early-stage basalts in the locality, whereas Australian sapphires are found in basalts from all stages, implying that the lithospheric mantle in eastern China was hydrated during a short period, while that beneath eastern Australia experienced hydration in multiple stages throughout the Cenozoic. CONCLUSIONS Based on experimental results and geological evidence, our study sheds light on the formation of sapphire in the lithospheric mantle. The incongruent dissolution of phlogopite under high-pressure conditions (> 1.5 GPa) leads to sapphire production and provides clues to the water migration in the big mantle wedge. This is important to understand the deep water cycle, mantle metasomatism, and spatial-temporal constraints on lithospheric hydration, which are crucial for interpreting Earth’s geological evolution and habitable environments. Declarations Acknowledgments We thank Zhuoyu Liu, Mei Mi, and Haibo Yan for their guidance and help in the laboratory; Jianxi Zhu for the provision of initial samples; and Lipeng Zhang for discussion. The study was supported by the National Natural Science Foundation of China (92258303) to W.D.S. Author contributions Conceptualization: WDS Visualization: XQS, WDS Funding acquisition: WDS Experimental implementation: XQS, XD Writing – original draft: XQS, WDS Writing – discussion: RFH, XD, SY, GZX, KW Writing – review & editing: XQS, WDS Competing interests The authors declare no competing interests. References Langmuir CH, Broecker W (2012) How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind. Princeton University Press Karato SI, Karki B, Park J (2020) Deep mantle melting, global water circulation and its implications for the stability of the ocean mass. Progress Earth Planet Sci 7 Schmidt MW, Poli S (2014) 4.19 - Devolatilization During Subduction. 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Comput Geosci 18:899–947 Pokrovskii VA, Helgeson HC (1995) Thermodynamic properties of aqueous species and the solubilities of minerals at high pressures and temperatures: The system Al 2 O 3 -H 2 O-NaCl. Am J Sci 295:1255–1342 Pokrovskii VA, Helgeson HC (1997) Thermodynamic properties of aqueous species and the solubilities of minerals at high pressures and temperatures: The system Al 2 O 3 -H 2 O-KOH. Chem Geol 137:221–242 Wohlers A, Manning CE (2009) Solubility of corundum in aqueous KOH solutions at 700°C and 1 GPa. Chem Geol 262:310–317 Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryDataset1.xlsx Dataset 1 ExtendedDataFigures.docx SupplementaryInformation.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4794218","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":334351691,"identity":"a6a3596b-4aee-4da6-b863-cc0406ae52ae","order_by":0,"name":"Wei-Dong Sun","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5ElEQVRIiWNgGAWjYFACxgYJCIP5AAOPAWla2BKI1cLAANUCVM9DjHKD482NNz7uqLU3uN3z8cabAhsG/vYDjJ8L8Gk5c7DZcuaZ44kb7pzdbDnHII1B4kwCs/QMPFrMbiS2SfO2HUswuJG7TZrH4DADww0GNmZ8LjS7/xCsxd7gRs4zsBZ5glpuMIK01DBuuJHDBtZiQEiL/ZlEoF/aDiTOvJFmDPILjyFQRBqfFsn24w9vfGyrs+e7kfzwxps/NnJyxw8f/ExEaB8Gk6AI4gFFLmENDAx1cC2jYBSMglEwCjAAAKF2TR+8f/2QAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-9003-9608","institution":"Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071","correspondingAuthor":true,"prefix":"","firstName":"Wei-Dong","middleName":"","lastName":"Sun","suffix":""}],"badges":[],"createdAt":"2024-07-24 09:40:50","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4794218/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4794218/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":70124611,"identity":"36b5c1f2-6e62-4e5d-8d78-abb04a74ab1b","added_by":"auto","created_at":"2024-11-28 14:53:19","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":356494,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4794218/v1/1c2a0705-0f29-4ec8-9f1d-5326625251b7.pdf"},{"id":61646087,"identity":"bb0827b8-9f9e-451c-a51b-58ba37cc66cb","added_by":"auto","created_at":"2024-08-02 11:10:19","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":73360,"visible":true,"origin":"","legend":"Dataset 1","description":"","filename":"SupplementaryDataset1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4794218/v1/a1d755748bf25a44b597264d.xlsx"},{"id":61646093,"identity":"671f32ef-b554-41dd-9b3c-f0152c0d33b3","added_by":"auto","created_at":"2024-08-02 11:10:19","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":14408814,"visible":true,"origin":"","legend":"","description":"","filename":"ExtendedDataFigures.docx","url":"https://assets-eu.researchsquare.com/files/rs-4794218/v1/6c83db2ed95cbf3fe9e5f123.docx"},{"id":61646644,"identity":"723ef294-d629-4d74-b36f-48d4576b004c","added_by":"auto","created_at":"2024-08-02 11:18:19","extension":"docx","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":46519,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4794218/v1/9e2eb63f44eaba3599e727ef.