The role of water in the deformation of dunite.

Abstract

Specimens of two natural dunites (Anita Bay, of 100 mu m grain size, and Aheim, of 900 mu m grain size) have been deformed in experiments at 1200-1400oC and constant strain rates between 10-4 and 10-6/s in a gas-medium deformation apparatus at 300 MPa confining P. Prior to these experiments the specimens were given a heat treatment in a controlled fO2 furnace at 1200oC for >60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water. The predried specimens are much stronger than specimens tested under wet conditions in an earlier study. IR absorption measurements on the present specimens and those of the earlier study indicate that for a given rock, flow stress can be correlated with the intensity of a residual 'water-related' broad absorption. The addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband IR component. The weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water-related species in the grain boundaries, but intragranular weakening may also be contributing. It is suggested that the concentration of the water-related species in the grain boundaries is governed by its concentration in the small melt fraction present. The amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0.01 wt.%, amounts much less than most estimates of the water content of the Earth's undepleted upper mantle. Such water may have an important role in determining upper mantle flow behaviour. (Authors' abstract)-A.W.H. Show more