Decarbonation embrittlement of scapolite as a trigger for earthquakes in the lower crust

Decarbonation embrittlement of scapolite as a trigger for earthquakes in the lower crust | 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 Decarbonation embrittlement of scapolite as a trigger for earthquakes in the lower crust Haemyeong Jung, Yoonhae Ha, Sejin Jung, Jung Hun Seo, Håkon Austrheim This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6694269/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 Recent studies and seismological data have reported numerous earthquakes in the lower continental crust 1–7 . However, the mechanisms triggering earthquakes in the lower crust remain unclear. Here, we present results from the experimental deformation of granulite in simple shear at the pressures of 0.7 to 1.5 GPa and temperatures of 600–830 ℃ using a modified Griggs apparatus. We found faults in the deformed granulite in which the scapolite grains were devolatilized. Notably, most Raman spectra of the deformed scapolites did not have peaks at 1100 cm − 1 due to the vibrations of carbonate ions, indicating that carbon dioxide gas was released following the breakdown of scapolite. Therefore, our data suggest that the decarbonation embrittlement of scapolite can trigger earthquakes in the continental lower crust. Earth and environmental sciences/Solid Earth sciences/Geophysics Earth and environmental sciences/Solid Earth sciences/Seismology Earth and environmental sciences/Solid Earth sciences/Geology/Structural geology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction The lower continental crust primarily consists of feldspar-rich granulites and amphibolites 8 . During orogeny, structural and metamorphic transformations in the lower crust are strongly associated with earthquakes 1,9,10 . An example of pseudotachylyte, which provides evidence of paleo-earthquakes, is shown in Fig. 1 . This pseudotachylyte was found in granulites within the Bergen Arcs, Norway 1,9,11 . Pseudotachylytes have also been identified in other areas of the Bergen Arcs, Norway, and the Musgrave Ranges in Central Australia 12,13 . Previous studies and seismological data have shown that many earthquakes occur at lower crustal depths across diverse tectonic settings 2,3,6,7,14,15 . In collision zones, lower crustal earthquakes at focal depths of 20–50 km occur in southern Tibet 2,4 , the Himalayas 7 , and beneath the northern foreland of the Central Alps 5 . In the active continental rifts, lower crustal earthquakes were also nucleated in the East African 14 and Baikal rifts 16 and the Taupo Volcanic Zone in New Zealand 6 . Lower crustal earthquakes were also reported at the continental margin in northwestern Canada 3 ; at the intraplates in Botswana, South Africa 15 and Bhuj, India 17 . The earthquakes occur in the lower crust in collision zones and other tectonic settings and play a key role in metamorphic processes that modify the physical properties of the lower crust 18,19 . Deep intracontinental earthquakes can also cause considerable destruction. Several potential explanations have been proposed, including shear instability caused by thermal runaway 20 , fluid and reaction-induced failure 21 , local stress amplification 22 , instabilities triggered by metamorphic transformation 2,23 , mantle upwelling 24 , and aftershocks of large earthquakes in the shallower seismogenic regime 1 . However, the mechanism of brittle failure in lower continental crust remains poorly understood. Scapolite is a mineral commonly found in granulites 25–30 . It contains volatile components—primarily sulfate ions (SO 4 2− ), carbonate ions (CO 3 2− ), and chloride ions (Cl − ) within its structure, which can release sulfur and carbon to form sulfides and carbonates. Notably, granulites comprise much of the lower continental crust 31 ; therefore, scapolite may constitute an essential reservoir for sulfur and carbon, which was not previously accounted for in the sulfur and carbon cycles 32 . In nature, the nucleation of earthquakes is influenced by the build-up of gases under high pressure 33,34 . This observation highlights the important role played by volatile compounds in triggering seismic activity. However, no direct evidence of CO 2 gas-triggering earthquake has been reported to date. Therefore, in this study, we conducted deformation experiments on granulite to investigate the relationship between granulite failure and degassing. The granulite samples showed faults during deformation experiments under lower crust conditions, and the scapolites exhibited clear evidence of the loss of volatile components, especially CO 3 2− . Starting material and experiment A granulite from Holsnøy, southwestern Norway, was used as the starting material. It comprises plagioclase (~ 70 vol.%), garnet (~ 15 vol.%), diopside (~ 10 vol.%), scapolite (~ 5 vol.%), and minor sulfide (~ 0.1 vol.%). Granulite powder (< 53 µm) was hot-pressed at a pressure (P) of 1.3 GPa and temperature (T) of 830°C near the scapolite stability field for 12 h. The scapolite stability field is shown in Fig. 2 . The hot-pressed granulite was then deformed in simple shear (Fig. 3 ) using a modified Griggs apparatus at P = 0.7–1.5 GPa and T = 600–830°C— conditions outside the scapolite stability field (Fig. 2 ). These deformation conditions represent the typical P–T conditions of the continental lower crust. The detailed experimental procedures are provided in the Methods section. Microstructures of deformed granulite Table 1 lists the experimental conditions for sample deformation and the results. Figure 4 a shows the hot-pressed sample (JH205). A representative microstructure of the deformed granulite at a high P of 1.0 GPa and T of 700 ℃ is shown in Fig. 4 b and c. Notably, a fault developed in the deformed granulite sample (Fig. 4 b), and sulfides were found around the fault (Fig. 4 b and c). The sulfides were irregular and observed in the pulverized garnet and pyroxene. In addition, the sulfide content increased in the deformed samples (~ 0.4%) compared with that in the starting material (~ 0.1%) (Fig. 5 ). Furthermore, many deformed samples showed strain localization on the scale of mm to µm, which is closely associated with fault generation and the comminution of grains (Fig. 6 a and 6 d). Fault structure and sulfides Fault zones showing sulfides around the fault were observed in more detail using the spherical aberration-corrected transmission electron microscope (Cs-TEM). A backscattered electron (BSE) image of sample JH201, deformed at P = 1.0 GPa and T = 700°C, showed sulfides infiltrating pulverized garnets near the fault (Fig. 6 a–c). Figures 6 d-f also showed a fault zone in the other sample (JH199), which was deformed at P = 0.7 GPa and T = 700 ℃, which exhibited sulfides along both sides of the fault in BSE (Fig. 6 d) and TEM