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Hydration of the lithospheric mantle above big mantle wedges indicated by sapphire deposits","fulltext":[{"header":"Main text","content":"\u003cp\u003eWater is requisite to form a habitable Earth\u003csup\u003e1\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e. Plate subduction transports surface water into the upper mantle or even the mantle transition zone (MTZ)\u003csup\u003e3\u003c/sup\u003e. The subducted slabs or MTZ then release water to hydrate the mantle wedge above\u003csup\u003e4\u003c/sup\u003e, a process known as the deep water cycle. This cycle directly induces magmatic activities and promotes ore formation and, consequently, impacts the stability of the ancient cratonic lithosphere\u003csup\u003e5\u003c/sup\u003e. The big mantle wedge beneath eastern China serves as a typical example, which has been influenced by the subduction of the western Pacific plate since the Jurassic\u003csup\u003e6\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e7\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e8\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e9\u003c/sup\u003e. However, there is a disagreement between geophysical and geochemical evidence regarding water migration beneath eastern China.\u003c/p\u003e\n\u003cp\u003eGeophysical researchers reported stagnant slabs beneath eastern China, which currently span a length of 1200\u0026minus;1600 km from the trenches to the west horizontally\u003csup\u003e10\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e11\u003c/sup\u003e. Seismic data shows that the subducted slab can disturb the hydrous MTZ\u003csup\u003e12\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e13\u003c/sup\u003e or cause the breakdown of hydrous phases in the slab\u003csup\u003e4\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e14\u003c/sup\u003e, releasing water to hydrate mantle atop 410 km\u003csup\u003e9\u003c/sup\u003e. However, geochemists argue that the Cenozoic lithosphere beneath eastern China is relatively dry as suggested by low water contents (0\u0026minus;90 ppm on average) of nominally anhydrous minerals (NAMs) in mantle xenoliths. In contrast, the lithospheric mantle source contained \u0026gt;1000 ppm of water during the Mesozoic era\u003csup\u003e15\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e16\u003c/sup\u003e. The lithospheric hydration beneath eastern China lacks geochemical evidence. Here we propose that the magmatic sapphire (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) deposits in eastern China indicate major Cenozoic lithospheric hydration (Fig. 1), based on our hydrothermal experiments. We synthesized sapphires using phlogopite and water under lithospheric mantle P-T conditions, linking sapphire formation with hydration events. This model can also be applied to the Cenozoic sapphire deposits distributed in eastern Australia and Southeast Asia, indicating lithospheric mantle hydration beneath those regions.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"METHODS","content":"\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Hydrothermal diamond anvil cell experiments and Raman spectroscopy\u003c/h2\u003e \u003cp\u003eThe experiments were conducted using a hydrothermal diamond-anvil cell (HDAC-V\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e) at the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry, China. The HDAC was equipped with two Ia-type diamonds (750 \u0026micro;m culet diameter) and rhenium gaskets with a 500-\u0026micro;m-diameter hole. Microscopic observations and in-situ laser Raman spectroscopic investigations were performed using a Confocal WITec Raman Microscope alpha300 R., experiments. Two diamonds sandwich the rhenium gaskets, forming a reaction cell. The cell can be considered as isometric while heated. Heating was achieved with molybdenum wires, controlled by the Hydrothermal Diamond Anvil Cell Controller 1300, with temperature errors of \u0026lt;\u0026thinsp;1\u0026deg;C. The cell\u0026rsquo;s pressure was calculated using an improved method\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e based on the water\u0026rsquo;s equation of state\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, with errors of \u0026lt;\u0026thinsp;0.1 GPa. Pure helium gas protected the diamond from oxidation.