The effect of water on high‐temperature deformation in olivine

Abstract

Numerous natural single crystal specimens of olivine were examined to assess the levels of intrinsic water (hydrogen) concentration and to measure their plastic deformation characteristics within controlled environments in constant strain‐rate experiments. Our efforts to characterize the "water" content of the natural specimens, as well as to artificially "wet" single crystals, have indicated that the olivine structure can retain "water" in very small amounts only (a maximum of 55 H/106 Si was observed). Moreover, compressional creep deformation experiments at differential stresses between 10 and 450 bars, with strain‐rates from 10−6 to 10−5 sec−1 under zero confining pressure, and at a temperature of 1575°C, demonstrate no loss in strength for single crystal specimens deformed in "wet", relative to "dry", experimental environments. Our "wet" environments were obtained from an H2O‐H2 gas mixture with controlled oxygen fugacity within the olivine stability region, whereas "dry" conditions were obtained by using a CO2‐CO gas flow. The results of this study, as well as those from other recent investigations, suggest strongly that the "water weakening" effect, which has been observed in natural olivine and dunite, is most likely due to the increase of oxygen partial pressure resulting from the high temperature decomposition of H2O, rather than the hydrolysis of crystal bonds.