Effect of water on compressibility of San Carlos olivine and its implications

Water has been proved playing an important role in effecting physical properties of minerals. Previous studies have demonstrated that incorporating small amounts of water significantly reduce bulk moduli of wadsleyite and ringwoodite by 10% and 16% respectively (Yusa and Inoue, 1997; Inoue et al. 1998). Although olivine takes much smaller amount of water (10 times less) than wadsleyite and ringwoodite, rheology experiments have revealed that olivine is considerably weakened by water (Chen et al. 1998). Here we report a comparative study of effect of water on compressibility of San Carlos olivine, indicating a 5% reduction in bulk modulus. Hydrous olivine sample is prepared by annealing San Carlos olivine in presence of water at 12.5GPa and 1200°C for an hour. Water content in the hydrous sample is about 0.1wt%, estimated according to water solubility in olivine at such P-T condition (Kohlstedt, Keppler and Rubie, 1996). Two small pieces of pressed hydrous and anhydrous sample are loaded in the same diamond anvil chamber; Ruby chips are placed between the two samples as a pressure marker. The specimens are compressed in solution of methanol and ethanol (4:1) pressure medium up to 11 GPa. Energy dispersive x-ray diffraction is conducted at the X17C beamline of the National Synchrotron Light Source. The diffraction data are processed using full spectrum Le Bail refinement method to derive accurate volumes of olivine. Fitting the experimental data to 3rd-order Birch-Murnaghan equation of state with fixed K0' = 4.29 (Abramson et al. 1997) yield bulk moduli of 130 ± 4 GPa and 123 ± 4 GPa for anhydrous and hydrous samples respectively. This result demonstrates that 0.1wt% of water reduces the bulk modulus of San Carlos olivine by 5%. As the hydrous sample is at water saturated condition, the 5% reduction represents the upper limit of water effect on the bulk modulus of olivine. Taking the effect of water on bulk moduli of both olivine and wadsleyite into account, if water presents at the boundary between upper mantle and transition zone, the amplitude of 410 km seismic discontinuity will be reduced, and therefore the mineral physics composition model will better agree with seismic observations - another evidence of presence of water in the mantle.