\u003c/p\u003e \u003cp\u003eA piece of natural phlogopite from Lixian-Dangchang of Gansu Province, in the western Qinling area, was used as the initial material (Extended Data Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). The phlogopite piece measured approximately 1\u0026times;2 cm with a uniform thickness of ~\u0026thinsp;2 mm. For each experiment, a small slice of phlogopite and deionized pure water were placed into the anvil. The volume of the phlogopite was estimated by measuring its area and thickness, and the diameter of the air bubble under the microscope. This allowed for estimating the volumes of water, the air bubble, and the phlogopite slice. In all experiments, the mass ratios of water/mineral ranged from 2 to 10, with errors of about\u0026thinsp;\u0026plusmn;\u0026thinsp;10%.\u003c/p\u003e \u003cp\u003eThe unpolarized Raman spectra were obtained during and after the experiments using a WITec confocal Raman microscope. The microscope utilized a 532 nm (frequency doubled Nd:YAG) laser excitation source operating at 20 mW, with a 100 mm confocal aperture and an 1800 gr/mm grating. Measurements were performed in backscattering geometry under an Olympus microscope, employing a Mitutoyo MPlan Apo SL 20\u0026times; long-working distance objective. The lateral spatial resolution of the microscope was less than 1 \u0026micro;m, and the spectrometer\u0026rsquo;s accuracy was \u0026plusmn;\u0026thinsp;1.5 cm\u003csup\u003e‒\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Each spectrum was collected for 180 seconds with three accumulations. The collected data from different spectral windows, each with a width of 500 cm\u003csup\u003e‒\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, was processed using Project Five software.\u003c/p\u003e \u003cp\u003eThe HDAC device was operated from room temperature up to 950\u0026deg;C. Each run was maintained at the target temperature (500\u0026thinsp;\u0026minus;\u0026thinsp;950\u0026deg;C) for at least 30 minutes to allow the system to achieve equilibrium, after which heating was abruptly stopped to induce a rapid decrease in temperature. Despite the low solubility of phlogopite and the fact that the working temperature was kept below its melting point\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e, only a few parts of the phlogopite piece were dissolved in our experiments. The products of phlogopite dissolution were investigated in situ under the microprobe (Extended Data Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Following the experiments, these products on the surface of the diamond anvil were carefully investigated using a 100\u0026times; objective lens to verify their mineralogy.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Electron microprobe analyses\u003c/h2\u003e \u003cp\u003eThe chemical compositions of the phlogopite used in our experiments were determined using an Electron Probe Micro-Analyzer (EPMA) (JEOL JXA-8230) at the Institute of Oceanology, Chinese Academy of Sciences. The analyses were conducted with a 15 kV acceleration voltage, a 20 nA beam current, and a defocused beam spot of 1\u0026thinsp;\u0026minus;\u0026thinsp;5 mm. Routine analyses for Na and K were obtained by counting for 10s at the peak and 5s on the background, while for other elements, they were obtained by counting for 20s at the peak and 5s on the background. Analytical standards such as jadeite (Na, Si, and Al), wollastonite (Ca), K-feldspar (K), magnetite (Fe), rutile (Ti), apatite (P), periclase (Mg), and manganese oxide (Mn) were used. The determination of ferric iron in minerals followed a previous method\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. Three different parts from the phlogopite piece were extracted for target fabrication and EPMA analysis.\u003c/p\u003e \u003c/div\u003e"},{"header":"RESULTS","content":"\u003cp\u003eThe EMPA analysis of the phlogopite indicates a homogeneous chemical composition (see Supplementary Information). The ideal chemical composition of phlogopite is KMg\u003csub\u003e3\u003c/sub\u003eAlSi\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e10\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e with a water content of 4.3%. Our initial sample inhibited a high loss on ignition (~\u0026thinsp;7.1 wt%), suggesting the presence of halogen elements. To estimate the relative proportions of each element, we performed a simple calculation: elements with low contents (\u0026lt;\u0026thinsp;0.1 wt%) were excluded, and the total electrovalence of all cations was set at 22. The chemical formula of our sample used in experiments can be expressed as K\u003csub\u003e0.887\u003c/sub\u003eNa\u003csub\u003e0.065\u003c/sub\u003eMg\u003csub\u003e2.632\u003c/sub\u003eFe\u003csub\u003e0.139\u003c/sub\u003eAl\u003csub\u003e1.296\u003c/sub\u003eSi\u003csub\u003e2.850\u003c/sub\u003eTi\u003csub\u003e0.054\u003c/sub\u003eO\u003csub\u003e10\u003c/sub\u003e(OH, F, Cl)\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eThe detailed results of HDAC experiments on the \u0026ldquo;phlogopite\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO\u0026rdquo; system at 500\u0026thinsp;\u0026minus;\u0026thinsp;950\u0026deg;C and 10\u0026thinsp;\u0026minus;\u0026thinsp;18 kbar are shown in Supplementary Information. Phlogopite remains stable below 500\u0026deg;C but dissolves at higher temperatures. At 950\u0026deg;C and 1.8 GPa, the sharp edges of the primary mineral become less defined, and acicular phlogopites grow around it (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003eA; Extended Data Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). This indicates the dissolution and recrystallization of phlogopite in water, with the fluids becoming over-saturated in major components like K, Mg, Al, and Si. Corundum precipitation occurs when phlogopite interacts with H\u003csub\u003e2\u003c/sub\u003eO at 1.5 GPa and 800\u0026deg;C. The quenched products after the experiments preserve corundum, with crystals exhibiting triangular or hexagonal cross-sections and sharp edges (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003eB, C), consistent with sapphire\u0026rsquo;s crystallographic properties\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Raman bands at 400 cm\u003csup\u003e‒\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, 580 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and 760 cm\u003csup\u003e‒\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e indicate the vibration modes of corundum, with slight differences in characteristic Raman peaks between natural and synthetic corundum due to the small crystal size (~\u0026thinsp;4 \u0026micro;m diameter) and strong fluorescence effects\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. The shapes of corundum crystal, combined with the Raman spectrum, suggest ongoing growth before quenching. Amorphous substances observed in the quenched products have a similar composition to natural pyroxene based on Raman spectrum analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003eC). These amorphous pyroxenes were not observed under the in situ microscope, suggesting its precipitation from the fluid during quenching. Elemental solubilities in the fluid decrease during rapid cooling, leading to precipitation of some components (like Mg, Fe, Si) as pyroxene, while soluble components (Na and K) remain in fluids, suggesting a silicate-rich component in the fluid at pressures\u0026thinsp;\u0026gt;\u0026thinsp;1.5 GPa.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe phenomenon described above can be attributed to the incongruent dissolution of Al-rich minerals. Due to the lower solubility of Al compared to other elements like K, Mg, and Si, most elements dissolve and are leached out completely except for Al, which forms corundum as a residue. Similar phenomena have been reported in other \u0026ldquo;Al-rich mineral\u0026thinsp;+\u0026thinsp;water\u0026rdquo; systems (Extended Data Fig. S2). For example, in the \u0026ldquo;K-feldspar\u0026thinsp;+\u0026thinsp;water\u0026rdquo; system, higher temperatures lead to the conversion of K-feldspar into muscovite and eventually into corundum\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, as in the \u0026ldquo;albite\u0026thinsp;+\u0026thinsp;water\u0026rdquo; system\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. However, K-feldspar and albite break down in natural mantle P-T conditions\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. This study demonstrates the incongruent dissolution of phlogopite, which can be stable in deep mantle up to 6 GPa.\u003c/p\u003e \u003cp\u003eIn summary, our experiments demonstrated that phlogopite incongruently dissolves at pressures\u0026thinsp;\u0026gt;\u0026thinsp;1.5 GPa, while Al remains partially dissolved due to its low solubility, resulting in the formation of corundum. Under higher pressure and higher temperatures, phlogopite is expected to dissolve further in water or even melt, leading to increased corundum precipitation. However, conducting hydrothermal experiments beyond 1000\u0026deg;C is challenging with the current HDAC setup, as the diamond cells would be oxidized to CO\u003csub\u003e2\u003c/sub\u003e at such high temperatures even when protected by helium gas. Therefore, corundum precipitation is predicted to favor even higher P-T conditions than those revealed in this study (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"DISCUSSION","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Sapphire formation in the lithospheric mantle\u003c/h2\u003e \u003cp\u003eThe corundum gem deposits (a brief introduction see Supplementary information) consist of ruby and sapphire deposits, and this study focuses on six Cenozoic magmatic sapphire deposits in eastern China: Muling, Kuandian, Changle, Luhe, Mingxi, and Penglai (details see Supplementary Dataset 1). In these deposits, sapphires are found in mantle xenoliths or as xenocrysts, not in the basaltic matrix. Previous works showed that sapphires did not crystallize in the basaltic magma but were trapped during magma upwelling\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. They initially formed in the lithospheric P-T conditions of 790\u0026thinsp;\u0026minus;\u0026thinsp;1200\u0026deg;C and 0.85\u0026thinsp;\u0026minus;\u0026thinsp;2.5 GPa.