images (Fig. 6 e and f). Focused ion beam (FIB) foils were prepared oblique or normal to the fault direction to observe both sides of the fault in depth and were analyzed using TEM-Energy-dispersive X-ray spectroscopy (TEM-EDS) (Fig. 6 a and d). The TEM-EDS mapping images of S show that elemental sulfur is distributed along the fault below the sample surface (Fig. 6 c and f). Devolatilization of scapolite The deformed samples with faults exhibited higher sulfide contents than the hot-pressed starting material (Fig. 5 ). The scanning electron microscope (SEM)-BSE and TEM-EDS images indicate that sulfur-rich scapolite released sulfur and formed sulfides during the deformation experiments (Figs. 4 c, 5 , and 6 ). To investigate this further, we analyzed and compared the chemical compositions of scapolite grains in the hot-pressed starting material and deformed samples using an electron probe microanalyzer (EPMA) and Raman spectroscopy. Representative scapolite compositions from the hot-pressed material (JH205) and deformed samples (JH191, JH201, and JH219) are presented in Extended Data Table 1 . Scapolite grains in JH205 contained high sulfur levels (4.42–5.21 wt% SO 3 ). However, about half of the scapolite grains analyzed in the deformed samples JH191 and JH201 had significantly lower sulfur content (0.11–1.05 wt% SO₃). Although sample JH219 also exhibited faults, its scapolite grains had a sulfur content of 4.01–4.83 wt% SO₃, slightly lower than that of the starting material. Variations in S content and meionite component (Me = Ca/ (Ca + Na)) are shown in Fig. 7 . Scapolite grains in the faulted samples (JH191, JH201) at T = 600 and 700 ℃ contained markedly less sulfur than those in the hot-pressed starting material (JH205). Scapolite is a silicate mineral that contains additional sulfate and carbonate groups in its structure, which can be detected using Raman spectroscopy. We analyzed scapolite grains using Raman spectroscopy to further study the devolatilization process because some scapolite grains in the deformed samples showed much lower sulfur content, indicative of the loss of volatile components (Extended Data Table 1 ). A previous study 35 showed that Raman bands at 993 and 1,095 cm − 1 represent the sulfate and carbonate ν 1 symmetric stretching vibrations, respectively. The scapolites in the hot-pressed starting material (JH205) and the deformed sample without faults (JH209) showed peaks at 982 and 1110 cm − 1 (Fig. 8 a and b), indicating the absence of devolatilization of the samples. However, many scapolites in the deformed samples with faults did not show a Raman peak at 1100 cm − 1 indicative of the loss of carbonate ions (Fig. 8 c-f), and some did not show a peak at 982 cm − 1 (SO 4 2− ) (Fig. 8 e and f, and Extended Data Fig. 1 ). Notably, most scapolites in the faulted samples showed an absence of the Raman peak at 1100 cm − 1 (CO 3 2− ) (Extended Data Table 2). Discussion Scapolite grains in granulites that exhibited faulting under high-pressure and high-temperature conditions showed clear evidence of breakdown. These grains had significantly reduced sulfur content compared to those in the starting material (Extended Data Table 1 and Fig. 7 ). In addition, the Raman spectra of most scapolites in the faulted samples lacked CO 3 2− carbonate peaks, indicating the devolatilization of CO 2 (Fig. 8 c–f, Extended Data Fig. 1 , and Extended Data Table 2). These results suggest that scapolite underwent devolatilization during the deformation experiment. In natural settings, the breakdown of S-rich scapolite during hydration and deformation under amphibolite-facies conditions is coupled with sulfide formation 32 . This previous study proposed that sulfur-rich scapolite was initially replaced by sulfur-poor scapolite, followed by complete replacement with other minerals such as epidote, amphibole, and sulfides 32 . Despite the many earthquakes observed at lower crustal depths, their underlying mechanisms remain unclear. Several possible causes of lower crustal seismicity have been proposed. Prior studies have linked such earthquakes to metamorphic transformations of granulite 2,23 . In these studies, granulite samples showed transformational faulting under the eclogite-facies conditions during deformation experiment. A study on the seismicity in the northern foreland of the Central Alps suggested that slab rollback of the Alpine lithosphere may have caused lower crustal earthquakes 5 . In addition, deep fluids and reaction-induced failure could also be one of potential mechanisms triggering earthquakes 6,15,21 . For example, numerous lower crustal earthquakes below the Taupo Volcanic Zone—an active continental rift in New Zealand—often occur in swarms, suggesting fluid movement in critically loaded fault zones 6 . These deep fluids are usually considered as mantle-derived CO 2 fluids because the depths at which lower crustal earthquakes occur are close to the depths of CO 2 emissions from basaltic melts 21,36,37 . Our experimental results suggest that the carbon dioxide released through scapolite devolatilization may be strongly linked to fault development. Previous studies have also proposed a global spatial correlation between seismically active regions and CO 2 emissions 38,39 . Furthermore, high-pressure CO 2 at depth has been reported to promote fault weakening 40,41 . A thermodynamic modeling study reported that the productivity of CO 2 -rich fluids is maximum for carbonate-bearing sediments originally containing low to moderate amounts of calcite (10–30 vol%) and metamorphosed at medium- to high-temperature (T > 600°C) and at medium pressure (P > 8 kbar) conditions 42 . The CO 2 component was generated by calcite and/or scapolite consumption 42 . Therefore, as the decarbonation reactions proceed, confined CO 2 - rich fluids would cause fluid overpressure, inducing carbo-fracturing of the host rocks and their upward migration. Notably, these processes can trigger crustal permeability, possibly resulting in earthquake nucleation and seismicity 33,39,43–46 . Thus, our experiments provide the first evidence of faulting by devolatilization of scapolite in the lower crust P-T conditions, especially by decarbonation embrittlement of scapolite in the deformed granulite. In the Lindas Nappe of the Bergen Arcs region of Western Norway, sulfur-carbon-scapolite completely replaces the original granulite mineralogy locally, creating an S-scapolite predominant rock 32 . In areas with complete metasomatic replacement, scapolite is the main rock-forming mineral, comprising over 70% of the rock volume 32 . Therefore, more CO 2 gas can be degassed due to the devolatilization of scapolite in this area. Figure 1 shows an example of a pseudotachylyte vein that follows a scapolite-rich area of granulites in the Lindas Nappe of the Bergen Arc. Furthermore, the observations