In Situ Studies of Iron under Pressure: New Windows on the Earth's Core

Abstract

Experiments on iron with recently developed high-pressure techniques are providing new insights into the nature of the Earth's core. In situ high P-T synchrotron x-ray diffraction studies to above 160 GPa and 3000 K with double-sided, laser-heated diamond-anvil cells demonstrate that the hexagonal closed packed phase (ϵ-Fe) has a wide stability field extending from deep mantle to core conditions. Direct measurements of the melting line of ϵ-Fe to above 100 GPa with these techniques provide constraints on the temperature and density at the inner core boundary. Radial x-ray diffraction measurements carried out at room temperature to 220 GPa have constrained the elasticity, rheology, and acoustic velocities of ϵ-Fe at core pressures. Vibrational properties providing information on thermodynamic and elastic parameters have been measured to above 150 GPa using both synchrotron x-ray and laser Raman spectroscopies. X-ray diffraction studies of liquid iron reveal changes in short-range structure as functions of pressure and temperature. Insight into the problem of the identity and abundance of other (e.g., light) elements in the core has been obtained from direct studies of the Fe-O, Fe-S, and Fe-H binary systems as well as observations for several other key elements. Progress using various new techniques is reviewed, and new directions and questions are examined. We also compare recent experimental findings with a variety of theoretical predictions.