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 |
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| Authors: | Norimasa Nishiyama Robert Paul Rapp Tetsuo Irifune Takeshi Sanehira Daisuke Yamazaki Ken-ichi Funakoshi |
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| Institution: | (1) Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan;(2) Japan Synchrotron Radiation Research Institute, Mikazuki, Sayo-gun, Hyogo 679-5198, Japan;(3) Present address: GeoSoilEnviroCARS, The University of Chicago, S. Ellis Ave., Chicago, IL 60637, USA;(4) Building 434A, 9700 South Cass Avenue, Argonne, IL 60439, USA |
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| Abstract: | 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) Å3, K 300,0=183(3) GPa, (?K T,0/?T) P =?0.033(2) GPa K?1, a 0=3.32(5)×10?5 K?1, and b 0=1.09(1)×10?8 K?2, where a 0 and b 0 are coefficients describing the zero-pressure thermal expansion: α 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. |
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| Keywords: | K-hollandite High pressure and temperature Phase boundary Equation of state Continental crust |
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