Flow behavior of fine-grained synthetic dunite in the presence of 0.5 wt% H2O

The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% e nstatite) with a grain size of similar to 1 mu m has been investigated in c ombined constant displacement rate and temperature stepping experiments at temperatures of 850 degrees to 1000 degrees C, strain rates of 10(-7) to 10 (-5) s(-1) and a confining pressure of 600 MPa. Nominally dry material beha ved purely elastically. In contrast, samples with 0.5 wt% added water flowe d at stresses between 9 and 81 MPa reaching strains up to 12%. Fitting a Do rn-type power law to the wet data yielded stress exponents (n) of 1.7 +/- 0 .4 and apparent activation energies in the range 302 +/- 22 kJ mol(-1). The wet-deformed samples showed polygonal grains, no crystallographic preferre d orientation, no subgrains, low dislocation densities, and evidence for gr ain boundary cavitation. Grain growth was miller. It is concluded that the wet samples probably deformed by a grain size sensitive, grain boundary sli ding dominated mechanism, though the nature of the accommodation process re mains unclear, and the possibility of transitional behavior between disloca tion creep and diffusion creep cannot be eliminated. Application of the res ults to upper mantle shear zones supports previous speculation that the pre sence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere.