High Pressure Electrical Conductivity in Naturally Occurring Silicate Liquids
- Robert N. Schock
Published Online: 18 MAR 2013
Copyright 1985 by the American Geophysical Union.
Point Defects in Minerals
How to Cite
Tyburczy, J. A. and Waff, H. S. (1985) High Pressure Electrical Conductivity in Naturally Occurring Silicate Liquids, in Point Defects in Minerals (ed R. N. Schock), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM031p0078
- Published Online: 18 MAR 2013
- Published Print: 1 JAN 1985
Print ISBN: 9780875900568
Online ISBN: 9781118664070
- Mineralogical chemistry—Congresses;
Electrical conductivities of molten Hawaiian rhyodacite and Yellowstone rhyolite obsidian were measured between 1200° C and 1400° C and at pressures up to 25 kilobars. The two melts exhibit similar trends. Arrhenius behavior is observed at all pressures studied. Isobaric activation enthalpies increase from about 0.5 eV at atmospheric pressure to about 0.9 eV at 25 kbars, and the magnitude of the conductivity decreases by about a factor of 4 between 0 and 25 kbar. At pressures between about 10 and 15 kbar an abrupt decrease in the slopes of isothermal log a versus pressure plots is observed. In each pressure range an equation of the form σ = σ′0 exp [− (E′σ + PΔV′σ)/kT], where σ′0, E′σ, and ΔV′σ, are constants, describes the polybaric, polythermal data. Comparison of these data with high pressure electrical conductivities of molten basalt and andesite reveals that relatively silica-rich melts, from andesitic to rhyolitic in composition, display similar trends, while the basaltic melt has analogous, but quantitatively different trends. Comparison of zero-pressure electrical conductivity and sodium diffusivity by means of the Nernst-Einstein relation indicates that sodium ion transport is the dominant mechanism of charge transport in the obsidian melt at zero pressure. The tholeiitic melt, on the other hand, displays only order of magnitude agreement between the electrical conductivity and sodium diffusivity, indicating that either ions other than sodium play a significant role in electrical transport or that the motions of the sodium ions are strongly correlated, or both. Comparison of the isobaric and isochoric activation enthalpies indicates that electrical conduction is energy restrained, as opposed to volume restrained. Conductivities in the andesitic, rhyodacitic, and rhyolitic melts conform to a single compensation law line, with no indication of the change in activation volume. The tholeiitic melt has a slightly different compensation line. In light of Jambon's (1982) semiempirical activation energy theory, this fact indicates that under all conditions studied ionic conduction is due to transport of cations of similar size, and that these conducting ions are small relative to the size of an oxygen anion. The changes in activation volume occur at roughly the same pressures that changes in liquidus mineral assemblage occur. Changes in melt polymerization may or may not be reflected in changes in liquidus phase degree of polymerization.