of numerous pseudotachylytes in the Bergen Arcs in western Norway 11,19,47 and the Musgrave Ranges in Central Australia 12,13 and many earthquakes occurring in the lower crust in various tectonic settings (such as in collision zones 2,4,7 , active continental rifts 6,14,16 , continental margins 3 , intraplates 15,21 , and along the major strike-slip fault 48 ) could be explained by faults induced by the decarbonation embrittlement of scapolite in the granulite if scapolite exists, as shown by our study. Our study was limited to the deformation experiments of granulite under dry conditions. In the future, it is of interest to understand the effects of fluid and mineral composition on fault generation in granulite. Methods Sample material and sample assembly A granulite from Holsnøy, southwestern Norway, was crushed and sieved to a grain size of < 53 µm. A simple shear type of flow was achieved by placing 0.018 g of the rock powder between cylindrical alumina forcing blocks precut at 45° to the load axis (Fig. 3 ). The samples and forcing blocks were placed in a platinum jacket. Weak sodium chloride was used as the pressure medium. Temperature was measured using two thermocouples (Pt–30% Rh and Pt–6% Rh) placed near the granulite specimen. The temperature uncertainty was approximately ± 10 ℃ without considering any effect of pressure on the electromotive force of the thermocouple. Experimental procedures Deformation experiments were conducted using a modified Griggs apparatus at the Tectonophysics Laboratory of Seoul National University (SNU). The confining pressure was increased to 1.3 GPa for 12 h, and the temperature was increased to 830 ℃ within 1 h. At P–T conditions near the scapolite stability field (Fig. 2 ), the sample was annealed for 12 h using a hot press. After sintering, the confining pressure and temperature were adjusted to the target experimental P-T conditions away from the scapolite stability field (except for sample JH219). This was followed by 1 h annealing to remove the possible defects generated during pressurization. Shear stress was applied to the sample by moving the alumina (Al 2 O 3 ) piston down at a constant axial displacement rate of 9 x 10 − 8 ms − 1 . The sample was quenched after the deformation experiments by turning off the electrical power to preserve the deformation microstructure, and the pressure was lowered for 12 h. The detailed procedures for the deformation experiments using this apparatus are presented in the study by Ko and Jung. 1 Microstructural analysis After deformation experiments, the samples were impregnated with epoxy, cut parallel to the shear direction, and polished for observation. The polished sample was observed using a JEOL JSM-7100F field-emission SEM (FE-SEM) housed at the School of Earth and Environmental Sciences (SEES) at SNU. Electron-transparent thin-foil specimens for TEM observations were prepared from experimentally deformed samples using a FIB system (Helios G4) housed at the National Center for Interuniversity Research Facilities (NCIRF) of SNU. The internal microstructures near the faults were observed using Cs-TEM (JEM-ARM200F), also located at the NCIRF at SNU. EDS mapping was performed using Cs-TEM operating at an accelerating voltage of 80 kV. A JEOL JXA-8900R EPMA at the National Center for NCIRF in SNU was used to analyze the chemical composition of the major minerals in the hot-pressed and deformed samples. Raman spectra of the scapolite grains were acquired using a Horiba XploRA instrument at the Economic Geology Laboratory of SEES, SNU. The samples were excited using a 532-nm laser. Methods reference Ko, B., Jung, H. Crystal preferred orientation of an amphibole experimentally deformed by simple shear. Nat Commun. 6 , 6586 (2015). Declarations Acknowledgments This research was funded by grants from the National Research Foundation of Korea (NRF: 2020R1A2C2003765 and 2022R1A5A1085103) to H.J. We thank Yuri Choi for her help with the Raman analysis. We also thank Jung-Woo Park, Hyunwoo Lee, and YoungHee Kim for helpful discussions. Author contributions H.J. conceived the study with H.A. H.A. provided the natural granulite used in the experiments. Y.H. conducted the experimentsand drafted the manuscript under the supervision of H.J. Y.H. analyzed the data with contributions from S.J., J.H.S., and H.J. All authors contributed to editing and discussion of the manuscript. Competing interests The authors declare no competing interests. Data availability statement The data supporting the findings of this study are available upon request from Haemyeong Jung ( [email protected] ). 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J., Elliott, J. R., Wang, H. & Ryder, I. Earthquake cycle deformation and the Moho: implications for the rheology of continental lithosphere. Tectonophys 609 , 504–523 (2013). Almeida, Kaléo MF, and David M. Jenkins. A comparison between the stability fields of a Cl-rich scapolite and the end-member marialite. American Mineralogist: Journal of Earth and Planetary Materials 104 , 1788-1799 (2019). Table 1 Table 1 is available in the Supplementary Files section. Additional Declarations There is NO Competing Interest. Supplementary Files Haetal250515NaturecommunicationsExtendeddata.docx Dataset 1 Table1.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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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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-6694269","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":462036396,"identity":"c07c983e-2862-4031-9fa0-e46f39b03eb7","order_by":0,"name":"Haemyeong Jung","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAt0lEQVRIiWNgGAWjYBACA4YDQLICxj1AtJYzpGkBAsY2UrSYM54xe/h1Xl2ewQHmhx8YztwjrMWy4Yy5sey2w8UGB9iMJRhuFBPhsANnzKQltx1I3HCAwYyB4UMCsVrm1AG1sH8jXovkxwZmoBYeoC03iNJyrEya4djhxJmHeYolEs4Qo+XG4W2SP2rqEvuOt2/88OEYEVoYJA4wMPOAGMxATIwGBgb+BgbGH0SpHAWjYBSMghELANR6P8Xw1ySvAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-4327-4701","institution":"Seoul National University","correspondingAuthor":true,"prefix":"","firstName":"Haemyeong","middleName":"","lastName":"Jung","suffix":""},{"id":462036397,"identity":"9e51a5b4-980d-4074-8419-1fc247fda93c","order_by":1,"name":"Yoonhae Ha","email":"","orcid":"","institution":"Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Yoonhae","middleName":"","lastName":"Ha","suffix":""},{"id":462036398,"identity":"270e753e-d52e-4700-8482-a19435cbacd7","order_by":2,"name":"Sejin Jung","email":"","orcid":"https://orcid.org/0000-0002-0790-9765","institution":"Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Sejin","middleName":"","lastName":"Jung","suffix":""},{"id":462036399,"identity":"11678a48-bf53-49a1-bebc-64e246694ca7","order_by":3,"name":"Jung Hun Seo","email":"","orcid":"","institution":"Seoul National