\u003c/p\u003e \u003cp\u003eIn these sapphire deposits, the sapphire gems have been widely discovered in mantle xenoliths of alkaline basalts\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, which typically originate from the lithosphere-asthenosphere boundary (LAB)\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, and are rarely found in the Mesozoic calc-alkaline basalts (lithospheric origin)\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. This suggests that the sapphire primarily formed in the deep lithosphere. Additionally, the MORB-like \u003csup\u003e3\u003c/sup\u003eHe/\u003csup\u003e4\u003c/sup\u003eHe and high \u003csup\u003e40\u003c/sup\u003eAr/\u003csup\u003e36\u003c/sup\u003eAr isotopic ratios\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e and the presence of multiple fluid inclusions\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e in these sapphires indicate the involvement of asthenosphere-derived melts or fluids in their formation. Previous models did not explain why sapphires form at such deep lithosphere and how water influences their production\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOur experiments show that sapphire forms when phlogopite interacts with water beyond 1.5 GPa, which equals\u0026thinsp;\u0026gt;\u0026thinsp;50 km depth in the lithospheric mantle (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This reaction answers two key questions about the sapphire deposits: how do sapphires form in the lithospheric mantle, and why do these deposits only form in the Cenozoic?\u003c/p\u003e \u003cp\u003eThe depth of sapphire formation, as indicated by this study, approximates the melting depth of Mesozoic calc-alkaline basalts\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e and can only be brought to the surface by Cenozoic alkaline basaltic magma from LAB\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. These sapphires are initially produced in the lithosphere, trapped by deep-origin magmas, and then transported to the surface. From a traditional viewpoint, an Al-saturated environment rarely occurs in the mantle. This study provides a relatively universal mechanism for sapphire formation in the lithospheric mantle (\u0026gt;\u0026thinsp;50 km depth) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe timescale of sapphire formation is controlled by water migration in the mantle. Before the Cenozoic era, LAB beneath eastern China was generally more than 90 km\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, and the upward-moving water was first captured by pargasite\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, which hindered the direct hydration of the lithospheric mantle. However, during the Cenozoic era, with the destruction of the cratonic lithosphere, LAB become less than 90 km, and thus pargasite breaks down. Abundant water can directly react with phlogopite in the lithospheric mantle to form sapphire. Phlogopite is frequently found in Paleozoic mantle xenoliths, even as phlogopitites (~\u0026thinsp;100% phlogopite), while Cenozoic xenoliths lack phlogopite-bearing rocks\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e, possibly due to mantle metasomatism consumption.\u003c/p\u003e \u003cp\u003eOur hydrothermal diamond-anvil cell experiments of the \u0026ldquo;phlogopite\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO\u0026rdquo; system at 500\u0026thinsp;\u0026minus;\u0026thinsp;950\u0026deg;C and 1.0\u0026thinsp;\u0026minus;\u0026thinsp;1.8 GPa simulate a typical mantle metasomatism process, essential to understanding sapphire formation. On the other hand, this finding highlights the role of P-T conditions in phlogopite dissolution and sapphire formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Thus, the presence of sapphire deposits provides unique information about hydration events.