A New Petrological and Geophysical Investigation of the Present-day Plumbing System of Mt. Vesuvius

A model of the electrical resistivity of Mt. Vesuvius has been elaborated to investigate the present structure of the volcanic edifice. The model is based on electrical conductivity measurements in the laboratory, on geophysical information, in particular, magnetotelluric (MT) data, and on petrological and geochemical constraints. Both 1-D and 3-D simulations explored the effect of depth, volume and resistivity of either one or two reservoirs in the structure. For each configuration tested, modeled MT transfer functions were compared to field transfer functions from field magnetotelluric studies. The field electrical data are reproduced with a shallow and very conductive layer (~0.5km depth, 1.2km thick, 5ohm.m resistive) that most likely corresponds to a saline brine present beneath the volcano. Our results are also compatible with the presence of cooling magma batches at shallow depths (<3-4km depth). The presence of a deeper body at ~8km depth, as suggested by seismic studies, is consistent with the observed field transfer functions if such a body has an electrical resistivity >~100ohm.m. According to a petro-physical conductivity model, such a resistivity value is in agreement either with a low-temperature, crystal-rich magma chamber or with a small quantity of hotter magma interconnected in the resistive surrounding carbonates. However, the low quality of MT field data at long periods prevent from placing strong constraints on a potential deep magma reservoir. A comparison with seismic velocity values tends to support the second hypothesis. Our findings would be consistent with a deep structure (8-10km depth) made of a tephriphonolitic magma at 1000°C, containing 3.5wt%H2O, 30vol.% crystals, and interconnected in carbonates in proportions ~45% melt 55% carbonates.