University","correspondingAuthor":false,"prefix":"","firstName":"Jung","middleName":"Hun","lastName":"Seo","suffix":""},{"id":462036400,"identity":"34dc3aa5-b186-4e93-80b8-62c634d8d159","order_by":4,"name":"Håkon Austrheim","email":"","orcid":"","institution":"University of Oslo","correspondingAuthor":false,"prefix":"","firstName":"Håkon","middleName":"","lastName":"Austrheim","suffix":""}],"badges":[],"createdAt":"2025-05-19 02:10:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6694269/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6694269/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":83536556,"identity":"900fdcc8-f7b4-47e3-91a5-1285ac1465c1","added_by":"auto","created_at":"2025-05-28 06:49:44","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":390450,"visible":true,"origin":"","legend":"\u003cp\u003eExamples of pseudotachylyte in granulite from the Lindås Nappe of the Bergen Arc, western Norway. \u003cstrong\u003ea\u003c/strong\u003e Hand specimen showing pseudotachylytes and an injection vein. \u003cstrong\u003eb\u003c/strong\u003eScanned thin section of granulite showing a pseudotachylyte and an injection vein following a scapolite-rich area.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/864cc306c77d968199c6c2d0.png"},{"id":83536557,"identity":"de6a83bd-e4fe-4c02-8938-2efd0cb656de","added_by":"auto","created_at":"2025-05-28 06:49:44","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":106484,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental conditions and phase diagram showing the stability field of scapolite. Red circles represent the experimental conditions for the faulted samples. Samples JH180 and JH205 are hot-pressed starting materials. Phase boundary after ref.\u003csup\u003e49\u003c/sup\u003e Ma = marialite; Me = meionite.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/7c862a1198fdd86aba04ec55.png"},{"id":83537397,"identity":"bd55b997-7391-49fe-945a-855ad5e3beac","added_by":"auto","created_at":"2025-05-28 06:57:45","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":119663,"visible":true,"origin":"","legend":"\u003cp\u003eSample assembly used for simple shear deformation experiments.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/b98c9db0033df472b94324d1.png"},{"id":83536562,"identity":"2e8e3eeb-ec95-4ea5-a9af-446c46453315","added_by":"auto","created_at":"2025-05-28 06:49:45","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":365051,"visible":true,"origin":"","legend":"\u003cp\u003eBackscattered electron images of hot-pressed and deformed samples. \u003cstrong\u003ea\u003c/strong\u003e Sample JH205, hot-pressed at a pressure of 1.3 GPa and temperature of 830 ℃. \u003cstrong\u003eb\u003c/strong\u003e Representative deformed sample showing a fault (JH201), deformed at P = 1.0 GPa, T = 700 ℃. \u003cstrong\u003ec\u003c/strong\u003eMagnified view of the yellow dashed rectangle in \u003cstrong\u003eb\u003c/strong\u003e. White arrows indicate the sense of shear during the experiment. Sulfides are distributed along the fault. pl: plagioclase; cpx: clinopyroxene; grt: garnet; sc: scapolite.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/f8f9b0f79450f1f9ebe7bfe8.png"},{"id":83536570,"identity":"92acbecc-90e8-45ca-b4cd-113e209e4a53","added_by":"auto","created_at":"2025-05-28 06:49:45","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":297808,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea \u003c/strong\u003eBackscattered electron images of a hot-pressed starting material and \u003cstrong\u003eb\u003c/strong\u003e a deformed sample, and \u003cstrong\u003ec\u003c/strong\u003e and \u003cstrong\u003ed\u003c/strong\u003e sulfide particles analyzed using ImageJ software. \u003cstrong\u003ea\u003c/strong\u003e Sample JH205 was hot-pressed at P = 1.3 GPa and T = 830 ℃. \u003cstrong\u003eb\u003c/strong\u003e Sample JH201 was deformed at P = 1.0 GPa, T = 700 ℃. Yellow arrows indicate a fault. \u003cstrong\u003ec\u003c/strong\u003e and \u003cstrong\u003ed\u003c/strong\u003e White particles are irregular sulfides analyzed using ImageJ software in \u003cstrong\u003ea\u003c/strong\u003e and \u003cstrong\u003eb\u003c/strong\u003e, respectively. The deformed sample showing a fault (JH201) exhibited a higher sulfide content than the hot-pressed sample (JH205).\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/55a05442310620ff1dd45626.png"},{"id":83536572,"identity":"e69427c1-584f-43dd-b2ee-cfa4996c7274","added_by":"auto","created_at":"2025-05-28 06:49:45","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":478900,"visible":true,"origin":"","legend":"\u003cp\u003eStrain localization and distribution of sulfides along faults. \u003cstrong\u003ea\u003c/strong\u003e BSE image of sample JH201, deformed at P = 1.0 GPa, T = 700 ℃, showing sulfides along the fault (yellow arrows).Sulfides are infiltrated into pulverized garnets near the fault. The red box indicates the location of the TEM foil. \u003cstrong\u003eb\u003c/strong\u003e Bright-field scanning transmission electron microscopy (STEM) image of the foil shown in \u003cstrong\u003ea\u003c/strong\u003e. \u003cstrong\u003ec\u003c/strong\u003eTEM–EDS sulfur mapping of the same area as in \u003cstrong\u003eb\u003c/strong\u003e. \u003cstrong\u003ed\u003c/strong\u003e BSE image of sample JH199, deformed at P = 0.7 GPa T = 700 ℃, showing sulfides along the fault (yellow arrows).The red box indicates the location of the TEM foil. \u003cstrong\u003ee\u003c/strong\u003e Bright-field STEM image of the foil shown in \u003cstrong\u003ed\u003c/strong\u003e. \u003cstrong\u003ef\u003c/strong\u003e TEM–EDS sulfur mapping of the same area as in \u003cstrong\u003ee\u003c/strong\u003e. In the TEM images (\u003cstrong\u003eb\u003c/strong\u003e, \u003cstrong\u003ec\u003c/strong\u003e, \u003cstrong\u003ee\u003c/strong\u003e, and \u003cstrong\u003ef\u003c/strong\u003e), faults are marked with red dashed lines. S is concentrated along the fault. S: sulfur.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/4d158bdaf017a844146a0903.png"},{"id":83537404,"identity":"f13933b0-7d50-44ce-807e-7ef77276811b","added_by":"auto","created_at":"2025-05-28 06:57:46","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":138931,"visible":true,"origin":"","legend":"\u003cp\u003eDiagram showing the relationship between the meionite component and sulfur content (S) from EPMA analysis of the samples. Some scapolite grains in the faulted deformed samples (JH191, JH201) contain low sulfur content.