An interdisciplinary approach is needed to fully understand the formation and evolution of the Earth’s crust. Different disciplines (geology, geophysics, geochemistry, etc.) provide complementary information that cover different depth and time scales. By combining this information, a better understanding of crustal processes becomes possible. Seismic methods, for example, are utilized to obtain structural images of the present state of the crust and derive its petrophysical properties. This knowledge can then be used as constraints in geodynamic modelling.
In this presentation, we will focus on the Ivrea Verbano Zone (IVZ) where lower crustal rocks and mantle peridotites are exposed at surface. The IVZ is the subject of recent studies and drilling projects to gain a better understanding of the crustal evolution and crust-mantle transition. In preparation of a proposed drilling campaign (ICDP-DIVE), seismic exploration surveys (fixed-spread and roll-along) were carried out across the Balmuccia peridotite body and the Insubric Zone. However, the seismic data show strong first break onsets of P- and S-waves and converted waves which interfere with signals from the peridotite body. Hence, we combined different seismic processing techniques to derive a structural image of the Balmuccia peridotite and its surroundings: Conventional seismic reflection imaging shows a rather diffusive image of the subsurface. The results can be improved by applying coherency-based Prestack Depth Migration and stacking which reveals reflective structures at the borders of intrusive bodies. Tomographic imaging of the fixed-spread data set mapped the 3D structure of an asymmetric high-velocity body that extends down to 3 km depth and is limited in the West by the Insubric Zone. The smooth 3D velocity model is supplemented by a high-resolution image of the near-surface structure that was obtained by inverting the travel times from the roll-along data set. By performing a machine-learning based cluster analysis the near-surface structure is subdivided into distinct model regions with well-defined seismic properties enabling now petrophysical interpretation.
To support the preparation of the proposed drilling campaign (ICDP-DIVE), our results together with the results from recent studies can now be implemented in a joined geomodel of the Balmuccia peridotite.
How to cite: Wawerzinek, B., Ryberg, T., Bauer, K., Haberland, C., Stiller, M., Weber, M., and Krawczyk, C. M.: Seismic imaging and petrophysical classification of the Balmuccia peridotite and surrounding upper crust, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17414, https://doi.org/10.5194/egusphere-egu25-17414, 2025.
The Drilling the Ivrea-Verbano zonE (DIVE) project focuses on the continental lower crust from petrological, geophysical, fluid and gas, as well as microbiological perspectives in the framework of ICDP expedition 5071. Two scientific boreholes of DIVE phase 1 have cored 578.5 and 909.5 metres of lower crustal rocks in Val d’Ossola, Italy, and preparations for DIVE phase 2 have already started. The primary goals are to continuously sample the crust–mantle transition, and to test the suitability of a natural peridotite body for serpentinization and hydrogen production.
The structural characterization of the drilling target and the assessment of the subsurface physical properties has been ongoing for several years, and at various spatial scales. Up to date, three active seismic campaigns, one passive seismic profile, regional and local gravity campaigns, and drone-based photogrammetry (digital outcrop model based fracture network analysis) have been undertaken under the umbrella of, or in connection with, project DIVE. Furthermore, aeromagnetic data is available over the region, and geological mapping is being refined in the area planned for drilling. This contribution will present the results reached so far, the differences between them as a function of spatial resolution, models of the Balmuccia peridotite body and related questions, as well as the currently ongoing efforts of geophysical imaging and modelling to reduce the uncertainties. Ultimately, we present the current drilling strategy of the 5071_2 borehole(s).
References
Hetényi G, Baron L, Scarponi M, et al. (2024) Report on an open dataset to constrain the Balmuccia peridotite body (Ivrea-Verbano Zone, Italy) through a participative gravity-modelling challenge. Swiss J Geosci 117:2. doi:10.1186/s00015-023-00450-3
Liu Y, Greenwood A, Hetényi G, Baron L, Holliger K (2021) High-resolution seismic reflection surveys crossing the Insubric Line into the Ivrea-Verbano Zone: Novel approaches for interpreting the seismic response of steeply dipping structures. Tectonophys 816:229035. doi:10.1016/j.tecto.2021.229035
Menegoni N, Panara Y, Greenwood A, Mariani D, Zanetti A, Hetényi G (2024) Fracture network characterisation of the Balmuccia peridotite using drone-based photogrammetry, implications for active-seismic site survey for scientific drilling. J Rock Mech Geotech 16:3961-3981. doi:10.1016/j.jrmge.2024.03.012
Pasiecznik D, Greenwood A, Bleibinhaus F, Hetényi G (2024) Seismic structure of the Balmuccia Peridotite from a high-resolution refraction and reflection survey. Geophys J Int 238:1612-1625. doi:10.1093/gji/ggae239
Ryberg T, Haberland C, Wawerzinek B, Stiller M, Bauer K, Zanetti A, Ziberna L, Hetényi G, Müntener O, Weber M, Krawczyk CM (2023) 3-D imaging of the Balmuccia peridotite body (Ivrea–Verbano zone, NW-Italy) using controlled source seismic data. Geophys J Int 234:1985-1998. doi:10.1093/gji/ggad182
Scarponi M, Hetényi G, Berthet T, Baron L, et al. (2020) New gravity data and 3D density model constraints on the Ivrea Geophysical Body (Western Alps). Geophys J Int 222:1977-1991. doi:10.1093/gji/ggaa263
Scarponi M, Hetényi G, Plomerová J, Solarino S, Baron L, Petri, B (2021) Joint seismic and gravity data inversion to image intra-crustal structures: the Ivrea Geophysical Body along the Val Sesia profile (Piedmont, Italy). Front Earth Sci 9:671412. doi:10.3389/feart.2021.671412
Scarponi M, Kvapil J, Plomerová J, Solarino S, Hetényi G (2024) New constraints on the shear-wave velocity structure of the Ivrea geophysical body from seismic ambient noise tomography (Ivrea-Verbano Zone, Alps). Geophys J Int 236:1089-1105. doi:10.1093/gji/ggad470
How to cite: Hetényi, G., Greenwood, A., Bauer, K., Bleibinhaus, F., Haberland, C., Holliger, K., Krawczyk, C., Menegoni, N., Panara, Y., and Wawerzinek, B. and the ICDP DIVE-2 Geophysics Team: Geophysical overview of the planned phase 2 drilling sites of projet DIVE in Val Sesia, Italy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12155, https://doi.org/10.5194/egusphere-egu25-12155, 2025.
The Ivrea-Verbano Zone (IVZ) is widely regarded as an archetypal outcrop of Phanerozoic lower continental crust due to its comprehensive exposure, near-vertical structural orientation, and far-reaching absence of deformation and alteration. As such, geological evidence from the IVZ and associated laboratory measurements of pertinent rock physical properties have been widely used to construct canonical seismic models of the lower continental crust. These endeavors were either based on deterministic or stochastic approaches. While deterministic models allow us to directly relate the seismic response to field observations, they are inherently hampered by the fact that geological and rock physical information of adequate quality is only available for part of the IVZ. These problems can be alleviated through stochastic approaches to model building. Efforts of this kind were so far based on standard covariance-type statistics and, hence, were unable to fully capture the prevailing structural complexity and spatial variability. Quite importantly, studies accounting for seismic anisotropy were so far largely limited to deterministic or stochastic 1D layered models. To address these challenges, we use a multi-point statistics (MPS) approach, which we train on detailed geological maps from the central IVZ. In addition to the spatial information associated with the categorized lithologies, we also use the orientation of the foliation as part of the underlying training information. Each grid point of the resulting MPS simulations is then assigned anisotropic P- and S-waves seismic velocities associated with the categorized lithology at the given location. The values of the seismic velocities are randomly chosen from corresponding distributions based on available laboratory measurements. The generic nature of the seismic anisotropy prevailing the IVZ is accounted for by aligning the fast axes of the P- and S-wave velocities with the orientation of the foliation. To emulate the short-range coherence of the seismic velocity variations observed in sonic logs from two recent boreholes drilled into the upper and lower parts of the IVZ in the framework of the ICDP DIVE program, we subject these so far locally uncorrelated models to an accordingly parameterized autoregressive moving average (ARMA) process. Finally, we evaluate and assess the seismic response of this new generation of lower crustal models by simulating near-vertical and wide-angle synthetic seismic reflection profiles using a finite-difference solution of the generically anisotropic viscoelastic equations.
How to cite: Holliger, K. and Luo, Z.: A new seismic model of Ivrea-type lower continental crust accounting for realistic structural complexity, spatial variability, and generic anisotropy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3693, https://doi.org/10.5194/egusphere-egu25-3693, 2025.
