First cosmic-ray muography of a crust-mantle transition zone

First cosmic-ray muography of a crust-mantle transition zone | 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 First cosmic-ray muography of a crust-mantle transition zone László Oláh, Yuki Kusano, Tomoaki Morishita, Said Mohammed Almusharafi, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8019975/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 10 You are reading this latest preprint version Abstract Studying the structure and composition of ophiolitic crustal-mantle boundaries provides insights into the nature of oceanic lithosphere. In this work, we conducted the first muography of an ophiolite segment at Wadi Fizh in the northern Samail Ophiolite, which represents an analogue of crust-mantle boundaries formed at fast-spreading mid-ocean ridges. A multi-wire-proportional-chamber-based muographic observation system was operated at a distance of 400 meters from the crust-mantle transition zone (Moho Transition Zone, MTZ) for 171 days. The resulting high-resolution muographic image resolves the internal density structure of the ophiolite segment with a spatial resolution of approximately 3.5 meters. Mean density was measured as 3.03 g cm-3 for the layered gabbros, consistent with weakly serpentinized gabbroic intrusions. A significantly lower value of 2.72 g cm-3 was obtained for the MTZ, indicating a highly serpentinized MTZ, in contrast with petrological profiles which previously revealed gradational transition from the mantle to the crust at Wadi Fizh. A mean density of 3.38 g cm-3 was revealed in the lower part of ophiolite rige that indicates the presence of fresh peridotites beneath the thin layer of gabbroic cover. These results demonstrate that muography can provide complementary information about the density structure of ophiolites and, by extension, on the architecture of the oceanic lithosphere. Earth and environmental sciences/Planetary science Earth and environmental sciences/Solid earth sciences oceanic lithosphere Moho ophiolite serpentinization muon muography Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Mohorovičić discontinuity (Moho) is the boundary of the lowermost crust and the uppermost solid mantle layers within Earth’s lithosphere 1 . Processes occurring near the Moho in the oceanic lithosphere, such as mantle melting, melt migration, and crustal accretion, drive a large-scale cycles of energy and matter that form natural resources and trigger the occurrence of geohazards 2 . The architecture of the Moho formed by several parameters including the spreading rate, lithologic heterogeneity and tectono-magmatic history 3-6 . For example, Ohira et al. 7 observed a variety of Moho in the northwestern Pacific, where the Moho appears to be a sharp seismic boundary in regions with intermediate to fast seafloor spreading rates. This was interpreted as a lithological contact between gabbros and peridotites. In contrast, the Moho in other sites was observed to be a multiple reflection boundary, or invisible and obscure due to the complex lithology. Tamura et al. 8 found that the Moho reflectivity increases beneath thicker oceanic crust and shallower water depth. However, factors such as the age of the lithosphere and local effects (e.g., off-axis magmatism, serpentinization) affect the seismic data. It is of prime interest what makes the variability of the oceanic Moho. To date, the Moho has not yet been directly reached by ocean drilling (e.g., Ref. 9 ). Nevertheless, intact structure of the oceanic lithosphere is preserved in ophiolites that are sequences of crust and mantle layers obducted onto continental margins 5,10 . The Samail Ophiolite, an analogue of huge blocks of the oceanic lithosphere formed on fast-spreading ridges, has the MTZs consisting of dunite intercalated with gabbro and chromitite layers up to several hundred meters above mantle diapirs, whereas the MTZs become thin and sharp away from mantle diapirs 11-13 . Thus, studying the structure and composition of the MTZs in Samail Ophiolite enables us to test the causal relationship between the physical properties and lithology of the MTZs and improve the models of formation and evolution of Moho discontinuities in oceanic lithosphere. Various geophysical, remote sensing, and laboratory-based techniques have been applied to study the structure and the composition of ophiolites. Gravity profiles measured across the Samail Ophiolite showed a strong correlation with the thickness of the ophiolitic sequences 14 . Anomalies in the gravity data were interpreted as serpentinization of the mantle peridotites and thinning of ophiolite nappes. Image processing of remote sensing data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) was utilized to delineate the Moho discontinuity and the associated rocks, as well as to map mineralogical variations 15 . Integrated geophysical studies combined seismic experiments, gravity modeling, and analysis of aeromagnetic data to enable the reconstruction of the structural architecture of the Troodos ophiolite 16 and the northern segment of the Samail Ophiolite 17 at a regional scale. In addition, ambient seismic noise tomography was applied to image the composition and structure of the continental crust underlying the Samail Ophiolite 18 . More recently, the Oman Drilling Project (ODP) sampled the crust and upper mantle sections of the ophiolite in situ with an unprecedented spatial resolution 19 . The extracted cores were characterized by different laboratory-based techniques. Here, we provide a brief description of a few results without claiming completeness. Micro-imaging infrared spectroscopy was applied to investigate the spatial patterns in hydrothermal alteration of oceanic crust 20 . The rock and mineral major and trace element compositions of the lower and the upper oceanic crust were quantified by X-ray fluorescence 21 . Permeability profiles across the crust-mantle sections were inferred from resistivity measurements to provide insights into fluid circulation in the oceanic lithosphere 22 . Gamma ray attenuation and P-wave velocity measurements were conducted on borehole cores to determine bulk density profiles and to evaluate pore structures and degree of cementation for studying the geological nature of the crust-mantle transition zone and the effects of the hydrothermal alteration processes that occurred in the lower crust 19 . Although applying different techniques to research the ophiolites and the nature of the oceanic lithosphere are effective approach, there are discrepancies between the different measurement techniques that lead to biased interpretations. For example, Swift et al. 23 measured the profiles of seismic velocity versus depth below the sea floor. Acoustic wave velocity measurements of drilling cores from the hole 1256D significantly differ from the sonic log data and the P-wave velocities measured by ocean bottom seismometers. Similarly, discrepancies were found between the drilling cores from the ophiolites and the seismic data. These discrepancies arise from the disparity between the size of specimens (on the order of centimeters) and the spatial resolution of seismic techniques (on the order of kilometers). In addition, lithological changes occur in both horizontally and vertically ranging from a few centimetres to several tens of meters. Therefore, the seismic velocity structure is biased by the limited sampling density from ophiolites and drilling holes. Reconstructing high-resolution mass density structure of crustal and Moho transition zones in ophiolites may advance our knowledge about the geologic nature of the crust-mantle transition zones. Here, we focus on the implementation of a novel remote geophysical method called muography, which offers unique spatial and density resolving capabilities for imaging the density structure of ophiolites 24 . Muography is a novel geophysical imaging technique that utilizes cosmic-ray muons to probe Earth's subsurface 25 . Cosmic-ray muons are charged elementary particles that are generated continuously in Earth’s atmosphere at altitudes of approximately 10-15 km above sea level as secondary products of particle showers resulting from the interaction of primary cosmic rays with atmospheric nuclei 26 . Muons are highly energetic and relativistic particles, allowing them to penetrate a few kilometers into rock with a measurable flux. Their high-penetration power, predictable attenuation rate, and negligible angular deviations from their original track make them applicable for passive imaging and monitoring subsurface density variations 27,28 . In practical applications 29 , muography is advantageous for structures where non-invasive, passive, and long-term monitoring is needed and in which large-scale, high-density contrasts are present relative to the surrounding media, e.g., magma in volcanic conduits, ore bodies above mine tunnels, karstic caves in mountains, etc. Here, we focus on muography of an ophiolite segment formed from crust, MTZ, and mantle layers. Geological setting The Samail Ophiolite is a mountain chain on the Arabian continental margin that extends over 400 km in length and approximately 80 km in width. This ophiolite preserves a complete oceanic crust-mantle sequence of pelagic sedimentary rocks, extrusive rocks, sheeted dike complex, gabbroic rocks, and ultramafic rocks, in descending order (Fig. 1) 5,11 . Five or more paleoridge segments are identified though the ophiolite. These are defined by the presence of diapiric structures within the mantle peridotite at the segment centers, and mutually intrusive sheeted dikes and gabbros and doubly superimposed upper crustal structures at the segment boundaries 3,4,30-33 . The measurement site is located at Wadi Fizh, corresponding to the northern end of a 60-km long paleoridge segment between wads Fizh and Ahin 33-35 . Here, the Moho is represented by a relatively thin transition zone of dunite intervening gabbro layers between the layered gabbro and the underlying harzburgite tectonite, in accordance with the MTZ at the segment end 36 . The boundary between the layered gabbro and