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/59736ba9df9d343f9f7a0466.png"},{"id":83537534,"identity":"7e455115-f010-4c36-a9e2-c29b74547825","added_by":"auto","created_at":"2025-05-28 07:05:45","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":215204,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative Raman spectra of scapolites in the hot-pressed starting material and deformed samples. Notably, most scapolites in the faulted deformed samples did not exhibit a peak at 1100 cm\u003csup\u003e-1\u003c/sup\u003e, indicating the loss of carbonate ions. \u003cstrong\u003ea \u003c/strong\u003eRaman spectrum of scapolite in the hot-pressed starting material (P = 1.3 GPa and T = 830 ℃), showing the presence of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e and CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e. \u003cstrong\u003eb\u003c/strong\u003e Deformed sample JH209 (P = 1.5 GPa, T = 600 ℃), which did not show faulting. \u003cstrong\u003ec\u003c/strong\u003e–\u003cstrong\u003ef\u003c/strong\u003e Deformed samples showing faults, in which scapolite lacked the CO₃²⁻ peak at 1100 cm\u003csup\u003e-1\u003c/sup\u003e. \u003cstrong\u003ec\u003c/strong\u003e Sample JH219 (P = 1.3 GPa, T = 830 ℃); \u003cstrong\u003ed\u003c/strong\u003e and \u003cstrong\u003ee\u003c/strong\u003e Sample JH191 (P = 1.0 GPa, T = 600 ℃); \u003cstrong\u003ef\u003c/strong\u003e Sample JH201 (P = 1.0 GPa, T = 700 ℃).\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/34f3befa8b4cc5cb9ef58099.png"},{"id":100356526,"identity":"87cea87c-ca7c-4b26-bfd0-347fc373ed6b","added_by":"auto","created_at":"2026-01-16 07:14:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2872422,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/b8add4d2-b729-414d-a744-35f6aa10314c.pdf"},{"id":83537533,"identity":"f953a2fe-db04-49f8-9c34-f7800b603d66","added_by":"auto","created_at":"2025-05-28 07:05:44","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":536988,"visible":true,"origin":"","legend":"Dataset 1","description":"","filename":"Haetal250515NaturecommunicationsExtendeddata.docx","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/74f65f0b72066b8c166839a1.docx"},{"id":83536559,"identity":"a754ad44-10a0-4068-80fb-176164dd1f01","added_by":"auto","created_at":"2025-05-28 06:49:44","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":137541,"visible":true,"origin":"","legend":"","description":"","filename":"Table1.docx","url":"https://assets-eu.researchsquare.com/files/rs-6694269/v1/f421f7e032baa7289c3e7a3e.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Decarbonation embrittlement of scapolite as a trigger for earthquakes in the lower crust","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe lower continental crust primarily consists of feldspar-rich granulites and amphibolites\u003csup\u003e8\u003c/sup\u003e. During orogeny, structural and metamorphic transformations in the lower crust are strongly associated with earthquakes\u003csup\u003e1,9,10\u003c/sup\u003e. An example of pseudotachylyte, which provides evidence of paleo-earthquakes, is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. This pseudotachylyte was found in granulites within the Bergen Arcs, Norway\u003csup\u003e1,9,11\u003c/sup\u003e. Pseudotachylytes have also been identified in other areas of the Bergen Arcs, Norway, and the Musgrave Ranges in Central Australia\u003csup\u003e12,13\u003c/sup\u003e. Previous studies and seismological data have shown that many earthquakes occur at lower crustal depths across diverse tectonic settings\u003csup\u003e2,3,6,7,14,15\u003c/sup\u003e. In collision zones, lower crustal earthquakes at focal depths of 20\u0026ndash;50 km occur in southern Tibet\u003csup\u003e2,4\u003c/sup\u003e, the Himalayas\u003csup\u003e7\u003c/sup\u003e, and beneath the northern foreland of the Central Alps\u003csup\u003e5\u003c/sup\u003e. In the active continental rifts, lower crustal earthquakes were also nucleated in the East African\u003csup\u003e14\u003c/sup\u003e and Baikal rifts\u003csup\u003e16\u003c/sup\u003e and the Taupo Volcanic Zone in New Zealand\u003csup\u003e6\u003c/sup\u003e. Lower crustal earthquakes were also reported at the continental margin in northwestern Canada\u003csup\u003e3\u003c/sup\u003e; at the intraplates in Botswana, South Africa\u003csup\u003e15\u003c/sup\u003e and Bhuj, India\u003csup\u003e17\u003c/sup\u003e. The earthquakes occur in the lower crust in collision zones and other tectonic settings and play a key role in metamorphic processes that modify the physical properties of the lower crust\u003csup\u003e18,19\u003c/sup\u003e. Deep intracontinental earthquakes can also cause considerable destruction. Several potential explanations have been proposed, including shear instability caused by thermal runaway\u003csup\u003e20\u003c/sup\u003e, fluid and reaction-induced failure\u003csup\u003e21\u003c/sup\u003e, local stress amplification\u003csup\u003e22\u003c/sup\u003e, instabilities triggered by metamorphic transformation\u003csup\u003e2,23\u003c/sup\u003e, mantle upwelling\u003csup\u003e24\u003c/sup\u003e, and aftershocks of large earthquakes in the shallower seismogenic regime\u003csup\u003e1\u003c/sup\u003e. However, the mechanism of brittle failure in lower continental crust remains poorly understood.\u003c/p\u003e \u003cp\u003eScapolite is a mineral commonly found in granulites\u003csup\u003e25\u0026ndash;30\u003c/sup\u003e. It contains volatile components\u0026mdash;primarily sulfate ions (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e), carbonate ions (CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e), and chloride ions (Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e) within its structure, which can release sulfur and carbon to form sulfides and carbonates. Notably, granulites comprise much of the lower continental crust\u003csup\u003e31\u003c/sup\u003e; therefore, scapolite may constitute an essential reservoir for sulfur and carbon, which was not previously accounted for in the sulfur and carbon cycles\u003csup\u003e32\u003c/sup\u003e. In nature, the nucleation of earthquakes is influenced by the build-up of gases under high pressure \u003csup\u003e33,34\u003c/sup\u003e. This observation highlights the important role played by volatile compounds in triggering seismic activity. However, no direct evidence of CO\u003csub\u003e2\u003c/sub\u003e gas-triggering earthquake has been reported to date.\u003c/p\u003e \u003cp\u003eTherefore, in this study, we conducted deformation experiments on granulite to investigate the relationship between granulite failure and degassing. The granulite samples showed faults during deformation experiments under lower crust conditions, and the scapolites exhibited clear evidence of the loss of volatile components, especially CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Starting material and experiment","content":"\u003cp\u003eA granulite from Holsn\u0026oslash;y, southwestern Norway, was used as the starting material. It comprises plagioclase (~\u0026thinsp;70 vol.%), garnet (~\u0026thinsp;15 vol.%), diopside (~\u0026thinsp;10 vol.%), scapolite (~\u0026thinsp;5 vol.%), and minor sulfide (~\u0026thinsp;0.1 vol.