Rock and mineral magnetic investigations of the DIVE (Drilling the Ivrea-Verbano zonE) drill cores: towards the magnetic petrology of the lower continental crust

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.