The role of water in the deformation of dunite

Abstract

Specimens of two natural dunites (Anita Bay, of 100 μm grain size, and Åheirn, of 900 μm grain size) have been deformed in experiments at temperatures between 1200° and 1400°C and constant strain rates between 10 −4 and 10 −6 s −1 in a gas‐medium deformation apparatus at 300 MPa confining pressure. Prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200°C 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. The predried specimens also do not show the apparent grain size dependence seen under wet conditions. Infrared 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 infrared 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 behavior. Stress‐strain curves for all experiments are available with entire manuscript on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009‐ Document B84‐008, $2.50. Payment must accompany order.