Detecting deeply subducted crust from the elasticity of hollandite

Subduction of differentiated continental and oceanic crusts through sediments and basalt to the deep mantle has been shown to be a likely source for the geochemical signature of ocean island basalts that are enriched in large ion lithophile elements such as K, Na, Rb, and Sr. At high pressure such a lithology will consist of stishovite, majorite and hollandite, where hollandite (KAlSi3O8) can readily host the large ion lithophile elements, and is hence a geochemically important phase. Here we study the elasticity of hollandite up to lower mantle pressure by electronic structure simulations and attempt to constrain the volume percent of hollandite in a subduction zone environment. In agreement with experiments we predict a phase transition from a low pressure tetragonal phase to a high pressure monoclinic phase at 33 GPa. The phase transition has significant effects on the elastic properties of hollandite, with an increase in shear modulus of 10%. Based on the computed reflection coefficient across the transition and observed reflectance for mid-mantle seismic scatterers (920 km discontinuity) we constrain the maximum volume of hollandite to be around 5% in a subduction zone environment.