The eastern margin of the Tibetan Plateau is a critical window through which to observe the northeastward extrusion and escape of material caused by the collision of Indian and Eurasian plates. Using 872 magnetotelluric stations, we obtained for the first time a high-resolution three-dimensional resistivity model of the lithosphere of the eastern margin of the Tibetan Plateau and its surrounding areas. The model shows that the near EW direction anomalies dominate the electrical structure of the middle-upper crust of the Qinling Orogenic Belt, and high-resistivity anomalies are arranged at intervals along regional faults and plate suture zones, which show obvious low resistivity. The lower crust and upper mantle show electrical structural characteristics mainly in the near SN direction. Based on these, we propose that the "overpass-structured" electrical structure indicating the crust-mantle decoupling of the Qinling Orogenic Belt was formed under the influence of the asthenospheric flow escaping from the northeast of the Tibetan Plateau. The three-dimensional lithospheric resistivity model also shows that the high-conductivity anomaly in the middle and lower crust of the northeastern margin of the Tibetan Plateau terminates in the West Qinling Orogenic Belt at about 106.5°E and continues to migrate eastward in the form of asthenospheric flow. High conductors penetrating the crust and mantle are also found in the Weihe Graben, the East Qinling Orogenic Belt, the circular rift zone around the Ordos Basin, and even the northern Ordos Block, considered a stable craton. This may represent upwelling of northeastward escaping asthenospheric flow along rift zones and areas of structural weakness. We propose that this asthenospheric flow and upwelling transformed the lithospheric mantle in the Qinling Mountains and the central and western parts of the North China Craton through thermal erosion.
How to cite: Ye, G., Liu, C., Jin, S., Yin, H., and Dong, H.: Asthenospheric Flow Beneath the Eastern Margin of the Tibetan Plateau: Evidence from a Three-dimensional Resistivity Model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14475, https://doi.org/10.5194/egusphere-egu25-14475, 2025.
The lithospheric deep architecture is generally uncovered through two primary avenues: geophysical exploration and the analysis of xenoliths. This article endeavors to construct an approach to lithospheric deep architecture through an investigation of magmatism based on big data. Our approach leverages rock probes and multi-isotopic mapping of igneous rocks, complemented by a synthesis of geophysical exploration and experimental simulations. We introduce several studies that have applied isotopic mapping (Sr, Nd, Hf, Pb) in conjunction with geophysical data to delineate the spatial distribution of juvenile, ancient, and reworked components within the deep lithosphere. The results demonstrate the consistency and effectiveness of the multi-isotopic systems in tracing deep materials and the correspondence between the isotopic mapping and geophysical investigation results.
The application of this methodology to various geological settings, such as the Central Asian Orogenic Belt (an accretionary orogen), the Tibetan Plateau (a collisional orogen), and the North China and Yangtze cratons, has yielded promising results. These outcomes highlight the significant potential of our approach. The achievements illustrate that our methodological system is adept at deciphering the three-dimensional material architecture of the lithosphere and its four-dimensional evolutionary narrative. This capability opens new avenues for the investigation of the deep lithosphere, offering insights that were previously inaccessible. Our methodological system enhances our understanding of the lithospheric architecture.
Key words: Magmatic rock; rock probe; isotopic mapping; deep compositional architecture.
How to cite: Wang, T., Huang, H., Zheng, Y., Yang, L., Xu, B., Zhang, J., Hou, Z., Yin, J., Wang, C., Bao, X., Tong, Y., and Zhu, X.: Probing deep 3D to 4D lithospheric architecture: Based on magmatic big data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20389, https://doi.org/10.5194/egusphere-egu25-20389, 2025.
The Ivrea-Verbano Zone (IVZ, western Alps) exposes an iconic middle-lower continental crustal section of the Adriatic Plate. This metamorphic sequence mainly consists mainly of metapelites/metapsammites and metabasites, with numerous lenses of metacarbonate rocks. The metamorphic grade of the crustal section increases progressively in P-T conditions, from amphibolite to granulite facies, with increasing crustal paleo-depth moving from East to West. The metamorphic overprinting renders particularly difficult determining the origin of metabasites (Leake, 1964). The amphibolite facies metabasites occur as numerous layers intercalated between siliciclastic metasediments. The mineralogical composition of metabasitesis is variable. These variations may be related to the original variability of the protolith or to interaction with surrounding metasediments and marble lenses. Despite this heterogeneity, pioneering studies identified a possible MORB signature from geochemistry (Sills & Tarney, 1984; Mazzucchelli & Siena, 1986).
In this contribution, we report new constraints on the geochemical affinity of the protoliths for all the amphibolite-facies metabasite paragenesis using major and trace elements and isotopes of the bulk rocks. To understand the primary signature of amphibolite and the metamorphic changes, we collected amphibolites from borehole 5071_1_B, drilled in Ornavasso (Val d’Ossola, Italy) in the frame of the DIVE-ICDP project (Pistone et al., 2017). We selected 13 samples representative of amphibolite sequences covering all different mineralogical parageneses, and one representative of siliciclastic metasediments. The selected metabasite samples are divided in the following four groups by mineralogy: (i) amphibolite s.s. (Amph+Cpx+Pl±Qz), (ii) garnet-bearing amphibolite, (iii) biotite-bearing amphibolite, and (iv) carbonate-rich amphibolite.
The results obtained from the characterization of bulk trace elements provide insights into the geochemical affinity of the sequence, revealing that it derives from two different protoliths with N-MORB and E-MORB affinities, respectively. Moreover, highly incompatible elements, which reflect the original signature in some samples, have been partially or totally modified by various high-temperature metamorphic events and contaminated by fluids migrating from surrounding metaterrigenous and metacarbonate lithologies. To better discriminate the original magmatic signature from the metamorphic overprinting, we used the isotopes data. The protholith signature is well-preserved in the garnet-bearing amphibolites and amphibolite s.s. by the Nd isotopes, which show a range of εNd values between 5.16 and 6.86, indicating a mantle-derived source for the parental melt that formed the amphibolite. In contrast, the 87Sr/86Sr400 (0.70492–0.71562) and 206Pb/204Pb400(18.4200–18.9536) ratios precisely track the remobilization of crustal signatures in all lithologies, reflecting the overprinting that occurred during collisional and post-collisional metamorphism.
The mineralogical evidence, in combination with trace element and geochemical data, suggests that the metabasites are volcanic sequence of rocks erupted in an extensional basin, being successively buried at ~20 km depth in the continental crust.
References:
Leake BE (1964), J Petrol, 5, 238-254
Mazzucchelli M & Siena RC (1986), TMPM, 35, 99-116
Pistone et al. (2017), Sci Dril, 23, 47-56
Sills JD & Tarney J (1984), Tectonophysics, 107, 187-206
How to cite: Bonazzi, M., Mariani, D., Agostini, S., Sanfilippo, A., and Zanetti, A.: New Insights into the Formation of Phanerozoic Continental Crust: Evidence from Metamorphosed Metabasites in the Ivrea-Verbano Zone, Italy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18674, https://doi.org/10.5194/egusphere-egu25-18674, 2025.
The factors and mechanisms related to magmatic underplating in continental settings remain a topic of active scientific debate. The Ivrea-Verbano Zone (IVZ), western Southern Alps, provides a rare opportunity to contribute to this debate, being an exhumed, nearly complete section of the continental crust that also include the roots of a completely exposed Permian magmatic system. Some parts of these roots are represented by the so-called Layered Series, a sequence of ultramafic and mafic rocks located between the localities of Balmuccia and Vocca in the Sesia valley. They have been interpreted as the first stages of magmatic underplating that later formed an 8-km thick sequence of gabbroic cumulates formed in the lower continental crust.
This work focuses on an 80 m wide outcrop within the Upper Zone (UZ) of the Layered Series, which is located between the villages of Isola and Sassiglioni. It includes a lithological sequence primarily composed of partially foliated gabbros with variable amounts of garnet, olivine, amphibole, and/or hercynite, along with layers of anorthosites and pyroxenites, pseudotachylites, and mafic pegmatites.
The objective of this work is to use this outcrop to constrain the P-T conditions of magmatic crystallization and subsequent metamorphic re-equilibration of the UZ and combine this to the existing data on the lower crustal rocks from IVZ. To reach this objective, a comprehensive characterization of the outcrop is ongoing, which include virtual outcrop modelling, structural and petrological field characterization and petrographic and geochemical analyses. These data are being used to decrypt the spatial relationships between the original magmatic units, which were possibly modified by post-Permian metamorphism and tectonics.
