Stability and P-V-T equation of state of KAlSi₃O₈ -hollandite determined by in situ X-ray observations and implications for dynamics of subducted continental crust material

In situ X-ray diffraction measurements of KAlSi₃O₈ -hollandite (K-hollandite) were performed at pressures of 15-27 GPa and temperatures of 300-1,800 K using a Kawai-type apparatus. Unit-cell volumes obtained at various pressure and temperature conditions in a series of measurements were fitted to the high-temperature Birch-Murnaghan equation of state and a complete set of thermoelastic parameters was obtained with an assumed K′_300,0 = 4. The determined parameters are V_300,0 = 237.6(2) ų, K_300,0=183(3) GPa, (∂K_T,0/∂T)_P = -0.033(2) GPa K^-1, a₀ = 3.32(5)×10^-5 K^-1, and b₀ = 1.09(1)×10^-8 K^-2, where a₀ and b₀ are coefficients describing the zero-pressure thermal expansion: α_T,0 = a₀ + b₀ T. We observed broadening and splitting of diffraction peaks of K-hollandite at pressures of 20-23 GPa and temperatures of 300-1,000 K. We attribute this to the phase transitions from hollandite to hollandite II that is an unquenchable high-pressure phase recently found. We determined the phase boundary to be P (GPa) = 16.6 + 0.007 T (K). Using the equation of state parameters of K-hollandite determined in the present study, we calculated a density profile of a hypothetical continental crust (HCC), which consists only of K-hollandite, majorite garnet, and stishovite with 1:1:1 ratio in volume. Density of HCC is higher than the surrounding mantle by about 0.2 g cm^-3 in the mantle transition zone while this relation is reversed below 660-km depth and HCC becomes less dense than the surrounding mantle by about 0.15 g cm^-3 in the uppermost lower mantle. Thus the 660-km seismic discontinuity can be a barrier to prevent the transportation of subducted continental crust materials to the lower mantle and the subducted continental crust may reside at the bottom of the mantle transition zone.