Quantitative interpretation of MT anomalies in volcanic regions requires laboratory measurements of electrical conductivities of natural magma compositions. The study was applied to Mt. Vesuvius (Italy), a dangerous strato- volcano surrounded by a densely populated area, characterised by several violent eruptions and dormant since 1944. Geophysical measurements (MT as well as seismic measurements) revealed an anomaly located at a depth of 5-8 km and interpreted as a magma reservoir. We present here laboratory measurements of magma electrical conductivity allowing a discussion on the nature of the geophysical anomaly and the possibility of magma storage below Mt. Vesuvius. Experiments were performed on three compositions of eruptive products of Mt. Vesuvius, representative of the entire spectrum of differentiation: a tephrite from VIIIth century's eruption ; a phonotephrite from the eruption of Pollena (472AD) and a phonolite from the eruption of Pompei (79AD), from the less to the most evolved product. Conductivity experiments were conducted on melts using an internally heated pressure vessel connected to an impedance spectrometer. The P-T conditions overlap magma storage conditions determined by independent petrologic studies. Dry and hydrous (up to 5.6 wt% H2O) conditions were investigated. The effect of P, T, water content and composition was discriminated for this database. An empirical equation was established to calculate the electrical conductivity of Mt. Vesuvius melts ranging from tephritic to phonolitic. The modified Archie's law permitted the application of this equation to magmas. Considering pre-eruptive conditions determined by independent phase relation studies, we discuss the nature of the electrical anomaly revealed by MT data at 5-8km depth. A scenario similar to one of the three eruptions studied here is unlikely, as their electrical responses do not match. However, the present electrical anomaly is close to the one expected from a phonotephritic magma with high solid fraction. Thus, the electrical resistivities measured in MT studies would be compatible with a low temperature crystal rich magmatic system, consistent with the interpretations of seismic tomography data.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 8439 Physics and chemistry of magma bodies;
- 8445 Experimental volcanism