In this first stage, we are focusing on the garnet-, amphibole- and olivine-bearing gabbros of this outcrop. They have been characterized in detail through petrography and electron probe microanalysis. Their main petrological features can be summarized as follows:
- Granoblastic to polygonal textures formed by pyroxenes, plagioclase, olivine, amphibole and hercynite-magnetite assemblages;
- Significant variability in the presence of garnet (Alm50–0.52, Py0.31–0.34, Gr0.15–0.17) among the analyzed samples, which mostly occur as coronitic textures, sometimes associate with vermicular clinopyroxene, suggesting a metamorphic origin;
- Plagioclase (An50-70) shows slight optical zoning and presence of exsolution lamellae of hercynite-rich spinel within the cores;
- Olivine with composition from Fo5 to Fo0.6, clino- and orthopyroxene with mg# in the range 0.68–0.81 and 0.65–0.81, and Al2O3 in the range 4.5–8.1 wt% and 2.2–3.8 wt%, respectively.
The presence of olivine-bearing gabbros allows to apply the recently developed geobarometer for the assemblage olivine + clinopyroxene + plagioclase + spinel. An initial application attempt indicates a pressure of 6.7 ± 1.8 kbar, consistent or slightly lower with respect to existing estimates from the metapelitic lithologies of nearby outcrops. Further calculations are ongoing, which are part of an approach that include multiple-reaction thermobarometry, pseudosection modelling and petrographic constraints to decrypt the P-T path underwent by the gabbros.
How to cite: Del Rio, M., Ziberna, L., Corradetti, A., and Černok, A.: Petrological investigation of garnet- and olivine-bearing mafic crustal rocks in the Sesia Valley (Ivrea-Verbano Zone, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-961, https://doi.org/10.5194/egusphere-egu25-961, 2025.
The lower continental crust is a critical component of the deep carbon cycle, serving as a long-term reservoir for carbon (C) in the form of residual carbonates and graphite. Yet, the extent of C storage remains poorly understood. The Ivrea-Verbano Zone in northern Italy exposes lower crustal mafic and metasedimentary lithologies, providing a unique natural laboratory to investigate C retention. As part of the ICDP-funded DIVE project (Drilling the Ivrea-Verbano Zone), this study focuses on quantifying the C budget and exploring the isotopic composition of C phases.
The upper portion of the lower continental crust (borehole 5071-1_B, Ornavasso) consists primarily of felsic metasedimentary rocks (kinzigites, 73 vol-%) alongside amphibolites (13 vol-%) and calcsilicate rocks (11 vol-%), metamorphosed under upper amphibolite facies conditions (~750 ± 50°C, 7.5 ± 1.5 kbar). C is hosted in the form of graphite (Gr) and calcite (Cc). Gr occurs as inclusions in garnet and in the matrix of kinzigites, while Cc is observed in calcsilicate rocks and occasionally in amphibolites and leucosomes. Notably, a single marble layer has been identified.
The isotopic composition of C (Gr) and C-O (Cc) is being investigated to provide insights into the origin and evolution of C. Preliminary results range from -11.8 ‰ to -13.8 ‰ δ13CGr in the kinzigites and -0.7 to ‰ to -5.6 ‰ δ13CCc as well as 11.0-15.0 ‰ δ18OCc in the calcsilicate rocks.The marble layer from the borehole exhibits δ¹³C- and δ¹⁸O-values of -8.22 ‰ and 12.5‰, respectively, while marbles from nearby outcrops show a broader range of -0.7 to 1.19 ‰ δ13CCc and 13.6 to 22.6 δ18OCc. The isotope data supports the field observations suggesting that the sequence formed at the surface before burial and metamorphism.
Two sampling approaches were employed to determine the average and the local variability of C concentrations. (i) A broad approach, where samples of 6-12 cm length were taken from each rock type at approximately 10-meter intervals throughout the entire borehole, providing a comprehensive overview of the C distribution across different lithologies. Carbon-Nitrogen-Sulfur analyses from this approach revealed that kinzigites contain an average of 0.26 wt.-% C, while amphibolites and calcsilicate rocks average 0.07 and 0.73 wt.-% C, respectively. For the 578.7m deep borehole, the overall carbon concentration reaches an average of 0.23 wt.-%. (ii) A complementary microbulk sampling approach was specifically designed compare the variability across different scales, which is especially relevant in heterogeneous rock types. This method involves extracting core segments perpendicular to the foliation and subdividing them into centimetre-scale slices to capture fine-scale heterogeneities. While detailed results from the microbulk approach are pending, preliminary observations reveal notable intra-rock variability in C content. For example, a single kinzigite segment analysed using the microbulk approach covers the range in which 63% of the total number of kinzigite samples from the more extensive broad approach dataset are contained (n=27).
Our findings underline the importance of metasedimentary rocks at lower crustal depths (~25 km, ~750°C) as C reservoirs which enhances our understanding of the carbon cycle in deep crustal environments.
How to cite: Degen, S., Secrétan, A., Rubatto, D., and Hermann, J.: Carbon Storage and Isotopic Variability in the Deep Crust of the Ivrea-Verbano Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17519, https://doi.org/10.5194/egusphere-egu25-17519, 2025.
The deep subsurface is a dynamic and biologically active environment that harbors a vast array of microbial communities, accounting for a substantial fraction of Earth’s biomass. Most of the available information about subsurface ecosystems in continental regions is derived from studies on sedimentary rock formations and the analysis of groundwater and deep fluids accessed through boreholes and mines. Research on microbial life within crystalline bedrock has historically been more limited, primarily focusing on rocks such as granites, schists, and serpentinized ophiolites. Nevertheless, several studies have demonstrated that fractured crystalline rocks can host unique and diverse microbial ecosystems. In this study, we present the microbiological characterization of a water overflow at the borehole 5071_1_B (IGSN: ICDP5071EH30001) in the context of the ICDP-sponsored DIVE (Drilling the Ivrea-Verbano zonE) project (expedition number 5071) aiming for a full geophysical and petrological characterization of the continental lower crust in the Ivrea-Verbano Zone and for the identification of microbial communities inhabiting the different lithologies encountered in borehole 5071_1_B. During the drilling operation, a water overflow was observed at a depth of 300-316 m below current surface, due to the presence of a deep aquifer. The fluids were analysed geochemically and through a combination of 16S rRNA gene amplicon sequencing, metagenomic analysis, and epifluorescence microscopy. By using the waters of the nearby Toce River and on-site contamination tracking procedures we provide hypotheses on the origins of the rising fluids, as well as insights into the microbial taxonomic and functional diversity within the deep aquifer fluids.
How to cite: Tonietti, L., Corso, D., Cascone, M., Esposito, M., Brusca, J., Longo, A., Barosa, B., Eckert, E. M., Venier, M., Cordone, A., Kallmeyer, J., Greenwood, A., Hetényi, G., Müntener1, O., Pistone, M., Zanetti, A., Ziberna, L., and Giovannelli, D.: Microbial characterization of deep-waters from a borehole within the Ivrea-Verbano Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17249, https://doi.org/10.5194/egusphere-egu25-17249, 2025.
Constraining the bulk composition of the lower continental crust is important for understanding the evolution and dynamics of Earth's lithosphere. Lower crustal P-wave velocities inferred from seismic wide-angle profiles tend to be significantly higher than their upper crustal counterparts, which, in turn, points towards a rather mafic composition of the lower continental crust. There is, however, also geochemical evidence to suggest that the bulk composition of the lower continental crust could be intermediate to felsic, which would imply that the seismic evidence is biased towards the mafic side. A likely reason for such a potential bias could be the fact that the interpretation of seismic wide-angle data tends to ignore the effects of anisotropy in the lower continental crust. To explore this question, we have constructed canonical models of Phanerozoic lower continental crust based on the comprehensive cross-section exposed in the Ivrea-Verbano Zone (IVZ). These models simulate a 1D stochastic interlayering of the prevailing metapelitic and metamafic lithologies, to which we assign anisotropic P- and S-wave velocities based on published laboratory measurements. The effective elastic properties of these stochastically layered sequences are calculated using a Backus averaging scheme accounting for intrinsic anisotropy in conjunction with a Monte Carlo procedure to comprehensively explore the model space. Our analysis reveals that seismic anisotropy is primarily governed by the alignment of anisotropic minerals, such as mica and hornblende, while the associated influence of macroscopic layering is rather benign. Synthetic wide-angle seismic data generated using a finite-difference solution of the anisotropic elastodynamic equations show that isotropic interpretations of such data essentially provide the effective horizontal P-wave velocities of the underlying 1D layered lower crustal models. We find that the SiO2 content inferred from these effective lower crustal velocities generally agrees well with the actual values based on the underlying samples. Quite interestingly, the most significant discrepancies in terms of the predicted and observed SiO2 content, which are on the order of 4%, seem to be largely unrelated to the prevailing seismic anisotropy. Our results therefore indicate that, despite the rather pronounced intrinsic anisotropy of the metapelitic lithologies prevailing in the IVZ, estimates of lower crustal bulk composition based on seismic wide-angle measurements are unlikely to be systematically biased towards mafic side.
