THERMODYNAMIC PROPERTIES FOR IRON AT VERY HIGH PRESSURES: IMPLICATIONS FOR THE EARTH'S CORE

The most widely held hypothesis is that the earth's core is predominately iron. However, since temperatures and pressures within the core are well above those normally achieved in experimental work, the behavior of pure iron under core conditions has been somewhat uncertain. Results from two shock wave experimental programs are reported, giving new data for the thermodynamics, elastic properties, and melting behavior of iron under pressures in excess of 100 GPa. These data are combined in a preliminary thermal model for the core.;Using a new experimental technique, melting in iron has been detected at high pressure. Release wave velocities were measured with the overtaking rarefaction method. Upon melting the shear modulus becomes zero leading to a decrease in the acoustic wave velocity behind the shock front. Two velocity discontinuities have been detected: (1) 200 (+OR-) 2 GPa at a calculated temperature near 4400 K and (2) 250 (+OR-) 10 GPa at a calculated temperature between 5000 and 6300 K. The most plausible interpretation is that the (epsilon)-(gamma) transition occurs at 200 GPa while (gamma)-iron melts near 250 GPa along the iron Hugoniot.;Elastic constants for (epsilon)-iron increase along the Hugoniot between 40 and 200 GPa. The bulk modulus ranges from 331 (+OR-) 9 GPa to 913 (+OR-) 53 GPa while the shear modulus increases from 137 (+OR-) 16 to 183 (+OR-) 54 over this pressure range. The fairly high value of Poisson's ratio ((sigma) = .41) for (epsilon)-iron at 200 GPa is consistent with earth model values for the solid inner core.;A tentative phase diagram gives a melting temperature for pure iron between 5500 K and 7200 K at a pressure equivalent to that at the inner core boundary. The Lindemann melting estimate of 6600 K, using porous iron thermodynamic data, is intermediate to these bounds. The ((epsilon)-(gamma)- ) triple point falls at a pressure near that of the inner core boundary, suggesting that the solid iron phase in the inner core is likely to be (epsilon)-iron.;Hugoniot measurements on porous iron are used to define thermodynamic properties. By differencing pressure and internal energy states between iron Hugoniots of different initial density, the quantity dE/dP(VBAR)(,v) is determined. Over a range of stock densities between 8 and 12 Mg/m('3) a constant value for dE/dP(VBAR)(,v) of 0.06 (+OR-) .01 m('3)/Mg is suggested. Trends in the data may reflect systematic thermal effects and a simple model for the temperature dependence is applied to the data. A change in dE/dP(VBAR)(,v) of between 2 and 4% per 1000 K is noted. Thermodynamic properties calculated using the temperature dependent model are only marginally different from those calculated with a constant value for dE/dP(VBAR)(,v).;Since the outer core is 6 to 10% less dense than pure iron under equivalent conditions, the melting temperature of pure iron is an upper bound for temperatures at the inner core boundary. A temperature near 5000 K is considered more reasonable. This leads to an estimate of 3800 (+OR-) 1000 K for the temperature at the coremantle boundary.