EQUATION OF STATE OF HYDROUS RINGWOODITE FO89 TO 50 GPA DETERMINED BY SYNCHROTRON X-RAY POWDER DIFFRACTION

Using synchrotron X-ray powder diffraction and diamond-anvil cell techniques, pressure-volume (P-V) measurements have been carried out to 50 GPa on a hydrous Fo₈₉ ringwoodite (Mg _1.70 Fe_0.20 ²⁺ Fe_0.02 ³⁺ H _0.13 SiO₄) synthesized at 20 GPa and 1400° C and containing 0.79 wt. % H₂O, at GSECars, Advanced Photon Source, Argonne National Laboratory.  Two runs were made using different pressure media: 4:1 methanol-ethanol mixture and Ar. In general, the pressure-volume data obtained in the Ar pressure medium showed more scatter than in the case of methanol-ethanol pressure medium, mainly as a result of some overlapping  of the Ar and the ringwoodite diffraction peaks.

The crystal structure of this specimen has been defined and its compression measured by single-crystal X-ray diffraction by Smyth et al. (in press). They have reported the measured bulk modulus K_o and its pressure derivative K_o´ as 169.0 ± 3.4 and 7.9 ± 0.9, respectively, based on measurements to 11.2 GPa. Using the Birch-Murnaghan equation of state and fixing V_o=535.39ų, we obtain from the compression data in methanol-ethanol K_o=182 GPa and K_o´=5.54. Further, by fixing V_o=535.39 ų and K_o=167 GPa (ultrasonic value, Jacobson et al., in press), we obtain K_o´=6.18± 0.16. The lowering of the K_o´ at higher pressure would imply that the volume occupied by water, principally as octahedral cation vacancy, is highly compressible at low to moderate pressures, but less compressible at higher pressures. The implications for this effect are discussed in terms of water in the mantle.