Shear modulus and Q of forsterite and dunite near partial melting from forced-oscillation experiments |
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| Authors: | H Berckhemer W Kampfmann E Aulbach H Schmeling |
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| Institution: | Institut für Meteorologie und Geophysik, Frankfurt University, Feldbergst. 47, 6 Frankfurt a.M. F.R.G. |
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| Abstract: | An apparatus designed to determine the complex shear modulus of rock samples by forced torsion oscillations at high temperature and in the seismic frequency band 0.003–30 Hz is briefly described. Measurements were performed on natural dunite from Åheim (Norway) up to 1400°C and on polycrystalline forsterite up to 1500°C at 1 atm pressure. The two materials were chosen to study, by comparison, the effect of melt on the elasticity and anelasticity of mantle rocks.Between 1000 and 1200°C the absolute values of the shear modulus G are almost equal for both materials. Above 1200°C G for natural dunite decreases progressively with temperature and at 1400°C and 1 Hz reaches of its value at 1100°C. In contrast, G of pure forsterite depends little on temperature. For petrological reasons, supported by simultaneous measurements of the electric resistivity, there is strong evidence that the decrease of G in dunite above 1200°C is due to melt from the lower melting components of the dunite. Based on different models estimates of the melt fraction are made.At high temperature, in both materials Q?1 is characterized by a monotonic decrease with frequency according to ω?α, with α ≈ 0.25. An apparent activation energy of 38±5 kcal mol?1 for forsterite and 48±8 kcal mol?1 for dunite was found with no significant change in the regime of partial melting. From this it is concluded that Q?1, even at partial melting, is dominated by solid state high temperature background absorption. There is no indication from these experiments for a constant-Q-band at low seismic frequencies or an increase of Q proportional to frequency as suggested by some seismologists. The present results are in good qualitative agreement with those for Young's modulus obtained previously by strain retardation experiments. |
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