Specimens of two natural dunites (Anita Bay, of 100 •m grain size, and •heim, of 900 •m grain size) have been deformed in these dunites are of the order of 0.01 wt %, 1000 ø to 1300øC and constant strain rates between amounts much less than most estimates of the water 10 -3 and 10 -6 s -1 at a confining pressure of 300 content of the earth's undepleted •upper mantle. MPa [Chopra and Paterson, 1981•. We report here Such water may have an important role in determining upper mantle flow behavior. in which this water resides in the rocks of the upper mantle is unclear. It ha s been proposed on the one hand that such small amounts of water could be incorporated into nominally anhydrous silicates such as olivine and pyroxene by atomic substitution [e.g., Martin and Donnay, 1972]. On the other hand, Wyllie [1971] suggests the Experimental Procedures New rheological experiments were performed with the gas apparatus and procedures described previously •Paterson, 1970; Chopra and Paterson, 1981]. The apparatus uses argon as the confining medium and incorporates an internal furnace and internal load cell. These features permit the establishment of very low thermal gradients within the specimen region and a high sensitivity in the measurement of applied load. With only a few exceptions, as indicated in the text, the 7861 7862 Chopra and Paterson: Role of Water in Deformation of Dunite experiments reported herein used long sealing jackets of iron to prevent ingress of argon to the specimens [Paterson et al., 1982]. Solid alumina spacers and thin platinum or iron foils were interposed between the specimens and the pistons. The specimens of 10-or 7-mm diameter and 20-mm length were diamond core drilled from blocks of Anita Bay dunite and •heim dunite perpendicular to their respective foliation planes. The cores were subsequently dehydrated by heating them to 1200øC for >60 hours in a controlled oxygen fugacity furnace by using a mixture of C02 and CO gases producing an fo 2 of 10-5 Pa. Following this treatment, the cores and specimen assembly ß ß ß ß ß ß ß ß ß ß Am. Mineral., 59, 81 1-819, 1974. Wyllie, P. J., The D•namic Earth: Textbook in Geosciences,

New microstructural data on experimentally deformed “wet” and “dry” natural olivine rocks (Anita Bay and Åheim dunite), together with the other reliable experimental data, indicate that the experimental stress‐recrystallized grain size relationship in olivine‐rocks is largely independent of water content and temperature, and is only slightly dependent on the flow properties of the material. The experimental data cover a stress range of 30–300 MPa, water contents from <30 ppm to 300 ppm, and temperatures in the range 1100–1650°C. Local melt contents of up to 10 volume% cannot be demonstrated to have a significant effect on the stress—grain size relationship.

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