Sound velocity and density of liquid Fe-Ni-Si under pressure: Application to the composition of planetary molten core

Abstract:

The cores of Mercury, Mars and Moon are reported to be partially/totally molten (e.g., Margot et al. 2007, Yoder et al. 2003, Williams et al. 2001). In order to constrain the core compositions of those bodies from observed and future-planned seismic data, sound velocity and density of the core material, i.e., liquid Fe-alloy, are necessary. In this study, we have performed simultaneous measurements on these physical properties of liquid Fe-Ni–Si alloys, which is one of the major candidates for the core constituent. The effects of pressure and Si content on these properties were studied.

High pressure experiments were performed using 80-ton uniaxial press designed for CT measurement or 180-ton cubic type multi-anvil press installed at BL20XU and BL22XU beamlines of SPring-8 synchrotron facility, respectively. Used samples were Fe-Ni-Si with Si content of 10-30 at%. The sample pellet was sandwiched by the single crystal sapphire buffer rod for sound velocity measurement. P-wave sound velocity was measured using pulse-echo overlapping ultrasonic method. LiNbO₃ transducer was attached to the backside of the anvil to generate and receive elastic wave signals. Density was determined based on 3D volume data obtained from CT measurement or X-ray absorption profile.

The P-wave velocity (V_P) and density of liquid Fe-Ni-Si were successfully measured up to 2.5 GPa and 1773 K. Obtained V_P of the Fe-Ni-Si is found to increase rapidly with pressure below 1 GPa and increase gradually above 1 GPa. It is also found that V_P increases slightly with Si content on the density-V_P plot. These trends provide a constraint on the core composition of the planets and moon by comparing with observed data.