the uppermost dunite layer in MTZ gently dips to the east, undulating with a wavelength of 500-600 m and a wave height of ~100 m (Fig. 2). Comparison of the muographic data collected at the lithologic Moho of the ophiolite and the seismic data acquired from an oceanic Moho (e.g., at the Pacific Plate) may enable us to infer the physical and geological nature of the Moho at fast-spreading MORs. Results Experimental setting Figure 2a shows the topographic map of the measurement site with contours of 20-m intervals based on the 1:50,000 topomap of the National Survey Authority (NSA), Sultunate of Oman. A Multi-Wire-Proportional-Chamber-based Muography Observation System (MMOS) was installed on the Moho plane at latitude of 24.45655º, longitude of 56.29703º, and at an altitude of 467 m above sea level, marked as red-colored × in Fig. 2a. The MMOS was oriented toward an azimuth 299º from geographic north (defined as tan( θ x) = 0, and shown by the red-colored arrow in Fig. 2a), with tracking detector layers set vertically (i.e. perpendicular to the horizontal plane, defined as tan( θ y) = 0). The MMOS tracked muons within ± 0.55 rad in both the horizontal and the vertical directions. The green-colored hatched area and the purple-colored hatched area show the layered gabbro (LGb) and the dunite/harzburgite (Hz/Dn) lithologic layers, respectively. The blue-colored lines (Profile-1 & -2) indicate selected cross-sectional profiles across the ophiolite segment in Figs. 2b and 2c, respectively. Orange-colored dashed lines show the boundary between the LGb and Dn/Hz layers. The red-colored arrows point towards the location of observatory. The horizontal distance between the MMOS and the ridge of imaged ophiolite was about 400 m. Figure 2. The experimental arrangement and the observed lithologic layers are shown in the topographic map of Wadi Fizh. (a) The muon tracking system was installed at latitude of 24.45655º and longitude of 56.29703º at 467 m above sea level (red-colored ×). The ridge of the muographically imaged ophiolite is located at a distance of about 400 m from the observational instrument. The red-colored arrow shows the azimuthal orientation of the tracker that was set to 299 deg from north. The green-colored hatched area and the purple-colored hatched area shown the layered gabbroes (LGb) and the dunite/harzburgite (Dn/Hz) lithologic layers, respectively. The blue-colored lines show two selected cross-sections across the ophiolite segment. (b) The cross-section is shown along the Profile-2 of Fig. 2a. The solid line shows the altitude above sea level. The dashed-line shows the boundary between the layered gabbro and Dunite/Harzburgite. (c) The cross-section is shown along the Profile-1 of Fig. 2a. The lines show the same information as in Fig. 2b. It is worth noting that the lower left ridge beyond the layered gabbro in Fig. 3a corresponds to that of layered gabbro. However, because the boundary between the MTZ and the layered gabbro is undulating, the layered gabbro layer is thin and covers only the surface of the ridge, which is underlain by the dunite-rich MTZ. Figure 3b shows the rock thicknesses along the muon paths calculated using the 5-m DEM data of NSA from the location of the MMOS across the ophiolite within the black rectangle of Fig. 3a. The red-colored dased line shows the boundary between the LGb and the MTZ. Data collection and processing The data collection campaign was conducted between 27 February 2024 and 22 January 2025. We conducted an off-line data analysis for detector calibration, data quality assurance, and muographic image processing (Methods). The detector calibration procedures involved offline alignment of detector layers and removing of noise and dead electronics channels. After the data quality assurance, t he effective time of data collection was totaled 171 days due to detector malfunctions, power cuts, and the maintenance works. In the muographic images , each pixel corresponds to a pair of slopes, tan(θx)-tan(θy), which represent the tangents of the horizontal and vertical projection angles of muon tracks, measured relative to the orientation of the MMOS (Fig. 2). Each image has 65 × 65 pixels with a size of 0.0086 in both horizontal and vertical directions. Each angular bin corresponds to an approximately 3.5 m × 3.5 m area at the peak of the ophiolite 400 m away from the MMOS. The measured flux, the modeled flux and the average mass density were determined for each angular bin. The first muographic images Figures 4a and 4b show the measured and modeled muon fluxes, respectively. The modeled flux was calculated assuming a uniform rock density of 3 g cm − 3 within each slope bin. Both flux maps reflect the structure of the ophiolite segment and the surrounding topography thanks to the excellent spatial resolution of approximately 3.5 meters. The corresponding density image of the ophiolite segment was reconstructed with the same spatial resolution, as shown in Fig. 5a. Here the white-shaded region indicate slope bins without density values because these bins correspond to the open sky where the path lengths of muons was assumed to be zero. The densities were under-measured in angular bins where the path lengths of muons across the ophiolite segment exceeded 500 meters. This underestimation was caused by the contamination from background muons in the selected track set. Figure 5b shows the density error values which were quantified by averaging the upper and lower densities calculated for the F-ΔF and F + ΔF values, respectively. Three regions were designated across the ophiolite segment: a high density-region of the ridge behind in the lower left (Dn/Hz), a region across the upper part of the ridge (LGb), and a region across the MTZ. The averaged rock thicknesses were estimated as 165 m, 286 m and 61 m across the Dn/Hz, the MTZ, and the LGb, respectively. Figures 5 c, 5d, and 5e show the density distributions extracted from the regions Dn/Hz (blue-colored histogram), MTZ (green-colored histogram), and LGb (orange-colored histogram), respectively. A density bin size of 0.1 g cm − 3 was used for each histogram. The density values ranged from 2.3 g cm − 3 to 6 g cm − 3 with a mean of 3.38 g cm − 3 for the Dn/Hz (Fig. 5c). The density values above 4.5 were considered non-physical and were excluded from the mean calculation. The densities were measured between 1.7 g cm − 3 and 4 g cm − 3 with a mean of 2.72 g cm − 3 for the MTZ (Fig. 5d). For the LGb (Fig. 5e), the density values ranged from 1.6 g cm − 3 to 5.5 g cm − 3 with a mean of 3.03 g cm − 3 . Discussion The first muographic density measurement demonstrates that muon imaging allows resolving the internal structure and composition of rocks forming ophiolite segments, from the upper mantle to the lower crust. At Wadi Fizh, the density across the lower crust layered gabbro (LGb) is consistent with the typical densities of layered gabbros (2.95-3 g cm − 3 ). This density value indicates that the layered gabbro is generally fresh, lack of hidden fractures and not affected by alteration. The mean density measured across the MTZ is significantly lower (2.72 g cm − 3 ) than typical density values of reported for other MTZs in the Samail Ophiolite (e.g., Ref. 14 ). This muographic observations is not consistent with lithological profile described by Akizawa & Arai 36 , which reveals a gradual lithological transition from harzburgite through dunite and wehrlite layers to layered gabbro. Similar MTZ contacts were observed in other segments of the northern Samail Ophiolite, where gabbro sills intrude progressively into dunite 37 . Such transitions between the lower crust and upper mantle are attributed to melt infiltration and subsequent crystallization in the Moho transition zone 32 , 38 , 39 . Here, the significantly lower density of MTZ reflects the highly serpentinized nature of the rocks as observed at outcrops. The density values were measured significantly higher (3.38 g cm − 3 ) across the lower left ridge (Dn/Hz) on the back. Although the ridge is covered by the LGb layer, its mean density indicates that the ridge is primarily composed of harzburgite. This means that the LGb covering the ridge is very thin and mainly underlain by relatively fresh mantle peridotite (Fig. 2c). This supports the field observations indicating that the Moho surface is not a flat plane but exhibits undulations with a wavelength of 500 to 600 m (Fig. 2). The applicability of muography for studying ophiolites has several limitations, and possible upgrades are discussed as follows. (A) Constraints on local environment (such as harsh and varying climate or topography) and logistic (e.g., limited resources for power supplying) may not allow multi-month measurements at each ophiolite segment. (B) The current spatial resolution of a few meters does not allow to resolve fine-scale structures. For example, the gabbro sills in size range from a few millimeters to a few tens of centimeters, whose sizes are well below the current spatial resolution. To improve the spatial resolution of muography to an order of a few tens of centimeters would need the operation of a larger muographic observation system at a distance of a few meters from the studied segment for a longer period. (C) The penetration range of muons is limited due to their finite yield and energy distribution. As shown in Fig. 4, the flux of penetrated muons decreasing significantly with the thickness of the imaged structure and the background contamination is increasing above the thickness of approximately 500 meters. Studying ophiolite segments with thickness in order of a few hundred meters will require the development of modular muography observatories, similarly to volcano monitoring. (D) Muography enables studying the structure and constituents of ophiolites just in a presence of significant (> 0.1 g cm − 3 ) density contrast. For example, muography cannot distinguish the serpentinised peridotites from the crustal gabbro layers. (D) Muography reconstructs integrated density along the path of muons. Multi-directional muography (e.g., Ref. 40 ) and joint inversion of muographic and gravimetric