The Importance of Chemical Compostion on the Electrical Conductivity of Silicate Melts

The electrical conductivity of geomaterials can vary by several orders of magnitude over small changes in temperature and/or chemical composition. The variation in electrical conductivity of Earth's mantle is largely dependent on temperature as it increases from approximately 0.01 S/m in the upper mantle to 1 S/m in the shallow depths of the warmer lower mantle. Activation energies for electrical conduction of iron-bearing silicate minerals are on the order of 1 eV and depend mostly on Fe and H2O contents. Over a more restricted temperature range, the importance of chemical composition on the conductivity of natural silicate melts is much more evident. From a database of several experimental data we have generated an empirical model for the electrical conductivity of natural silicate melts as a function of chemical composition and temperature. The data include those obtained in our laboratory and others taken from the literature. All melts are anhydrous and we take into account seven major chemical components (SiO2, Al2O3, Na2O, K2O, MgO, CaO, and Fetot) in addition to temperature. Various forms of equations are utilized (Arrhenian-based, VTF-based) with the aim of minimizing both standard error and number of adjustable parameters. In some cases, the resulting fit parameters show strong correlations to other properties (e.g. cation field strength) of their respective chemical components. We also address partially molten systems and how both melt composition and volume fraction affect bulk conductivity. Interestingly, in a basaltic system melt fraction appears to be more important than melt composition in affecting bulk conductivity. However, in more alkali-rich systems, melt composition plays a much more important role and the variation of electrical conductivity with increased melting can be complex. These results can be potentially useful for locating and estimating the size of magmatic reservoirs in regions where magnetotelluric data are available.