High pressure and high temperature equation of state of gamma and liquid iron

Abstract

Shock-wave experiments on pure iron preheated to 1573 K were conducted in the 17–73 GPa range. The shock-wave equation of state of γ-iron at an initial temperature of 1573 K can be fit with u_s = 4.102 (0.015) km/s + 1.610(0.014) u_p with ρ_o = 7.413±0.012 Mg/m^3 We obtain for γ-iron's bulk modulus and pressure derivative the values: 124.7±1.1 GPa and 5.44±0.06, respectively. We present new data for sound velocities in the γ- and liquid-phases. In the γ-phase, to a first approximation, the longitudinal sound velocity is linear with respect to density: V_p = −3.13 (0.72) + 1.119(0.084) p(units for V_p and p are km/s and Mg/m^3, respectively). Melting was observed in the highest pressure (about 70–73 GPa) experiments at a calculated shock temperature of 2775±160 K. This result is consistent with a previously calculated melting curve (for ε-iron) which is close to those measured by Boehler [1] and Saxena et al. [2]. The liquid iron sound velocity data yields a Grüneisen parameter value of 1.63±0.28 at 9.37±0.02 Mg/m^3 at 71.6 GPa. The quantity γ_ρ is 15.2±2.6 Mg/m^3, which agrees with the uncertainty bounds of Brown and McQueen [3] (13.3–19.6 Mg/m^3). Based on upward pressure and temperature extrapolation of the melting curve of γ-iron, the estimated inner core-outer core boundary temperature is 5500±400 K, the temperature at the core-mantle boundary on the outer core side is 3930±630 K.