How to cite: Luo, Z. and Holliger, K.: Assessing the Impact of Seismic Anisotropy on Estimates of Lower Continental Crust Bulk Composition: Insights from the Ivrea-Verbano Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3239, https://doi.org/10.5194/egusphere-egu25-3239, 2025.
Since the inception of the ICDP expedition 5071, the Drilling the Ivrea-Verbano zonE (DIVE) project, geophysical surveys have played a key role in the planning and operational stages of phase 1. DIVE aims to investigate the continental lower crust with a focus on the physical and petrological transition towards the crust–mantle boundary at key sites in the Ivrea Zone of the Italian Alps. Phase 1 drilling site selection was strengthened with several 2D seismic profiles in the Ornavasso, Megolo and Premosello municipalities of Val d’Ossola, characterizing the complexity of the underlying lower crustal rock of interest and the overlying sedimentary cover. Preliminary site surveys identified near surface features deemed either disadvantageous or advantageous to drilling, subsequently refining the exact drill collar locations for the 5071_1_B and 1_A drill holes. During drilling pauses of 1_A and 1_B, dominant fracture orientations were determined with borehole acoustic methods, which influenced drilling decisions and the strategic termination of 1_B. Physical rock properties, magnetic susceptibility, and natural gamma radiation, were measured on rock cores as part of the on-site core characterization process. Additionally, seismic activity during the drilling periods was monitored through three approaches: seismic-while-drilling arrays located immediately around the drilling sites; a buried fiber optic cable in the nearby area; and a seismic monitoring network (DIVEnet) of a broader area covering the northeastern part of the Ivrea-Verbano Zone.
Physical rock properties have been determined along the length of each borehole through open-hole wireline logging, during and at the end of the drilling, establishing clear relationships between the different core-lithologies. Vertical seismic profiles (VSP), and reverse VSP check shot surveys have been conducted at the conclusion of drilling, including the use of a novel bare-fiber optic cable deployed in 5071_1_A, to determine the seismic velocity structure and seismic reflectivity. Additionally, gamma-gamma density measurements have been acquired at 10 cm intervals on all the recovered core using a multi-sensor-core-logger allowing the computation of elastic rock properties. Further rock physics experiments are ongoing and nearing completion and, together with a wealth of chemical analyses, start to reveal the fine details of lower crustal variability.
All the above surveys have contributed to an extensive geophysical data set across all scales that will be analyzed in the coming years. These data sets will briefly be introduced in the presentation.
How to cite: Greenwood, A., Hetényi, G., Baron, L., Trabi, B., Li, J., Caspari, E., Bleibinhaus, F., Kueck, J., Pierdominici, S., Tertyshnikov, K., Pevzner, R., and Pondrelli, S.: Overview of geophysical surveys conducted during ICDP-DIVE phase 1 in Val d’Ossola, Italy., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15226, https://doi.org/10.5194/egusphere-egu25-15226, 2025.
Our understanding of the Earth's interior - its physical structure, geochemical composition, and dynamic evolution - largely relies on seismic observations, particularly seismic wave velocities. Evidence derived from seismic P-wave velocities and heat flow measurements suggests that the chemical composition of the lower continental crust (LCC) ranges from predominantly mafic to felsic. More recent models, however, suggest an intermediate to felsic compositional range, raising the question of the significance of felsic components. Therefore, metasediments play a critical role in deciphering the LCC’s composition and evolution.
The Ivrea-Verbano Zone in the Alps offers insights into the lithological variability from a pre-Permian felsic lower crust then modified by Lower Permian mafic underplating. This study presents initial whole-rock data from the ICDP-funded DIVE project (Drilling the Ivrea-Verbano zonE), with drill cores from the first drilling target 5071_1_B (Ornavasso). The whole sequence of drill cores (578 m) is representative of the upper Ivrea LCC and consists of amphibolite facies rocks.
To estimate the bulk rock composition and volatile budget (e.g. Degen et al.) of the lower crust, a systematic sampling strategy was employed. Results presented are from a broad sampling approach, with 6–12 cm long samples collected from each lithology at approximately 10-meter intervals along the entire borehole. In order of lithological abundance, the retrieved lithologies include metasediments (kinzigites, ~73 vol-%), metamafic rocks (~13 vol-%), and calcsilicates (~11 vol-%):
- Kinzigites (Qz + Pl + Bt ± Gt ± Kfs ± Sil), local name for felsic gneisses characterized by biotite, range from pelites to psammites and are predominantly peraluminous. They exhibit LREE enrichment, slight HREE depletion, and a negative Eu anomaly.
- Metamafic rocks, primarily amphibolites (Amp + Pl + Qz ± Px ± Bt ± Gt), are locally interlayered with kinzigites. They present a flat REE pattern with a subtle negative Eu anomaly. A distinct subgroup, enriched in K2O and CaO, occurs at contacts/transitional zone between kinzigites, amphibolites and calc-silicates, reflecting increasing modal biotite and Ca-rich minerals. This subgroup has a REE pattern similar to
- Calcsilicate rocks occur as heterogenous layers of cm to dm scale aggregates of Ca-rich minerals (i.e. grossular-rich Gt, Pl, Scp, Ttn) ± Cpx ± Amp, and up to 14% carbonate minerals. These rocks are metaluminous and exhibit significant variability in their chemistry as a result of the mineral modal proportion, leading to highly variable major oxide and trace elements.
No distinct chemical trends are observed with increasing depth along the borehole. However, elements such as K, Th, and U differ between units and align with gamma-ray logging data. Intensities are notably higher in the kinzigite units compared to the more mafic units reflecting higher amounts of biotite and accessory phases (monazite, zircon). The weighted calculated bulk composition of 5071_1_B aligns with the upper end of LCC literature estimates.
Bulk trace element ratios (Th/La, Sm/La, Sm/Nd) suggest that the metasediments likely originated from (Paleozoic?) turbidites. Subduction and accretion processes may explain the dominance of metasediments in this section of the Ivrea-Verbano Zone LCC.
How to cite: Secrétan, A., Degen, S., Pacchiega, L., Li, J., Pistone, M., Hermann, J., and Müntener, O. and the DIVE Drilling Project Science team: Geochemical characteristics of lower continental crust metasediments: insights from the DIVE Project (5071_1_B, Val d’Ossola, Ivrea-Verbano Zone, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15873, https://doi.org/10.5194/egusphere-egu25-15873, 2025.
One of the aims of the ICDP Drilling the Ivrea-Verbano zonE project (DIVE) is to unravel lithological drivers of geophysical observations in the lower continental crust. During Phase I of the project, two boreholes, 5071_1_B and 5071_1_A, have been completed at depths of 578.5 and 909.5 m in December 2022 and March 2024, respectively. Borehole 5071_1_B is drilled into the upper part of the lower continental crust, while borehole 5071_1_A extends deeper into the lower crust. Core descriptions identified the lithologies in borehole 5071_1_B as kinzigite, amphibolite, calcsilicate and leucosome. In contrast, borehole 5071_1_A encountered a variety of lithologies including stronalite, anorthosite, gabbro, garnetite, gabbronorite, garnet granulite and pyroxenite. Together, these two boreholes represent a comprehensive cross-section of the lower continental crust in the Ivrea-Verbano zone. To understand the geophysical characteristics and their correlation to lithologies, a comprehensive set of geophysical borehole logs was acquired, including among others spectral gamma ray, magnetic susceptibility, sonic, acoustic and optical televiewer data. To complement the downhole data set, core density and magnetic susceptibility measurements were conducted using a multi-sensor core logger.
