Shock-induced superheating and melting curves of geophysically important minerals

Shock-state temperature and sound-speed measurements on crystalline materials, demonstrate superheating-melting behavior distinct from equilibrium melting. Shocked solid can be superheated to the maximum temperature, T-c'. At slightly higher pressure, P-c shock melting occurs, and induces a lower shock temperature, T-c. The Hugoniot state, (P-c, T-c), is inferred to fie along the equilibrium melting curve. The amount of superheating achieved on Hugoniot is, Theta(H)(+) = T-c'/T-c - 1. Shock-induced H superheating for a number of silicates, alkali halides and metals agrees closely with the predictions of a systematic framework describing superheating at various heating rates [Appl. Phys. Lett. 82 (12) (2003) 1836]. High-pressure melting curves are constructed by integration from (P-c, T-c) based on the Lindemann law. We calculate the volume and entropy changes upon melting at (P-c, T-c) assuming the R In 2 rule (R is the gas constant) for the disordering entropy of melting [J. Chem. Phys. 19 (1951) 93; Sov. Phys. Usp. 117 (1975) 625; Poirier, J.P., 1991. Introduction to the Physics of the Earth's Interior. Cambridge University Press, Cambridge, 102 pp.]. (P-c, T-c) and the Lindemann melting curves are in excellent accord with diamond-anvil cell (DAC) results for NaCl, KBr and stishovite. But significant discrepancies exist for transition metals. If we extrapolate the DAC melting data [Phys. Rev. B 63 (2001) 132104] for transition metals (Fe, V, Mo, W and Ta) to 200-400 GPa where shock melting occurs, shock temperature measurement and calculation would indicate Theta(H)(+) similar to 0.7-2.0. These large values of superheating are not consistent with the H superheating systematics. The discrepancies could be reconciled by possible solid-solid phase transitions at high pressures. In particular, this work suggests that Fe undergoes a possible solid-solid phase transition at similar to200 GPa and melts at similar to270 GPa upon shock wave loading, and the melting temperature is similar to6300 K at 330 GPa. (C) 2004 Elsevier B.V. All rights reserved.