%). Granulite powder (\u0026lt;\u0026thinsp;53 \u0026micro;m) was hot-pressed at a pressure (P) of 1.3 GPa and temperature (T) of 830\u0026deg;C near the scapolite stability field for 12 h. The scapolite stability field is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The hot-pressed granulite was then deformed in simple shear (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) using a modified Griggs apparatus at P\u0026thinsp;=\u0026thinsp;0.7\u0026ndash;1.5 GPa and T\u0026thinsp;=\u0026thinsp;600\u0026ndash;830\u0026deg;C\u0026mdash; conditions outside the scapolite stability field (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). These deformation conditions represent the typical P\u0026ndash;T conditions of the continental lower crust. The detailed experimental procedures are provided in the Methods section.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMicrostructures of deformed granulite\u003c/h2\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e lists the experimental conditions for sample deformation and the results. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea shows the hot-pressed sample (JH205). A representative microstructure of the deformed granulite at a high P of 1.0 GPa and T of 700 ℃ is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and c. Notably, a fault developed in the deformed granulite sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), and sulfides were found around the fault (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and c). The sulfides were irregular and observed in the pulverized garnet and pyroxene. In addition, the sulfide content increased in the deformed samples (~\u0026thinsp;0.4%) compared with that in the starting material (~\u0026thinsp;0.1%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Furthermore, many deformed samples showed strain localization on the scale of mm to \u0026micro;m, which is closely associated with fault generation and the comminution of grains (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eFault structure and sulfides\u003c/h3\u003e\n\u003cp\u003eFault zones showing sulfides around the fault were observed in more detail using the spherical aberration-corrected transmission electron microscope (Cs-TEM). A backscattered electron (BSE) image of sample JH201, deformed at P\u0026thinsp;=\u0026thinsp;1.0 GPa and T\u0026thinsp;=\u0026thinsp;700\u0026deg;C, showed sulfides infiltrating pulverized garnets near the fault (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea\u0026ndash;c). Figures\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed-f also showed a fault zone in the other sample (JH199), which was deformed at P\u0026thinsp;=\u0026thinsp;0.7 GPa and T\u0026thinsp;=\u0026thinsp;700 ℃, which exhibited sulfides along both sides of the fault in BSE (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed) and TEM images (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee and f). Focused ion beam (FIB) foils were prepared oblique or normal to the fault direction to observe both sides of the fault in depth and were analyzed using TEM-Energy-dispersive X-ray spectroscopy (TEM-EDS) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea and d). The TEM-EDS mapping images of S show that elemental sulfur is distributed along the fault below the sample surface (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and f).\u003c/p\u003e\n\u003ch3\u003eDevolatilization of scapolite\u003c/h3\u003e\n\u003cp\u003eThe deformed samples with faults exhibited higher sulfide contents than the hot-pressed starting material (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The scanning electron microscope (SEM)-BSE and TEM-EDS images indicate that sulfur-rich scapolite released sulfur and formed sulfides during the deformation experiments (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). To investigate this further, we analyzed and compared the chemical compositions of scapolite grains in the hot-pressed starting material and deformed samples using an electron probe microanalyzer (EPMA) and Raman spectroscopy. Representative scapolite compositions from the hot-pressed material (JH205) and deformed samples (JH191, JH201, and JH219) are presented in Extended Data Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Scapolite grains in JH205 contained high sulfur levels (4.42\u0026ndash;5.21 wt% SO\u003csub\u003e3\u003c/sub\u003e). However, about half of the scapolite grains analyzed in the deformed samples JH191 and JH201 had significantly lower sulfur content (0.11\u0026ndash;1.05 wt% SO₃). Although sample JH219 also exhibited faults, its scapolite grains had a sulfur content of 4.01\u0026ndash;4.83 wt% SO₃, slightly lower than that of the starting material. Variations in S content and meionite component (Me\u0026thinsp;=\u0026thinsp;Ca/ (Ca\u0026thinsp;+\u0026thinsp;Na)) are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Scapolite grains in the faulted samples (JH191, JH201) at T\u0026thinsp;=\u0026thinsp;600 and 700 ℃ contained markedly less sulfur than those in the hot-pressed starting material (JH205).\u003c/p\u003e \u003cp\u003eScapolite is a silicate mineral that contains additional sulfate and carbonate groups in its structure, which can be detected using Raman spectroscopy. We analyzed scapolite grains using Raman spectroscopy to further study the devolatilization process because some scapolite grains in the deformed samples showed much lower sulfur content, indicative of the loss of volatile components (Extended Data Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). A previous study\u003csup\u003e35\u003c/sup\u003e showed that Raman bands at 993 and 1,095 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e represent the sulfate and carbonate ν\u003csub\u003e1\u003c/sub\u003e symmetric stretching vibrations, respectively. The scapolites in the hot-pressed starting material (JH205) and the deformed sample without faults (JH209) showed peaks at 982 and 1110 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea and b), indicating the absence of devolatilization of the samples. However, many scapolites in the deformed samples with faults did not show a Raman peak at 1100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicative of the loss of carbonate ions (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec-f), and some did not show a peak at 982 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ee and f, and Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Notably, most scapolites in the faulted samples showed an absence of the Raman peak at 1100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) (Extended Data Table\u0026nbsp;2).