On the (?, ?, ?) Triple Point of Iron and the Earth's Core

Abstract

Summary The triple point (γ, ɛ, l) of iron has been located at 0.94±0.20 Mbar and 2970 ± 200°C by means of extrapolations of recent data on the melting temperature of the γ phase of iron to 200 kbar and of the revised γ - ɛ phase boundary. The slope for the ɛ - l boundary is calculated to be 1.4±0.3 deg/kbar at the triple point. For a core composition dominated by iron and for all realistic estimates of the slope of the γ - ɛ phase boundary, the ɛ phase appears to be the appropriate iron phase for the inner core. Using the linear relationship between melting temperature and volume compression of the solid phase proposed by Kraut and Kennedy, the melting temperature of the ɛ iron at a pressure corresponding to the mantle-core boundary is calculated to be 3500 ± 200°C and that at a pressure corresponding to the inner-outer core boundary to be 4650 ± 500°C. The present calculated melting temperature difference throughout the outer core is more than twice as great as that estimated by Higgins and Kennedy who have not considered the effect of the γ - ɛ transformation on the melting curve. Hence, the obstacle preventing adiabatic convection in the liquid outer core has been removed without resort to the suggestions of other investigators.