Comparative compressibility of hydrous wadsleyite and ringwoodite: Effect of H₂O and implications for detecting water in the transition zone

Review of recent mineral physics literature shows consistent trends for the influence of Fe and H₂O on the bulk modulus (K₀) of wadsleyite and ringwoodite, the major phases of Earth's mantle transition zone (410-660 km). However, there is little consensus on the first pressure derivative, K₀′ = (dK/dP)_P=0, which ranges from about 4 to >5 across experimental studies and compositions. Here we demonstrate the importance of K₀′ in evaluating the bulk sound velocity of the transition zone in terms of water content and provide new constraints on the effect of H₂O on K₀′ for wadsleyite and ringwoodite by conducting a comparative compressibility study. In the experiment, multiple crystals of hydrous Fo₉₀ wadsleyite containing 2.0 and 0.25 wt % H₂O were loaded into the same diamond anvil cell, along with hydrous ringwoodite containing 1.4 wt % H₂O. By measuring their pressure-volume evolution simultaneously up to 32 GPa, we constrain the difference in K₀′ independent of the pressure scale, finding that H₂O has no effect on K₀′, whereas the effect of H₂O on K₀ is significant. The fitted K₀′ values of hydrous wadsleyite (0.25 and 2.0 wt % H₂O) and hydrous ringwoodite (1.4 wt % H₂O) examined in this study were found to be identical within uncertainty, with K₀′ ~3.7(2). New secondary-ion mass spectrometry measurements of the H₂O content of these and previously investigated wadsleyite samples shows the bulk modulus of wadsleyite is reduced by 7.0(5) GPa/wt % H₂O, independent of Fe content for upper mantle compositions. Because K₀′ is unaffected by H₂O, the reduction of bulk sound velocity in very hydrous regions of transition zone is expected to be on the order of 1.6%, which is potentially detectible in high-resolution, regional seismology studies.