Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001pepi..128..179s&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 128, Issue 1-4, p. 179-193.
Physics
12
Scientific paper
A critical appraisal of finite strain theories used in geophysics, emphasising the importance of derivative properties, especially /K'=dK/dP (/K is bulk modulus and /P is pressure), leaves only two survivors, Keane's equation and the reciprocal /K' equation, by which /1/K' varies linearly with /P/K. The two most used relationships, by Birch and Rydberg, are fundamentally flawed, as is the argument that the infinite pressure asymptote, K∞', should have the value 5/3, corresponding to a Thomas-Fermi gas. A study of the PREM data for the core restricts K∞' for core material to a value close to 3.0, confirming an earlier suggestion. Extrapolation of core data to /P=0 gives zero pressure values of /K', /K and density, /ρ at the foot of the core adiabat: 5.2<=K0'<=5.4, 108GPa<=K0 <=123GPa, 6420kgm-3<=ρ0<=6570kg m-3. These values do not agree well with reports on K0' and K0 for laboratory liquid iron. At least part of the explanation appears to be a structural difference between liquid iron at core pressures and /P=0, mirroring the different structures of the solid. This limits the accuracy with which equations of state can, at present, be fitted to outer core data. Inner core properties, apart from density, must be very similar to those of /ɛ-iron but comparison with high pressure measurements on iron reveals an incompatibility in bulk moduli that is most easily explained as pressure calibration errors in the laboratory data. This means that the equation of state for platinum, used as a reference standard to estimate pressure, requires reconsideration. This could involve comparison of laboratory data for iron with seismological data for the core, where pressure is more precisely known. The estimated zero pressure density for inner core material is 7850kgm-3, at least 5% less than the extrapolated density of /ɛ-iron, indicating a light element content more than half of that in the outer core.
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