High Temperature Flow of Dunite and Peridotite
Deformation experiments on dunite and peridotite, both in the presence and absence of externally released H 2 O, have been carried out in solid pressure media apparatus in the confining pressure range 5 to 30 kb, temperature interval 300°C to 1400 ° C at constant strain rates ranging from 10 −3 to 10 −8 /sec. At the lower temperatures and higher rates, olivine deforms by plastic flow on the system T = (110), t = [001], the slip systems changing with increasing temperature and decreasing strain rate through {0kl} [100] to (010) [100]. At a strain rate of 10 −3 /sec, polygonization (dislocation climb) is first observed at about 1000 ° C and recrystallization first appears at about 1050 ° C; these temperatures decrease by about 50 ° C with a ten-fold reduction in strain rate. Preliminary mechanical results indicate that steady state deformation takes place only in the temperature-strain rate range in which the diffusion controlled processes of polygonization and recrystallization are important or dominant. The steady state data are best fit by a power creep equation with the stress exponent, n ≃ 2.4 and 4.8 and the creep activation energy, Q ≃ 80 and 120 Kcal/mole for deformation in the presence and absence of externally released H 2 O, respectively. Extrapolation of the equation for the dunite deformed dry to a representative geological strain rate of 10 −14 /sec gives shear stresses in the range 100 bars to 1 bar and effective viscosities from 10 23 to 10 20 poise for the temperature interval 1000 ° C to 2000 ° C. The estimated variation in shear stress over most of the upper mantle, where T/Tm (the ratio of ambient to melting temperature) is greater than 0.75, is about 6 to 15 bars, and in viscosity in the range 10 20 to 10 21 poise. These inferences are in accord with estimates based on other geophysical observations, and our results suggest that flow in the upper mantle may be governed primarily by a non-linear creep law.