data (e.g., Ref. 41 ) enable the reconstruction of the three-dimensional density structure of ophiolite sequences. This will reveal the heterogeneity of the oceanic lithosphere including lithological heterogeneity and the presence of fractures and alteration zones. (E) Different lithologies may have similar mass densities. The integrated processing of muographically measured densities and seismic velocities 42 will allow to determine the elastic constants (bulk and shear moduli) of the rocks in ophiolites. We conducted the first muography of a Moho transition zone at Wadi Fizh in the northern Samail Ophiolite. The reconstructed mass densities indicate that the mantle and the lower crustal gabbro unaltered, however the MTZ dunite is largely altered to serpentinite. A gradational Moho transition zone is revealed between the peridotites to the layered gabbros. The obtained results demonstrate that muography can reveal density contrast within ophiolite segment that allows measuring the crustal thickness, localizing and exploring the depth of the crust-mantle boundary, revealing fluid-rock interactions in the mantle, such as degree of serpentinization or melt impregnation. Measurement of 3-dimensional density distribution will allow to investigate the diversity of Moho along the paleoridge segment and explore whether the discontinuity forms either sharp, blurred or multiple seismic reflections, thus inferring to the physical processes occurred between the diverse oceanic Moho. Methods Muon detector The technical details of the applied tracking detector has already been presented in our earlier works 43 – 45 . This technology has successfully been applied in various environments, including underground mines and near active volcanoes 29 . In this study, we used an MMOS consisting of seven MWPCs, each with a surface area of 0.8 m × 0.8 m and a thickness of 0.02 m . Each MWPC provided a 1 + 1 dimensional positional information by two perpendicular wires planes. Wire spacing was 0.012 m in both wire planes, which allowed a spatial resolution of approximately 4 mm for each MWPC. Five of the seven detectors operated reliably during the measurement period, thus we used the data from these detectors for this study. The five MWPCs spanned a length of 1.397 m in the tracking detector. Figure 6 shows a photograph and a schematic drawing of the experimental setup with five MWPCs (gray-colored rectangles) and two 2-cm-thick lead walls (hatched rectangles). The lead walls were used to absorb and deflect low-energy muons, which may scatter from the surface of ophiolite or from the atmosphere into the detectors (e.g., Ref. 46 ). The numbers shown below the MWPCs and lead walls indicate their distance from the first tracking layer in millimeters. The tracking system was powered by the local electricity network, supplied from an apartment located approximately 300 meters from the MMOS. The detector layers were continuously flushed with an argon-carbon-dioxide (Ar-CO 2 ) gas mixture in a proportion of 80%:20%. This gas mixture enabled signal generation on the wires via ionisation of argon atoms by penetrating charged particles, typically producing 100 ionisation electrons per centimeter. A high-voltage of + 1,700 V was applied to the wires to amplify the signal to a measurable signal in the order of 10,000 electrons. Custom-designed front-end electronics amplified the analogue signals by a factor of ten and discriminated them by shift registers, converting them to binary output on each electronics channel. Detector operation and data collection were controlled by a microcomputer. Data collection was triggered by triple coincidence of MWPCs. The recorded data included time stamps, analogue signal amplitudes (ADCs), trigger patterns, high-voltage values, temperature, humidity, pressure, and the hit coordinates of the detected particles. These data were stored into ASCII files on an event-by-event basis. The trigger was temporarily blocked during the data readout. The trigger frequency ranged from 5 Hz to 35 Hz, with an average of 10.4 Hz. A significant reduction in the trigger rate was observed after the removal of uninterruptible power supply from the power supplying system. Muon flux measurement The muon flux values were calculated for each slope bin by dividing the number of reconstructed tracks (N) by the measurement time (T), corrected for the dead time ( t dead ), the trigger efficiency ( ε trigger ), the tracking efficiency ( ε tracking ), and the detector acceptance factor, which accounts for the sensitive surface area (A) and the solid angle (Ω): F = N / [ T × (1 - t dead ) × ε trigger × ε tracking × A × Ω ] . (1) The tracks were counted as a function of horizontal and vertical slopes on event-by-event basis. The reconstruction of particle trajectories was conducted independently in the horizontal and vertical directions. For each MWPC, the centroids, sizes and multiplicities of signal clusters were determined. A combinatorial algorithm then produced track candidates by merging one cluster centroid from each MWPC. A straight line was fitted on the clusters of each track candidate. The track candidates were ranked according to the goodness of fit (χ 2 /ndf), and the best-fitting one was selected for further analysis. To suppress tracks originate from electronic noise and low-energy scattered particles, only tracks with clusters in at least four MWPCs, analogue signal amplitudes above 200 ADC in at least four MWPCs, and a goodness of fit χ 2 /ndf < 2 were retained. The measurement time T was 171 days after quality assurance of the track data. Figures 7a and 7b show the distributions of the time differences between consecutive events. Assuming that the detected particles arrived from different air showers with uniform probability over time, the interarrival times follow an exponential distribution, consistent with a Poisson arrival process. As shown in Fig. 7b, the distribution starts from 112.5 microseconds due to the data readout, which blocked the trigger for this period after each event. This dead time resulted in only a negligible reduction (0.12%) of the muon yield over the entire data acquisition period. The trigger efficiency was evaluated for each chamber by calculating the ratio of tracks that produced trigger signal in the investigated chamber to the total number of tracks. Figure 7c shows the time evolution of the trigger efficiency with a 6-hour bin size. The trigger efficiencies exceeded 93% for every MWPC throughout th e measurement period. The overall trigger efficiency of the tracking system approached 100%. The tracking efficiency was measured for each MWPC as a function of time and of tracks slopes. A tracklet was constructed from clusters detected in four MWPCs. This tracklet was extrapolated to the fifth MWPC. The tracking efficiency of the investigated chamber was given by the ratio of the cases in which a cluster was found on the fifth MWPC to the number of tracklets. The overall tracking efficiency of the detector system was calculated from the individual tracking efficiencies of MWPCs by the following formula: ε tracking = ∏ i=1:5 ε i + Σ i=1:5 ∏ j=1:5 [ (1 - δ ij ) × ε i + δ ij × (1 - ε i ) ] , (2) where the δ ij Kronecker delta gives δ ij = 1 for i = j , and 0 otherwise. Figure 8 summarizes the tracking performance. A 6-hour time binning was applied for monitoring the time evolution of tracking efficiencies. Figure 8a shows the track rate as a function of time, represented by black points with one standard deviation error bars. The mean track rate was 0.163 Hz (red-colored line). Tracking efficiencies exceeded 92% for each MWPC throughout the data collection period (Fig. 8b). The overall tracking efficiency of the detector system is shown as a function of horizontal and vertical slopes in Fig. 8c. These efficiencies were found well above 95% in the studied angular regions. Muon flux modeling The density-lengths (i.e., integrated densities along muon paths) were determined for each slope bin by minimizing the difference between the modeled and measured muon fluxes. The modeled muon fluxes ( F calc ) were calculated as follows. Zenith-angle ( θ ) and energy ( E ) dependent muon spectra were integrated over energy to simulate the attenuation of muon flux after traversing different density-lengths. In this work, the energy spectra were parametrised with an empirical formula based on a modified Gaisser model 47 . The altitude ( A ) dependence of modeled fluxes was taken into account by a factor F calc (A) = F calc (0 m) × e [−A/K(E)] , where F calc (0 m) is the flux at sea level and the K(E) is an energy-dependent parameter, given by K(E) = 4,900 + 750 × E (E in GeV , K in m ) . The difference in integrated fluxes calculated between standard rock and peridotite for the same density-length is < 2%, due to their different chemical compositions 28 . The E min minimum energy were calculated from the X density-lengths expressed in meter-water-equivalent (m.w.e.) units based on the following parametrization 48 : E min = (e X/a0 -a 2 )/a 1 (3) where the parameters a 0 , a 1 and a 2 were 2298.2 m.w.e., 0.001920 1/GeV, 0.99809, respectively. The muon fluxes were modeled for different density-lengths up to 2,000 m.w.e. with a 2 m.w.e. stepping. The corresponding density values were then obtained by dividing the density-lengths by the path-lengths. Declarations Data availability The data sets and software of this study are available in a data repository in Open Science Framework (https://doi.org/10.17605/OSF.IO/5YX8U). Raw data is available from LO upon reasonable request. Acknowledgements This project is supported by the ERI JURP 2023-H-01, ERI JURP 2023-H-03 and ERI JURP 2024-H-01 grants of Earthquake Research Institute, the University of Tokyo, MEXT Grant-in-Aid for Scientific Research (KAKENHI) under project ID 24K00731 and HUN-REN Welcome Home and Foreign Researcher Recruitment Programme KSZF-144/2023. Instrumentation is constructed in the Vesztergombi Laboratory for High Energy Physics (VLAB) with the support of Hungarian NKFIH research grant under identification number TKP2021-NKTA-10. Author contributions SU, LO, HKMT, DV, SMA, NSSA, IAMAS, YK, TM, TL designed the muographic survey. TL, DV constructed the muon detectors. LO, TL, SU installed the muon detectors. NSSA maintenanced the muon detectors. LO analysed the muon data. SU, YK, TM conducted the geological survey. SU prepared figures 1, 2, 3a. LO prepared figures 3b, 4, 5, 6, 7, 8. LO, SU wrote the manuscript. All authors reviewed the manuscript. Funding Funding was not provided. 