Effect of water on the spinel-postspinel transformation in Mg2SiO4

The phase boundary between ringwoodite and perovskite plus periclase in Mg2 SiO4 was determined at 1600 degreesC under hydrous conditions with 1 simila r to3 wt% H2O. We found that the boundary moved to high-pressure side by si milar to0.2 GPa in these hydrous conditions compared with that in anhydrous condition. The SIMS measurements confirmed that the ringwoodite in hydrous condition contained a significant amount (similar to1 wt%) of water, where as the perovskite accepted only very little (similar to0.05 wt%) water in i ts crystal structure. The discrepancy between the pressure of 660 kin seism ic discontinuity and that of the postspinel transformation reported in a re cent in situ x-ray study is not explained by the effect of water on the tra nsformation.