Effect of boron, phosphorus and fluorine on shear stress relaxation in haplogranite melts
Bagdassarov, Nikolai SH.; Dingwell, Donald B.; Webb, Sharon L.
European Journal of Mineralogy Volume 5 Number 3 (1993), p. 409 - 426
published: Jun 14, 1993
manuscript accepted: Feb 2, 1993
manuscript received: Aug 17, 1992
ArtNo. ESP147050503001, Price: 29.00 €
Abstract Shear stress relaxation in haplogranite melts has been investigated using torsional deformation. Sinusoidal shear strains of small amplitude (10-3-10-5) have been generated over 4 orders of magnitude of strain rate and a wide range of temperatures. The viscoelastic behavior of the investigated melts can be characterized in terms of the complex shear modulus, the complex shear viscosity, and the internal friction. With addition of B, P, F to a haplogranite melt, the relaxation spectrum becomes broader, skewing further towards shorter relaxation times. Thus, the solution of volatiles in highly polymerized melts leads to a broadening, in frequency-temperature space, of the viscoelastic region which separates liquid behavior from glassy behavior. The relaxation spectrum of pure haplogranite melt has an asymmetrical form which can be fitted by a stretched exponent with parameter ß ~0.5. The boron-containing melts are characterized by ß~0.4, the phosphorus- and fluorine-containing melts yield ß < 0.4. The unrelaxed shear modulus of the liquids obtained at high frequencies and low temperatures are in agreement with the results of new high-frequency (20 MHz) ultrasonic measurements performed at room temperature. The additions of boron, phosphorus, and fluorine all result in decreases in the unrelaxed shear modulus. The relaxed (or Newtonian) shear viscosity obtained from this study at low frequencies and high temperatures compares well with the data obtained by micropenetration viscometry on the same samples. The present low-temperature viscosity data together with high-temperature concentric cylinder viscometry measurements describe an Arrhenian relationship for all investigated compositions in the temperature range of 650 -1650 °C. The activation energy of viscous flow decreases with B, P and F content.