M411_31c 577..581

NATURE | VOL 411 | 31 MAY 2001 | www.nature.com 577 The circle shown in Fig. 3 shows an area where all three phases have been identi®ed in different runs. The centre of this circle is in excellent agreement with the g±b±perovskite+MgO triple point reported at 22.9 GPa and 2,260 K. The present data strongly support earlier estimates of the phaseboundary temperature between ringwoodite and perovskite of about 1,900 K at 24 GPa, the pressure at a depth of 660 km in the Earth's mantle. The measurements also show that phase relations in simple systems can be measured more reliably in laser-heated diamond cells than by using multi-anvil methods; this is because measurements of pressure and temperature are more accurate, and the method provides the capability to use an inert and nearly hydrostatic pressure environment. Moreover, phase boundaries can be measured over much larger temperature ranges. Additional constraints on the phase diagram will have to come from calorimetric and spectroscopic measurementsÐas shown for the MgSiO3 high-pressure polymorphs. Larger samples and multi-anvil devices will be required for more complex systems, but pressure estimates will need to be improved by the systematic investigation of high-temperature equations of state of inert pressure calibrants such as gold. These could be provided by synchrotron X-ray measurements in laser-heated diamond cells. M