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,V-0 = 237.6( 2) angstrom(3), K-300,K-0 = 183(3) GPa, (partial derivative K-T,K-0/partial derivative T)(P) = - 0.033( 2) GPa K-1, a(0) = 3.32(5) x 10(-5) K-1, and b(0) = 1.09(1) x 10(-8) K-2, where a(0) and b(0) are coefficients describing the zero-pressure thermal expansion: alpha(T,0) = a(0) + b(0) 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.