In our previous study, fuzzy c-means clustering of magnetic susceptibility and natural gamma logs from borehole 5071_1_B demonstrated an excellent agreement with the lithological core description, despite notable spatial variability. In this study, we integrate the petrophysical data from both boreholes revealing significant contrasts in petrophysical properties between them. Preliminary results indicate that the rocks in 5071_1_A generally exhibit lower gamma radiation, higher densities and higher velocities compared to those in 5071_1_B, with the exception of some amphibolite intervals in 5071_1_B. With respect to their magnetic susceptibilities the lithologies of both boreholes partially overlap; however, gabbros, gabbronorites and garnet granulites exhibit significantly higher average susceptibilities with values up to 10-1 SI. Most of the stronalites in borehole 5071_1_A exhibit gamma ray values comparable to the lower range observed in 5071_1_B, whereas gamma ray values for all other lithologies in 5071_1_A are generally lower than those in 5071_1_B. These findings suggest that gamma ray and magnetic susceptibility data may also act as good lithological indicators when analysing the combined data set. Core density measurements further complement this analysis, with values ranging between 2.8 and 3.4 g/cm3 in 5071_1_A, compared to 2.5 to 2.8 g/cm3 in 5071_1_B. The P-wave velocity of 5071_1_A predominantly ranges from 6000 to 7000 m/s, exceeding those observed in borehole 5071_1_B where velocities are strongly influenced by brittle deformation rather than lithological factors. Although numerous fractures are encountered in 5071_1_A, an initial analysis suggests correlations with lithological variations, as evidenced by high P-wave velocities exceeding 7000 m/s in a pyroxenite section. This implies that seismic reflections in 5071_1_A may be attributable to lithological velocity contrasts. To further investigate the origins of seismic reflectivity in these rocks, an acoustic impedance profile for both boreholes is required. This will help evaluate the influence of brittle deformation and lithological variations on seismic reflectivity.
How to cite: Li, J., Caspari, E., Greenwood, A., and Pierdominici, S. and the DIVE Drilling Project Science Team: Characterization of the Lower Continental Crust in the Ivrea-Verbano Zone from the Well Logging and Core Data of ICDP-DIVE Boreholes 5071_1_B and 5071_1_A, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6534, https://doi.org/10.5194/egusphere-egu25-6534, 2025.
The deep drilling project Drilling the Ivrea-Verbano zone (DIVE) is a scientific and multidisciplinary project that focuses on investigating a section of the lower continental crust and upper mantle, exposed in northwest Italy. The exposure of the crust to mantle transition represents one of the few places around the world, thus providing a unique possibility for study. As a consequence, during 2022-2024 two phases of drilling took place near Megolo (5071_1_A) and Ornavasso (5071_1_B). The tectonic evolution that led to exposure of the crust–mantle section in this area are still not fully understood, and thus forms a central part of scientific questions of DIVE. The drilling is part of the International Continental Scientific Drilling Program (ICDP) and involves a broad range of topics, such as the tectonic, petrological and geochronological development of the geological setting, geophysical and petrophysical investigations, and the deep biosphere.
The Ivrea Zone record complex processes from convergence during the Variscan orogeny followed by extensional processes in the Permian and decompression during the Jurassic periods. The zone has been a focus area for understanding how the continental lower crust and upper mantle is magnetized (e.g., Lanza et al., 1982; Belluso, 1990; Minelli et al., 2024), and why the main magnetic anomalies do not coincide with the source of the main gravimetric and seismic anomalies. The two, fully cored, scientific boreholes provide a unique opportunity to investigate such questions at the rock and mineral scale based on nearly 1.5 km of continuous fresh cores. Here we present preliminary results of detailed rock magnetic measurements, including magnetic susceptibility and remanent magnetization for more than 500 cylindrical specimens (20 mm diameter, 17 mm length), extracted from more than 150 core pieces that were systematically sampled from the two drill core sections (6-12 cm long core pieces, sampled at ~10-meter intervals along the entire borehole). Future work will focus on integrating magnetic properties with geochemical and petrological characteristics, in order to obtain a record of the magnetic petrology of the drill cores i.e., the chemical requirements and conditions to form iron oxides and other ferromagnetic phases in the continental lower crust and upper mantle. Additionally, detailed magnetic properties data will help in untangling the contrasts observed in magnetic anomalies, with gravitational and seismic anomalies. This joint magnetic and chemical analysis should eventually help in understanding how the continental crust is magnetized, and why there are similarities but also differences in crustal magnetizations in different continental crust settings.
References
Belluso, E., Biino, G., Lanza, R. (1990), New data on the rock magnetism in the Ivrea-Verbano Zone (Northern Italy) and its relationships to the magnetic anomalies. Tectonophysics, 182 (1-2), 79-89.
Lanza, R. (1982), Models for interpretation of the magnetic anomaly of the Ivrea body. Géologie Alpine, 58, 85-94.
Minelli, L., Gaia, S., Speranza, F., Caricchi, C., Fazio, E., Silvia, P., Michele, Z. (2024), Magnetic characterization of the Ivrea-Verbano zone (NW Italy): A key to understand the magnetism and structure of the continental lower crust. EGU General Assembly, doi.org/10.5194/egusphere-egu24-1523.
How to cite: Almqvist, B., Secrétan, A., Petri, B., Pistone, M., Hetényi, G., and Müntener, O.: Rock and mineral magnetic investigations of the DIVE (Drilling the Ivrea-Verbano zonE) drill cores: towards the magnetic petrology of the lower continental crust, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15616, https://doi.org/10.5194/egusphere-egu25-15616, 2025.
This study investigates the deformation mechanisms of olivine in ultramafic rocks from the Balmuccia and Finero massifs in the Ivrea-Verbano Zone, emphasizing the influence of dry and hydrous conditions on deformation behavior, microstructural features, and geochemical compositions. Advanced techniques, including EBSD, HR-EBSD, and EPMA, were utilized to provide detailed insights into the processes shaping these regions.
Our findings reveal significant chemical and microstructural distinctions between the Balmuccia and Finero peridotites. Olivine grains in the Balmuccia massif are smaller (~67 µm on average) and exhibit higher internal distortion. They display an A-type CPO pattern characterized by the alignment of the [100] axes with the stretching direction, the [010] axes perpendicular to the foliation plane, and the [001] axes within the foliation plane but normal to the lineation direction. The distribution of misorientation axes along the [010] direction and the alignment of Weighted Burgers Vectors (WBVs) along both [100] and [001] directions suggest the activation of both (001)<100> and (100)<001> slip systems, with the latter being more prominent in olivine grains in contact with orthopyroxene grains.
Conversely, olivine grains in the Finero massif are larger (~137 µm on average) and exhibit less internal deformation. These grains show a complex deformation history, with grain-size-dependent variations in CPO patterns. Larger grains predominantly display A-type CPO, while smaller grains exhibit a mixed B-type and A-type CPO pattern. The clustering of misorientation axes along the [001] direction in fine grains suggests the activation of the (010)<100> slip system in fine-grained olivine from Finero.
Geochemical analyses indicate that Balmuccia retains primary mantle characteristics with minimal metasomatic alteration. Spinels in this region have low Cr# (10–30) and high Al, indicative of a refractory mantle origin. In contrast, Finero samples exhibit strong evidence of metasomatism, with spinels enriched in Cr (Cr# 60–80) and TiO₂, reflecting interactions with subduction-related melts and fluids. Chemical profiles of Finero spinels show Cr enrichment and Al depletion along grain boundaries, pointing to chemical redistribution during deformation.
HR-EBSD analysis reveals that the maximum GND density in Balmuccia samples is two orders of magnitude higher than in Finero samples. In Balmuccia, areas with olivine grains in contact with orthopyroxene and clinopyroxene grains exhibit a higher frequency of subgrains with GND densities exceeding 10¹⁴ m⁻². Finero samples exhibit a relatively homogeneous stress distribution, with an average stress of approximately 24 MPa. Balmuccia samples show a more heterogeneous stress distribution. Stress maps align with GND density distribution patterns, and stress magnitudes in regions where olivine grains contact opx grains range from 3 to 4 GPa at subgrain boundaries.
We propose that Finero and Balmuccia initially experienced similar conditions at the onset of rifting. However, subsequent detachment faults amplified their divergence, displacing Finero into a foredeep position analogous to the Banda Sea foredeep and Balmuccia into a magmatic arc environment similar to the Banda Sea magmatic arc.
How to cite: Mansouri, H., Elyaszadeh, R., Toy, V., Pistone, M., and Wheeler, J.: Deformation Mechanisms and Strain Localization in Ultramafic Rocks: Insights from the Balmuccia and Finero Peridotites, Ivrea-Verbano Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19933, https://doi.org/10.5194/egusphere-egu25-19933, 2025.
