Temperatures in the Earth's core from melting-point measurements of iron at high static pressures

Abstract

THE temperature distribution in the Earth's core places important constraints on the Earth's internal heat budget and on models of the geodynamo. The solid inner core crystallizes from a liquid outer core, consisting mainly of iron alloyed with a lighter element, at a depth of about 5,100 km (corresponding to a pressure of about 3.3 Mbar). Thus, the most reliable means of determining the temperature gradient in the core is to estimate the melting temperature of iron and iron-rich compounds at the pressure of the inner core boundary. Current estimates range from about 4,000 to 8,000 K; but these estimates, obtained from shock compression^1–3, theory (discussed in ref. 4) and extrapolation of static pressure data^2,3,5, are poorly constrained. Here I present melting-point measurements on iron and iron–oxygen compounds at static pressures of up to Mbar. Extrapolation of these results to 3.3 Mbar yields a temperature at the inner-core boundary of 4,850±200 K. A weak change in optical absorption observed above 2,000 K may correspond to the solid–solid phase transition found in shock experiments at 2 Mbar (ref. 1).