From the lower crust towards the crust–mantle transition zone: Initial results from the ICDP DIVE project

We report initial results from the first phase of the ICDP-funded ‘Drilling the Ivrea-Verbano zonE‘ (DIVE) project in Val d’Ossola (northern Italy). Characterized by pronounced geophysical anomalies, the exposed Ivrea-Verbano Zone offers unique opportunities to test geophysical and petrologic models about the lower continental crust (LCC) and its transition to the upper mantle. From October 2022 to April 2024 two boreholes of respectively 578.5 and 909.5 m depth were drilled using continuous diamond double tube wireline coring. Core recovery was ~100% for both boreholes. During and after drilling, geophysical logs were acquired, providing natural and spectral gamma ray, magnetic susceptibility, electrical resistivity (SPR and DLL), spontaneous potential, sonic, acoustic and optic televiewer data. Retrieved rock cores were described and classified by the DIVE drilling project science team and later shipped to the BGR Rock Core Repository in Spandau-Berlin, where core density and magnetic susceptibility were measured with a multi-sensor core logger followed by XRF scans.

Here we summarize core descriptions, initial geochemical results, geophysical logging data, drill hole fluid chemistry and gas compositions, and preliminary microbiological investigations. The two boreholes sampled two fundamentally different compositions of the lower continental crust: one (5071_1_B) mostly consists of metasedimentary rocks and a few amphibolites, and the second hole (5071_1_A) mostly captures a variety of gabbroic rocks with intercalations of granulite facies metasediments, pyroxenite, and intrusive gabbronorite. This is in agreement with the expected structural positions but allow to study the continental lower crust across numerous spatial scales.

In borehole 5071_1_A, several zones of ultramylonites, cataclasites, fault gauges and pseudotachylites were recovered documenting important episodes of semi-brittle behaviour of the LCC after assembly in the Lower Permian. Along the entire drillholes fractures and open cracks were observed and sampled, some of them filled with precipitates of quartz, carbonates, sulfides, graphite, and oxides.

Continuous monitoring of borehole fluids and gases provide evidence of varying gas mixtures including H2, CH4, and CO2, indicating diverse fluid sources and microbial activities in the deep crust. At the current stage, we are evaluating the biotic and abiotic contributions. Some of these open fractures are potentially promising hosts for microbial communities and are currently under investigation. Additional samples for microbiological studies were taken every 20 m from the drillcores and are currently cultivated for further investigations and also analyzed for bulk rock major and trace elements.

The two drillholes of DIVE provide unprecedented details of the variability of lower continental crust. Metasedimentary sections of the drilled LCC are important reservoirs for volatile and radiogenic heat producing elements, while dominantly mafic sections of the lower continental crust are depleted in these elements. Measured seismic velocities and densities are affected by numerous fractures but metasedimentary rocks are uniformly lower in density (2.5-2.8 g/cm3) compared to the mafic section (2.8-3.4 g/cm3) indicating that the lowermost part of the drilled section enters the continental crust–mantle transition zone.