High-pressure experiments and the phase diagram of lower mantle and core materials

The interpretation of seismic data and computer modeling requires increased accuracy in relevant material properties in order to improve our knowledge of the structure and dynamics of the Earth's deep interior. To obtain such properties, a complementary method to classic shock compression experiment s and theoretical calculations is the use of laser-heated diamond cells, wh ich are now producing accurate data on phase diagrams, equations of state, and melting. Data on one of the most important measurements, the melting te mperatures of iron at very high pressure, are now converging. Two other iss ues linking core properties to those of iron are investigated in the diamon d cell: One is the melting point depression of iron in the presence of ligh t elements, and the other is the structure of iron at the conditions of the inner core. First measurements on eutectic systems indicate a significant decrease in the melting point depression with increasing pressure, which is in contrast to previous predictions. X-ray diffraction measurements at sim ultaneously high pressure and high temperature have improved significantly with the installation of high-pressure "beam lines" at synchrotron faciliti es, and structural measurements on iron are in progress. Considerable effor ts have been made to develop new techniques to heat minerals at the conditi ons of the deep mantle in the diamond cell and to measure their phase relat ions reliably. Even measurements of the melting behavior of realistic rock compositions at high pressure, previously considered to be impossible in th e diamond cell, have been reported. The extrapolated solidus of the lower m antle intersects the geotherm at the core-mantle boundary, which may explai n the seismically observed ultra low velocity zone. The diamond cell has gr eat potential for future development and broad application, as new measurem ents on high-pressure-geochemistry at deep mantle and core conditions have opened a new field of research. There are, however, strict experimental req uirements for obtaining reliable data, which are summarized in the present paper. Results from recent measurements of melting temperatures and phase d iagrams of lower mantle and core materials at very high pressure are review ed.