High temperature experiments on the elastic and anelastic behaviour of magmatic rocks

Details High temperature experiments on the elastic and anelastic behaviour of magmatic rocks High temperature experiments on the elastic and anelastic behaviour of magmatic rocks

Nov 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985pepi...40..223k&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 40, Issue 3, p. 223-247.

Physics

56

Scientific paper

Measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks (dunite, peridotite, gabbro, and basalts) and a synthetic forsterite at temperatures between 600 and 1500°C (depending on the sample material) and in the frequency range from 0.003 to 30 Hz under forced torsional oscillations. The maximum shear stress was 0.3 MPa and the strains were of the order 5 . 10-6 to 5 . 10-5. Measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting.
The absolute values of the shear modulus below Tm are comparable with the results of other authors. The onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity. At high temperatures (T > 0.8 . Tm) the absorption factor Q-1 follows a power law Q-1∈ω-α exp(-A'/RT) with α = 0.15-0.30 and A' between 100 and 200 kJ mol-1. Towards lower temperatures this ``High Temperature Background'' is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature. This peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time τ ~ exp(A/RT) with A ~ 400-600 kJ mol-1. Above Tm a third contribution to Q-1 is observed that can be regarded as independent of frequency between 1 and 30 Hz.
This last component which contributes < 50% to the total absorption is attributed to the melt phase in the samples. An explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines. By some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features. At very high temperatures and low frequencies a small influence of steady state flow on Q-1 is observed and below T = 0.6 Tm thermal cracking or thermoelastic losses may contribute to Q-1.
The influence of lithostatic pressure on Q-1 is estimated and the extrapolation of the experimental data to the p, T conditions of the uppermost mantle are in good agreement with published Earth models. An attempt is made to establish a general Q-T relationship from the experimental data.

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