Some New Rheological Experiments on Igneous Rocks at Temperatures up to 1120°C

Summary The stress‐strain relations and creep behaviour of a dolerite, a microgranodiorite, and a dunite have been determined in uniaxial compression at temperatures up to 1120°C. All three rocks remained brittle at temperatures up to 1050°C, with approximately linear stress‐strain relations and small fracture strains. Creep tests were carried out at stresses (< 10 8 Nm ‐2 )* which were low compared to the uniaxial compressive strength at the same temperature. At these stresses and in experiments with a duration of 1 d only transient creep was observed, with the creep strain proportional to a fractional power of the time (Andrade creep), as was observed in earlier experiments at lower temperatures, and the creep rate decreased with the elapse of time. The apparent viscosity, therefore, increases with time, having a value in the range 10 12 ‐ 10 15 Nsm ‐2 after times of 10 3 s and 10 15 ‐10 20 Nsm –2 after times of ˜ 3y (by extrapolation)†. The activation energy for creep at a low stress of 130 X 10 5 Nm ‐2 determined for microgranodiorite at temperatures greater than 800°C and for dolerite at temperatures greater than 940°C indicated that the creep was probably diffusion controlled. Using a theory due to Mott, the steady‐state creep rate and the corresponding equivalent viscosity were calculated for the conditions of the experiments. The viscosities for pressures on the geotherms of Ringwood were then calculated, by making use of the proportionality between activation energy for diffusion and the absolute melting temperature. For stresses in the range 10 6 – 10 8 Nm ‐2 dolerite has a viscosity of 10 19 ‐10 21 Nsm ‐2 at at depths of 60–80 kin in oceanic areas, and at depths greater than 120 km in shield areas; and microgranodiorite at the same stresses has a similar viscosity at depths in the range 80–130 km in shield areas. These estimates are in satisfactory agreement with present concepts of the lithosphere. At temperatures above 1050°C the dolerite and microgranodiorite exhibited partial melting. The dolerite remained brittle, with the partial melt being extruded from microfractures, but the microgranodiorite showed some ductility. We can postulate that even rocks deep in the mantle may be brittle if their temperatures are not more than ˜1000°C. A discussion is given of the apparent conflict between the experimental observation of hot brittleness and the geological observation that even some comparatively cool silicate rocks have apparently deformed in a ductile manner.