الملخص الإنجليزي
Torsion oscillatory deformation experiments have been performed at high temperatures (600-1170°C) and over a wide range of low frequencies (20-2.10-3 Hz) on fine-grained gabbronorite samples from the Oman ophiolite in order to determine the shear wave attenuation as a function of temperature and melt fraction. The specimens have a small and uniform grain size (0.25-0.3 mm) and do not contain secondary, hydrated minerals. Measurements of internal friction (Q-1) were performed using a forced oscillatory torsion apparatus at small strains (∼10-7), and with increasing small temperature steps to reduce thermal microcracking. The general dependence of Q-1 to frequency is Q-1 α ω-α, where ω is the angular velocity of forced oscillations and α is an empirical exponent. Below the melting temperature (∼1050°C), α has average values of ∼0.15 at low frequency (≤0.5 Hz) and 0.06 at higher frequency. Above the melting temperature, α has average values of ∼0.22 at low frequency and -0.02 at higher frequency. This frequency dependence of Q-1 is attributed to a viscoelastic behaviour due to the diffusion controlled grain boundary sliding, and partially to the squirt flow of the melt-phase wetting grain boundaries. The onset of melting is associated with a markedly higher Q-1 and a stronger dependence of -1 on temperature. The melt-related mechanical dissipation process could be a melt squirt flow. The characteristic frequency for the melt squirt flow is ωm ∼ 0.15-300 Hz when the melt pocket aspect ratio is ∼ 10-3-10-2. Around the melting temperature the internal friction can be approximated by an experimental power law Q-1 = A· [ω-1· d-1 · exp(-Ea/RT)]α with α ∼0.08, A = 34.72 s-α μm-α and Ea ∼ 873 kJ mol-1.