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08:27:29","extension":"xml","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":118386,"visible":true,"origin":"","legend":"","description":"","filename":"7e3b3fafb7ca432b90561c834ec2002a1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/b4c0c3d53e503fc5cf85cb01.xml"},{"id":97142511,"identity":"098d9034-0125-435e-836a-262ff7c6b633","added_by":"auto","created_at":"2025-12-01 10:07:41","extension":"html","order_by":20,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":130829,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/85a87a2f45ea502bbd0acee9.html"},{"id":97127334,"identity":"a9f4b3f3-b2c7-4dfd-983b-5f70a69e8b6b","added_by":"auto","created_at":"2025-12-01 08:27:28","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":416096,"visible":true,"origin":"","legend":"\u003cp\u003eA simplified geologic map of northern Samail ophiolite. The solid star indicates the location of the measurement site.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/b55e1bffeda9faeaf390272c.png"},{"id":97142749,"identity":"129b0361-e5c7-46b6-8c51-81cce480f5fa","added_by":"auto","created_at":"2025-12-01 10:07:56","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":741506,"visible":true,"origin":"","legend":"\u003cp\u003eThe experimental arrangement and the observed lithologic layers are shown in the topographic map of Wadi Fizh. (a) The muon tracking system was installed at latitude of 24.45655º and longitude of 56.29703º at 467 m above sea level (red-colored ×). The ridge of the muographically imaged ophiolite is located at a distance of about 400 m from the observational instrument. The red-colored arrow shows the azimuthal orientation of the tracker that was set to 299 deg from north. The green-colored hatched area and the purple-colored hatched area shown the layered gabbroes (LGb) and the dunite/harzburgite (Dn/Hz) lithologic layers, respectively. The blue-colored lines show two selected cross-sections across the ophiolite segment. (b) The cross-section is shown along the Profile-2 of Fig. 2a. The solid line shows the altitude above sea level. The dashed-line shows the boundary between the layered gabbro and Dunite/Harzburgite. (c) The cross-section is shown along the Profile-1 of Fig. 2a. The lines show the same information as in Fig. 2b.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/752ac933acc13b90a194f644.png"},{"id":97141683,"identity":"e9ae5f64-4cb1-423d-ac93-7e21238c8eb7","added_by":"auto","created_at":"2025-12-01 10:06:53","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":809736,"visible":true,"origin":"","legend":"\u003cp\u003eThe studied ophiolite sections at Wadi Fizh segment. (a) A photograph of the ophiolite is shown. Red dashed lines indicate the boundary of the lower crustal layered gabbro (LGb) and the Moho Transition Zone (MTZ). The white dashed line shows the location of a fault. The black rectangle highlight a region which was explored by muography. The tracking system operated in the hut. (b) The path-lengths of muons calculated from the location of observation system are shown in the natural coordinate system of observation instrument with a slope bin size of 0.0086 by 0.0086. This bin corresponds to a spatial resolution of 3.5 m from a distance of 400 m. The red dashed line visualizes the boundary between the sections LGb and MTZ.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/e6a9f2479b37654aecba046c.png"},{"id":97127344,"identity":"1ce2edb7-d7bc-4a18-b5a3-e1e559b76a12","added_by":"auto","created_at":"2025-12-01 08:27:28","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":135913,"visible":true,"origin":"","legend":"\u003cp\u003eMuon fluxes are shown after the ophiolite segment in the natural coordinate system of observation instrument with a slope bin size of 0.0086 by 0.0086. (a) The measured muon flux is shown. (b) The muon fluxes were calculated by assumming a uniform density of 3 g cm\u003csup\u003e-3\u003c/sup\u003e for the ophiolite segment.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/737d8c41ac038b8aadf47c4a.png"},{"id":97142916,"identity":"ac1f152a-ec35-4cc7-a293-ef59e177f1b3","added_by":"auto","created_at":"2025-12-01 10:08:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":161630,"visible":true,"origin":"","legend":"\u003cp\u003eThe mass densities of rocks measured across the ophiolite segment are shown in the natural coordinate system of observation instrument with a slope bin size of 0.0086 by 0.0086. (a) The density map of ophiolite segment is shown. (b) The errors of densities measured across the ophiolite. (c)-(d) The distributions of densities across the regions Dn/Hz (blue-colored histogram), the MTZ (green-colored histogram) and the LGb (orange-colored histogram) are shown.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/02df885fde48f26701196b79.png"},{"id":97127341,"identity":"353ff017-1a77-4e3b-9034-11ab71585fca","added_by":"auto","created_at":"2025-12-01 08:27:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":488819,"visible":true,"origin":"","legend":"\u003cp\u003eThe structure of the muon tracking system. (a) A photograph of the tracker built up from seven MWPC detectors and two 2-cm-thick lead plates is shown. White-colored coordinate system shows the orientation of the tracker. The gas system and the power system are shown in the foreground. (b) A schematic drawing about the arrangement of the MWPCs (grey-colored rectangles) and the Pb plates (hatched rectangles) is shown. The coordinate system corresponds to the coordinate system shown in Fig. 3a. The numbers from 1 to 5 show order of MWPCs. The numbers from 0 to 1397 show the horizontal distance of detectors from the first MWPC in mm units. Black arrows aim to visualize the penetration of low-energy (upper) and high-energy (lower) muons throughout the tracking system.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/8fa28206d2692d46d567066b.png"},{"id":97142455,"identity":"bf7e9814-70d7-4b29-8127-110cb6a4febf","added_by":"auto","created_at":"2025-12-01 10:07:37","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":129403,"visible":true,"origin":"","legend":"\u003cp\u003eThe performance of the triggering of data collection. (a) The distribution of time difference between the consecutive events is shown in second units. The Poission nature of muons is demonstrated here. (b) The same data are shown with a time binning of 25 microsendonds. The dead time of the tracking system was quantified to 112.5 microseconds. (c) Trigger efficiencies are shown as a function of time for the entire data collection period. The trigger efficiences were measured above 93% for each detector.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/bdc76e1189164861123a0e90.png"},{"id":97127345,"identity":"01bfe470-76fb-46eb-8699-8a9b2658a145","added_by":"auto","created_at":"2025-12-01 08:27:28","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":158385,"visible":true,"origin":"","legend":"\u003cp\u003eThe performance of track reconstruction. (a) Track rate is shown for the entire data collection period. The red-colored line shows the aritmetic mean track rate of 0.163 Hz. (b) Tracking efficiencies are shown as a function of time. (c) Tracking efficiency is shown as a function of horizontal and vertical slopes in the natural coordinate system of observation instrument with a slope bin size of 0.0086 by 0.0086.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/29a77120271818d1b6f7ce6f.png"},{"id":97249090,"identity":"7f1b8e11-bda4-4e20-8e1a-629e9ea5067e","added_by":"auto","created_at":"2025-12-02 13:10:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3610715,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8019975/v1/4b65080f-2d72-46af-bffe-a788c0739844.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"First cosmic-ray muography of a crust-mantle transition zone","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMohorovičić discontinuity (Moho) is the boundary of the lowermost crust and the uppermost solid mantle layers within Earth\u0026rsquo;s lithosphere\u003csup\u003e1\u003c/sup\u003e. Processes occurring near the Moho in the oceanic lithosphere, such as mantle melting, melt migration, and crustal accretion, drive a large-scale cycles of energy and matter that form natural resources and trigger the occurrence of geohazards\u003csup\u003e2\u003c/sup\u003e. The architecture of the Moho formed by several parameters including the spreading rate, lithologic heterogeneity and tectono-magmatic history\u003csup\u003e3-6\u003c/sup\u003e. For example, Ohira et al.\u003csup\u003e7\u003c/sup\u003e observed a variety of Moho in the northwestern Pacific, where the Moho appears to be a sharp seismic boundary in regions with intermediate to fast seafloor spreading rates. This was interpreted as a lithological contact between gabbros and peridotites. In contrast, the Moho in other sites was observed to be a multiple reflection boundary, or invisible and obscure due to the complex lithology. Tamura et al.\u003csup\u003e8\u003c/sup\u003e found that the Moho reflectivity increases beneath thicker oceanic crust and shallower water depth. However, factors such as the age of the lithosphere and local effects (e.g., off-axis magmatism, serpentinization) affect the seismic data. It is of prime interest what makes the variability of the oceanic Moho. To date, the Moho has not yet been directly reached by ocean drilling (e.g., Ref.