The variability of lower continental crust: Initial and advanced results from the ICDP DIVE project

Understanding the chemical and physical processes governing the formation and evolution of the Earth’s continental crust is fundamental for the Earth system and other planets. The upper crust is accessible to direct geological observation and sampling, but deeper portions, especially the lower crust and the crust–mantle transition zone (“Moho”) are usually beyond reach. The lower continental crust (LCC) is one of the most important, but also most enigmatic regions of Earth’s lithosphere and its composition and physical properties are strongly debated. Here we report some of the initial results from the first phase of the ICDP-funded “Drilling the Ivrea-Verbano zonE” (DIVE) project (site 5071_1) in Val d’Ossola (northern Italy). From October 2022 to April 2024 two boreholes of respectively 578.5 (Ornavasso, 5071_1_B) and 909.5 m (Megolo, 5071_1_A) depth were drilled using continuous diamond double tube wireline coring. 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, complemented by multi-sensor core logging data (with focus on density) acquired in the core repository of the BGR in Berlin-Spandau (Germany). In addition, continuous real-time mud gas logging provides evidence of varying gas mixtures including He, H₂, CH₄, and CO₂, indicating diverse fluid sources and possible microbial activities in the deep crust.

The two drillholes sampled two fundamentally different compositions of the lower continental crust: the first hole (5071_1_B) drilled the upper part of the lower continental crust and mostly consists of metasedimentary rocks and a few amphibolites. The second hole (5071_1_A) drilled the lowermost continental crust and mostly captured a variety of garnet and/or orthopyroxene bearing gabbroic rocks with intercalations of garnet granulite facies metasediments, pyroxenite, and intrusive gabbronorite including frequent pseudotachylites. Combining multi-sensor core-logging data with petrophysical information and whole rock geochemical data provides mineral modes of the drilled cores, which can be used to calculate densities and seismic velocities. These calculations together with direct observations of drilled rock types indicate that the lowermost part of the Ivrea Verbano Zone continental crust is enriched in garnet.

Bulk compositions of the two different drillholes of the lower crust show fundamental differences. 5071_1_B is felsic, similar to global upper crust, while 5071_1_A is dominantly mafic, and similar to the more depleted estimates of global lower continental crust. There is about a 10-fold difference in radiogenic heat producing and volatile elements between the two drillholes, and highly variable thermal properties. Extrapolating the observed datasets beyond the scale of the drillholes suggests both intrinsic and structural variability caused anisotropy of the continental lower crust.