Pressure Dependence of the Liquidus and Solidus Temperatures in the Fe-P Binary System Determined by In Situ Ultrasonics: Implications to the Solidification of Fe-P Liquids in Planetary Cores

Abstract

We have developed a new technique for determining the liquidus and eutectic (or solidus) temperatures of Fe-light element alloys at high pressures in a multianvil apparatus, by studying ultrasonic wave propagation through the sample. While the onset of melting is manifested by the loss of both compressional (P-) and shear (S-) wave signals due to the scattering of sound waves by partial melts, the completion of melting is confirmed by the reappearance of the P wave signal when the scattering due to residual crystals disappears. By applying this technique to the Fe-P binary system with three different phosphorus contents, we were able to constrain the Fe-rich portion of the phase diagram up to 7 GPa and 1,733 K. Our results show that for phosphorus-poor compositions, ranging from Fe-5wt%P to the eutectic composition, the liquidus temperature exhibits a weak negative pressure dependence (dT/dP = −10.4 K GPa−1 for Fe-5wt%P). While for the phosphorus-richer compositions, including Fe-10wt%P and Fe3P, the liquidus temperature increases significantly with pressure (dT/dP = 71.3 and 62.5 K GPa−1, respectively). This indicates a shift of the eutectic composition to lower phosphorus contents with increasing pressure. Consequently, molten metallic cores of planetary bodies with phosphorus contents ranging from Fe-5wt%P to the eutectic composition would start crystallization from the top of the core and proceed downward. Whereas cores with phosphorus-richer compositions (Fe-10wt%P to Fe3P) would undergo a bottom-up crystallization, resulting in a growing solid inner core.