\u003csup\u003e9\u003c/sup\u003e). Nevertheless, intact structure of the oceanic lithosphere is preserved in ophiolites that are sequences of crust and mantle layers obducted onto continental margins\u003csup\u003e5,10\u003c/sup\u003e. The Samail Ophiolite, an analogue of huge blocks of the oceanic lithosphere formed on fast-spreading ridges, has the MTZs consisting of dunite intercalated with gabbro and chromitite layers up to several hundred meters above mantle diapirs, whereas the MTZs become thin and sharp away from mantle diapirs\u003csup\u003e11-13\u003c/sup\u003e. Thus, studying the structure and composition of the MTZs in Samail Ophiolite enables us to test the causal relationship between the physical properties and lithology of the MTZs and improve the models of formation and evolution of Moho discontinuities in oceanic lithosphere.\u003c/p\u003e\n\u003cp\u003eVarious geophysical, remote sensing, and laboratory-based techniques have been applied to study the structure and the composition of ophiolites. Gravity profiles measured across the Samail Ophiolite showed a strong correlation with the thickness of the ophiolitic sequences\u003csup\u003e14\u003c/sup\u003e. Anomalies in the gravity data were interpreted as serpentinization of the mantle peridotites and thinning of ophiolite nappes. Image processing of remote sensing data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) was utilized to delineate the Moho discontinuity and the associated rocks, as well as to map mineralogical variations\u003csup\u003e15\u003c/sup\u003e. Integrated geophysical studies combined seismic experiments, gravity modeling, and analysis of aeromagnetic data to enable the reconstruction of the structural architecture of the Troodos ophiolite\u003csup\u003e16\u003c/sup\u003e and the northern segment of the Samail Ophiolite\u003csup\u003e17\u003c/sup\u003e at a regional scale. In addition, ambient seismic noise tomography was applied to image the composition and structure of the continental crust underlying the Samail Ophiolite\u003csup\u003e18\u003c/sup\u003e. More recently, the Oman Drilling Project (ODP) sampled the crust and upper mantle sections of the ophiolite in situ with an unprecedented spatial resolution\u003csup\u003e19\u003c/sup\u003e. The extracted cores were characterized by different laboratory-based techniques. Here, we provide a brief description of a few results without claiming completeness. Micro-imaging infrared spectroscopy was applied to investigate the spatial patterns in hydrothermal alteration of oceanic crust\u003csup\u003e20\u003c/sup\u003e. The rock and mineral major and trace element compositions of the lower and the upper oceanic crust were quantified by X-ray fluorescence\u003csup\u003e21\u003c/sup\u003e. Permeability profiles across the crust-mantle sections were inferred from resistivity measurements to provide insights into fluid circulation in the oceanic lithosphere\u003csup\u003e22\u003c/sup\u003e. Gamma ray attenuation and P-wave velocity measurements were conducted on borehole cores to determine bulk density profiles and to evaluate pore structures and degree of cementation for studying the geological nature of the crust-mantle transition zone and the effects of the hydrothermal alteration processes that occurred in the lower crust\u003csup\u003e19\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eAlthough applying different techniques to research the ophiolites and the nature of the oceanic lithosphere are effective approach, there are discrepancies between the different measurement techniques that lead to biased interpretations. For example, Swift et al.\u003csup\u003e23\u003c/sup\u003e measured the profiles of seismic velocity versus depth below the sea floor. Acoustic wave velocity measurements of drilling cores from the hole 1256D significantly differ from the sonic log data and the P-wave velocities measured by ocean bottom seismometers. Similarly, discrepancies were found between the drilling cores from the ophiolites and the seismic data. These discrepancies arise from the disparity between the size of specimens (on the order of centimeters) and the spatial resolution of seismic techniques (on the order of kilometers). In addition, lithological changes occur in both horizontally and vertically ranging from a few centimetres to several tens of meters. Therefore, the seismic velocity structure is biased by the limited sampling density from ophiolites and drilling holes. Reconstructing high-resolution mass density structure of crustal and Moho transition zones in ophiolites may advance our knowledge about the geologic nature of the crust-mantle transition zones. Here, we focus on the implementation of a novel remote geophysical method called muography, which offers unique spatial and density resolving capabilities for imaging the density structure of ophiolites\u003csup\u003e24\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eMuography is a novel geophysical imaging technique that utilizes cosmic-ray muons to probe Earth\u0026apos;s subsurface\u003csup\u003e25\u003c/sup\u003e. Cosmic-ray muons are charged elementary particles that are generated continuously in Earth\u0026rsquo;s atmosphere at altitudes of approximately 10-15 km above sea level as secondary products of particle showers resulting from the interaction of primary cosmic rays with atmospheric nuclei\u003csup\u003e26\u003c/sup\u003e. Muons are highly energetic and relativistic particles, allowing them to penetrate a few kilometers into rock with a measurable flux. Their high-penetration power, predictable attenuation rate, and negligible angular deviations from their original track make them applicable for passive imaging and monitoring subsurface density variations\u003csup\u003e27,28\u003c/sup\u003e. In practical applications\u003csup\u003e29\u003c/sup\u003e, muography is advantageous for structures where non-invasive, passive, and long-term monitoring is needed and in which large-scale, high-density contrasts are present relative to the surrounding media, e.g., magma in volcanic conduits, ore bodies above mine tunnels, karstic caves in mountains, etc. Here, we focus on muography of an ophiolite segment formed from crust, MTZ, and mantle layers.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGeological setting\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Samail Ophiolite is a mountain chain on the Arabian continental margin that extends over 400 km in length and approximately 80 km in width. This ophiolite preserves a complete oceanic crust-mantle sequence of pelagic sedimentary rocks, extrusive rocks, sheeted dike complex, gabbroic rocks, and ultramafic rocks, in descending order (Fig. 1)\u003csup\u003e5,11\u003c/sup\u003e. Five or more paleoridge segments are identified though the ophiolite. These are defined by the presence of diapiric structures within the mantle peridotite at the segment centers, and mutually intrusive sheeted dikes and gabbros and doubly superimposed upper crustal structures at the segment boundaries\u003csup\u003e3,4,30-33\u003c/sup\u003e. The measurement site is located at Wadi Fizh, corresponding to the northern end of a 60-km long paleoridge segment between wads Fizh and Ahin\u003csup\u003e33-35\u003c/sup\u003e. Here, the Moho is represented by a relatively thin transition zone of dunite intervening gabbro layers between the layered gabbro and the underlying harzburgite tectonite, in accordance with the MTZ at the segment end\u003csup\u003e36\u003c/sup\u003e. The boundary between the layered gabbro and the uppermost dunite layer in MTZ gently dips to the east, undulating with a wavelength of 500-600 m and a wave height of ~100 m (Fig. 2). Comparison of the muographic data collected at the lithologic Moho of the ophiolite and the seismic data acquired from an oceanic Moho (e.g., at the Pacific Plate) may enable us to infer the physical and geological nature of the Moho at fast-spreading MORs.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cb\u003eExperimental setting\u003c/b\u003e\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eFigure\u0026nbsp;2a shows the topographic map of the measurement site with contours of 20-m intervals based on the 1:50,000 topomap of the National Survey Authority (NSA), Sultunate of Oman. A Multi-Wire-Proportional-Chamber-based Muography Observation System (MMOS) was installed on the Moho plane at latitude of 24.45655º, longitude of 56.29703º, and at an altitude of 467 m above sea level, marked as red-colored × in Fig.\u0026nbsp;2a. The MMOS was oriented toward an azimuth 299º from geographic north (defined as tan(\u003c/span\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eθ\u003c/span\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003ex) = 0, and shown by the red-colored arrow in Fig.\u0026nbsp;2a), with tracking detector layers set vertically (i.e. perpendicular to the horizontal plane, defined as tan(\u003c/span\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eθ\u003c/span\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003ey) = 0). The MMOS tracked muons within ± 0.55 rad in both the horizontal and the vertical directions. The green-colored hatched area and the purple-colored hatched area show the layered gabbro (LGb) and the dunite/harzburgite (Hz/Dn) lithologic layers, respectively. The blue-colored lines (Profile-1 \u0026amp; -2) indicate selected cross-sectional profiles across the ophiolite segment in Figs.\u0026nbsp;2b and 2c, respectively. Orange-colored dashed lines show the boundary between the LGb and Dn/Hz layers. The red-colored arrows point towards the location of observatory. The horizontal distance between the MMOS and the ridge of imaged ophiolite was about 400 m.\u003c/span\u003e\u003c/p\u003e\u003cp\u003e\u003cspan type=\"BoldUnderline\" class=\"BoldUnderline\" name=\"Emphasis\"\u003eFigure\u0026nbsp;2.\u003c/span\u003e \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eThe experimental arrangement and the observed lithologic layers are shown in the topographic map of Wadi Fizh. (a) The muon tracking system was installed at latitude of 24.45655º and longitude of 56.29703º at 467 m above sea level (red-colored ×). The ridge of the muographically imaged ophiolite is located at a distance of about 400 m from the observational instrument. The red-colored arrow shows the azimuthal orientation of the tracker that was set to 299 deg from north. The green-colored hatched area and the purple-colored hatched area shown the layered gabbroes (LGb) and the dunite/harzburgite (Dn/Hz) lithologic layers, respectively. The blue-colored lines show two selected cross-sections across the ophiolite segment. (b) The cross-section is shown along the Profile-2 of Fig.