HIGH-TEMPERATURE RHEOLOGY OF ENSTATITE - IMPLICATIONS FOR CREEP IN THE MANTLE

High-temperature deformation experiments have been performed on oriented single crystals of enstatite at 1 atm under controlled conditions of oxygen fugacity and silica activity. The results indicate that creep on the weakest high temperature slip system in enstatite within the protoenstatite stability field is independent of oxygen fugacity and obeys a power law relation with a stress exponent of n = 3.8 +/- 0.5 and activation energy of Q = 820 +/- 80 kJ/mol. A decrease in creep rate with increasing aluminum and iron content suggests that aluminum or iron ions play an active role in maintaining charge neutrality within the crystals. The single crystal strengths are greater than those reported for polycrystalline enstatite deforming predominantly on the same slip system, suggesting that the polycrystalline samples may have been water weakened. A comparison of the single crystal results with those for olivine deformed at 1 atm indicates that enstatite is probably stronger than olivine under upper mantle conditions, unless the presence of water results in a greater weakening of enstatite than olivine.