Borehole 5071_1_A of the ICDP-DIVE project intersects the lower part of the lower continental crust and is drilled to a depth of 909.5 m. Several fracture zones are encountered, which not only exert control on the hydrological characteristics of the very low permeability formations in the presence of strong topographic relief, but also affect their mechanical properties. In this study we characterize the fracture network in borehole 5071_1_A with respect to its flow characteristics using a suite of geophysical borehole data. Acoustic and optical televiewer data, as well as normal resistivity logs, are utilized to locate fluid-bearing fracture zones and delineate their geometrical characteristics. Most natural fractures have azimuthal orientations between NNW to NE and exhibit a wide range of dips between 10° – 80°. According to their appearance in the televiewer data they can be divided into three classes, whereby Class 1 consists of the largest aperture fractures and Class 2 and 3 of smaller aperture fractures. Class 1, and clusters of Class 2 and 3 fractures correlate with resistivity anomalies suggesting open fluid-bearing natural fractures, which are encountered along the length of the borehole. To gain further insights into the flow characteristics, a combined analysis of self potential, temperature and mud parameters (conductivity and pressure), as well as flow meter logs, is on-going to locate in- and out-flow zones and to identify water of different compositions and temperature in the borehole. Preliminary results show that in-flow and out-flow zones can be correlated with fractures along the borehole track, whereby three strong in-flow zones around 700 m and 850 m depths correlate with changes in the fluid conductivity. This suggests that different types of water may enter the system, hinting at a compartmentalized complex system with distinct hydraulic zones. The strong in-flow zone at a depth around 850 m is also picked up in passive borehole fiber optic data. Identification of these different flow paths and their correlation to fracture zones provide important information for understanding potential diversity in microbiology in these lower crustal rocks and, support the interpretation of mud gas logging results, allowing a better understanding of the nature and origin of these geofluids.
How to cite: Caspari, E., Li, J., Fuetsch, M., Pierdominici, S., and Greenwood, A.: Fracture and flow characterization of ICDP-DIVE Borehole 5071_1_A from geophysical well logging data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15416, https://doi.org/10.5194/egusphere-egu25-15416, 2025.
Pseudotachylytes are commonly interpreted as evidence of seismic rupture, and there are numerous studies documenting their occurrence under a wide range of P-T conditions throughout the entire lithosphere. Pseudotachylytes, which appear to have formed under different ambient conditions, have previously been reported in exhumed fault zones within the mafic and ultramafic rocks of the lower crustal section of the Ivrea-Verbano Zone. However, an unexpectedly high abundance of pseudotachylytes was encountered in core DT-1A, obtained at Megolo as part of the Drilling the Ivrea-Verbano Zone (DIVE) project. Along its total length of 900 m, pseudotachylytes can be observed as distinct generation surfaces, anastomosing networks and breccias, not only in major fault zones but also in virtually undeformed parts of the core. While commonly found pristine, some pseudotachylyte are overprinted by ductile shearing, and in a few cases exhibiting cross-cutting relationships with both other pseudotachylytes and ultramylonites.
Samples (up to 20 cm long and 9 cm wide) collected from different depths and with different characteristics have been polished and prepared for SEM analysis, including BSE, EDS, CL, possibly EBSD and CT. By carrying out microstructural and petrological investigations on these large samples, the context of the observations is preserved and the results can be reliably interpreted on a larger scale.
The study of a wide range of pseudotachylytes within DT-1A aims to: (1) improve our understanding of the (metamorphic) conditions that govern the formation of pseudotachylytes and (2) better understand the interaction between different deformation mechanisms within the continental crust.
How to cite: Ferenczy, S., Hawemann, F., and Toy, V.: Pseudotachylyte Diversity in the Ivrea-Verbano Zone: Insights into Crustal Rheology from DIVE Drill Core DT-1A, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-855, https://doi.org/10.5194/egusphere-egu25-855, 2025.
Knowledge of the thermal conductivity (TC) and internal heat production (A) of rocks forming the lower continental crust (LCC) is essential for any thermal study of the Earth's lithosphere, as they define the geotherm. Many heat flow models use sparsely sampled data of thermal properties along with simplified layer structures, leading to poorly constrained geotherms and large uncertainties in temperature calculations. To improve heat flow calculations of the LCC, we present new high-resolution TC data (total data points: 13080) of felsic, (meta-)mafic and ultramafic lithologies of the LCC of the Ivrea-Verbano Zone (IVZ, Northern Italy). The measurements are done on a representative set of drill cores from the scientific drilling project DIVE (Drilling the Ivrea Verbano ZonE; ICDP; www.dive2ivrea.org; Pistone et al. 2017) in which two boreholes 5071_1_A (final depth: 909.5 m), located in Megolo, and 5071_1_B (final depth: 578.5 m), located in Ornavasso, have been realised. A total number of 74 drill core samples with a total length of ~ 28 m from the two boreholes have been measured at high spatial resolution using an optical TC scanner (OTCS, Popov et al., 1999). Measured TC for lithologies of the upper part of the LCC (1_B) ranges between 1.79 – 4.97 W/m·K for amphibolites, 2.02 – 6.63 W/m•K for kinzigites, 1.53 – 5.34 W/m•K for calcsilicates, and 2.12 – 5.70 W/m•K for leucocratic veins. The lithologies of the lower part of the LCC (1_A) show TC that ranges between 1.71 – 2.75 W/m•K for stronalites, 1.83 – 2.54 W/m•K for gabbros, 1.55 – 2.11 W/m•K for gabbronorites, 1.70 – 2.36 W/m•K for garnet granulites, 1.66 – 2.40 W/m•K for intermediate gabbronorites, 1.56 – 2.13 W/m•K for anorthosites, and 2.16 – 3.54 W/m•K for pyroxenites. The results show a significant variability of TC within the same lithology and between different lithologies, explained by the spatially variable mineral contents and grain sizes. Measured concentrations of heat-producing elements (U, Th and K) of 33 selected drill cores were obtained using powder-based gamma spectrometry. The results show that the concentrations are lithology-dependent and decrease towards mafic and ultramafic rocks. These data are compared to spectral-gamma borehole logs to evaluate the radiogenic heat production along both boreholes. TC and A are used as input parameters for 7 types of probabilistic, steady-state 1D heat flow models with synthetic lithology columns with variable layer thicknesses (d) that are randomly assigned and emulate the lithology characteristics seen in the boreholes and the IVZ. By carrying out many realisations, the effect of high-resolution TC sampling on heat flow uncertainties is quantified. The adaptive nature of our models allows us to test the parameter sensitivity of TC, A, and d. The first results show that higher spatial variability on thermal properties structure cause larger model uncertainties in the temperature calculations compared to cases with more homogeneous structure.
How to cite: Lemke, K. and Hetényi, G. and the ICDP DIVE Science Team: Thermal structure and variability of the lower continental crust: dataset and models based on project DIVE boreholes 5071_1_A and 5071_1_B, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11341, https://doi.org/10.5194/egusphere-egu25-11341, 2025.
The geochemical differentiation of the Earth’s crust is influenced by high-temperature metamorphic processes. Partial melting of the lower continental crust leads to the redistribution of the heat-producing elements (HPEs), which are responsible for ca. 60% of the crustal heat flux. The major carriers of HPEs in typical lower crustal rocks are accessory minerals, such as zircon and monazite. During partial melting, the solubility of these accessory phases in anatectic melts controls the redistribution of the HPEs. The felsic rocks of the lower continental crust exposed in the Ivrea-Verbano Zone (IVZ) host significant amounts of HPEs and they can be used as a natural laboratory to investigate the link between melting reactions, solubility of accessory minerals and migration of HPEs.
This study focuses on the felsic metasediments of boreholes 5071-1A and 5071-1B, which have been recently drilled in the framework of the ICDP-DIVE project. Metapelites and metapsammites show macroscopic and microscopic signs of partial melting, such as the segregation of leucocratic domains. Petrographic observations, geothermobarometric calculations and thermodynamic modeling show that partial melting in 5071-1B rocks occurred at upper amphibolite facies conditions, at P-T conditions of ca. 7 kbar and 750°C and is predominantly controlled by muscovite dehydration melting. Instead, 5071-1A lithologies experienced temperatures in excess of 900°C, associated with extensive anatexis related to biotite dehydration melting.
The spatial distribution of the accessory minerals has been determined with a combination of SEM chemical mapping and BSE imaging, supervised classification of minerals by XMapTools and counting statistics by image analyses techniques. Furthermore, a full trace element budget has been performed by means of LA-ICPMS analyses on major minerals and accessory phases.
Our results indicate that the HPEs budget is high in the amphibolite facies part of the sequence, with values that are around five times greater than average lower crustal values and comparable to values typical for middle and upper crust (Rudnick and Gao, 2014). The higher-grade felsic rocks are relatively depleted and more similar to previous estimates for the lower crust. The primary hosts of U and Th at amphibolite facies are monazite, allanite, and, to a lesser extent, zircon and apatite. The U-Th budget is shared between zircon, monazite and rutile in the higher-grade equivalents.
