Laboratory measurements of electrical conductivities of hydrous and dry Mount Vesuvius melts under pressure

Abstract

[1] Quantitative interpretation of MT anomalies in volcanic regions requires laboratory measurements of electrical conductivities of natural magma compositions. The electrical conductivities of three lava compositions from Mount Vesuvius (Italy) have been measured using an impedance spectrometer. Experiments were conducted on both glasses and melts between 400 and 1300°C, at both ambient pressure in air and high pressures (up to 400 MPa). Both dry and hydrous (up to 5.6 wt % H 2 O) melt compositions were investigated. A change of the conduction mechanism corresponding to the glass transition was systematically observed. The conductivity data were fitted by samplespecific Arrhenius laws on either side of Tg. The electrical conductivity increases with temperature and is higher in the order tephrite, phonotephrite to phonolite. For the three investigated compositions, increasing pressure decreases the conductivity, although the effect of pressure is relatively small. The three investigated compositions have similar activation volumes (DV = 16-24 cm 3 mol À1 ). Increasing the water content of the melt increases the conductivity. Comparison of activation energies (Ea) from conductivity and sodium diffusion and use of the Nernst-Einstein relation allow sodium to be identified as the main charge carrier in our melts and presumably also in the corresponding glasses. Our data and those of previous studies highlight the correlation between the Arrhenius parameters Ea and s 0 . A semiempirical method allowing the determination of the electrical conductivity of natural magmatic liquids is proposed, in which the activation energy is modeled on the basis of the Anderson-Stuart model, s 0 being obtained from the compensation law and DV being fitted from our experimental data. The model enables the electrical conductivity to be calculated for the entire range of melt compositions at Mount Vesuvius and also satisfactorily predicts the electrical response of other melt compositions. Electrical conductivity data for Mount Vesuvius melts and magmas are slightly lower than the electrical anomaly revealed by MT studies.Citation: Pommier, A., F. Gaillard, M. Pichavant, and B. Scaillet (2008), Laboratory measurements of electrical conductivities of hydrous and dry Mount Vesuvius melts under pressure,

Geophysical and petrological constraints on ocean plate dynamics

“…Melt present in the upper asthenosphere would provide a sharp decrease in mantle seismic velocity and viscosity at the LAB, effectively decoupling the lithosphere and asthenosphere. While various melts are thought to be stable within the LVZ in the upper asthenosphere [Gaillard et al, 2008;Hirschmann, 2010;Ni et al, 2011], it is unclear if enough melt is present and interconnected to generate the necessary decreases in velocity and viscosity [Takei, 2002].…”

“…Partial melts of mantle materials have conductivities in the range of 1-10 S/m, substantially higher than anhydrous subsolidus olivine, although the conductivity of a particular melt depends on composition (H2O and CO2 in particular) [Yoshino et al, 2012;Sifre et al, 2014], temperature, and pressure [Roberts and Tyburczy, 1999;Toffelmier and Tyburczy, 2007;Yoshino et al, 2012;Sifre et al, 2014]. For example, at constant pressure the addition of H2O to melt promotes an increase in conductivity [e.g., Ni et al, 2011;Pommier et al, 2008].…”

“…Silicate, carbonated silicate, carbonatite melts [Hirschmann, 2010;Gaillard et al, 2008], and hydrous silicate melts [Ni et al, 2011] could be stable in the LVZ depending on the pressure and temperature. Estimated melt fractions range from 0.05-0.1% if the melt is carbonatite [Gaillard et al, 2008;Yoshino et al, 2010;, ~0.3% if carbonated silicate melt [Hirschmann, 2010], and 0.3-1% if hydrous silicate melt [Ni et al, 2011]. At higher melt fraction the melt interconnectivity increases, which in combination with the melt buoyancy can provide pathways for melt to migrate [Karato, 2012].…”

“…Because of their high conductivity, volume fractions as small as 0.01% have the potential to impact bulk conductivity at measurable levels, and carbonate melt is interconnected to fractions as low as 0.05wt% [Minarik and Watson, 1995]. Silicate, carbonated silicate, carbonatite melts [Hirschmann, 2010;Gaillard et al, 2008], and hydrous silicate melts [Ni et al, 2011] could be stable in the LVZ depending on the pressure and temperature. Estimated melt fractions range from 0.05-0.1% if the melt is carbonatite [Gaillard et al, 2008;Yoshino et al, 2010;, ~0.3% if carbonated silicate melt [Hirschmann, 2010], and 0.3-1% if hydrous silicate melt [Ni et al, 2011].…”

“…Carbonatite melts are at least 2 orders of magnitude more conductive than silicate melts [Gaillard et al, 2008;Yoshino et al, 2010]. Carbonatite melts can form in trace amounts deep in the mantle [Dasgupta and Hirschmann, 2006] and are thought to be stable in the upper asthenosphere beneath seafloor older than about 45 Ma [Hirschmann, 2010].…”