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

Norimasa Nishiyama, Robert Paul Rapp, Tetsuo Irifune, Takeshi Sanehira, Daisuke Yamazaki, Ken Ichi Funakoshi

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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 V300,0 = 237.6(2) Å3, K300,0=183(3) GPa, (∂KT,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: αT,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.

Original languageEnglish
Pages (from-to)627-637
Number of pages11
JournalPhysics and Chemistry of Minerals
Volume32
Issue number8-9
DOIs
Publication statusPublished - Dec 1 2005
Externally publishedYes

Keywords

  • Continental crust
  • Equation of state
  • High pressure and temperature
  • K-hollandite
  • Phase boundary

ASJC Scopus subject areas

  • Materials Science(all)
  • Geochemistry and Petrology

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