Abstract
AbstractWe 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.
Highlights
Terrestrial planets and several rocky/icy satellites have metallic cores composed mainly of iron with small amounts of non-metallic light elements such as S, C, Si, O, H, etc. (Birch, 1947)
We have developed a new technique for determining the liquidus and eutectic temperatures of Fe-light element alloys at high pressures in a multi-anvil apparatus, by studying ultrasonic wave propagation through the sample
5wt%P, Fe-10wt%P, and Fe3P (Fe-15.6wt%P). These compositions were chosen to fall within the expected iron-rich and iron-poor liquidus loops in the Fe-P binary phase diagram at the experimental conditions, with Fe-10wt%P being close to the expected eutectic composition
Summary
Terrestrial planets and several rocky/icy satellites have metallic cores composed mainly of iron with small amounts of non-metallic light elements such as S, C, Si, O, H, etc. (Birch, 1947). Phase diagrams involving melting have been investigated for several Fe-alloying systems including Fe-Ni (Zhang et al, 2016), Fe-S (e.g., Stewart and Schmidt 2007; Chen et al 2008; Buono and Walker 2011), Fe-C (e.g., Chabot et al, 2008; Fei and Brosh, 2014), and Fe-Si (e.g., Kuwamaya and Hirose 2004), at pressure and temperature conditions relevant to planetary cores. These diagrams were mostly constrained by quench experiments conducted in a large volume hydraulic press such as a multianvil apparatus. Phase diagrams of Fe-alloys are only available at a very limited number of pressure conditions
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