TY - JOUR
T1 - 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
AU - Nishiyama, Norimasa
AU - Rapp, Robert Paul
AU - Irifune, Tetsuo
AU - Sanehira, Takeshi
AU - Yamazaki, Daisuke
AU - Funakoshi, Ken Ichi
PY - 2005/12/1
Y1 - 2005/12/1
N2 - 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.
AB - 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.
KW - Continental crust
KW - Equation of state
KW - High pressure and temperature
KW - K-hollandite
KW - Phase boundary
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U2 - 10.1007/s00269-005-0037-y
DO - 10.1007/s00269-005-0037-y
M3 - Article
AN - SCOPUS:29544445057
VL - 32
SP - 627
EP - 637
JO - Physics and Chemistry of Minerals
JF - Physics and Chemistry of Minerals
SN - 0342-1791
IS - 8-9
ER -