THERMODYNAMICS OF IRON AND EARTH’S CORE
1999 (English)In: Technical Program of the 128 TMS Annual Meeting, 1999, 110- p.Conference paper (Refereed)
There are four well-known structural polymorphs of iron, namely alfa and delta (body centerd cubic), gamma (face centered cubic) and epsilon (hexagonal closest packed). The possible occurrence of a new iron phase was suggested by (1); the evidence was indirect, obtained through laser heating of iron above pressures of about 40 gigapascal (GPa). We found that the FCC phase (face centered cubic), when heated in its stability field, is quenchable at high pressures. Using this technique, we determined that the structure of the quenched phase at high pressures is DHCP (double hexagonal closest packed). Several experiments done using in-situ heating and x-ray have now confirmed the presence of this new phase (2). Although experimental data are sparse, it is possible to obtain a fairly quantitative thermodynamic description of all the iron phases and the double hexagonal closest packed (DHCP) beta iron. Earth’s core beginning at depths of 2900 Km (133 GPa) and continuing to the center (6730 Km, 360 GPa) consists principally of iron. Until experiments become feasible on iron above 200 GPa, extrapolation of thermodynamic data on iron is necessary for the study of the core. The assessed data on iron is based on experimental data on a) melting to 200 GPa, b) the location of the triple point HCP-DHCP-FCC at 36 GPa and 1450K and c) the location of the triple point DHCP-FCC-melt close to 60 GPa and 2800K. If no other phase transition intervenes, the melting of beta-iron at 360 GPa takes place at temperatures less than 5000K, which constrains the temperature of Earth’s center to be less than that. However there are some shock-wave data that cannot be easily reconciled with the current data obtained with the diamond-anvil cell technique unless there is yet another high P phase transition of the beta phase at about 200 GPa to increase the melting temperature by about 1000 at the center. A search for such a phase is now on.
1. S. K. Saxena, Shen, G. & Lazor, P. Science, 260, 1312-1314, (1993).
2. S. K. Saxena, L. S. Dubrovinsky and Haggkvist, P. Geophys. Res. Lett., 23, 2441-2444 (1996).
3. O.L. Anderson, Rev. Geophys. Suppl., 429-441, (1995).
Place, publisher, year, edition, pages
1999. 110- p.
IdentifiersURN: urn:nbn:se:uu:diva-22232OAI: oai:DiVA.org:uu-22232DiVA: diva2:50005