Carbonated ultramafic rocks in the Balmuccia layered series, Ivrea-Verbano Zone, Italy

The Ivrea-Verbano Zone (IVZ) in the western Southern Alps offers a unique opportunity to study an exhumed, almost complete section of continental crust, including a Lower Permian transcrustal magmatic system. Among these features is the Layered Series, a sequence of ultramafic and mafic cumulates located between the localities of Balmuccia and Vocca in the Sesia Valley. These rocks formed during the early stages of magmatic underplating in the Lower Permian. Here we report previously undocumented occurrences of carbonated peridotites and pyroxenites in this area.

High resolution drone mapping and field observations reveal carbonation zones associated with fault-controlled fluid flow. Petrographic analysis shows the formation of serpentine, talc and carbonate minerals (magnesite, siderite and calcite) replacing olivine and pyroxene. In peridotites, reaction fronts between replacing phases and olivine and pyroxenes show sharp, well-defined boundaries, whereas in pyroxenites they show more gradual transitions, indicating differences in fluid reactivity between lithologies. The rocks are also characterized by volume expansion, which seems to have induced fracturing and facilitated further fluid-rock interactions, possibly creating a feedback loop that promotes alteration. Furthermore, the presence of talc in meter scale fault zones may affect their mechanics by promoting aseismic slip.

Serpentinization and carbonation could have taken place either during (a) Jurassic rifting, (b) final exhumation related to tectonic activity along the Insubric line, or (c) recent, near surface alteration. Detailed mineralogical and microstructural studies are underway to quantify the origin, timing and evolution of the CO₂-rich fluids and the temperature/pressure conditions under which these transformations occurred.