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eScapolite grains in granulites that exhibited faulting under high-pressure and high-temperature conditions showed clear evidence of breakdown. These grains had significantly reduced sulfur content compared to those in the starting material (Extended Data Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). In addition, the Raman spectra of most scapolites in the faulted samples lacked CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e carbonate peaks, indicating the devolatilization of CO\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec\u0026ndash;f, Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, and Extended Data Table\u0026nbsp;2). These results suggest that scapolite underwent devolatilization during the deformation experiment. In natural settings, the breakdown of S-rich scapolite during hydration and deformation under amphibolite-facies conditions is coupled with sulfide formation\u003csup\u003e32\u003c/sup\u003e. This previous study proposed that sulfur-rich scapolite was initially replaced by sulfur-poor scapolite, followed by complete replacement with other minerals such as epidote, amphibole, and sulfides\u003csup\u003e32\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eDespite the many earthquakes observed at lower crustal depths, their underlying mechanisms remain unclear. Several possible causes of lower crustal seismicity have been proposed. Prior studies have linked such earthquakes to metamorphic transformations of granulite\u003csup\u003e2,23\u003c/sup\u003e. In these studies, granulite samples showed transformational faulting under the eclogite-facies conditions during deformation experiment. A study on the seismicity in the northern foreland of the Central Alps suggested that slab rollback of the Alpine lithosphere may have caused lower crustal earthquakes\u003csup\u003e5\u003c/sup\u003e. In addition, deep fluids and reaction-induced failure could also be one of potential mechanisms triggering earthquakes\u003csup\u003e6,15,21\u003c/sup\u003e. For example, numerous lower crustal earthquakes below the Taupo Volcanic Zone\u0026mdash;an active continental rift in New Zealand\u0026mdash;often occur in swarms, suggesting fluid movement in critically loaded fault zones\u003csup\u003e6\u003c/sup\u003e. These deep fluids are usually considered as mantle-derived \u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eCO\u003c/span\u003e\u003csub\u003e2\u003c/sub\u003e fluids because the depths at which lower crustal earthquakes occur are close to the depths of CO\u003csub\u003e2\u003c/sub\u003e emissions from basaltic melts\u003csup\u003e21,36,37\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOur experimental results suggest that the carbon dioxide released through scapolite devolatilization may be strongly linked to fault development. Previous studies have also proposed a global spatial correlation between seismically active regions and CO\u003csub\u003e2\u003c/sub\u003e emissions\u003csup\u003e38,39\u003c/sup\u003e. Furthermore, high-pressure CO\u003csub\u003e2\u003c/sub\u003e at depth has been reported to promote fault weakening\u003csup\u003e40,41\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eA thermodynamic modeling study reported that the productivity of CO\u003csub\u003e2\u003c/sub\u003e-rich fluids is maximum for carbonate-bearing sediments originally containing low to moderate amounts of calcite (10\u0026ndash;30 vol%) and metamorphosed at medium- to high-temperature (T\u0026thinsp;\u0026gt;\u0026thinsp;600\u0026deg;C) and at medium pressure (P\u0026thinsp;\u0026gt;\u0026thinsp;8 kbar) conditions\u003csup\u003e42\u003c/sup\u003e. The CO\u003csub\u003e2\u003c/sub\u003e component was generated by calcite and/or scapolite consumption\u003csup\u003e42\u003c/sup\u003e. Therefore, as the decarbonation reactions proceed, confined CO\u003csub\u003e2\u003c/sub\u003e- rich fluids would cause fluid overpressure, inducing carbo-fracturing of the host rocks and their upward migration. Notably, these processes can trigger crustal permeability, possibly resulting in earthquake nucleation and seismicity\u003csup\u003e33,39,43\u0026ndash;46\u003c/sup\u003e. Thus, our experiments provide the first evidence of faulting by devolatilization of scapolite in the lower crust P-T conditions, especially by decarbonation embrittlement of scapolite in the deformed granulite.\u003c/p\u003e \u003cp\u003eIn the Lindas Nappe of the Bergen Arcs region of Western Norway, sulfur-carbon-scapolite completely replaces the original granulite mineralogy locally, creating an S-scapolite predominant rock\u003csup\u003e32\u003c/sup\u003e. In areas with complete metasomatic replacement, scapolite is the main rock-forming mineral, comprising over 70% of the rock volume\u003csup\u003e32\u003c/sup\u003e. Therefore, more CO\u003csub\u003e2\u003c/sub\u003e gas can be degassed due to the devolatilization of scapolite in this area. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows an example of a pseudotachylyte vein that follows a scapolite-rich area of granulites in the Lindas Nappe of the Bergen Arc. Furthermore, the observations of numerous pseudotachylytes in the Bergen Arcs in western Norway\u003csup\u003e11,19,47\u003c/sup\u003e and the Musgrave Ranges in Central Australia\u003csup\u003e12,13\u003c/sup\u003e and many earthquakes occurring in the lower crust in various tectonic settings (such as in collision zones\u003csup\u003e2,4,7\u003c/sup\u003e, active continental rifts\u003csup\u003e6,14,16\u003c/sup\u003e, continental margins\u003csup\u003e3\u003c/sup\u003e, intraplates\u003csup\u003e15,21\u003c/sup\u003e, and along the major strike-slip fault\u003csup\u003e48\u003c/sup\u003e) could be explained by faults induced by the decarbonation embrittlement of scapolite in the granulite if scapolite exists, as shown by our study. Our study was limited to the deformation experiments of granulite under dry conditions. In the future, it is of interest to understand the effects of fluid and mineral composition on fault generation in granulite.\u003c/p\u003e "},{"header":"Methods","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSample material and sample assembly\u003c/h2\u003e \u003cp\u003eA granulite from Holsn\u0026oslash;y, southwestern Norway, was crushed and sieved to a grain size of \u0026lt;\u0026thinsp;53 \u0026micro;m. A simple shear type of flow was achieved by placing 0.018 g of the rock powder between cylindrical alumina forcing blocks precut at 45\u0026deg; to the load axis (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The samples and forcing blocks were placed in a platinum jacket. Weak sodium chloride was used as the pressure medium. Temperature was measured using two thermocouples (Pt\u0026ndash;30% Rh and Pt\u0026ndash;6% Rh) placed near the granulite specimen. The temperature uncertainty was approximately\u0026thinsp;\u0026plusmn;\u0026thinsp;10 ℃ without considering any effect of pressure on the electromotive force of the thermocouple.