PROPERTIES OF IRON AT HIGH-PRESSURES AND THE STATE OF THE CORE

Shock-wave (Hugoniot) data for initially porous and nonporous samples of iron are inverted to yield values of the Gruneisen parameter (gamma), adiabatic bulk modulus (Ks), and coefficient of thermal expansion (alpha), along the Hugoniot to pressures of about 150 GPa (1.5 Mbar). This represents the first reliable estimate of thermal properties (e.g., alpha) to such high pressures based directly on experimental data. The values of gamma fit the conventional function gamma=gammao(v/vo)n but with n significantly larger than 1, while extrapolation formulas suggested to date for appear not to provide the best fit to the data. The present analysis yields values of gamma between about 1.4 and 1.0 (essentially temperature independent) and alpha between about 1.0 and 0.4 x 10-5 K-1 throughout the earth's core, therefore implying that simple dynamical models of the core are quite vaiable. These results provide experimental confirmation of the Vaschenko-Zubarev/Irvine-Stracey (or 'free-volume') formulation for gamma of iron at high pressures, as well as support for Stacey's recent model of the thermal state of the core. -Author