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3.9 b. Viscoelastic behavior of partially molten granites (N.S. Bagdassarov and A. Dorfman)

Effects of partial melting on the shear moduli and internal friction of granites have been investigated using a torsion deformation apparatus in the frequency range 5 mHz - 20 Hz at 650° - 1250 °C. Granite samples from Kirchberg/W-Erzgebirge, Germany (OG3) and a fine-grain quartz-feldspar porphyry from Åland/S.W.-Finland (QFP) have been annealed for ~ 30 h at temperature 1175° - 1150 °C, 1 bar and log[fo2] ≈ -11. In OG3 samples the crystal volume percent (φ ) = 40 ± 2, and in QFP samples φ = 55 - 60 of predominantly quartz crystals. Two different viscoelastic behaviours have been observed: (1) melt dominated viscoelasticity below a critical concentration of crystals (φ crit ≈ 40) for OG3 samples and weak elastic behaviour caused by mechanical interaction between crystals above a critical concentration of them (φ = 55 - 60) in QPF samples. For OG3 samples, Newtonian relaxed viscosity is observable at high temperature/low frequency conditions. For QFP samples the relaxed viscosity is unobservable, in both torsional deformation or by dilatometry. The internal friction log[Q-1(ωτ )] for partially molten samples plotted on a double-log plot vs the dimensionless variable ωτ (the product of the experimental frequency and the relaxation time) has two different slope values above and below ωτ = 1, respectively. For OG3 and QFP samples at ωτ >> 1, the slope of log[Q-1(ωτ )] is about -0.5 and does not depend on the volume fraction of crystals. In the OG3 sample at ωτ <<1, this slope is - 0.45. In QFP samples, the slope is ~ 0 and the internal friction is practically independent of ωτ in the high temperature-low frequency limit, Q-1(ωτ ⇒ > 0) ≈ 1. The shape of the Cole-Cole diagrams (G"/G∞ vs. G´/G∞ , where G`, G" and G∞ are the real, imaginary and high frequency shear moduli) changes with the increase of crystal content in partially molten granites. In OG3 samples where the crystal content lies below the rheological critical concentration, the shape is asymmetrical and is related to a ß -relaxation law (Kohlrausch stretched exponential function) of shear stresses with ß ~ 0.5. In QFP samples with crystal contents above the rheological critical value, the mechanical contacts between crystals determine the elastic behaviour of the suspension and the Cole-Cole diagram is symmetrical and described by a Caputo body stress relaxation function. The imaginary component of shear modulus G" for the OG3 sample is asymmetrical with extended shoulder when ωτ >>1. In contrast, for the QFP sample, G" has a symmetrical shape.

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