ULTRASONIC STUDIES OF SILICATE LIQUIDS (COMPRESSIBILITY, MELTING, BULK MODULUS, DENSITY)

The ultrasonic velocity and absorption of 32 silicate melts have been determined as a function of temperature (1175-1925 K) and frequency (3-12 MHz). The samples include 14 oxide components and range from simple alkali silicates to multicomponent synthetic and natural melts. An isothermal algorithm for the compositional dependence of the ultrasonic velocity: c = (SIGMA) x(,i) (.) c(,i) where x(,i) is the mole fraction of component i and c(,i) is independent of composition, is capable of predicting relaxed melt velocities to within 3%. The temperature dependence of the sound speeds in the non-dispersive region is quite small, (DBLTURN)1 in 10('-4) K('-1). The contributions of the changes in sound speed and density with temperature are nearly the same in their influence on the temperature dependence of the bulk modulus of natural melts.;The measured zero pressure ultrasonic velocities are combined with high pressure melting curve and falling sphere experiments to estimate the effect of pressure on the volume of silicate melts. Values of K(,0)', the pressure derivative of the bulk modulus, are derived. The fusion curves of aluminum free silicates (fayalite, diopside, and pseuodo-wollastonite) can be fit well using the zero pressure thermodynamic properties and the Birch-Murnaghan equation of state with large values of K(,0)', greater than 9. The fusion curves of albite, and possibly anorthite and sanidine, require the melt densities to increase more rapidly below 1 GPa than the ultrasonic data would predict.;Measurements were made with a novel variable path length acoustic interferometer, which employs a molybdenum buffer rod and a single ultrasonic transducer. The theoretical response of the interferometer for fluids of arbitrary acoustic impedance is derived. The precision and accuracy of the ultrasonic velocity measurements range from (+OR-)0.3% for liquids with Q values greater than about 20, to 1-2% for highly attenuating samples. Attenuation determinations, expressed as (alpha)/f('2), have a precision and accuracy of 0.5 - 2 x10('-12) s('2) m('-1).