Effect of Water on the Sound Velocities of Ringwoodite in the Transition Zone

Abstract

High-pressure elasticity studies will play a central role in efforts to constrain the potential hydration state of the Earth's mantle from seismic observations. Here we report the effects of 1 wt% H 2 O (as structurally bound OH) on the sound velocities and elastic moduli of singlecrystal ringwoodite of Fo 90 composition, thought to be the dominant phase in the deeper part of the transition zone between 520 and 660-km depth. The experiments were made possible through development of a GHz-ultrasonic interferometer used to monitor P and S-wave travel times through micro-samples (30-50 μm thickness) under hydrostatic compression in the diamond-anvil cell. The velocity data to ~10 GPa indicate that hydrous ringwoodite supports 1-2% lower shear-wave velocities than anhydrous ringwoodite at transition zone pressures, though elevated pressure derivatives (K' = 5.3 ± 0.4 and G' = 2.0 ± 0.2) bring calculated hydrous P-velocities close to anhydrous values within their mutual uncertainties above ~12 GPa. Corresponding V P /V S ratios are elevated by ~2.3% and not strongly dependent on pressure. Velocities for hydrous ringwoodite are calculated along a 1673 K adiabat using finite-strain theory and compared with existing data on anhydrous ringwoodite and various radial seismic models. It may be possible to distinguish hydration from temperature anomalies by low S-velocities associated with "normal" P-velocities and accompanying high V P /V S ratios. The presence of a broadened and elevated 410-km discontinuity, together with depressed 660-km discontinuities and intervening low S-wave anomalies along with high V P /V S ratios are the most seismologically diagnostic features of hydration considering the available information from mineral physics.

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

“…[], (12) Fei et al . [], (13) Sinogeikin et al ., [], (14) Jacobsen and Smyth [], (15) Manghnani et al . [], (16) Mao et al .…”

“…[], implying a difference of 20% between K 0 ′ of hydrous wadsleyite and hydrous ringwoodite. Ultrasonic measurements by Jacobsen and Smyth [] reported an extremely elevated K S 0 ′ = 5.3(4) for hydrous Fo 89 ringwoodite (0.89 wt % H 2 O). We have found through comparative compressibility studies that the difference between K 0 ′ of hydrous wadsleyite and hydrous ringwoodite is indistinguishable.…”

Multiple transition zone seismic discontinuities and low velocity layers below western United States

“…; Ricard et al .,; Stixrude and Lithgow‐Bertelloni , ] and seismology [e.g., Shearer , ] indicate that for a pyrolitic bulk mantle composition, the wadsleyite to ringwoodite transition is not associated with an important change of elastic properties (about 5 times smaller than for the 410 and the 660). The detection of a clear interface may also be difficult because of the presence of other transitions (dissolution of pyroxene into garnet slightly above and exsolution of calcium perovskite slightly deeper), of laterally varying basalt fraction [e.g., Xu et al ., ] and water content [e.g., Jacobsen and Smyth , ; Mao et al ., ]. The wadsleyite to ringwoodite transition might contribute, however, to the broad maximum observed between 510 and 550 km depth in Figure a and in Figure a.…”

Oligomerization and carbonization of polycyclic aromatic hydrocarbons at high pressure and temperature

“…These fluids can play a significant role in the modification of mantle physical properties (e.g., viscosity, electrical conductivity, seismic wave velocities, and melting temperature) [1][2][3][4][5][6][7]. The fluids also influence most of the geologic processes in the Earth's mantle such as mantle metasomatism, magma generation, and diamond formation [8][9][10][11][12].…”