The Melting Relations of Iron, and Temperatures in the Earth's Core

Abstract

Thermodynamic data on the phases of iron are assembled in the form of speculative projections on the temperature-volume plane. Following a proposal by Kraut and Kennedy, the melting curves, for each phase, are assumed to be straight lines in this representation. As the position of the triple point, gamma-epsilon-liquid, is unknown, two hypothetical locations are considered: (1) at about 4 Mb, (2) at about 1 Mb. In the former case, the gamma phase is in equilibrium with the liquid at core pressures, in the latter, the epsilon phase. Nothing is known directly about the slopes of these melting curves, but the gamma melting temperatures are estimated to lie some 700° above the projection given by Higgins and Kennedy, with epsilon melting temperatures still higher. The relation of the isentropes of the liquid to these melting curves is also unknown, but ‘conventional’ isentropes, originating on the melting curve, may be found which lie in the liquid field at the pressures of the outer core. Application to the Earth is complicated by the requirement for a light alloying element in the outer core; if the alloying element is silicon, a proportion of about 15 per cent by weight accounts satisfactorily for the density-velocity relation. The phase relations may be considerably modified by the presence of so much silicon: it is conjectured that the gamma phase is suppressed or confined to low pressures, that the epsilon phase will be the stable solid phase at core pressures, and that the isentrope originating at the melting temperature of the inner core-outer core boundary lies entirely in the liquid phase.