Our results offer novel insights on the factors controlling the behavior of the accessory minerals during partial melting and permit to investigate the applicability of models and solubility equations in comparison with natural rocks. At UHT conditions, the partial preservation of zircons and monazites (such as inherited zircon cores and metamorphic monazites) proves that these minerals are not fully dissolved in partial melts even at extreme crustal temperatures. A representative migmatite shows that biotite-sillimanite melanocratic domains, interpreted as restites formed after melt loss, are relatively enriched in Th, U, and K compared with the interlayered leucocratic domains that have experienced net melt gain. In particular, monazite is enriched by approximately one order of magnitude in the melanosome, in contradiction with predictions from models, indicating that the redistribution of HPEs in the crust is more complicated than previously thought.
How to cite: Pacchiega, L., Degen, S., Secrétan, A., Hawemann, F., Tholen, S., Hermann, J., and Rubatto, D.: Redistribution of heat-producing elements during partial melting of felsic rocks of the lower continental crust, Ivrea Verbano Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16075, https://doi.org/10.5194/egusphere-egu25-16075, 2025.
The crust-mantle transition zone in continental settings is a key and still unexplored part of our planet. It can help to understand the pathways of magmas from their source to the surface, the formation of continental crust and its past and present architecture. The Ivrea-Verbano Zone (IVZ, Southern Alps) is a well known exposed section of lower continental crust and includes units of mantle peridotites that could testify the presence of a crust-mantle transition zone at or near the surface. This is well exposed along the Sesia river, where a km-sized mantle peridotite body (i.e., Balmuccia peridotite body) is in contact with a sequence of pegmatoid clinopyroxenites, websterites, cumulus peridotites and gabbros. This study focuses on a pegmatoid clinopyroxenite outcrop showing a possible magmatic contact with the Balmuccia peridotite to the west and a transition to a gabbro-pyroxenite sequence to the east. The objectives are to determine the origin of this clinopyroxenite and decrypt its metamorphic history through petrographic analyses and thermodynamic modelling.
Field mapping has shown that the pegmatoid clinopyroxenite is a relatively heterogeneous body, being variably rich in mm- to cm-sized patches rich in garnet, plagioclase and spinel. In some parts of the outcrop these patches are larger and can be defined as a mineralogical banding. The pegmatoid clinopyroxenes show evidence of ductile deformation and exhibit exsolution lamellae of orthopyroxene ± garnet ± spinel ± plagioclase. The texture suggests a cumulus origin for the pegmatoid clinopyroxenes, with the garnet-bearing assemblages possibly representing a recrystallized intercumulus assemblage. In particular, the texture of the garnets suggests a metamorphic origin and may be related to high bulk aluminium content (clinopyroxene Al2O3 is up to 8.1 wt%) or high equilibration pressure, or a combination of both. Further work is being devoted to the garnet-bearing exsolution lamellae in the pegmatoid clinopyroxenes, as these might provide insights into the P-T path undergone by the original assemblage. We are investigating these aspects through phase equilibrium calculations using a set of bulk compositions assumed to represent the original pegmatoid clinopyroxenite and the garnet-bearing assemblage. Altogether, the results are will shed light on the petrological history of this pegmatoid clinopyroxenite and provide new insights into the magmatic and metamorphic evolution of this sector of the IVZ.
How to cite: Ziberna, L., Terranova, K. G., Narduzzi, F., Venier, M., Del Rio, M., and Černok, A.: Garnet-bearing clinopyroxenites in the lower crustal units of Ivrea Verbano Zone, Italian Alps, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17690, https://doi.org/10.5194/egusphere-egu25-17690, 2025.
The source of high-intensity magnetic anomalies from cratons has long been debated, as it requires speculative rocks yielding 2-6 A/m magnetization. Magnetic properties of the lower crust lithology are generally poorly constrained, considering their low exposure at surface. Here we report on the magnetic and paleomagnetic investigation of the Ivrea-Verbano Zone (IVZ), Western Alps, where both metamorphic and intrusive lower crust rocks of Late Variscan-Permian ages are spectacularly exposed. We sampled 312 oriented cores at 39 sites along the Cannobina, Ossola, Strona, and Sesia valleys/sections. Low (0.27-2.1·10-3) magnetic susceptibility (k) values were routinely measured in metamorphic rocks from the Ossola and Strona valleys. There, only two metabasite (one in amphibolite and one in granulite metamorphic grade) out of 25 metamorphic sites containing pseudo single domain (PSD) magnetite yield 0.48-1.1·10-1 k values that remain strikingly constant until 550°C heating. K values of gabbros and granodiorites from Sesia valley mimic low values from metamorphic rock, whereas at Cannobina valley one gabbro and one mafic granulite display k values comparable to the two strongly magnetic sites from Ossola/Strona valleys. Peridotite lenses embedded in gabbros at Balmuccia and Finero similarly yielded low (0.24-5.5·10-3) k values, consistently with their low (<20%) serpentinization degree. Results indicate that remanence contribution is negligible, as 1) Q <1 values imply remanent magnetization subordinate to induced magnetization, 2) paleomagnetic directions are generally scattered consistently with PSD magnetite grain size, and 3) remanence is notoriously unstable at lower crust temperatures. We conclude that IVZ lower crust could not yield cratonic magnetic anomalies, and similar conclusions might stand for other Variscan-age lower crust sections. Scattered high-intensity metabasites could be candidates, if their PSD-MD magnetite-rich mineralogy dominates pre-Cambrian lower crust.
How to cite: Minelli, L., Siravo, G., Speranza, F., Zucali, M., Fazio, E., and Caricchi, C.: Magnetic and paleomagnetic characterization of the Ivrea-Verbano lower crust body (NW Italy): Assessing the magnetization of Variscan-age lower crust, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5843, https://doi.org/10.5194/egusphere-egu25-5843, 2025.
Probing the deep lithosphere remains a key objective in earth sciences. The present-day lithosphere architecture is a terminal, time-integrated image shaped by long-term geological processes, among which magmatism plays a crucial role. Consequently, a causal relation exists between deep-time magmatism and present-day lithospheric architecture. This study employs multi-proxy isotopic and elemental mapping of Late Carboniferous to Middle Permian magmatic rocks in West Tianshan, SW Central Asian Orogenic Belt. The mapping unravels two distinct lithospheric domains, i.e., an isotopically depleted domain in the north and an isotopically enriched domain in the south. Seismic and gravity data suggest significant differences in geophysical properties across different domains. By integrating phase equilibrium modeling, we further indicate that the present-day geophysical disparities can be attributed to deep-time magmatism involving differential mantle sources and infracrustal differentiation. The lower crust of the northern part was built by magmatic processes starting from intermediate magmas ultimately derived from a subduction-modified mantle. In contrast, the lower crust of the southern part was constructed through the interaction between ancient crust-metasomatized mantle-derived magmas and supracrustal relaminant. Our findings suggest a novel methodological approach for utilizing geochemical data from deep-time magmatic rocks to decipher present-day deep lithospheric architecture.
How to cite: Huang, H., Wang, T., Gómez-Frutos, D., Castro, A., Zhu, X., and Bao, X.: Linking deep-time magmatism to present-day lithospheric architecture through isotopic and elemental mapping, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14928, https://doi.org/10.5194/egusphere-egu25-14928, 2025.
Granitoids, being the most abundant lithologies of the Earth’s upper continental crust, are key source to study the crustal evolutionary history and the associated tectonic activities producing them. The Southern Granulite Terrane (SGT) comprises various granitoid intrusions during Neoproterozoic time that caused due to the subduction-accretion-collision processes during the Gondwana supercontinental assembly. This study aims to provide the origin, basement-cover relationship between the Sankari Granite (SG) and the Basement Hornblende-Biotite Gneisses (HBG) in the Namakkal block, Southern Granulite Terrane. A detailed field study suggests that the Sankari granite occurs as small to medium isolated hills and is mostly massive, leucocratic, pegmatoidal at some places. It also shows little deformation at the marginal area due to presence of shear zone. Field study suggests that SG intrudes into the basement HBG as the SG is seen in contact with the HBG where the HBG are getting migmatised. At some places, HBG can be seen as a caught-up and engulfed fragments within the SG. This evidence suggest that the HBG are older, and SG is younger. The migmatisation of HBG at the contact with SG suggests that SG is derived from the partial melting of the basement HBG. The petrographic study suggests that Sankari granite is composed mainly quartz, k-feldspar, plagioclase with little amount of amphibole and biotite with minor accessory phases like calcite, apatite and Fe-Ti oxide. Mineralogically, it falls into monzo-granite, alkali feldspar granite to granite in the QAP diagram. A whole rock major element chemistry suggests that all the samples fall into the granite field and alkali to alkali-calcicferroan to peraluminous in nature. Trace element study suggests that SG is poorly enriched in REE (∑REE = 18.41–52.62 ppm) and show slight flat pattern with negative europium anomalies (EuN/Eu∗N = 0.75–2, av. 2.21) on average. It shows enrichment of Rb, Th, U and depleted Ti, Sr, P, Eu anomalies which is the characteristic of A-type granite (David and Chappell 1992). The Zircon U-Pb age of SG suggests it’s emplacement age of 559.1 ± 3.5 Ma (Glorie et al., 2014) which is younger than the HBG. The overall study suggests the emplacement of Sankari Granite, which is a A-type granite, during Neoproterozoic time by partial melting of basement Hornblende-Biotite Gneisses prior the amalgamation of early stage of Gondwana supercontinental assembly.