\u0026nbsp;2a. The solid line shows the altitude above sea level. The dashed-line shows the boundary between the layered gabbro and Dunite/Harzburgite. (c) The cross-section is shown along the Profile-1 of Fig.\u0026nbsp;2a. The lines show the same information as in Fig.\u0026nbsp;2b.\u003c/span\u003e\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eIt is worth noting that the lower left ridge beyond the layered gabbro in Fig.\u0026nbsp;3a corresponds to that of layered gabbro. However, because the boundary between the MTZ and the layered gabbro is undulating, the layered gabbro layer is thin and covers only the surface of the ridge, which is underlain by the dunite-rich MTZ. Figure\u0026nbsp;3b shows the rock thicknesses along the muon paths calculated using the 5-m DEM data of NSA from the location of the MMOS across the ophiolite within the black rectangle of Fig.\u0026nbsp;3a. The red-colored dased line shows the boundary between the LGb and the MTZ.\u003c/span\u003e\u003c/p\u003e\u003cp\u003e\u003cspan type=\"BoldUnderline\" class=\"BoldUnderline\" name=\"Emphasis\"\u003eData collection and processing\u003c/span\u003e\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eThe data collection campaign was conducted between 27 February 2024 and 22 January 2025.\u003c/span\u003e We conducted an off-line data analysis for detector calibration, data quality assurance, and muographic image processing (Methods). The detector calibration procedures involved offline alignment of detector layers and removing of noise and dead electronics channels. After the data quality assurance, t\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003ehe effective time of data collection was totaled 171 days due to detector malfunctions, power cuts, and the maintenance works. In the muographic images\u003c/span\u003e, each pixel corresponds to a pair of slopes, tan(θx)-tan(θy), which represent the tangents of the horizontal and vertical projection angles of muon tracks, measured relative to the orientation of the MMOS (Fig.\u0026nbsp;2). Each image has 65 × 65 pixels with a size of 0.0086 in both horizontal and vertical directions. Each angular bin corresponds to an approximately 3.5 m × 3.5 m area at the peak of the ophiolite 400 m away from the MMOS. The measured flux, the modeled flux and the average mass density were determined for each angular bin.\u003c/p\u003e\u003cp\u003e\u003cb\u003eThe first muographic images\u003c/b\u003e\u003c/p\u003e\u003cp\u003eFigures\u0026nbsp;4a and 4b show the measured and modeled muon fluxes, respectively. The modeled flux was calculated assuming a uniform rock density of 3 g cm\u003csup\u003e− 3\u003c/sup\u003e within each slope bin. Both flux maps reflect the structure of the ophiolite segment and the surrounding topography thanks to the excellent spatial resolution of approximately 3.5 meters.\u003c/p\u003e\u003cp\u003eThe corresponding density image of the ophiolite segment was reconstructed with the same spatial resolution, as shown in Fig.\u0026nbsp;5a. Here the white-shaded region indicate slope bins without density values because these bins correspond to the open sky where the path lengths of muons was assumed to be zero. The densities were under-measured in angular bins where the path lengths of muons across the ophiolite segment exceeded 500 meters. This underestimation was caused by the contamination from background muons in the selected track set. Figure\u0026nbsp;5b shows the density error values which were quantified by averaging the upper and lower densities calculated for the F-ΔF and F + ΔF values, respectively. \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eThree regions were designated across the ophiolite segment: a high density-region of the ridge behind in the lower left (Dn/Hz), a region across the upper part of the ridge (LGb), and a region across the MTZ. The averaged rock thicknesses were estimated as 165 m, 286 m and 61 m across the Dn/Hz, the MTZ, and the LGb, respectively. Figures\u0026nbsp;5\u003c/span\u003ec, 5d, and 5e show the density distributions extracted from the regions Dn/Hz (blue-colored histogram), MTZ (green-colored histogram), and LGb (orange-colored histogram), respectively. A density bin size of 0.1 g cm\u003csup\u003e− 3\u003c/sup\u003e was used for each histogram. The density values ranged from 2.3 g cm\u003csup\u003e− 3\u003c/sup\u003e to 6 g cm\u003csup\u003e− 3\u003c/sup\u003e with a mean of\u003c/p\u003e\u003cp\u003e3.38 g cm\u003csup\u003e− 3\u003c/sup\u003e for the Dn/Hz (Fig.\u0026nbsp;5c). The density values above 4.5 were considered non-physical and were excluded from the mean calculation. The densities were measured between 1.7 g cm\u003csup\u003e− 3\u003c/sup\u003e and\u003c/p\u003e\u003cp\u003e4 g cm\u003csup\u003e− 3\u003c/sup\u003e with a mean of 2.72 g cm\u003csup\u003e− 3\u003c/sup\u003e for the MTZ (Fig.\u0026nbsp;5d). For the LGb (Fig.\u0026nbsp;5e), the density values ranged from 1.6 g cm\u003csup\u003e− 3\u003c/sup\u003e to 5.5 g cm\u003csup\u003e− 3\u003c/sup\u003e with a mean of 3.03 g cm\u003csup\u003e− 3\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe first muographic density measurement demonstrates that muon imaging allows resolving the internal structure and composition of rocks forming ophiolite segments, from the upper mantle to the lower crust. At Wadi Fizh, the density across the lower crust layered gabbro (LGb) is consistent with the typical densities of layered gabbros (2.95-3 g cm\u003csup\u003e− 3\u003c/sup\u003e). This density value indicates that the layered gabbro is generally fresh, lack of hidden fractures and not affected by alteration. The mean density measured across the MTZ is significantly lower (2.72 g cm\u003csup\u003e− 3\u003c/sup\u003e) than typical density values of reported for other MTZs in the Samail Ophiolite (e.g., Ref.\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e). This muographic observations is not consistent with lithological profile described by Akizawa \u0026amp; Arai\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e, which reveals a gradual lithological transition from harzburgite through dunite and wehrlite layers to layered gabbro. Similar MTZ contacts were observed in other segments of the northern Samail Ophiolite, where gabbro sills intrude progressively into dunite\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e. Such transitions between the lower crust and upper mantle are attributed to melt infiltration and subsequent crystallization in the Moho transition zone\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e. Here, the significantly lower density of MTZ reflects the highly serpentinized nature of the rocks as observed at outcrops. The density values were measured significantly higher (3.38 g cm\u003csup\u003e− 3\u003c/sup\u003e) across the lower left ridge (Dn/Hz) on the back. Although the ridge is covered by the LGb layer, its mean density indicates that the ridge is primarily composed of harzburgite. This means that the LGb covering the ridge is very thin and mainly underlain by relatively fresh mantle peridotite (Fig.\u0026nbsp;2c). This supports the field observations indicating that the Moho surface is not a flat plane but exhibits undulations with a wavelength of 500 to 600 m (Fig.\u0026nbsp;2).\u003c/p\u003e\u003cp\u003eThe applicability of muography for studying ophiolites has several limitations, and possible upgrades are discussed as follows. (A) Constraints on local environment (such as harsh and varying climate or topography) and logistic (e.g., limited resources for power supplying) may not allow multi-month measurements at each ophiolite segment. (B) The current spatial resolution of a few meters does not allow to resolve fine-scale structures. For example, the gabbro sills in size range from a few millimeters to a few tens of centimeters, whose sizes are well below the current spatial resolution. To improve the spatial resolution of muography to an order of a few tens of centimeters would need the operation of a larger muographic observation system at a distance of a few meters from the studied segment for a longer period. (C) The penetration range of muons is limited due to their finite yield and energy distribution. As shown in Fig.\u0026nbsp;4, the flux of penetrated muons decreasing significantly with the thickness of the imaged structure and the background contamination is increasing above the thickness of approximately 500 meters. Studying ophiolite segments with thickness in order of a few hundred meters will require the development of modular muography observatories, similarly to volcano monitoring. (D) Muography enables studying the structure and constituents of ophiolites just in a presence of significant (\u0026gt; 0.1 g cm\u003csup\u003e− 3\u003c/sup\u003e) density contrast. For example, muography cannot distinguish the serpentinised peridotites from the crustal gabbro layers. (D) Muography reconstructs integrated density along the path of muons. Multi-directional muography (e.g., Ref.\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e) and joint inversion of muographic and gravimetric data (e.g., Ref.\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e) enable the reconstruction of the three-dimensional density structure of ophiolite sequences. This will reveal the heterogeneity of the oceanic lithosphere including lithological heterogeneity and the presence of fractures and alteration zones. (E) Different lithologies may have similar mass densities. The integrated processing of muographically measured densities and seismic velocities\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e will allow to determine the elastic constants (bulk and shear moduli) of the rocks in ophiolites.