THERMAL PRESSURE IN THE LASER-HEATED DIAMOND-ANVIL CELL - AN X-RAY-DIFFRACTION STUDY

Using X-ray diffraction with synchrotron radiation, we have studied th e pressure changes induced by laser heating on samples compressed in a diamond-anvil cell. The method has been to compare experimentally obs erved phase transitions of Mg2SiO4 and SiO2 polymorphs with well-const rained phase diagrams and equations of state reported in the literatur e. Our results clearly demonstrate an increase of pressure in the lase r hot spot with respect to the nominal pressure measured from the ruby fluorescence at room temperature. At 2200 +/- 100 K, for instance, wa dsleyite has been synthesized from forsterite at a nominal pressure of 11 Gpa, which is 4 GPa lower than the reported transition pressure. I n addition, the measured high-pressure, high-temperature molar volumes of forsterite and wadsleyite appear much smaller than those calculate d from available thermoelastic data. Taking into account this pressure increase, we reconcile conflicting experimental determinations of the coesite-stishovite transition made with multi-anvil press and diamond -anvil cell. The pressure change induced by laser heating is a functio n of the product of the thermoelastic coefficients alpha (thermal expa nsion) and K-T (bulk modulus) of the sample. We thus stress the need f or an internal pressure standard, such as Pt, Au or MgO for determinin g equations of state and phase equilibria under the P-T conditions pre vailing in the Earth's mantle and core.