The Phase Diagram of Iron and the Temperature of the Inner Core

Birch was the pioneer of understanding the earth's core: that the core was mostly iron; that the solid inner core was nearly pure iron; that the liquid core had most of the impurities; and that the phase diagram (p.d.) of pure iron was complicated with several phases and triple points (t.p.). In 1972 he said, “5000°K, for the central temperature ... may have some standing as an upper limit.” I reconstruct the phase diagram accounting for new experiments and theories arising in the subsequent two decades. I stretch Birch's p.d. to account for the experiments of BOEHLER et al. (1990), BROWN and McQUEEN (B&M) (1982, 1986), and Yoo et al. (1992, 1993). BOEHLER (1993) reports the ε-γ-liquid t.p. at 100 GPa. This t.p. plus the solid-solid (ss) transition at 200 GPa reported by B&M require an additional high T phase, α', which has been suggested to be bcc. The new α' phase requires another t.p. near 190 GPa, which implies that the inner core is made largely of bcc iron. I suggest a Tm of about 6500°K for pure iron at the inner core-outer core boundary pressure. The Tm of the inner core itself will be affected by possible inner core impurities and by outer core impurities of larger concentration. To paraphrase Birch, 5700°K for the central temperature may have some standing as an upper limit.