\u003c/p\u003e\u003cp\u003eWe conducted the first muography of a Moho transition zone at Wadi Fizh in the northern Samail Ophiolite. The reconstructed mass densities indicate that the mantle and the lower crustal gabbro unaltered, however the MTZ dunite is largely altered to serpentinite. A gradational Moho transition zone is revealed between the peridotites to the layered gabbros. The obtained results demonstrate that muography can reveal density contrast within ophiolite segment that allows measuring the crustal thickness, localizing and exploring the depth of the crust-mantle boundary, revealing fluid-rock interactions in the mantle, such as degree of serpentinization or melt impregnation. Measurement of 3-dimensional density distribution will allow to investigate the diversity of Moho along the paleoridge segment and explore whether the discontinuity forms either sharp, blurred or multiple seismic reflections, thus inferring to the physical processes occurred between the diverse oceanic Moho.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cb\u003eMuon detector\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe technical details of the applied tracking detector has already been presented in our earlier works\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan additionalcitationids=\"CR44\" citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e–\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e. This technology has successfully been applied in various environments, including underground mines and near active volcanoes\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e. In this study, we used an MMOS consisting of seven MWPCs, each with a surface area of \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e0.8 m × 0.8 m and a thickness of 0.02 m\u003c/span\u003e. Each MWPC provided a 1 + 1 dimensional positional information by two perpendicular wires planes. Wire spacing was 0.012 m in both wire planes, which allowed a spatial resolution of approximately 4 mm for each MWPC. Five of the seven detectors operated reliably during the measurement period, thus we used the data from these detectors for this study. The five MWPCs spanned a length of 1.397 m in the tracking detector.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;6 shows a photograph and a schematic drawing of the experimental setup with five MWPCs (gray-colored rectangles) and two 2-cm-thick lead walls (hatched rectangles). The lead walls were used to absorb and deflect low-energy muons, which may scatter from the surface of ophiolite or from the atmosphere into the detectors (e.g., Ref.\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e). The numbers shown below the MWPCs and lead walls indicate their distance from the first tracking layer in millimeters. The tracking system was powered by the local electricity network, supplied from an apartment located approximately 300 meters from the MMOS. The detector layers were continuously flushed with an argon-carbon-dioxide (Ar-CO\u003csub\u003e2\u003c/sub\u003e) gas mixture in a proportion of 80%:20%. This gas mixture enabled signal generation on the wires via ionisation of argon atoms by penetrating charged particles, typically producing 100 ionisation electrons per centimeter. A high-voltage of + 1,700 V was applied to the wires to amplify the signal to a measurable signal in the order of 10,000 electrons. Custom-designed front-end electronics amplified the analogue signals by a factor of ten and discriminated them by shift registers, converting them to binary output on each electronics channel.\u003c/p\u003e\u003cp\u003eDetector operation and data collection were controlled by a microcomputer. Data collection was triggered by triple coincidence of MWPCs. The recorded data included time stamps, analogue signal amplitudes (ADCs), trigger patterns, high-voltage values, temperature, humidity, pressure, and the hit coordinates of the detected particles. These data were stored into ASCII files on an event-by-event basis. The trigger was temporarily blocked during the data readout. The trigger frequency ranged from 5 Hz to 35 Hz, with an average of 10.4 Hz. A significant reduction in the trigger rate was observed after the removal of uninterruptible power supply from the power supplying system.\u003c/p\u003e\u003cp\u003e\u003cb\u003eMuon flux measurement\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe muon flux values were calculated for each slope bin by dividing the number of reconstructed tracks (N) by the measurement time (T), corrected for the dead time (\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003et\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003edead\u003c/span\u003e\u003c/sub\u003e), the trigger efficiency (\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eε\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003etrigger\u003c/span\u003e\u003c/sub\u003e), the tracking efficiency (\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eε\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003etracking\u003c/span\u003e\u003c/sub\u003e), and the detector acceptance factor, which accounts for the sensitive surface area (A) and the solid angle (Ω):\u003c/p\u003e\u003cp\u003e\u003cem\u003eF = N / [ T\u003c/em\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e× (1 - t\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003edead\u003c/span\u003e\u003c/sub\u003e\u003cem\u003e)\u003c/em\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e× ε\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003etrigger\u003c/span\u003e\u003c/sub\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e× ε\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003etracking\u003c/span\u003e\u003c/sub\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e× A × Ω\u003c/span\u003e \u003cem\u003e]\u003c/em\u003e. (1)\u003c/p\u003e\u003cp\u003eThe tracks were counted as a function of horizontal and vertical slopes on event-by-event basis. The reconstruction of particle trajectories was conducted independently in the horizontal and vertical directions. For each MWPC, the centroids, sizes and multiplicities of signal clusters were determined. A combinatorial algorithm then produced track candidates by merging one cluster centroid from each MWPC. A straight line was fitted on the clusters of each track candidate. The track candidates were ranked according to the goodness of fit (χ\u003csup\u003e2\u003c/sup\u003e/ndf), and the best-fitting one was selected for further analysis. To suppress tracks originate from electronic noise and low-energy scattered particles, only tracks with clusters in at least four MWPCs, analogue signal amplitudes above 200 ADC in at least four MWPCs, and a goodness of fit χ\u003csup\u003e2\u003c/sup\u003e/ndf \u0026lt; 2 were retained.\u003c/p\u003e\u003cp\u003eThe measurement time \u003cem\u003eT\u003c/em\u003e was 171 days after quality assurance of the track data. Figures\u0026nbsp;7a and 7b show the distributions of the time differences between consecutive events. Assuming that the detected particles arrived from different air showers with uniform probability over time, the interarrival times follow an exponential distribution, consistent with a Poisson arrival process. As shown in Fig.\u0026nbsp;7b, the distribution starts from 112.5 microseconds due to the data readout, which blocked the trigger for this period after each event. This dead time resulted in only a negligible reduction (0.12%) of the muon yield over the entire data acquisition period.\u003c/p\u003e\u003cp\u003eThe trigger efficiency was evaluated for each chamber by calculating the ratio of tracks that produced trigger signal in the investigated chamber to the total number of tracks. Figure\u0026nbsp;7c shows the time evolution of the trigger efficiency with a 6-hour bin size. The trigger efficiencies exceeded 93% for every MWPC throughout th\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003ee measurement period.\u003c/span\u003e The overall trigger efficiency of the tracking system approached 100%.\u003c/p\u003e\u003cp\u003eThe tracking efficiency was measured for each MWPC as a function of time and of tracks slopes. A tracklet was constructed from clusters detected in four MWPCs. This tracklet was extrapolated to the fifth MWPC. The tracking efficiency of the investigated chamber was given by the ratio of the cases in which a cluster was found on the fifth MWPC to the number of tracklets. The overall tracking efficiency of the detector system was calculated from the individual tracking efficiencies of MWPCs by the following formula:\u003c/p\u003e\u003cp\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eε\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003etracking\u003c/span\u003e\u003c/sub\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e=\u003c/span\u003e \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e∏\u003c/span\u003e\u003csub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003ei=1:5\u003c/span\u003e\u003c/sub\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eε\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003ei\u003c/span\u003e\u003c/sub\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e+\u003c/span\u003e \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eΣ\u003c/span\u003e\u003csub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003ei=1:5\u003c/span\u003e\u003c/sub\u003e \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e∏\u003c/span\u003e\u003csub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003ej=1:5\u003c/span\u003e\u003c/sub\u003e \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e[ (1 - δ\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eij\u003c/span\u003e\u003c/sub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e)\u003c/span\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e× ε\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003ei\u003c/span\u003e\u003c/sub\u003e \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e+ δ\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eij\u003c/span\u003e\u003c/sub\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e× (1 - ε\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003ei\u003c/span\u003e\u003c/sub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e)\u003c/span\u003e \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e]\u003c/span\u003e, (2)\u003c/p\u003e\u003cp\u003ewhere the \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eδ\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eij\u003c/span\u003e\u003c/sub\u003e Kronecker delta gives \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eδ\u003c/span\u003e\u003csub\u003e\u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eij\u003c/span\u003e\u003c/sub\u003e \u003cem\u003e= 1\u003c/em\u003e for \u003cem\u003ei = j\u003c/em\u003e, and \u003cem\u003e0\u003c/em\u003e otherwise. Figure\u0026nbsp;8 summarizes the tracking performance. A 6-hour time binning was applied for monitoring the time evolution of tracking efficiencies. Figure\u0026nbsp;8a shows the track rate as a function of time, represented by black points with one standard deviation error bars. The mean track rate was 0.163 Hz (red-colored line). Tracking efficiencies exceeded 92% for each MWPC throughout the data collection period (Fig.