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Spatial and temporal constraints on hydration\u003c/h2\u003e \u003cp\u003eThe distribution and age of sapphire deposits provide spatial and temporal constraints on hydration events. The six Cenozoic sapphire deposits are all distributed along the eastern coast of China (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e1\u003c/span\u003e), roughly parallel to the Pacific subduction zone. The horizontal distance between these deposits and the subduction trench ranges from 1200 to 1600 kilometers, suggesting that plate subduction has triggered the hydration of the entire mantle wedge. This big mantle wedge extends to a maximum depth of 410 km, with a latitudinal and longitudinal distance of 1600 km and 2000 km, respectively. Notably, Cenozoic basalts in Kuandian and Chifeng have similar petrologic features, approximate ages, and close latitude\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. However, sapphires-bearing xenoliths are only reported in Kuandian and rarely in Chifeng. This is probably due to the deeper LAB beneath Chifeng\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Deeper LAB means less destruction of the lithospheric mantle, less pargasite breakdown, and thus less water release.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThere are two timelines for these sapphire deposits: the ages of sapphire formation (T\u003csub\u003es\u003c/sub\u003e) represent when sapphires initially appeared at the lithospheric mantle, while the ages of sapphire-bearing host rocks (T\u003csub\u003eh\u003c/sub\u003e) record when those sapphires were transported to the surface\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. Thus, sapphire formation cannot occur later than the eruption of associated alkaline basaltic magma (T\u003csub\u003eh\u003c/sub\u003e\u0026le;T\u003csub\u003es\u003c/sub\u003e). These basaltic magmas in eastern China erupted from ~\u0026thinsp;19.2 Ma to Pleistocene (T\u003csub\u003eh\u003c/sub\u003e), which represents the deadlines of hydration events. Recent studies reported the accurate dating of zircon inclusions inside the sapphire crystals or associated zircon megacrysts\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. However, whether these zircons\u0026rsquo; ages can represent when sapphire formed remains controversial. The relatively high temperature of alkaline basaltic magmas may lead to Pb diffusion in zircons and reset the U-Pb ages\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. However, the preservation of ancient zircons (e.g., ~\u0026thinsp;400 Ma zircons inclusions) in Cenozoic sapphire deposits suggests the lack of age resetting in zircons\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. A recent study\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e suggests that the zircon inclusions trapped in Changle sapphires can be used as the age of sapphire formation (~\u0026thinsp;15.5 Ma), which is consistent with the eruption age of the host basalts. This indicates that sapphires formed contemporary to the eruption of basaltic magma. This model can also be applied to Cenozoic magmatic sapphire deposits in Southeastern Asia and eastern Australia.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e4.3 A globally applicable model?\u003c/h2\u003e \u003cp\u003eEastern China and Australia are both typical examples of the destruction of ancient craton lithosphere due to the subduction of the western Pacific Plate (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Extended Data Fig. S4, S5). Extensive magmatic sapphire deposits in Southeast Asia are also distributed parallel to the subduction zone. In these regions, sapphire deposits are all related to alkaline basalts and have the same metallogenic features\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. All the sapphire deposits can be explained by the model presented in this study. These sapphire deposits provide constraints on the hydration of the lithospheric mantle beneath these areas, respectively.\u003c/p\u003e \u003cp\u003eSapphire deposits in these three regions are all distributed along the coastlines. The deposits in Australia are located farther from the subduction trench (~\u0026thinsp;3400 km on average) compared to those in China (~\u0026thinsp;1400 km on average), while the sapphire deposits in Southeast Asia are closest to the subduction zone trenches (~\u0026thinsp;650 km on average). Some of the early sapphires in eastern Australia may have been influenced by fluids from the subducting slab before 80 Ma. However, with the Tasman Sea spreading\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e, the impact of the subducting slab on eastern Australia has diminished. Seismic profiles reveal that stagnant slabs beneath the lithosphere are only present in northeastern Australia\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. This suggests that the mantle transition zone or mantle plumes, rather than the slab itself, contribute to the water, needed for forming eastern and southeastern sapphire deposits. The shallow LAB beneath the three regions\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e indicates the destruction of the thick pargasite layer, releasing fluids that interacted with phlogopite in the ancient craton lithosphere to form mantle sapphire.