Deciphering the petrophysics and petrology of the Ivrea Geophysical Body (Alps) by combining laboratory data and thermodynamic modelling

Geophysical and field data collectively provide insights into properties of the Earth's lithosphere. Linking the two needs up/downscaling across several orders of magnitude, calling for tighter constraints about the actual physical and chemical architecture of the lithosphere. The Ivrea-Verbano Zone (Southern Alps, Italy) represents one of the most complete, time-integrated crust-upper mantle archive that acts as a petro-geophysical benchmark of the Earth's continental lithosphere. Mantle peridotite slivers embedded in lower crustal rocks at the surface and large gravimetric and seismic velocity anomalies of the sub-surface Ivrea geophysical body suggest that dense, "mantle-like" rocks are possibly located as shallow as ~3 km beneath the surface. However, the spatial resolution of the currently available geophysical data does not allow to establish constraints on the chemistry of these dense rocks of the Ivrea body. Here we build on the petrological and geophysical data and present calculations on the density and seismic properties to get tighter constraints on the composition of the Ivrea geophysical body. We combine published laboratory measurements of compressional wave velocities of lower crust and upper mantle rocks and Perple_X thermodynamic modelling to calculate key physical properties (bulk rock compressional wave velocities and density) along a representative vertical profile, and compare them to in situ large-scale geophysical data. Using correlations between seismic velocities and rock densities, we find that primitive gabbros represent the best fit to the anomalies of the Ivrea geophysical body. Intriguingly, hydrous gabbros containing amphibole tend to better fit the constraints detailing the Ivrea geophysical body. Peridotites and pyroxenites containing amphibole and/or serpentine do not satisfy the available constraints.