The Earth's core and the phase diagram of iron

Abstract

The phase diagram of iron is presented for P < 330 GPa. The melting curve is derived from Stevenson’s generalized form of Lindemann’s law, successfully connecting the low-pressure (5-20 GPa) measurements to the new shock-wave measurements of 250 GPa. The isothermal equation of state of e-iron (h.c.p.) and y-iron (f.c.c.), indicate that the inner core density is that of pure solid iron. The present experiments cannot distinguish between the e or y phase for the inner core, but preference is given to y-iron. From these constructions, it is concluded that the melting temperature of iron at the inner core - outer core boundary pressure, T (i.c.b.), is 5200-6600 K. A likely model of the outer core temperature is presented by taking 5800 K as the probable value of T (i.c.b.), and assuming a temperature drop of 1000 K due to chemically induced melting point depression. This yields 3620 K for the T of the core side of the core-mantle boundary (c.m.b.). This model results in a large AjT(D"), (700 K), at the c.m.b., but the shock-wave data also allow other models where A!T(D") is less. A numerical experiment reveals that the value for A T(D") of 700 K does not lead to distortion of the density profile. The (y-8-liquid) triple point is beyond the i.c.b. Thus, diluted y-iron in the liquid phase constitutes the outer core. The experiments support a thermally driven model of the geomagnetic dynamo, and further support a model of a slowly freezing inner core for the energy source.