\u003c/p\u003e \u003cp\u003eThe formation age of sapphire deposits (T\u003csub\u003eh\u003c/sub\u003e) in eastern Australia varies from 74.5 Ma to 2 Ma, while those in eastern China and Southeast Asia are concentrated in the Late Cenozoic. Moreover, Asian sapphires are limited to early-stage basalts in the locality, whereas Australian sapphires are found in basalts from all stages, implying that the lithospheric mantle in eastern China was hydrated during a short period, while that beneath eastern Australia experienced hydration in multiple stages throughout the Cenozoic.\u003c/p\u003e "},{"header":"CONCLUSIONS","content":"\u003cp\u003eBased on experimental results and geological evidence, our study sheds light on the formation of sapphire in the lithospheric mantle. The incongruent dissolution of phlogopite under high-pressure conditions (\u0026gt;\u0026thinsp;1.5 GPa) leads to sapphire production and provides clues to the water migration in the big mantle wedge. This is important to understand the deep water cycle, mantle metasomatism, and spatial-temporal constraints on lithospheric hydration, which are crucial for interpreting Earth\u0026rsquo;s geological evolution and habitable environments.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank Zhuoyu Liu, Mei Mi, and Haibo Yan for their guidance and help in the laboratory; Jianxi Zhu for the provision of initial samples; and Lipeng Zhang for discussion. The study was supported by the National Natural Science Foundation of China\u0026nbsp;(92258303)\u0026nbsp;to W.D.S.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization: WDS\u003c/p\u003e\n\u003cp\u003eVisualization: XQS, WDS\u003c/p\u003e\n\u003cp\u003eFunding acquisition: WDS\u003c/p\u003e\n\u003cp\u003eExperimental implementation: XQS, XD\u003c/p\u003e\n\u003cp\u003eWriting \u0026ndash; original draft: XQS,\u0026nbsp;WDS\u003c/p\u003e\n\u003cp\u003eWriting \u0026ndash;\u0026nbsp;discussion:\u0026nbsp;RFH, XD, SY, GZX, KW\u003c/p\u003e\n\u003cp\u003eWriting \u0026ndash; review \u0026amp; editing:\u0026nbsp;XQS,\u0026nbsp;WDS\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLangmuir CH, Broecker W (2012) How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind. 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Chem Geol 262:310\u0026ndash;317\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4794218/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4794218/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe deep water cycle is pivotal in shaping Earth’s habitable environments. A fundamental process of this cycle is upward migration of water from Earth’s deep interior. A significant inquiry concerns how water released from the deep mantle hydrates the lithosphere. Here we report hydrothermal experiments of the “phlogopite + H\u003csub\u003e2\u003c/sub\u003eO” system, showing that the incongruent dissolution of phlogopite in water produces sapphire (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) under lithospheric mantle P-T conditions. Our results suggest that sapphire can be leached from phlogopite in the lithospheric mantle by excess water, and subsequently transported to the surface by basaltic magmas. We propose that the magmatic sapphire deposits in eastern China, Southeastern Asia, and eastern Australia provide evidence of lithospheric mantle hydration. The water that leached the lithospheric mantle mainly originated from the mantle transition zone or subducted slabs, while in eastern Australia and Hainan Island, mantle plumes may also contribute. The occurrence of sapphire deposits indicates extensive hydration of lithospheric mantle in big mantle wedges.\u003c/p\u003e","manuscriptTitle":"Hydration of the lithospheric mantle above big mantle wedges indicated by sapphire deposits","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-08-02 11:10:14","doi":"10.21203/rs.3.rs-4794218/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"be15be5a-cd6b-49ec-aaca-6d6e0f4ac5e0","owner":[],"postedDate":"August 2nd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":35408365,"name":"Earth and environmental sciences/Solid Earth sciences/Geochemistry"},{"id":35408366,"name":"Earth and environmental sciences/Solid Earth sciences/Petrology"}],"tags":[],"updatedAt":"2024-11-28T14:45:13+00:00","versionOfRecord":[],"versionCreatedAt":"2024-08-02 11:10:14","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4794218","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4794218","identity":"rs-4794218","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}