The Fate of Subducted Sediment at the Base of the Mantle Transition Zone: Water and the K-hollandite → NAL Phase Transformation
Abstract:Seismic tomography of subduction zones suggests that slabs of oceanic lithosphere can accumulate at the base of the mantle transition zone (MTZ), and primary mantle plumes originating in the lower mantle often pick up a geochemical signature of deeply recycled crust as they transit through the MTZ on their way to form hot-spot related chains of ocean-islands. Here we report the results of phase equilibria experiments on natural starting materials representative of "continental sediment" with variable amounts of H2O present (0.8, 3.3, and 6.0 wt%), carried out in the multi-anvil apparatus at pressures appropriate to the base of the MTZ (~23-24 GPa), and temperatures ~1200-1600°C. At the lowest H2O contents , the phase assemblage consists of K-hollandite (KAlSi3O8)+Ti-perovskite (Mg,Fe)(Si,Al,Ti)O3+Stishovite (SiO2)+Corundum (Al2O3)+H2O-Fluid, but in the presence of modest amounts of water (~3-6 wt%), K-hollandite and Ti-perovskite break down to form an assemblage comprised of potassic NAL-phase (KAlSiO4)+Stish+H2O-rich melt. This suggests that K-rich NAL-phase will form in deeply subducted continental material by a peritectic melting reaction of the general form: KAlSi3O8 ↔ KAlSiO4 + 2·SiO2. Ti-perovskite also participates as a reactant phase, and significant substitution of Fe, Mg, and Ti into the crystal structure of the new NAL-phase is observed. The reconstructed compositions of K-rich NAL-phase inclusions in (ultradeep) diamonds from the base of the MTZ closely resemble those of the potassic NAL-phases produced in our experiments, providing direct evidence for recycling of "continental" material to the base of the MTZ and a hydrous MTZ.
The elastic properties of K- and Na-bearing hollandite and NAL phases were also investigated using first principles simulations. Although preliminary calculations indicate that these phases have similar bulk and shear modulus at ambient conditions, the pressure derivative of the shear modulus for K-hollandite is lower than that for the K-NAL phase, due to the 2nd order phase-transition (from tetragonal to monoclinic space group symmetry) that K-hollandite undergoes at ~23-25 GPa. Thus, the breakdown of K-hollandite to NAL phase and stishovite is likely to cause an increase in the shear wave velocity of sediment lithologies at the base of the MTZ.