Hydrous SiO2 in subducted oceanic crust and H2O transport to the core-mantle boundary

Subduction of oceanic lithosphere transports surface H2O into the mantle. Recent studies show that dense SiO2 in the form of stishovite, an abundant mineral in subducted oceanic crust at depths greater than ∼270 km, has the potential to host and transport a considerable amount of H2O into the lower mantle, but the H2O storage capacity of SiO2 phases at high pressure and temperature remains uncertain. We investigate the hydration of stishovite and its higher-pressure polymorphs, β-stishovite and seifertite, with in situ X-ray diffraction experiments at high pressures and temperatures. The H2O contents in SiO2 phases are quantified based on observed increases in unit cell volume relative to the anhydrous SiO2 system. Density functional theory (DFT) computations permit calibration of water content as a function of volume change based on interstitial substitution of H2O. Regression of our experimental data indicates an H2O storage capacity in stishovite of ∼3.5 wt% in the transition zone and shallow lower mantle, decreasing to about 0.8 wt% at the base of the mantle. We find that SiO2-bearing subducted oceanic crust can accommodate all the H2O in slab lithosphere that survives sub-arc dehydration. Hydration of silica phases in subducted oceanic crust and their unparalleled capacity to host significant amounts of H2O even at high mantle temperatures provides a unique mechanism for transport and storage of water in the deepest mantle.