A new petrological and geophysical investigation of the present‐day plumbing system of Mount Vesuvius

Abstract

[1] 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 (∼0.5 km depth, 1.2 km thick, 5 ohm.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 (<3-4 km depth). The presence of a deeper body at ∼8 km depth, as suggested by seismic studies, is consistent with the observed field transfer functions if such a body has an electrical resistivity > ∼100 ohm.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 (8-10 km depth) made of a tephriphonolitic magma at 1000°C, containing 3.5 wt%H 2 O, 30 vol.% crystals, and interconnected in carbonates in proportions ∼45% melt −55% carbonates.

Prediction of silicate melt viscosity from electrical conductivity: A model and its geophysical implications

“…As a consequence, typical MT measurements would be hard pressed to discriminate between these models, even if they imply strong chemical variations in melt composition. This statement is in agreement with previous findings by Pommier et al [2010], who performed 3-D forward modeling considering different magma reservoirs (chamber volume up to 1000 km 3 and magma resistivity varying from 2 to 2000 ohm-m) and showed that the electrical response was only slightly affected by magma conductivity.…”

“…Hypothetical conductivity properties of a volcanic edifice [e.g., Pommier et al ., ] are presented in Table and highlight the fairly small range of conductivity variations expected. The models consider three types of melt at different temperatures, including: (1) a basaltic andesite, (2) a rhyolitic melt, and (3) a hybrid melt resulting from mixing.…”

“…This statement is in agreement with previous findings by Pommier et al . [], who performed 3‐D forward modeling considering different magma reservoirs (chamber volume up to 1000 km 3 and magma resistivity varying from 2 to 2000 ohm‐m) and showed that the electrical response was only slightly affected by magma conductivity.…”

“…Each scenario is expressed as a hypothetical conductivity structure, to which we apply a forward modeling to simulate the corresponding field electrical response, and therefore estimate the magnitude of the conductivity anomaly. [15] Hypothetical conductivity properties of a volcanic edifice [e.g., Pommier et al, 2010] are presented in Table 1 and highlight the fairly small range of conductivity variations expected. The models consider three types of melt at different temperatures, including: (1) a basaltic andesite, (2) a rhyolitic melt, and (3) a hybrid melt resulting from mixing.…”

Where is the Hot Rock and Where is the Ground Water – Using CSAMT to Map Beneath and Around Mount St. Helens

“…The strong conductor in this figure appears to be two conductors, but this is likely an artifact caused by the 3-D nature of the dome. The conductor likely represents hot brine near the boundary of still-hot dacite intrusive rock (Palacky, 1989;Pommier et al, 2010). This finding supports a water-modulated conductiveconvective process for cooling of the intrusive body over time, a process actually measured for the 1980-86 dome by Dzurisin et al (1990).…”