How to cite: Nema, S. and D'Souza, J.: Neoproterozoic A-type granite magmatism in the Southern Granulite Terrane, India: Constrain on the Genesis and Basement- Cover relationship, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20129, https://doi.org/10.5194/egusphere-egu25-20129, 2025.
The Arabian-Nubian Shield is a vast, juvenile continental crust province that formed during the Neoproterozoic by accretion of intra-oceanic island arcs. Sediments eroded from these arcs are preserved in the Eilat Metamorphic Complex in the northernmost part of the Arabian-Nubian Shield. We present a coupled U-Pb-Hf-O study of detrital zircons from metasedimentary units from Eilat area, intended to assess the juvenile nature of the island arcs and to detect crustal recycling processes involved in their formation. Detrital zircon geochronology places island arcs magmatism in this region at 1040-740 Ma, peaking between 850-750 Ma. Arc crustal evolution is demonstrated by coupling Hf and O isotopes in the detrital zircons. Zircons with mantle-like δ18O (5.0-6.5‰) have predominantly positive εHf(t) values of +6 to +12 that principally reflect late-Stenian to late-Tonian juvenile crust formation. A temporally decreasing trend in εHf(t) values implies ~80 m.y. of crustal reworking of the juvenile arcs. The contribution of reworked older crust was minor, as just two Paleoproterozoic grains and a small number of Neoproterozoic grains with lower εHf(t) values were detected. Crustal reworking is further demonstrated by abundant zircons with elevated δ18O values of mostly 6.5‰ to 9‰, indicating assimilation of 18O-rich supracrustal components in the arc magmas starting from ~930 Ma. Since the εHf(t) values of zircon grains with elevated δ18O are positive and high, we assign this 18O-enrichment to juvenile sediments that were recycled shortly after crust formation, suggesting a self-recycling island arcs system. Mixing calculations show that at least 20% and up to 40% of juvenile sediments with δ18O of 14‰ were assimilated in the melts sampled by our zircons. These results imply that reworking of arc terranes and the incorporation of supracrustal components played a fundamental role in the evolution of island arcs in the northern Arabian-Nubian Shield.
How to cite: Vardi, C., Avigad, D., Glazer, A., Gerdes, A., Li, S., Wang, T., Albert, R., and Geller Lutzky, Y.: Neoproterozoic island arcs evolution and recycling in the northern Arabian-Nubian Shield: U-Pb-Hf-O isotopes in detrital zircon from Eilat metasediments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10059, https://doi.org/10.5194/egusphere-egu25-10059, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 1
EGU25-9306 | ECS | Posters virtual | VPS22
Geochemical characterisation of Permian lower continental crust: case study from Ivrea-Verbano Zone (NW Italy)Tue, 29 Apr, 14:00–15:45 (CEST) | vP1.17
Investigating the main geochemical characteristics of the lower continental crust is essential to understand its formation and evolution, identifying crustal differentiation processes and possible crust-mantle interactions. We performed bulk rock (major and trace elements), noble gases isotopes (He, Ne, Ar), and fluid inclusions (Raman spectroscopy) analyses on metamorphic rocks from Ivrea-Verbano Zone (Southern Italian Alps). Specifically, we studied various lithologies (metapelite, metagabbro, mafic and felsic granulite, amphibolite, and gneiss) to analyse the continuous metamorphic gradient from amphibolite- to granulite-facies.
Bulk rock analyses confirm the mafic nature of the protoliths for metagabbros (MgO = 5.36-10.25 wt.%), mafic granulites (MgO = 8.32-25.80 wt.%) and amphibolite (MgO = 7.98 wt.%) plotting in the metabasite field of the ACF chemographic diagram. Felsic granulite and sillimanite-gneiss fall within metamorphosed quartz-feldspar rocks, except for metapelite, which approaches the metacarbonate field, due to the presence of secondary carbonates. Metagabbros, mafic granulites and amphibolite show low REE concentrations (∑REE between 3 and 25 ppm) and high Cr and Ni contents (up to 1865 and 265 ppm respectively in mafic granulite), reflecting the mafic/ultramafic nature of the protoliths, whereas felsic granulite, sillimanite-gneiss and metapelite show higher REE contents (∑REE between 48 and 197 ppm).
3He/4He isotope ratios in metamorphosed quartz-feldspar rocks (0.06-0.30 Ra) and metabasites (0.15 and 0.45 Ra) are significantly radiogenic, although the metabasites show slightly higher values, corroborating a more primitive component in their source. Most samples plot near the air component in the 20Ne/22Ne vs 21Ne/22Ne diagram, except for mafic granulites which show a crustal-air mixing trend. As regards the Ar isotope ratios, all samples appear rich in radiogenic component (40Ar/36Ar up to 2645 in metagabbros).
Raman spectroscopy analyses on fluid inclusions in orthopyroxene from mafic granulites show the coexistence of talc, graphite and magnesite with methane, providing direct evidence of a complex history in terms of post-metamorphic reactions and P-T-fO2 conditions.
Our preliminary results show the compositional diversity and evolution of the lower continental crust, highlighting the interplay between mafic and sedimentary sources and the importance of fluid interactions and post-metamorphic processes.
How to cite: Carnevale, G., Caracausi, A., Correale, A., Fazio, E., Paonita, A., Romano, P., and Zucali, M.: Geochemical characterisation of Permian lower continental crust: case study from Ivrea-Verbano Zone (NW Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9306, https://doi.org/10.5194/egusphere-egu25-9306, 2025.
EGU25-7536 | ECS | Posters virtual | VPS22
Water-fluxed melting and back-arc extension in the continental arc: Evidence from I-type granites, adakitic rocks and high-Nb mafic rocks at the western margin of the Yangtze Block, South ChinaTue, 29 Apr, 14:00–15:45 (CEST) | vP1.18
The Neoproterozoic western margin of the Yangtze Block in South China records significant continental crust-forming and modification processes through two distinct magmatic episodes. Using integrated geochemical and petrological approaches, we demonstrate that the 811-802 Ma Yuanmou Complex comprises alkaline high-Nb mafic rocks characterized by high Nb (15.7-41.9 ppm), TiO2 (2.13-3.39 wt%) contents and positive εNd(t) (+4.8 to +6.9), coupled with adakitic granodiorites showing high Sr/Y (17.4-49.0), (La/Yb)N (16.3-52.6) and consistent bulk rock εNd(t) (-0.5 to -1.5) and zircon εHf(t) (0.0 to +2.3). The younger 750 Ma Jinping I-type granites exhibit high SiO2 (71.2-73.5 wt%) and alkalis contents, enriched LREE patterns and depleted isotopic signatures (εNd(t): -0.4 to +1.3; zircon εHf(t): +4.83 to +8.37). Thermodynamic modeling reveals how crustal water content-controlled magma generation at different depths - low water-fluxed melting (2.0-3.5 wt% H2O) produced I-type granites at medium pressure (6-9 kbar), while deeper settings with higher water content generated adakitic melts. The high-Nb mafic rocks in the Yuanmou Complex, derived from metasomatized mantle wedge, provide evidence for crustal-mantle interaction during back-arc extension. These coupled magmatic processes demonstrate how water content variations with depth influenced continental crust formation and evolution in arc settings.
How to cite: Huang, B., Wang, W., Zhao, J., Khattak, N. U., Li, R., Huang, S.-F., Lu, G.-M., Sun, L., Xue, E.-K., Zhang, Y., and Cai, X.-Y.: Water-fluxed melting and back-arc extension in the continental arc: Evidence from I-type granites, adakitic rocks and high-Nb mafic rocks at the western margin of the Yangtze Block, South China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7536, https://doi.org/10.5194/egusphere-egu25-7536, 2025.
