Elasticity of superhydrous phase B, seismic anomalies in cold slabs and implications for deep water transport

Abstract

Seismic anomalies in the vicinity of cold slabs, including low velocity zones and enhanced seismic shear wave splitting have been commonly attributed to the presence of hydrated slab material. The dense hydrous magnesium silicate (DHMS) superhydrous phase B (ShyB, Mg10Si3H4O18) is considered an important water carrier to transition zone depth due to its large pressure stability and abundance (up to 20vol.%) in hydrous peridotites. To interpret the observed seismic anomalies in terms of hydration and to assess the role of ShyB in deep water recycling, we have investigated the sound velocities and single-crystal elasticity of ShyB to pressures of 17.5GPa at ambient temperature by Brillouin scattering spectroscopy in diamond anvil cells. The Voigt–Reuss–Hill averages for the adiabatic bulk moduli KS, shear moduli μ and their pressure derivatives yield KS=150(2)GPa, μ=99(1)GPa, (∂KS/∂P)=4.7(2) and (∂μ/∂P)=1.44(5). The aggregate compressional and shear wave velocities of ShyB at transition zone pressures are comparable to those of hydrous iron-bearing ringwoodite but are significantly lower than anhydrous phases in peridotites. The calculated velocity contrast between dry and ShyB-bearing hydrous peridotite (containing 1.2wt.% of water) indicate that 17vol.% of ShyB decreases both compressional VP and shear VS wave velocities by at most 2.5% at transition zone depths. Our results, combined with data for the deformation mechanisms of ShyB, indicate that ShyB aggregates develop strong textures under down-dip compression regime and yield a maximum shear wave splitting of 1.1% in the plane perpendicular to the compression axis at 17.5GPa. Although ShyB in hydrous subducted peridotite would reduce its seismic velocities, the splitting geometry VSV>VSH generated by ShyB fabrics in peridotite is incompatible with the pattern observed in Tonga and Sangihe, and would require the contribution of other phases to be explained. Textured phase D is a plausible candidate to explain the shear wave splitting in the lower transition zone, while at intermediate transition zone depths, the coexistence of textured phase D and ShyB in hydrous deformed peridotite would generate the observed array polarization. However, the later scenario will require locally very high water concentrations at depth.