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eExperimental procedures\u003c/h3\u003e\n\u003cp\u003eDeformation experiments were conducted using a modified Griggs apparatus at the Tectonophysics Laboratory of Seoul National University (SNU). The confining pressure was increased to 1.3 GPa for 12 h, and the temperature was increased to 830 ℃ within 1 h. At P\u0026ndash;T conditions near the scapolite stability field (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), the sample was annealed for 12 h using a hot press. After sintering, the confining pressure and temperature were adjusted to the target experimental P-T conditions away from the scapolite stability field (except for sample JH219). This was followed by 1 h annealing to remove the possible defects generated during pressurization. Shear stress was applied to the sample by moving the alumina (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) piston down at a constant axial displacement rate of 9 x 10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e ms\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The sample was quenched after the deformation experiments by turning off the electrical power to preserve the deformation microstructure, and the pressure was lowered for 12 h. The detailed procedures for the deformation experiments using this apparatus are presented in the study by Ko and Jung.\u003csup\u003e1\u003c/sup\u003e\u003c/p\u003e\n\u003ch3\u003eMicrostructural analysis\u003c/h3\u003e\n\u003cp\u003eAfter deformation experiments, the samples were impregnated with epoxy, cut parallel to the shear direction, and polished for observation. The polished sample was observed using a JEOL JSM-7100F field-emission SEM (FE-SEM) housed at the School of Earth and Environmental Sciences (SEES) at SNU. Electron-transparent thin-foil specimens for TEM observations were prepared from experimentally deformed samples using a FIB system (Helios G4) housed at the National Center for Interuniversity Research Facilities (NCIRF) of SNU. The internal microstructures near the faults were observed using Cs-TEM (JEM-ARM200F), also located at the NCIRF at SNU. EDS mapping was performed using Cs-TEM operating at an accelerating voltage of 80 kV. A JEOL JXA-8900R EPMA at the National Center for NCIRF in SNU was used to analyze the chemical composition of the major minerals in the hot-pressed and deformed samples. Raman spectra of the scapolite grains were acquired using a Horiba XploRA instrument at the Economic Geology Laboratory of SEES, SNU. The samples were excited using a 532-nm laser.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMethods reference\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eKo, B., Jung, H. Crystal preferred orientation of an amphibole experimentally deformed by simple shear. \u003cem\u003eNat Commun.\u003c/em\u003e \u003cb\u003e6\u003c/b\u003e, 6586 (2015).\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was funded by grants from the National Research Foundation of Korea (NRF: 2020R1A2C2003765 and 2022R1A5A1085103) to H.J. We thank Yuri Choi for her help with the Raman analysis. We also thank Jung-Woo Park, Hyunwoo Lee, and YoungHee Kim for helpful discussions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003cbr\u003eH.J. conceived the study with H.A. H.A. provided the natural granulite used in the experiments. Y.H. conducted the experimentsand drafted the manuscript under the supervision of H.J. Y.H. analyzed the data with contributions from S.J., J.H.S., and H.J. All authors contributed to editing and discussion of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003cbr\u003e The authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability statement\u003c/strong\u003e\u003cbr\u003e The data supporting the findings of this study are available upon request from Haemyeong Jung ([email protected]).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorrespondence and requests for materials should be addressed to Haemyeong Jung ([email protected]).\u003c/strong\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eJamtveit, B., Ben-Zion, Y., Renard, F. \u0026amp; Austrheim, H. Earthquake-induced transformation of the lower crust. \u003cem\u003eNature\u003c/em\u003e \u003cstrong\u003e556\u003c/strong\u003e, 487\u0026ndash;491 (2018).\u003c/li\u003e\n\u003cli\u003eShi, F., Wang, Y., Yu, T. et al. 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Jenkins. A comparison between the stability fields of a Cl-rich scapolite and the end-member marialite. \u003cem\u003eAmerican Mineralogist: Journal of Earth and Planetary Materials\u003c/em\u003e \u003cstrong\u003e104\u003c/strong\u003e, 1788-1799 (2019).\u003c/li\u003e\n\u003c/ol\u003e\n"},{"header":"Table 1","content":"\u003cp\u003eTable 1 is available in the Supplementary Files section.\u003c/p\u003e\n"}],"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-6694269/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6694269/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eRecent studies and seismological data have reported numerous earthquakes in the lower continental crust\u003csup\u003e1\u0026ndash;7\u003c/sup\u003e. However, the mechanisms triggering earthquakes in the lower crust remain unclear. Here, we present results from the experimental deformation of granulite in simple shear at the pressures of 0.7 to 1.5 GPa and temperatures of 600\u0026ndash;830 ℃ using a modified Griggs apparatus. We found faults in the deformed granulite in which the scapolite grains were devolatilized. Notably, most Raman spectra of the deformed scapolites did not have peaks at 1100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e due to the vibrations of carbonate ions, indicating that carbon dioxide gas was released following the breakdown of scapolite. Therefore, our data suggest that the decarbonation embrittlement of scapolite can trigger earthquakes in the continental lower crust.\u003c/p\u003e","manuscriptTitle":"Decarbonation embrittlement of scapolite as a trigger for earthquakes in the lower crust","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-28 06:49:40","doi":"10.21203/rs.3.rs-6694269/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":"61ade988-ccf3-4f10-a5ed-cbf27b8cf582","owner":[],"postedDate":"May 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":49061333,"name":"Earth and environmental sciences/Solid Earth sciences/Geophysics"},{"id":49061334,"name":"Earth and environmental sciences/Solid Earth sciences/Seismology"},{"id":49061335,"name":"Earth and environmental sciences/Solid Earth sciences/Geology/Structural geology"}],"tags":[],"updatedAt":"2026-03-25T10:30:30+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-28 06:49:40","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6694269","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6694269","identity":"rs-6694269","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}