\u0026nbsp;8b). The overall tracking efficiency of the detector system is shown as a function of horizontal and vertical slopes in Fig.\u0026nbsp;8c. These efficiencies were found well above 95% in the studied angular regions.\u003c/p\u003e\u003cp\u003e\u003cb\u003eMuon flux modeling\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe density-lengths (i.e., integrated densities along muon paths) were determined for each slope bin by minimizing the difference between the modeled and measured muon fluxes. The modeled muon fluxes (\u003cem\u003eF\u003c/em\u003e\u003csub\u003e\u003cem\u003ecalc\u003c/em\u003e\u003c/sub\u003e) were calculated as follows. Zenith-angle (\u003cem\u003eθ\u003c/em\u003e) and energy (\u003cem\u003eE\u003c/em\u003e) dependent muon spectra were integrated over energy to simulate the attenuation of muon flux after traversing different density-lengths. In this work, the energy spectra were parametrised with an empirical formula based on a modified Gaisser model\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e. The altitude (\u003cem\u003eA\u003c/em\u003e) dependence of modeled fluxes was taken into account by a factor \u003cem\u003eF\u003c/em\u003e\u003csub\u003e\u003cem\u003ecalc\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e(A) = F\u003c/em\u003e\u003csub\u003e\u003cem\u003ecalc\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e(0 m)\u003c/em\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e× e\u003c/span\u003e\u003csup\u003e\u003cem\u003e[−A/K(E)]\u003c/em\u003e\u003c/sup\u003e, where \u003cem\u003eF\u003c/em\u003e\u003csub\u003e\u003cem\u003ecalc\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e(0 m)\u003c/em\u003e is the flux at sea level and the \u003cem\u003eK(E)\u003c/em\u003e is an energy-dependent parameter, given by \u003cem\u003eK(E) = 4,900 + 750\u003c/em\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003e×\u003c/span\u003e \u003cem\u003eE\u003c/em\u003e (E in \u003cem\u003eGeV\u003c/em\u003e, \u003cem\u003eK\u003c/em\u003e in m\u003cem\u003e)\u003c/em\u003e. The difference in integrated fluxes calculated between standard rock and peridotite for the same density-length is \u0026lt; 2%, due to their different chemical compositions\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e. The \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003emin\u003c/em\u003e\u003c/sub\u003e minimum energy were calculated from the \u003cem\u003eX\u003c/em\u003e density-lengths expressed in meter-water-equivalent (m.w.e.) units based on the following parametrization\u003csup\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/span\u003e\u003c/sup\u003e:\u003c/p\u003e\u003cp\u003eE\u003csub\u003emin\u003c/sub\u003e= (e\u003csup\u003eX/a0\u003c/sup\u003e-a\u003csub\u003e2\u003c/sub\u003e)/a\u003csub\u003e1\u003c/sub\u003e (3)\u003c/p\u003e\u003cp\u003ewhere the parameters a\u003csub\u003e0\u003c/sub\u003e, a\u003csub\u003e1\u003c/sub\u003e and a\u003csub\u003e2\u003c/sub\u003e were 2298.2 m.w.e., 0.001920 1/GeV, 0.99809, respectively. The muon fluxes were modeled for different density-lengths up to 2,000 m.w.e. with a 2 m.w.e. stepping. The corresponding density values were then obtained by dividing the density-lengths by the path-lengths.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data sets and software of this study are available in a data repository in Open Science Framework (https://doi.org/10.17605/OSF.IO/5YX8U). Raw data is available from LO upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis project is supported by the ERI JURP 2023-H-01, ERI JURP 2023-H-03 and ERI JURP 2024-H-01 grants of Earthquake Research Institute, the University of Tokyo, MEXT Grant-in-Aid for Scientific Research (KAKENHI) under project ID 24K00731 and HUN-REN Welcome Home and Foreign Researcher Recruitment Programme KSZF-144/2023. Instrumentation is constructed in the Vesztergombi Laboratory for High Energy Physics (VLAB) with the support of Hungarian NKFIH research grant under identification number TKP2021-NKTA-10.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSU, LO, HKMT, DV, SMA, NSSA, IAMAS, YK, TM, TL designed the muographic survey. TL, DV constructed the muon detectors. LO, TL, SU installed the muon detectors. NSSA maintenanced the muon detectors. LO analysed the muon data. SU, YK, TM conducted the geological survey. SU prepared figures 1, 2, 3a. LO prepared figures 3b, 4, 5, 6, 7, 8. LO, SU wrote the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFunding was not provided.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe corresponding authors state that there is no conflict of interest.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eTeagle, D.A.H. Re-energizing the quest of drilling to the mantle. \u003cem\u003eNature reviews earth \u0026amp; environment\u003c/em\u003e 4, 207\u0026ndash;208. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/s43017-023-00413-0\u003c/span\u003e\u003cspan address=\"10.1038/s43017-023-00413-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2023).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKoppers, A.A.P. \u0026amp; Coggon, R. 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Calculation of the Underground Muon Intensity Crouch Curve from a Parameterization of the Flux at Surface. \u003cem\u003eProceedings of 30th International Cosmic Ray Conference\u003c/em\u003e 5, 1241\u0026ndash;1244. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://indico.nucleares.unam.mx/event/4/session/40/contribution/707/material/paper/0.pdf\u003c/span\u003e\u003cspan address=\"https://indico.nucleares.unam.mx/event/4/session/40/contribution/707/material/paper/0.pdf\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2008).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"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":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"oceanic lithosphere, Moho, ophiolite, serpentinization, muon, muography","lastPublishedDoi":"10.21203/rs.3.rs-8019975/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8019975/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Studying the structure and composition of ophiolitic crustal-mantle boundaries provides insights into the nature of oceanic lithosphere. In this work, we conducted the first muography of an ophiolite segment at Wadi Fizh in the northern Samail Ophiolite, which represents an analogue of crust-mantle boundaries formed at fast-spreading mid-ocean ridges. A multi-wire-proportional-chamber-based muographic observation system was operated at a distance of 400 meters from the crust-mantle transition zone (Moho Transition Zone, MTZ) for 171 days. The resulting high-resolution muographic image resolves the internal density structure of the ophiolite segment with a spatial resolution of approximately 3.5 meters. Mean density was measured as 3.03 g cm-3 for the layered gabbros, consistent with weakly serpentinized gabbroic intrusions. A significantly lower value of 2.72 g cm-3 was obtained for the MTZ, indicating a highly serpentinized MTZ, in contrast with petrological profiles which previously revealed gradational transition from the mantle to the crust at Wadi Fizh. A mean density of 3.38 g cm-3 was revealed in the lower part of ophiolite rige that indicates the presence of fresh peridotites beneath the thin layer of gabbroic cover. These results demonstrate that muography can provide complementary information about the density structure of ophiolites and, by extension, on the architecture of the oceanic lithosphere.","manuscriptTitle":"First cosmic-ray muography of a crust-mantle transition zone","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-01 08:27:23","doi":"10.21203/rs.3.rs-8019975/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-24T05:32:12+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-07T12:22:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"21837711383472071589021378224217828400","date":"2026-02-07T11:35:13+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-24T01:06:09+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"64376840197827590714176795477559626468","date":"2026-01-15T11:40:03+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-25T02:18:46+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-11-14T08:03:54+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-05T09:12:23+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-05T09:09:41+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-11-03T13:45:01+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"0e2b4004-025a-437b-a394-baaf3db6aea4","owner":[],"postedDate":"December 1st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":58683496,"name":"Earth and environmental sciences/Planetary science"},{"id":58683497,"name":"Earth and environmental sciences/Solid earth sciences"}],"tags":[],"updatedAt":"2026-05-11T18:23:15+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-01 08:27:23","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8019975","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8019975","identity":"rs-8019975","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}