Asami Sano

Influence of Water on Major Phase Transitions in the Earth's Mantle

In this chapter we summarize recent results on the influence of water on major phase transformations in the Earths mantle with implications for seismic discontinuity structure and mantle dynamics. The experimental data are based on quench multianvil and in situ X-ray diffraction studies. Differences in water solubility between olivine and wadsleyite, and between ringwoodite and Mg-perovskite + ferropericlase may displace the phase transition boundaries, which are responsible for the 410- and 660-km discontinuities, respectively. The results show that water expands the stability field of wadsleyite to lower pressures, which is consistent with broadening of the 410-km discontinuity in some regions of the mantle. A significant shift of the wadsleyite-ringwoodite phase transition to higher pressure caused by water may also be responsible for depth variations or absence of the 520-km discontinuity. Study of the post-spinel transformation in hydrous pyrolite indicates that the phase boundary also shifts to higher pressures. Displacement of this boundary with ∼2 wt.% H 2 O corresponds to about 15 km at 1473 K. Thus, presence of water could account for half of the observed 30-40 km depressions at the 660-km discontinuity in subduction zones at this temperature. Study of the post-garnet transformation in anhydrous and hydrous MORB show that this phase boundary shifts to the lower pressures by ∼2 GPa with the addition of 2-5 wt.% water. This observation demonstrates that the density crossover between peridotite and basaltic components near 660 km might be absent under hydrous conditions, inhibiting the separation of these components at the 660-km discontinuity.

Metastable garnet in oceanic crust at the top of the lower mantle

As oceanic tectonic plates descend into the Earths lower mantle, garnet (in the basaltic crust) and silicate spinel (in the underlying peridotite layer) each decompose to form silicate perovskite—the ‘post-garnet’ and ‘post-spinel’ transformations, respectively. Recent phase equilibrium studies have shown that the post-garnet transformation occurs in the shallow lower mantle in a cold slab, rather than at ∼800 km depth as earlier studies indicated, with the implication that the subducted basaltic crust is unlikely to become buoyant enough to delaminate as it enters the lower mantle. But here we report results of a kinetic study of the post-garnet transformation, obtained from in situ X-ray observations using sintered diamond anvils, which show that the kinetics of the post-garnet transformation are significantly slower than for the post-spinel transformation. Although metastable spinel quickly breaks down at a temperature of 1,000 K, we estimate that metastable garnet should survive of the order of 10 Myr even at 1,600 K. Accordingly, the expectation of where the subducted oceanic crust would be buoyant spans a much wider depth range at the top of the lower mantle, when transformation kinetics are taken into account.

Redetermination of the high-pressure modification of AlOOH from single-crystal synchrotron data

The single-crystal synchrotron study of the high-pressure modification of aluminium oxide hydroxide, δ-AlOOH, confirms the previous structure determination in the space group P21nm, which was based on X-ray powder data [Suzuki, Ohtani & Kamada (2000). Phys. Chem. Miner. 27, 689–693]. The present study includes the determination of the H-atom parameters, which revealed a strong asymmetric hydrogen bond with an O⋯O distance of 2.5479 (12) A. The δ-AlOOH structure is isotypic with that of β-CrOOH and may be considered as a distorted rutile type with all atoms located on mirror planes.

Crystal structures of high-pressure phases in the alumina-water system: II. Powder X-ray diffraction study of a new dense aluminum deuteroxide, δ-Al(OD)3

In this study we report the solution of the structure of δ-Al(OD)3. The crystal structure was determined using powder X-ray diffraction and direct methods, while the positions of deuterium atoms were estimated by difference-Fourier methods and MEM/Rietveld analysis. In the A-site deficient hydroxy-perovskite-type structure of δ-Al(OD)3, we find two types of one-dimensional H-bonding network.

Determination of Stability Filed of Delta‐AlOOH Under High Pressure and Temperature

We have investigated the stability field of delta‐AlOOH, which is high‐pressure polymorph of diaspore, using a laser‐heated diamond‐anvil cell. Delta‐AlOOH exists in the recovered samples from the pressure range of 40 to 130GPa, and it seems to decompose into Al2O3+ H2O above 1800K.