High-pressure elasticity of δ-(Al,Fe)OOH single crystals and seismic detectability of hydrous MORB in the shallow lower mantle

Oxyhydroxides like δ-(Al,Fe)OOH may stabilize “water” in Mid-Ocean Ridge Basalts (MORB) subducted into the Earth’s lower mantle. The single crystal elasticity of δ-(Al,Fe)OOH has not been experimentally constrained, hampering an accurate evaluation of the seismic detectability of this high-pressure solid solution, and by extrapolation the presence of “water”, in the deep Earth. Here, we report the first experimental single-crystal elasticity results of δ-(Al0.97Fe0.03)OOH measured by X-ray diffraction and Brillouin spectroscopy. Based on our results, seismic properties of hydrous and anhydrous MORB were computed at pressures and temperatures expected in slabs at shallow lower mantle conditions. We show that hydrous MORB are less dense than anhydrous MORB, but have faster aggregate seismic velocities. This suggests that hydration in MORB has an effect opposite to that observed in other lithologies, and further indicates that hydration of MORB increases the seismic contrast to the background mantle.