Compressibility of hydrous wadsleyite (<i>β</i>‐phase) in Mg₂SiO₄ by high pressure X‐ray diffraction

Abstract

Isothermal compressibility of hydrous wadsleyite has been measured for the first time by using in‐situ X‐ray diffraction in a diamond anvil apparatus at pressures up to 8.5 GPa of a sample which was synthesized at 15.5 GPa and 1300°C under hydrous conditions. The hydrous wadsleyite sample included 2.5 ± 0.3 wt% of H 2 O as determined by secondary ion mass spectroscopy. The cell parameters of the present hydrous wadsleyite are fit to the orthorhombic symmetry within error. The linear compressibilities along a ‐ and c ‐axes of the present hydrous wadsleyite are very similar to those of the anhydrous wadsleyite, but the liner compressibility along the b ‐axis of the present hydrous wadsleyite is ∼15% larger than that of the anhydrous wadsleyite. Consequently, the calculated bulk modulus (K 0 ) is 155 ± 2 GPa with the pressure derivative (K′ 0 ) fixed to 4.3. This value is 5∼11% smaller than that of anhydrous wadsleyite. On the basis of the above result, the seismic discontinuity around 400 km is discussed.