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. Key-words: haplogranite melt, shear viscosity, complex shear modulus, shear stress relaxation spectrum, internal friction.
Introductionrelaxed loss modulus (Newtonian viscosity × strain rate) to the unrelaxed storage (elastic) modThe nature of viscous deformation in silicate ulus. Many aspects of transport in silicate melts, melts has emerged, in recent years, as a central including Si self diffusivity (Dingwell & Webb, theme, linking the macroscopic transport proper-1990), nonbridging oxygen lifetimes (Liu et ai, ties of silicate melts to the microscopic structure 1988; Farnan & Stebbins, 1990a, b), backreaction of such materials (Sato & Manghnani, 1985; Liu of hydrous species during quenching (Silver, et al. 9 1988; Rivers & Carmichael, 1987; Ding-1988) and the onset of non-Newtonian viscosity well & Webb, 1989McMillan et al, 1992; are usefully scaled by Stebbins et al., 1992). Our current knowledge of such an approximation. This modulus ratio and the spectrum of relaxation processes in silicate the corresponding mechanical model (the Maxmelts is, however, incomplete. We know for ex-well model) remain nevertheless an approximaample that the high-temperature relaxation mode tion of the more detailed picture ...