Experimental methods for determination of transport properties of magma

Abstract

The experimental techniques for measuring at high temperatures and pressures the transport properties of magma, a concentrated solution of silicates, are at a primitive stage. The fluxes of energy, mass, and momentum are usually interrelated in nature and are difficult to isolate in the laboratory. The measurement of thermal conductivity, a property of magma related to heat transport, is usually obtained by observing the temperature at two points in a cylindrical sample at different distances from a coaxial line heat source. There are no reported measurements of thermal conductivity in molten rocks under pressure. Electrical conductivity is obtained by measuring the flow of electricity between two conductors imbedded in the sample at various stages of melting under pressure. Elastic energy is transmitted and collected by means of transducers attached through suitable refractory extension rods to the ends of a cylindrical specimen held at specified pressures and temperatures to measure seismic wave velocities. The diffusion coefficient, the property of a chemical constituent of magma related to mass transport, has been measured for the self-diffusion of elements in multicomponent liquids, diffusion of elements between dissimilar liquids, diffusion of volatiles into or out of liquids, and isotopic exchange. At least five methods have been used at atmospheric pressure to obtain diffusion coefficients: (1) diffusion couple; (2) thin source; (3) thick source; (4) capillary-reservoir; and (5) bulk gain or loss. The results of only one method, involving a thin source, have been reported for an investigation carried out at a series of high pressures in the melt region. The variation of composition has been determined with measurements of index of refraction, autoradiography, ion and electron microprobe traverses, radioactivity of sections or their successive remainders, mass spectrometric analysis of sections, and weight gain or loss. Viscosity, the property of magma related to momentum transport, has been measured in magmas at high pressure with a sinking- or floating-sphere technique, rotating concentric cylinder, and capillary extrusion. Both viscosity and density, hence the kinematic viscosity, can be determined for the applied conditions from the Stokes relation by varying the density and size of the spheres in the sink-float technique. The Stokes-Einstein relationship between diffusivity and viscosity does not appear to hold for silicate magmas. Only one attempt has been made to study the flow of fluid through porous media, infiltration, at high pressures and temperatures. Convection, which appears to be the major transport process in magma, is amenable to laboratory investigation. Opportunities for research still lie in the investigation of the primary transport properties of magma at high pressures. The coupling of the fluxes will subsequently require evaluation.