Grain boundary processes from the deep continental crust to the surface (ICDP-DIVE, Drilling the Ivrea-Verbano Zone)

Fluid transport, precipitation and accompanying mineral reactions along grain boundaries are among the most important processes impacting the rheology of the crust and the formation of mineral deposits. Porosity and permeability that constantly evolve during fluid flow govern major petrophysical properties of the rock. Most commonly, rocks are investigated in two-dimensional sections, where grain boundaries appear as thin lines and the three-dimensional structure cannot be captured. Computed tomography allows for a quantitative assessment of pore space but has a limited resolution. Additionally, it is difficult to assess the origin of pores, which may have been formed primarily in the crust or during near surface weathering or sampling.

In this study, we investigated grain boundaries directly using the “broken surface” technique: A cm-sized rock slice was broken and platinum-coated for scanning electron microscopy. In favorable cases, the rock slice broke along grain boundaries and pre-existing small-scale fractures, exposing these structures directly as true surfaces rather than sectioned traces. The samples investigated are from the ICDP-DIVE drilling project in the Ivrea Verbano Zone (Italy), an exhumed section of the lower continental crust, spanning the range of tens of meters to hundreds of meters of depth below surface; thus offer information about which grain boundary decorations can be clearly related to near-surface alteration. In addition, we compare samples from both amphibolite and granulite facies rocks to explore variations between supposedly fluid-rich and fluid-poor conditions.

Our observations contribute to the understanding of grain boundary processes through a catalogue of different features observed and interpreted, including, among other processes: formation of clays near the surface, sulfide precipitation, quartz recrystallisation and sericitization of feldspars.