Laboratory Study Of Electrical Conductivity Of Shoshonitic Melts

In order to better assess the likelihood of volcanic eruptions which greatly threaten the safety of human being and environment,it is crucial to unravel the status of the subvolcanic magma reservoir.Geophysical surveys and petrological researches can provide us with some physicochemical parameters(e.g.depth,temperature and volatile content)and electrical structure of magma reservoirs.Electrical conductivity is an important physical property of silicate melt,which depends on temperature,pressure and melt composition.Recently,in-situ electrical conductivity measurements on silicate melts have provided important data for interpreting geophysical results and grasping the conditions of magma reservoirs.A majority of previous electrical conductivity studies focused on felsic to mafic calc-alkaline melts,with little attention paid to alkali-rich silicate melts,especially to K-rich melts.With impedance analysis,this study measured the electrical conductivity of shoshonitic melt with H2O up to 2.59 wt%at 1073-1823 K and 0.5-1.8 GPa in a piston cylinder apparatus.Similar to other silicate melts,the electrical conductivity of shoshonitic melt increases with increasing temperature and H2O content but decrease with pressure.But the activation volume of the shoshonitic melt,which characterizes the pressure effect,is at the lower end of the previously reported values for other silicate melts,while the temperature and H2O effects are among the strongest.Based on the experimental data,we established a quantitative electrical conductivity model accounting for temperature,pressure and H2O content:log?=4.76-0.67w-6763.0-1565.2w+294.7P/T where ? is electrical conductivity in S/m,w is water content in wt%,P is pressure in GPa,and T is temperature in K.The strong temperature and H2O effects are related to the contributions of cations(K+,Mg2+and Ca2+)other than Na+ to the electrical conductivity of shoshonitic melt,but OH-is still not a main charge carrier.The addition of H2O mainly decreases the melt viscosity and increases the ionic mobility to enhance melt electrical conductivity.The electrical conductivity model is used to interpret the MT results from the Wudalianchi volcanic field in northeast China.We infer that the magma reservoir beneath the Huoshaoshan volcano contains 2-17 vol%shoshonitic melt and that beneath the Weishan volcano is 12 vol%.