The Earth's Core and the Phase Diagram of Iron

Details The Earth's Core and the Phase Diagram of Iron The Earth's Core and the Phase Diagram of Iron

Aug 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982rspta.306...21a&link_type=abstract

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, Volume 306, Issue 1492,

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38

Scientific paper

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 ɛ -iron (h.c.p.) and γ -iron (f.c.c.), indicate that the inner core density is that of pure solid iron. The present experiments cannot distinguish between the ɛ or γ phase for the inner core, but preference is given to γ -iron. From these constructions, it is concluded that the melting temperature of iron at the inner core - outer core boundary pressure, Tmi (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 Tmi (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 Δ T (D' '), (700 K), at the c.m.b., but the shock-wave data also allow other models where Δ T (D' ') is less. A numerical experiment reveals that the value for Δ T (D' ') of 700 K does not lead to distortion of the density profile. The (γ -ɛ -liquid) triple point is beyond the i.c.b. Thus, diluted γ -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.

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