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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
Author(s) -
Chantel Julien,
Jing Zhicheng,
Xu Man,
Yu Tony,
Wang Yanbin
Publication year - 2018
Publication title -
journal of geophysical research: planets
Language(s) - English
Resource type - Journals
eISSN - 2169-9100
pISSN - 2169-9097
DOI - 10.1029/2017je005376
Subject(s) - liquidus , solidus , eutectic system , phase diagram , analytical chemistry (journal) , materials science , scattering , phase (matter) , mineralogy , chemistry , metallurgy , optics , microstructure , alloy , physics , organic chemistry , chromatography
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 Fe 3 P, 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 Fe 3 P) would undergo a bottom‐up crystallization, resulting in a growing solid inner core.